GURU AAK DEV EGIEERIG COLLEGE, LUDHIAA
TO WHOM IT MAY COCER
I hereby certify that Pushkar Shanker Roll No 6140508670 of Guru Nanak Dev Engineering College, Ludhiana has undergone six weeks industrial training from 23rd June, 2008 to 1st August, 2008 at our organization to fulfill the requirements for the award of degree of B.Tech Electrical Engineering. He works on Power Plant Overview project during the training under the supervision of Mr. G. D. Sharma. During his tenure with us we found him sincere and hard working. We wish him a great success in the future.
Signature of the Student
Signature of the SUPERVISOR (S)
(Seal of Organization)
ACKOWLEDGEMET The authors are highly grateful to the Dr. M. S. Saini, Principal, Guru Nanak Dev Engineering College (GNDEC), Ludhiana, for providing this opportunity to carry out the six weeks industrial training at National Thermal Power Corporation, New Delhi. The constant guidance and encouragement received from Er. K. S. Mann Dean T&P, GNDEC Ludhiana has been of great help in carrying out the project work and is acknowledged with reverential thanks. The authors would like to express a deep sense of gratitude and thanks profusely to Mr. R. S. Sharma, CMD of the Company, without the wise counsel and able guidance, it would have been impossible to complete the report in this manner. The help rendered by Ms Rachana Singh Bhal, Supervisor, National Thermal Power Corporation for experimentation is greatly acknowledged. The author expresses gratitude to the HOD and other faculty members of Department of Electrical Engineering of GNDEC for their intellectual support throughout the course of this work. Finally, the authors are indebted to all whosoever have contributed in this report work and friendly stay at Badarpur Thermal Power Station, New Delhi.
Pushkar Shanker
CONTENT 1. Introduction to the Company a. About the Company b. Vision c. Strategies d. Evolution 2. Introduction to the Project 3. Project Report a. Operation i. Introduction ii. Steam Boiler iii. Steam Turbine iv. Turbine Generator b. EMD – I i. Coal Handling Plant ii. Motors iii. Switchgear iv. High Tension Switchgear v. Direct On Line Starter c. EMD – II i. Generator ii. Protection iii. Transformer 4. Reference
ITRODUCTIO TO THE COMPAY • About the Company • Vision • Strategies • Evolution
ational Thermal Power Corporation Limited Badarpur Thermal Power Station Badarpur, ew Delhi
ABOUT THE COMPANY NTPC, the largest power Company in India, was setup in 1975 to accelerate power development in the country. It is among the world’s largest and most efficient power generation companies. In Forbes list of World’s 2000 Largest Companies for the year 2007, NTPC occupies 411th place.
A View of Badarpur Thermal Power Station, ew Delhi NTPC has installed capacity of 29,394 MW. It has 15 coal based power stations (23,395 MW), 7 gas based power stations (3,955 MW) and 4 power stations in Joint Ventures (1,794 MW). The company has power generating facilities in all major regions of the country. It plans to be a 75,000 MW company by 2017. NTPC has gone beyond the thermal power generation. It has diversified into hydro power, coal mining, power equipment manufacturing, oil & gas exploration, power trading & distribution. NTPC is now in the entire power value chain and is poised to become an Integrated Power Major. NTPC's share on 31 Mar 2008 in the total installed capacity of the country was 19.1% and it contributed 28.50% of the total power generation of the country during 2007-08. NTPC has set
new benchmarks for the power industry both in the area of power plant construction and operations. With its experience and expertise in the power sector, NTPC is extending consultancy services to various organizations in the power business. It provides consultancy in the area of power plant constructions and power generation to companies in India and abroad. In November 2004, NTPC came out with its Initial Public Offering (IPO) consisting of 5.25% as fresh issue and 5.25% as offer for sale by Government of India. NTPC thus became a listed company with Government holding 89.5% of the equity share capital and rest held by Institutional Investors and Public. The issue was a resounding success. NTPC is among the largest five companies in India in terms of market capitalization.
Recognizing its excellent performance and vast potential, Government of the India has identified NTPC as one of the jewels of Public Sector 'Navratnas'- a potential global giant. Inspired by its glorious past and vibrant present, NTPC is well on its way to realize its vision of being "A world class integrated power major, powering India's growth, with increasing global presence".
VISION A world class integrated power major, powering India's growth with increasing global presence.
Mission Develop and provide reliable power related products and services at competitive prices, integrating multiple energy resources with innovative & Eco-friendly technologies and contribution to the society
View of a well flourished power plant
Core Values - BCOMIT • Business ethics • Customer Focus • Organizational & Professional Pride • Mutual Respect & Trust • Innovation & Speed • Total Quality for Excellence
STRATEGIES
Technological Initiatives
Introduction of steam generators (boilers) of the size of 800 MW
Integrated Gasification Combined Cycle (IGCC) Technology
Launch of Energy Technology Center -A new initiative for development of technologies with focus on fundamental R&D
The company sets aside up to 0.5% of the profits for R&D
Roadmap developed for adopting ‘Clean Development
Mechanism’ to help get / earn ‘Certified Emission Reduction
Corporate Social Responsibility
As a responsible corporate citizen NTPC has taken up number of CSR initiatives
NTPC Foundation formed to address Social issues at national level
NTPC has framed Corporate Social Responsibility Guidelines committing up to 0.5% of net profit annually for Community Welfare Measures on perennial basis
The welfare of project affected persons and the local population around NTPC projects are taken care of through well drawn Rehabilitation and Resettlement policies
The company has also taken up distributed generation for remote rural areas
Environment Management Management
All stations of NTPC are ISO 14001 certified
Various groups to care of environmental issues
The Environment Management Group
Ash Utilization Division
Afforestation Group
Centre for Power Efficiency & Environment Protection
Group on Clean Development Mechanism
TPC is the second largest owner of trees in the country after the Forest department.
Partnering government in various initiatives
Consultant role to modernize and improvise several plants across the country
Disseminate technologies to other players in the sector
Consultant role “Partnership in Excellence” Programme for improvement of PLF of 15 Power Stations of SEBs.
Rural Electrification work under Rajiv Gandhi Grameen Vidyutikaran Yojana
EVOLUTION 1975
1997
2004
NTPC was set up in 1975 with 100% ownership by the Government of India. In the last 30 years, NTPC has grown into the largest power utility in India.
In 1997, Government of India granted NTPC status of “Navratna’ being one of the nine jewels of India, enhancing the powers to the Board of Directors.
NTPC became a listed company with majority Government ownership of 89.5%. NTPC becomes third largest by Market Capitalization of listed companies
2005
The company rechristened as NTPC Limited in line with its changing business portfolio and transforms itself from a thermal power utility to an integrated power utility.
2008
National Thermal Power Corporation is the largest power generation company in India. Forbes Global 2000 for 2008 ranked it 411th in the world.
TPC is the largest power utility in India, accounting for about 20% of India’s installed capacity.
ITRODUCTIO TO THEMAL POWER PLAT • Introduction • Classification • Functioning
INTRODUCTION Power Station (also referred to as generating station or power plant) is an industrial facility for the generation of electric power. Power plant is also used to refer to the engine in ships, aircraft and other large vehicles. Some prefer to use the term energy center because it more accurately describes what the plants do, which is the conversion of other forms of energy, like chemical energy, gravitational potential energy or heat energy into electrical energy. However, power plant is the most common term in the U.S., while elsewhere power station and power plant are both widely used, power station prevailing in many Commonwealth countries and especially in the United Kingdom.
A coal-fired Thermal Power Plant At the center of nearly all power stations is a generator, a rotating machine that converts mechanical energy into electrical energy by creating relative motion between a magnetic field and a conductor. The energy source harnessed to turn the generator varies widely. It depends chiefly on what fuels are easily available and the types of technology that the power company has access to. In thermal power stations, mechanical power is produced by a heat engine, which transforms thermal energy, often from combustion of a fuel, into rotational energy. Most thermal power stations produce steam, and these are sometimes called steam power stations. About 80% of all electric power is generated by use of steam turbines. Not all thermal energy can be transformed to mechanical power, according to the second law of thermodynamics. Therefore, there is always heat lost to the environment. If this loss is employed as useful heat, for industrial processes or district heating, the power plant is referred to as a cogeneration power plant or CHP (combined heat-and-power) plant. In countries where district heating is common, there are dedicated heat plants called heat-only boiler stations. An important class of power stations in the Middle East uses byproduct heat for desalination of water.
CLASSIFICATION By fuel •
Nuclear power plants use a nuclear reactor's heat to operate a steam turbine generator.
•
Fossil fuelled power plants may also use a steam turbine generator or in the case of natural gas fired plants may use a combustion turbine.
•
Geothermal power plants use steam extracted from hot underground rocks.
•
Renewable energy plants may be fuelled by waste from sugar cane, municipal solid waste, landfill methane, or other forms of biomass.
•
In integrated steel mills, blast furnace exhaust gas is a low-cost, although low-energydensity, fuel.
•
Waste heat from industrial processes is occasionally concentrated enough to use for power generation, usually in a steam boiler and turbine.
By prime mover •
Steam turbine plants use the dynamic pressure generated by expanding steam to turn the blades of a turbine. Almost all large non-hydro plants use this system.
•
Gas turbine plants use the dynamic pressure from flowing gases to directly operate the turbine. Natural-gas fuelled turbine plants can start rapidly and so are used to supply "peak" energy during periods of high demand, though at higher cost than base-loaded plants. These may be comparatively small units, and sometimes completely unmanned, being remotely operated. This type was pioneered by the UK, Prince town being the world's first, commissioned in 1959.
•
Combined cycle plants have both a gas turbine fired by natural gas, and a steam boiler and steam turbine which use the exhaust gas from the gas turbine to produce electricity. This greatly increases the overall efficiency of the plant, and many new base load power plants are combined cycle plants fired by natural gas.
•
Internal combustion Reciprocating engines are used to provide power for isolated communities and are frequently used for small cogeneration plants. Hospitals, office buildings, industrial plants, and other critical facilities also use them to provide backup power in case of a power outage. These are usually fuelled by diesel oil, heavy oil, natural gas and landfill gas.
•
Micro turbines, Sterling engine and internal combustion reciprocating engines are low cost solutions for using opportunity fuels, such as landfill gas, digester gas from water treatment plants and waste gas from oil production.
FUNCTIONING Functioning of thermal power plant: In a thermal power plant, one of coal, oil or natural gas is used to heat the boiler to convert the water into steam. The steam is used to turn a turbine, which is connected to a generator. When the turbine turns, electricity is generated and given as output by the generator, which is then supplied to the consumers through high-voltage power lines.
Process of a Thermal Power Plant
Detailed process of thermal power plant:
power
generation
in
a
1) Water intake: Firstly, water is taken into the boiler through a water source. If water is available in a plenty in the region, then the source is an open pond or river. If water is scarce, then it is recycled and the same water is used over and over again. 2) Boiler heating: The boiler is heated with the help of oil, coal or natural gas. A furnace is used to heat the fuel and supply the heat produced to the boiler. The increase in temperature helps in the transformation of water into steam.
3) Steam Turbine: The steam generated in the boiler is sent through a steam turbine. The turbine has blades that rotate when high velocity steam flows across them. This rotation of turbine blades is used to generate electricity. 4) Generator: A generator is connected to the steam turbine. When the turbine rotates, the generator produces electricity which is then passed on to the power distribution systems. 5) Special mountings: There is some other equipment like the economizer and air pre-heater. An economizer uses the heat from the exhaust gases to heat the feed water. An air pre-heater heats the air sent into the combustion chamber to improve the efficiency of the combustion process. 6) Ash collection system: There is a separate residue and ash collection system in place to collect all the waste materials from the combustion process and to prevent them from escaping into the atmosphere.
Apart from this, there are various other monitoring systems and instruments in place to keep track of the functioning of all the devices. This prevents any hazards from taking place in the plant.
PROJECT REPORT • OPERATIO • EMD – I • EMD – II
Module - I
OPERATIO • Introduction • Steam Generator or Boiler • Steam Turbine • Electric Generator
Introduction The operating performance of NTPC has been considerably above the national average. The availability factor for coal stations has increased from 85.03 % in 1997-98 to 90.09 % in 200607, which compares favourably with international standards. The PLF has increased from 75.2% in 1997-98 to 89.4% during the year 2006-07 which is the highest since the inception of NTPC.
Operation Room of Power Plant In a Badarpur Thermal Power Station, steam is produced and used to spin a turbine that operates a generator. Water is heated, turns into steam and spins a steam turbine which drives an electrical generator. After it passes through the turbine, the steam is condensed in a condenser; this is known as a Rankine cycle. Shown here is a diagram of a conventional thermal power plant, which uses coal, oil, or natural gas as fuel to boil water to produce the steam. The electricity generated at the plant is sent to consumers through high-voltage power lines. The Badarpur Thermal Power Plant has Steam Turbine-Driven Generators which has a collective capacity of 705MW. The fuel being used is Coal which is supplied from the Jharia Coal Field in Jharkhand. Water supply is given from the Agra Canal.
Table: Capacity of Badarpur Thermal Power Station, New Delhi Sr. o.
Capacity
o. of Generators
Total Capacity
1.
210 MW
2
420 MW
2.
95 MW
3
285 MW
Total
705 MW
There are basically three main units of a thermal power plant: 1. Steam Generator or Boiler 2. Steam Turbine 3. Electric Generator We have discussed about the processes of electrical generation further. A complete detailed description of the three units is given further.
Typical Diagram of a Coal based Thermal Power Plant
1. Cooling tower 2. Cooling water pump 3. Transmission line (3-phase) 4. Unit transformer (3-phase) 5. Electric generator (3-phase) 6. Low pressure turbine 7. Condensate extraction pump 8. Condensor 9. Intermediate pressure turbine
10. Steam governor valve 11. High pressure turbine 12. Deaerator 13. Feed heater 14. Coal conveyor 15. Coal hopper 16. Pulverised fuel mill 17. Boiler drum 18. Ash hopper
19. Superheater 20. Forced draught fan 21. Reheater 22. Air intake 23. Economiser 24. Air preheater 25. Precipitator 26. Induced draught fan 27. Chimney Stack
Coal is conveyed (14) from an external stack and ground to a very fine powder by large metal spheres in the pulverised fuel mill (16). There it is mixed with preheated air (24) driven by the forced draught fan (20). The hot air-fuel mixture is forced at high pressure into the boiler where it rapidly ignites. Water of a high purity flows vertically up the tube-lined walls of the boiler, where it turns into steam, and is passed to the boiler drum, where steam is separated from any
remaining water. The steam passes through a manifold in the roof of the drum into the pendant superheater (19) where its temperature and pressure increase rapidly to around 200 bar and 540°C, sufficient to make the tube walls glow a dull red. The steam is piped to the high pressure turbine (11), the first of a three-stage turbine process. A steam governor valve (10) allows for both manual control of the turbine and automatic set-point following. The steam is exhausted from the high pressure turbine, and reduced in both pressure and temperature, is returned to the boiler reheater (21). The reheated steam is then passed to the intermediate pressure turbine (9), and from there passed directly to the low pressure turbine set (6). The exiting steam, now a little above its boiling point, is brought into thermal contact with cold water (pumped in from the cooling tower) in the condensor (8), where it condenses rapidly back into water, creating near vacuum-like conditions inside the condensor chest. The condensed water is then passed by a feed pump (7) through a deaerator (12), and pre-warmed, first in a feed heater (13) powered by steam drawn from the high pressure set, and then in the economiser (23), before being returned to the boiler drum. The cooling water from the condensor is sprayed inside a cooling tower (1), creating a highly visible plume of water vapor, before being pumped back to the condensor (8) in cooling water cycle. The three turbine sets are sometimes coupled on the same shaft as the three-phase electrical generator (5) which generates an intermediate level voltage (typically 20-25 kV). This is stepped up by the unit transformer (4) to a voltage more suitable for transmission (typically 250-500 kV) and is sent out onto the three-phase transmission system (3). Exhaust gas from the boiler is drawn by the induced draft fan (26) through an electrostatic precipitator (25) and is then vented through the chimney stack (27).
Steam Generator or Boiler The boiler is a rectangular furnace about 50 ft (15 m) on a side and 130 ft (40 m) tall. Its walls are made of a web of high pressure steel tubes about 2.3 inches (60 mm) in diameter. Pulverized coal is air-blown into the furnace from fuel nozzles at the four corners and it rapidly burns, forming a large fireball at the center. The thermal radiation of the fireball heats the water that circulates through the boiler tubes near the boiler perimeter. The water circulation rate in the boiler is three to four times the throughput and is typically driven by pumps. As the water in the boiler circulates it absorbs heat and changes into steam at 700 °F (370 °C) and 3,200 psi (22.1 MPa). It is separated from the water inside a drum at the top of the furnace. The saturated steam is introduced into superheat pendant tubes that hang in the hottest part of the combustion gases as they exit the furnace. Here the steam is superheated to 1,000 °F (540 °C) to prepare it for the turbine. The steam generating boiler has to produce steam at the high purity, pressure and temperature required for the steam turbine that drives the electrical generator. The generator includes the economizer, the steam drum, the chemical dosing equipment, and the furnace with its steam generating tubes and the superheater coils. Necessary safety valves are located at suitable points to avoid excessive boiler pressure. The air and flue gas path equipment include: forced draft (FD) fan, air preheater (APH), boiler furnace, induced draft (ID) fan, fly ash collectors (electrostatic precipitator or baghouse) and the flue gas stack.
Schematic diagram of a coal-fired power plant steam generator
For units over about 210 MW capacity, redundancy of key components is provided by installing duplicates of the FD fan, APH, fly ash collectors and ID fan with isolating dampers. On some units of about 60 MW, two boilers per unit may instead be provided.
Boiler Furnace and Steam Drum Once water inside the boiler or steam generator, the process of adding the latent heat of vaporization or enthalpy is underway. The boiler transfers energy to the water by the chemical reaction of burning some type of fuel. The water enters the boiler through a section in the convection pass called the economizer. From the economizer it passes to the steam drum. Once the water enters the steam drum it goes down the down comers to the lower inlet water wall headers. From the inlet headers the water rises through the water walls and is eventually turned into steam due to the heat being generated by the burners located on the front and rear water walls (typically). As the water is turned into steam/vapor in the water walls, the steam/vapor once again enters the steam drum.
External View of an Industrial Boiler at Badarpur Thermal Power Station, ew Delhi
The steam/vapor is passed through a series of steam and water separators and then dryers inside the steam drum. The steam separators and dryers remove the water droplets from the steam and the cycle through the water walls is repeated. This process is known as natural circulation. The boiler furnace auxiliary equipment includes coal feed nozzles and igniter guns, soot blowers, water lancing and observation ports (in the furnace walls) for observation of the furnace interior. Furnace explosions due to any accumulation of combustible gases after a tripout are avoided by flushing out such gases from the combustion zone before igniting the coal. The steam drum (as well as the superheater coils and headers) have air vents and drains needed for initial startup. The steam drum has an internal device that removes moisture from the wet steam entering the drum from the steam generating tubes. The dry steam then flows into the superheater coils.
Geothermal plants need no boiler since they use naturally occurring steam sources. Heat exchangers may be used where the geothermal steam is very corrosive or contains excessive suspended solids. Nuclear plants also boil water to raise steam, either directly passing the working steam through the reactor or else using an intermediate heat exchanger.
Fuel Preparation System In coal-fired power stations, the raw feed coal from the coal storage area is first crushed into small pieces and then conveyed to the coal feed hoppers at the boilers. The coal is next pulverized into a very fine powder. The pulverizers may be ball mills, rotating drum grinders, or other types of grinders. Some power stations burn fuel oil rather than coal. The oil must kept warm (above its pour point) in the fuel oil storage tanks to prevent the oil from congealing and becoming unpumpable. The oil is usually heated to about 100°C before being pumped through the furnace fuel oil spray nozzles.
Boiler Side of the Badarpur Thermal Power Station, ew Delhi
Boilers in some power stations use processed natural gas as their main fuel. Other power stations may use processed natural gas as auxiliary fuel in the event that their main fuel supply (coal or oil) is interrupted. In such cases, separate gas burners are provided on the boiler furnaces.
Fuel Firing System and Igniter System From the pulverized coal bin, coal is blown by hot air through the furnace coal burners at an angle which imparts a swirling motion to the powdered coal to enhance mixing of the coal powder with the incoming preheated combustion air and thus to enhance the combustion. To provide sufficient combustion temperature in the furnace before igniting the powdered coal, the furnace temperature is raised by first burning some light fuel oil or processed natural gas (by using auxiliary burners and igniters provide for that purpose).
Air Path External fans are provided to give sufficient air for combustion. The forced draft fan takes air from the atmosphere and, first warming it in the air preheater for better combustion, injects it via the air nozzles on the furnace wall. The induced draft fan assists the FD fan by drawing out combustible gases from the furnace, maintaining a slightly negative pressure in the furnace to avoid backfiring through any opening. At the furnace outlet, and before the furnace gases are handled by the ID fan, fine dust carried by the outlet gases is removed to avoid atmospheric pollution. This is an environmental limitation prescribed by law, and additionally minimizes erosion of the ID fan.
Auxiliary Systems Fly Ash Collection Fly ash is captured and removed from the flue gas by electrostatic precipitators or fabric bag filters (or sometimes both) located at the outlet of the furnace and before the induced draft fan. The fly ash is periodically removed from the collection hoppers below the precipitators or bag filters. Generally, the fly ash is pneumatically transported to storage silos for subsequent transport by trucks or railroad cars.
Bottom Ash Collection and Disposal At the bottom of every boiler, a hopper has been provided for collection of the bottom ash from the bottom of the furnace. This hopper is always filled with water to quench the ash and clinkers falling down from the furnace. Some arrangement is included to crush the clinkers and for conveying the crushed clinkers and bottom ash to a storage site.
Boiler Make-up Water Treatment Plant and Storage Since there is continuous withdrawal of steam and continuous return of condensate to the boiler, losses due to blow-down and leakages have to be made up for so as to maintain the desired water level in the boiler steam drum. For this, continuous make-up water is added to the boiler water system. The impurities in the raw water input to the plant generally consist of calcium and magnesium salts which impart hardness to the water. Hardness in the make-up water to the boiler will form deposits on the tube water surfaces which will lead to overheating and failure of
the tubes. Thus, the salts have to be removed from the water and that is done by a water demineralising treatment plant (DM).
Ash Handling System at Badarpur Thermal Power Station, ew Delhi
A DM plant generally consists of cation, anion and mixed bed exchangers. The final water from this process consists essentially of hydrogen ions and hydroxide ions which is the chemical composition of pure water. The DM water, being very pure, becomes highly corrosive once it absorbs oxygen from the atmosphere because of its very high affinity for oxygen absorption. The capacity of the DM plant is dictated by the type and quantity of salts in the raw water input. However, some storage is essential as the DM plant may be down for maintenance. For this purpose, a storage tank is installed from which DM water is continuously withdrawn for boiler make-up. The storage tank for DM water is made from materials not affected by corrosive water, such as PVC. The piping and valves are generally of stainless steel. Sometimes, a steam blanketing arrangement or stainless steel doughnut float is provided on top of the water in the tank to avoid contact with atmospheric air. DM water make-up is generally added at the steam space of the surface condenser (i.e., the vacuum side). This arrangement not only sprays the water but also DM water gets deaerated, with the dissolved gases being removed by the ejector of the condenser itself.
Steam Turbine Steam turbines are used in all of our major coal fired power stations to drive the generators or alternators, which produce electricity. The turbines themselves are driven by steam generated in 'Boilers' or 'Steam Generators' as they are sometimes called. Energy in the steam after it leaves the boiler is converted into rotational energy as it passes through the turbine. The turbine normally consists of several stages with each stage consisting of a stationary blade (or nozzle) and a rotating blade. Stationary blades convert the potential energy of the steam (temperature and pressure) into kinetic energy (velocity) and direct the flow onto the rotating blades. The rotating blades convert the kinetic energy into forces, caused by pressure drop, which results in the rotation of the turbine shaft. The turbine shaft is connected to a generator, which produces the electrical energy. The rotational speed is 3000 rpm for Indian System (50 Hz) systems and 3600 for American (60 Hz) systems.
In a typical larger power stations, the steam turbines are split into three separate stages, the first being the High Pressure (HP), the second the Intermediate Pressure (IP) and the third the Low Pressure (LP) stage, where high, intermediate and low describe the pressure of the steam. After the steam has passed through the HP stage, it is returned to the boiler to be re-heated to its original temperature although the pressure remains greatly reduced. The reheated steam then passes through the IP stage and finally to the LP stage of the turbine.
A distinction is made between "impulse" and "reaction" turbine designs based on the relative pressure drop across the stage. There are two measures for pressure drop, the pressure ratio and the percent reaction. Pressure ratio is the pressure at the stage exit divided by the pressure at the stage entrance. Reaction is the percentage isentropic enthalpy drop across the rotating blade or bucket compared to the total stage enthalpy drop. Some manufacturers utilise percent pressure drop across stage to define reaction. Steam turbines can be configured in many different ways. Several IP or LP stages can be incorporated into the one steam turbine. A single shaft or several shafts coupled together may be used. Either way, the principles are the same for all steam turbines. The configuration is decided by the use to which the steam turbine is put, co-generation or pure electricity production. For cogeneration, the steam pressure is highest when used as process steam and at a lower pressure when used for the secondary function of electricity production.
Nozzles and Blades Steam enthalpy is converted into rotational energy as it passes through a turbine stage. A turbine stage consists of a stationary blade (or nozzle) and a rotating blade (or bucket). Stationary blades convert the potential energy of the steam (temperature and pressure) into kinetic energy (velocity) and direct the flow onto the rotating blades. The rotating blades convert the kinetic energy into impulse and reaction forces caused by pressure drop, which results in the rotation of the turbine shaft or rotor. Steam turbines are machines which must be designed, manufactured and maintained to high tolerances so that the design power output and availability is obtained. They are subject to a number of damage mechanisms, with two of the most important being:
Erosion due to Moisture: - The presence of water droplets in the last stages of a turbine causes erosion to the blades. This has led to the imposition of an allowable limit of about 12% wetness in the exhaust steam;
Solid Particle Erosion: - The entrainment of erosive materials from the boiler in the steam causes wear to the turbine blades.
Cogeneration Cycles In cogeneration cycles, steam is typically generated at a higher temperature and pressure than required for a particular industrial process. The steam is expanded through a turbine to produce electricity and the resulting extractions at the discharge are at the temperature and pressure required by the process. Turbines can be condensing or non-condensing design typically with large mass flows and comparably low output. Traditionally, pressures were 6.21 MPa and below with temperatures 441º C or lower, although the trend towards higher levels of each continues. There are now a considerable number of co-generation steam turbines with initial steam pressures in the 8.63 to 10 MPa range and steam temperatures of 482 to 510º C.
Bearings and Lubrication Two types of bearings are used to support and locate the rotors of steam turbines:
Journal bearings are used to support the weight of the turbine rotors. A journal bearing consists of two half-cylinders that enclose the shaft and are internally lined with Babbitt, a metal alloy usually consisting of tin, copper and antimony; and
Thrust bearings axially locate the turbine rotors. A thrust bearing is made up of a series of Babbitt lined pads that run against a locating disk attached to the turbine rotor. High-pressure oil is injected into the bearings to provide lubrication. The oil is carefully filtered to remove solid particles. Specially designed centrifuges remove any water from the oil.
Shaft Seals The shaft seal on a turbine rotor consist of a series of ridges and groves around the rotor and its housing which present a long, tortuous path for any steam leaking through the seal. The seal therefore does not prevent the steam from leaking, merely reduces the leakage to a minimum. The leaking steam is collected and returned to a low-pressure part of the steam circuit.
Turning Gear Large steam turbines are equipped with "turning gear" to slowly rotate the turbines after they have been shut down and while they are cooling. This evens out the temperature distribution around the turbines and prevents bowing of the rotors.
Vibration The balancing of the large rotating steam turbines is a critical component in ensuring the reliable operation of the plant. Most large steam turbines have sensors installed to measure the movement of the shafts in their bearings. This condition monitoring can identify many potential problems and allows the repair of the turbine to be planned before the problems become serious.
Electric Generator The steam turbine-driven generators have auxiliary systems enabling them to work satisfactorily and safely. The steam turbine generator being rotating equipment generally has a heavy, large diameter shaft. The shaft therefore requires not only supports but also has to be kept in position while running. To minimize the frictional resistance to the rotation, the shaft has a number of bearings. The bearing shells, in which the shaft rotates, are lined with a low friction material like Babbitt metal. Oil lubrication is provided to further reduce the friction between shaft and bearing surface and to limit the heat generated.
A 95 MW Generator at Badarpur Thermal Power Station, ew Delhi
Barring Gear (or Turning Gear) Barring gear is the term used for the mechanism provided for rotation of the turbine generator shaft at a very low speed (about one revolution per minute) after unit stoppages for any reason. Once the unit is "tripped" (i.e., the turbine steam inlet valve is closed), the turbine starts slowing or "coasting down". When it stops completely, there is a tendency for the turbine shaft to deflect or bend if allowed to remain in one position too long. This deflection is because the heat inside the turbine casing tends to concentrate in the top half of the casing, thus making the top half portion of the shaft hotter than the bottom half. The shaft therefore warps or bends by millionths of inches, only detectable by monitoring eccentricity meters.
But this small amount of shaft deflection would be enough to cause vibrations and damage the entire steam turbine generator unit when it is restarted. Therefore, the shaft is not permitted to come to a complete stop by a mechanism known as "turning gear" or "barring gear" that automatically takes over to rotate the unit at a preset low speed. If the unit is shut down for major maintenance, then the barring gear must be kept in service until the temperatures of the casings and bearings are sufficiently low.
Condenser The surface condenser is a shell and tube heat exchanger in which cooling water is circulated through the tubes. The exhaust steam from the low pressure turbine enters the shell where it is cooled and converted to condensate (water) by flowing over the tubes as shown in the adjacent diagram. Such condensers use steam ejectors or rotary motor-driven exhausters for continuous removal of air and gases from the steam side to maintain vacuum.
A Typical Water Cooled Condenser
For best efficiency, the temperature in the condenser must be kept as low as practical in order to achieve the lowest possible pressure in the condensing steam. Since the condenser temperature can almost always be kept significantly below 100 oC where the vapor pressure of water is much less than atmospheric pressure, the condenser generally works under vacuum. Thus leaks of noncondensible air into the closed loop must be prevented. Plants operating in hot climates may have to reduce output if their source of condenser cooling water becomes warmer; unfortunately this usually coincides with periods of high electrical demand for air conditioning. The condenser generally uses either circulating cooling water from a cooling tower to reject waste heat to the atmosphere, or once-through water from a river, lake or ocean.
Feedwater Heater A Rankine cycle with a two-stage steam turbine and a single feedwater heater. In the case of a conventional steam-electric power plant utilizing a drum boiler, the surface condenser removes the latent heat of vaporization from the steam as it changes states from vapour to liquid. The heat content (btu) in the steam is referred to as Enthalpy. The condensate pump then pumps the condensate water through a feedwater heater. The feedwater heating equipment then raises the temperature of the water by utilizing extraction steam from various stages of the turbine.
A Rankine cycle with a two-stage steam turbine and a single feedwater heater
Preheating the feedwater reduces the irreversibilities involved in steam generation and therefore improves the thermodynamic efficiency of the system.[9] This reduces plant operating costs and also helps to avoid thermal shock to the boiler metal when the feedwater is introduced back into the steam cycle.
Superheater As the steam is conditioned by the drying equipment inside the drum, it is piped from the upper drum area into an elaborate set up of tubing in different areas of the boiler. The areas known as superheater and reheater. The steam vapor picks up energy and its temperature is now superheated above the saturation temperature. The superheated steam is then piped through the main steam lines to the valves of the high pressure turbine.
Deaerator A steam generating boiler requires that the boiler feed water should be devoid of air and other dissolved gases, particularly corrosive ones, in order to avoid corrosion of the metal. Generally, power stations use a deaerator to provide for the removal of air and other dissolved gases from the boiler feedwater. A deaerator typically includes a vertical, domed deaeration section mounted on top of a horizontal cylindrical vessel which serves as the deaerated boiler feedwater storage tank.
Boiler Feed Water Deaerator (with vertical, domed aeration section and horizontal water storage section)
There are many different designs for a deaerator and the designs will vary from one manufacturer to another. The adjacent diagram depicts a typical conventional trayed deaerator. If operated properly, most deaerator manufacturers will guarantee that oxygen in the deaerated water will not exceed 7 ppb by weight (0.005 cm³/L).
Auxiliary Systems Oil System An auxiliary oil system pump is used to supply oil at the start-up of the steam turbine generator. It supplies the hydraulic oil system required for steam turbine's main inlet steam stop valve, the governing control valves, the bearing and seal oil systems, the relevant hydraulic relays and other mechanisms. At a preset speed of the turbine during start-ups, a pump driven by the turbine main shaft takes over the functions of the auxiliary system.
Generator Heat Dissipation The electricity generator requires cooling to dissipate the heat that it generates. While small units may be cooled by air drawn through filters at the inlet, larger units generally require special cooling arrangements. Hydrogen gas cooling, in an oil-sealed casing, is used because it has the highest known heat transfer coefficient of any gas and for its low viscosity which reduces windage losses. This system requires special handling during start-up, with air in the chamber first displaced by carbon dioxide before filling with hydrogen. This ensures that the highly flammable hydrogen does not mix with oxygen in the air. The hydrogen pressure inside the casing is maintained slightly higher than atmospheric pressure to avoid outside air ingress. The hydrogen must be sealed against outward leakage where the shaft emerges from the casing. Mechanical seals around the shaft are installed with a very small annular gap to avoid rubbing between the shaft and the seals. Seal oil is used to prevent the hydrogen gas leakage to atmosphere. The generator also uses water cooling. Since the generator coils are at a potential of about 15.75 kV and water is conductive, an insulating barrier such as Teflon is used to interconnect the water line and the generator high voltage windings. Demineralized water of low conductivity is used.
Generator High Voltage System The generator voltage ranges from 10.5 kV in smaller units to 15.75 kV in larger units. The generator high voltage leads are normally large aluminum channels because of their high current as compared to the cables used in smaller machines. They are enclosed in well-grounded aluminum bus ducts and are supported on suitable insulators. The generator high voltage channels are connected to step-up transformers for connecting to a high voltage electrical substation (of the order of 220 kV) for further transmission by the local power grid. The necessary protection and metering devices are included for the high voltage leads. Thus, the steam turbine generator and the transformer form one unit. In smaller units, generating at 10.5 kV, a breaker is provided to connect it to a common 10.5 kV bus system.
Other Systems Monitoring and Alarm system Most of the power plant’s operational controls are automatic. However, at times, manual intervention may be required. Thus, the plant is provided with monitors and alarm systems that alert the plant operators when certain operating parameters are seriously deviating from their normal range.
An Engineer monitoring the various parameters at TPC, ew Delhi
Battery Supplied Emergency Lighting & Communication A central battery system consisting of lead acid cell units is provided to supply emergency electric power, when needed, to essential items such as the power plant's control systems, communication systems, turbine lube oil pumps, and emergency lighting. This is essential for a safe, damage-free shutdown of the units in an emergency situation.
Module - II
EMD - I • Coal Handling Plant • Motors • Switchgear • High Tension Switchgear • Direct On Line Starter
Coal Handling Plant Coal is delivered by highway truck, rail, barge or collier ship. Some plants are even built near coal mines and coal is delivered by conveyors. A large coal train called a "unit train" may be a kilometers (over a mile) long, containing 60 cars with 100 tons of coal in each one, for a total load of 6,000 tons. A large plant under full load requires at least one coal delivery this size every day. Plants may get as many as three to five trains a day, especially in "peak season", during the summer months when power consumption is high. A large thermal power plant such as the Badarpur Thermal Power Station, New Delhi stores several million tons of coal for use when there is no wagon supply.
Coal Handling Plant Layout
Modern unloaders use rotary dump devices, which eliminate problems with coal freezing in bottom dump cars. The unloader includes a train positioner arm that pulls the entire train to position each car over a coal hopper. The dumper clamps an individual car against a platform that swivels the car upside down to dump the coal. Swiveling couplers enable the entire
operation to occur while the cars are still coupled together. Unloading a unit train takes about three hours. Shorter trains may use railcars with an "air-dump", which relies on air pressure from the engine plus a "hot shoe" on each car. This "hot shoe" when it comes into contact with a "hot rail" at the unloading trestle, shoots an electric charge through the air dump apparatus and causes the doors on the bottom of the car to open, dumping the coal through the opening in the trestle. Unloading one of these trains takes anywhere from an hour to an hour and a half. Older unloaders may still use manually operated bottom-dump rail cars and a "shaker" attached to dump the coal. Generating stations adjacent to a mine may receive coal by conveyor belt or massive dieselelectric-drive trucks.
Layout of Coal Handling Plant at Badarpur Thermal Power Station, ew Delhi
Coal is prepared for use by crushing the rough coal to pieces less than 2 inches (50 mm) in size. The coal is then transported from the storage yard to in-plant storage silos by rubberized conveyor belts at rates up to 4,000 tons/hour. In plants that burn pulverized coal, silos feed coal pulverizers (coal mill) that take the larger 2 inch pieces grind them into the consistency of face powder, classify them, and mixes them with primary combustion air which transports the coal to the furnace and preheats the coal to drive off excess moisture content. In plants that do not burn pulverized coal, the larger 2 inch pieces may be directly fed into the silos which then feed the cyclone burners, a specific kind of combustor that can efficiently burn larger pieces of fuel.
Run-Of-Mine (ROM) Coal The coal delivered from the mine that reports to the Coal Handling Plant is called Run-of-mine, or ROM, coal. This is the raw material for the CHP, and consists of coal, rocks, middlings, minerals and contamination. Contamination is usually introduced by the mining process and may include machine parts, used consumables and parts of ground engaging tools. ROM coal can have a large variability of moisture and maximum particle size.
Coal Handling Coal needs to be stored at various stages of the preparation process, and conveyed around the CHP facilities. Coal handling is part of the larger field of bulk material handling, and is a complex and vital part of the CHP.
Stockpiles Stockpiles provide surge capacity to various parts of the CHP. ROM coal is delivered with large variations in production rate of tonnes per hour (tph). A ROM stockpile is used to allow the washplant to be fed coal at lower, constant rate.
Coal Handling Division of Badarpur Thermal Power Station, ew Delhi
A simple stockpile is formed by machinery dumping coal into a pile, either from dump trucks, pushed into heaps with bulldozers or from conveyor booms. More controlled stockpiles are
formed using stackers to form piles along the length of a conveyor, and reclaimers to retrieve the coal when required for product loading, etc. Taller and wider stockpiles reduce the land area required to store a set tonnage of coal. Larger coal stockpiles have a reduced rate of heat lost, leading to a higher risk of spontaneous combustion.
Stacking Travelling, lugging boom stackers that straddle a feed conveyor are commonly used to create coal stockpiles. Stackers are nominally rated in tph (tonnes per hour) for capacity and normally travel on a rail between stockpiles in the stockyard. A stacker can usually move in at least two directions typically: horizontally along the rail and vertically by luffing its boom. Luffing of the boom minimises dust by reducing the height that the coal needs to fall to the top of the stockpile. The boom is luffed upwards as the stockpile height grows.
Wagon Tripler at Badarpur Thermal Power Station, ew Delhi
Some stackers are able to rotate by slewing the boom. This allows a single stacker to form two stockpiles, one on either side of the conveyor. Stackers are used to stack into different patterns, such as cone stacking and chevron stacking. Stacking in a single cone tends to cause size segregation, with coarser material moving out towards the base. Raw cone ply stacking is when additional cones are added next to the first cone. Chevron stacking is when the stacker travels along the length of the stockpile adding layer upon layer of material.
Stackers and Reclaimers were originally manually controlled manned machines with no remote control. Modern machines are typically semi-automatic or fully automated, with parameters remotely set.
Reclaiming Tunnel conveyors can be fed by a continuous slot hopper or bunker beneath the stockpile to reclaim material. Front-end loaders and bulldozers can be used to push the coal into feeders. Sometimes front-end loaders are the only means of reclaiming coal from the stockpile. This has a low up-front capital cost, but much higher operating costs, measured in dollars per tonne handled.
Coal Storage Area of the Badarpur Thermal Power Station, ew Delhi
High-capacity stockpiles are commonly reclaimed using bucket-wheel reclaimers. These can achieve very high rates.
Coal Sampling Sampling of coal is an important part of the process control in the CHP. A grab sample is a oneoff sample of the coal at a point in the process stream, and tends not to be very representative. A routine sample is taken at a set frequency, either over a period of time or per shipment.
Screening
Screens are used to group process particles into ranges by size. These size ranges are also called grades. Dewatering screens are used to remove water from the product. Screens can be static, or mechanically vibrated. Screen decks can be made from different materials such as high tensile steel, stainless steel, or polyethelene.
Screening and Separation Unit of Coal Handling Division of a Thermal Power Plant
Magnetic Separation Magnetic separators shall be used in coal conveying systems to separate tramp iron (including steel) from the coal. Basically, two types are available. One type incorporates permanent or electromagnets into the head pulley of a belt conveyor. The tramp iron clings to the belt as it goes around the pulley drum and falls off into a collection hopper or trough after the point at which coal is charged from the belt. The other type consists of permanent or electromagnets incorporated into a belt conveyor that is suspended above a belt conveyor carrying coal. The tramp iron is pulled from the moving coal to the face of the separating conveyor, which in turn holds and carries the tramp iron to a collection hopper or trough. Magnetic separators shall be used just ahead of the coal crusher, if any, and/or just prior to coal discharge to the in-plant bunker or silo fill system.
Coal Crusher Before the coal is sent to the plant it has to be ensured that the coal is of uniform size, and so it is passed through coal crushers. Also power plants using pulverized coal specify a maximum
coal size that can be fed into the pulverizer and so the coal has to be crushed to the specified size using the coal crusher. Rotary crushers are very commonly used for this purpose as they can provide a continuous flow of coal to the pulverizer.
Pulverizer Most commonly used pulverizer is the Boul Mill. The arrangement consists of 2 stationary rollers and a power driven baul in which pulverization takes place as the coal passes through the sides of the rollers and the baul. A primary air induced draught fan draws a stream of heated air through the mill carrying the pulverized coal into a stationary classifier at the top of the pulverizer. The classifier separates the pulverized coal from the unpulverized coal.
An external view of a Coal Pulverizer
Advantages of Pulverized Coal •
Pulverized coal is used for large capacity plants.
•
It is easier to adapt to fluctuating load as there are no limitations on the combustion capacity.
•
Coal with higher ash percentage cannot be used without pulverizing because of the problem of large amount ash deposition after combustion.
•
Increased thermal efficiency is obtained through pulverization.
•
The use of secondary air in the combustion chamber along with the powered coal helps in creating turbulence and therefore uniform mixing of the coal and the air during combustion.
•
Greater surface area of coal per unit mass of coal allows faster combustion as more coal is exposed to heat and combustion.
•
The combustion process is almost free from clinker and slag formation.
•
The boiler can be easily started from cold condition in case of emergency.
•
Practically no ash handling problem.
•
The furnace volume required is less as the turbulence caused aids in complete combustion of the coal with minimum travel of the particles. The pulverized coal is passed from the pulverizer to the boiler by means of the primary air that is used not only to dry the coal but also to heat is as it goes into the boiler. The secondary air is used to provide the necessary air required for complete combustion. The primary air may vary anywhere from 10% to the entire air depending on the design of the boiler. The coal is sent into the boiler through burners. A very important and widely used type of burner arrangement is the Tangential Firing arrangement.
Tangential Burners: The tangential burners are arranged such that they discharge the fuel air mixture tangentially to an imaginary circle in the center of the furnace. The swirling action produces sufficient turbulence in the furnace to complete the combustion in a short period of time and avoid the necessity of producing high turbulence at the burner itself. High heat release rates are possible with this method of firing. The burners are placed at the four corners of the furnace. At the Badarpur Thermal Power Station five sets of such burners are placed one above the other to form six firing zones. These burners are constructed with tips that can be angled through a small vertical arc. By adjusting the angle of the burners the position of the fire ball can be adjusted so as to raise or lower the position of the turbulent combustion region. When the burners are tilted downward the furnace gets filled completely with the flame and the furnace exit gas temperature gets reduced. When the burners are tiled upward the furnace exit gas temperature increases. A difference of 100 degrees can be achieved by tilting the burners.
Ash Handling The ever increasing capacities of boiler units together with their ability to use low grade high ash content coal have been responsible for the development of modern day ash handling systems. The widely used ash handling systems are 1. Mechanical Handling System 2. Hydraulic System 3. Pneumatic System 4. Steam Jet System The Hydraulic Ash handling system is used at the Badarpur Thermal Power Station.
Ash Handling System of a Thermal Power Plant
Hydraulic Ash Handling System The hydraulic system carried the ash with the flow of water with high velocity through a channel and finally dumps into a sump. The hydraulic system is divided into a low velocity and high velocity system. In the low velocity system the ash from the boilers falls into a stream of water flowing into the sump. The ash is carried along with the water and they are separated at the sump. In the high velocity system a jet of water is sprayed to quench the hot ash. Two other jets force the ash into a trough in which they are washed away by the water into the sump, where they are separated. The molten slag formed in the pulverized fuel system can also be quenched and washed by using the high velocity system. The advantages of this system are that its clean, large ash handling capacity, considerable distance can be traversed, absence of working parts in contact with ash.
ELECTRIC MOTORS An electric motor uses electrical energy to produce mechanical energy. The reverse process that of using mechanical energy to produce electrical energy is accomplished by a generator or dynamo. Traction motors used on locomotives and some electric and hybrid automobiles often performs both tasks if the vehicle is equipped with dynamic brakes.
A High Power Electric Motor
Categorization of Electric Motors The classic division of electric motors has been that of Direct Current (DC) types vs Alternating Current (AC) types. The ongoing trend toward electronic control further muddles the distinction, as modern drivers have moved the commutator out of the motor shell. For this new breed of motor, driver circuits are relied upon to generate sinusoidal AC drive currents, or some approximation of. The two best examples are: the brushless DC motor and the stepping motor, both being polyphase AC motors requiring external electronic control. There is a clearer distinction between a synchronous motor and asynchronous types. In the synchronous types, the rotor rotates in synchrony with the oscillating field or current (eg. permanent magnet motors). In contrast, an asynchronous motor is designed to slip; the most ubiquitous example being the common AC induction motor which must slip in order to generate torque.
Comparison of Motor Types Type
Advantages
Disadvantages
Typical Application
Typical Drive
AC Induction (Shaded Pole)
Least expensive Long life high power
Rotation slips from frequency Low starting torque
Fans
Uni/Polyphase AC
Rotation slips from frequency
Appliances
Uni/Polyphase AC
More expensive
Clocks Audio turntables tape drives
Uni/Polyphase AC
Slow speed Requires a controller
Positioning in printers and floppy drives
Multiphase DC
High initial cost Requires a controller High maintenance (brushes) Low lifespan
Hard drives CD/DVD players electric vehicles
Multiphase DC
Treadmill exercisers automotive starters
Direct (PWM)
AC Induction (split-phase capacitor) AC Synchronous
Stepper DC
Brushless DC Brushed (PM) DC
High power high starting torque Rotation in-sync with freq long-life (alternator) Precision positioning High holding torque Long lifespan low maintenance High efficiency Low initial cost Simple speed control (Dynamo)
At Badarpur Thermal Power Station, New Delhi, mostly AC motors are employed for various purposes. We had to study the two types of AC Motors viz. Synchronous Motors and Induction Motor. The motors have been explained further.
AC Motor
Internal View of AC Motors
An AC motor is an electric motor that is driven by an alternating current. It consists of two basic parts, an outside stationary stator having coils supplied with AC current to produce a rotating magnetic field, and an inside rotor attached to the output shaft that is given a torque by the rotating field.
There are two types of AC motors, depending on the type of rotor used. The first is the synchronous motor, which rotates exactly at the supply frequency or a sub multiple of the supply frequency. The magnetic field on the rotor is either generated by current delivered through slip rings or a by a permanent magnet. The second type is the induction motor, which turns slightly slower than the supply frequency. The magnetic field on the rotor of this motor is created by an induced current.
Synchronous Motor A synchronous electric motor is an AC motor distinguished by a rotor spinning with coils passing magnets at the same rate as the alternating current and resulting magnetic field which drives it. Another way of saying this is that it has zero slip under usual operating conditions. Contrast this with an induction motor, which must slip in order to produce torque. Sometimes a synchronous motor is used, not to drive a load, but to improve the power factor on the local grid it's connected to. It does this by providing reactive power to or consuming reactive power from the grid. In this case the synchronous motor is called a Synchronous condenser. Electrical power plants almost always use synchronous generators because it's very important to keep the frequency constant at which the generator is connected.
Advantages Synchronous motors have the following advantages over non-synchronous motors: •
Speed is independent of the load, provided an adequate field current is applied.
•
Accurate control in speed and position using open loop controls, eg. Stepper motors.
•
They will hold their position when a DC current is applied to both the stator and the rotor windings.
•
Their power factor can be adjusted to unity by using a proper field current relative to the load. Also, a "capacitive" power factor, (current phase leads voltage phase), can be obtained by increasing this current slightly, which can help achieve a better power factor correction for the whole installation.
•
Their construction allows for increased electrical efficiency when a low speed is required (as in ball mills and similar apparatus).
Examples •
Brushless permanent magnet DC motor.
•
Stepper motor.
•
Slow speed AC synchronous motor.
•
Switched reluctance motor.
Induction Motor An induction motor (IM) is a type of asynchronous AC motor where power is supplied to the rotating device by means of electromagnetic induction.
Three Phase Induction Motors
An electric motor converts electrical power to mechanical power in its rotor (rotating part). There are several ways to supply power to the rotor. In a DC motor this power is supplied to the armature directly from a DC source, while in an AC motor this power is induced in the rotating device. An induction motor is sometimes called a rotating transformer because the stator (stationary part) is essentially the primary side of the transformer and the rotor (rotating part) is the secondary side. Induction motors are widely used, especially polyphase induction motors, which are frequently used in industrial drives. Induction motors are now the preferred choice for industrial motors due to their rugged construction, lack of brushes (which are needed in most DC Motors) and — thanks to modern power electronics — the ability to control the speed of the motor.
Construction The stator consists of wound 'poles' that carry the supply current that induces a magnetic field in the conductor. The number of 'poles' can vary between motor types but the poles are always in pairs (i.e. 2, 4, 6 etc). There are two types of rotor: 1. Squirrel-cage rotor 2. Slip ring rotor The most common rotor is a squirrel-cage rotor. It is made up of bars of either solid copper (most common) or aluminum that span the length of the rotor, and are connected through a ring at each end. The rotor bars in squirrel-cage induction motors are not straight, but have some skew to reduce noise and harmonics. The motor's phase type is one of two types: 1. Single-phase induction motor 2. 3-phase induction motor
Principle of Operation The basic difference between an induction motor and a synchronous AC motor is that in the latter a current is supplied onto the rotor. This then creates a magnetic field which, through magnetic interaction, links to the rotating magnetic field in the stator which in turn causes the rotor to turn. It is called synchronous because at steady state the speed of the rotor is the same as the speed of the rotating magnetic field in the stator. By way of contrast, the induction motor does not have any direct supply onto the rotor; instead, a secondary current is induced in the rotor. To achieve this, stator windings are arranged around the rotor so that when energised with a polyphase supply they create a rotating magnetic field pattern which sweeps past the rotor. This changing magnetic field pattern can induce currents in the rotor conductors. These currents interact with the rotating magnetic field created by the stator and the rotor will turn. However, for these currents to be induced, the speed of the physical rotor and the speed of the rotating magnetic field in the stator must be different, or else the magnetic field will not be moving relative to the rotor conductors and no currents will be induced. If by some chance this happens, the rotor typically slows slightly until a current is re-induced and then the rotor continues as before. This difference between the speed of the rotor and speed of the rotating magnetic field in the stator is called slip. It has no unit and the ratio between the relative speed of the magnetic field as seen by the rotor to the speed of the rotating field. Due to this an induction motor is sometimes referred to as an asynchronous machine. Types: •
Based on type of phase supply 1. three phase induction motor (self starting in nature) 2. single phase induction motor (not self starting)
•
Other 1. Squirrel cage induction motor 2. Slip ring induction motor
SWITCHGEAR The term switchgear, used in association with the electric power system, or grid, refers to the combination of electrical disconnects, fuses and/or circuit breakers used to isolate electrical equipment. Switchgear is used both to de-energize equipment to allow work to be done and to clear faults downstream. The very earliest central power stations used simple open knife switches, mounted on insulating panels of marble or asbestos. Power levels and voltages rapidly escalated, making open manually-operated switches too dangerous to use for anything other than isolation of a deenergized circuit. Oil-filled equipment allowed arc energy to be contained and safely controlled. By the early 20th century, a switchgear line-up would be a metal-enclosed structure with electrically-operated switching elements, using oil circuit breakers. Today, oil-filled equipment has largely been replaced by air-blast, vacuum, or SF6 equipment, allowing large currents and power levels to be safely controlled by automatic equipment incorporating digital controls, protection, metering and communications.
A View of Switchgear at a Power Plant
Types A piece of switchgear may be a simple open air isolator switch or it may be insulated by some other substance. An effective although more costly form of switchgear is "gas insulated switchgear" (GIS), where the conductors and contacts are insulated by pressurized (SF6) sulfur hexafluoride gas. Other common types are oil [or vacuum] insulated switchgear. Circuit breakers are a special type of switchgear that are able to interrupt fault currents. Their construction allows them to interrupt fault currents of many hundreds or thousands of amps. The quenching of the arc when the contacts open requires careful design, and falls into four types: Oil circuit breakers rely upon vaporization of some of the oil to blast a jet of oil through the arc. Gas (SF6) circuit breakers sometimes stretch the arc using a magnetic field, and then rely upon the dielectric strength of the SF6 to quench the stretched arc.
Vacuum circuit breakers have minimal arcing (as there is nothing to ionize other than the contact material), so the arc quenches when it is stretched a very small amount (<2-3 mm). Vacuum circuit breakers are frequently used in modern medium-voltage switchgear to 35,000 volts. Air circuit breakers may use compressed air to blow out the arc, or alternatively, the contacts are rapidly swung into a small sealed chamber, the escaping of the displaced air thus blowing out the arc. Circuit breakers are usually able to terminate all current flow very quickly: typically between 30 ms and 150 ms depending upon the age and construction of the device. Several different classifications of switchgear can be made:
By the current rating:
By interrupting rating (maximum short circuit current that the device can safely interrupt)
Circuit breakers can open and close on fault currents
Load-break/Load-make switches can switch normal system load currents
Isolators may only be operated while the circuit is dead, or the load current is very small.
By voltage class:
Low Tension (less than 440 volts AC)
High Tension (more than 6.6 kV AC)
By insulating medium:
Air
Gas (SF6 or mixtures)
Oil
Vacuum
By construction type:
Indoor (further classified by IP (Ingress Protection) class or NEMA enclosure type)
Outdoor
Industrial
Utility
Marine
Draw-out elements (removable without many tools)
Fixed elements (bolted fasteners)
Live-front
Dead-front
Open
Metal-enclosed
Metal-clad
Metal enclose & Metal clad
Arc-resistant
High Tension Switchgear at Thermal Power Plant
By IEC degree of internal separation:
No Separation
Bus bars separated from functional units
Terminals for external conductors separated from bus bars
Terminals for external conductors separated from functional units but not from each other
Functional units separated from each other
Terminals for external conductors separated from each other
Terminals for external conductors separate from their associated functional unit
By interrupting device:
Fuses
Air Blast Circuit Breaker
Minimum Oil Circuit Breaker
Oil Circuit Breaker
Vacuum Circuit Breaker
Gas (SF6) Circuit breaker
By operating method:
Manually-operated
Motor-operated
Solenoid/stored energy operated
By type of current:
Alternating current
Direct current
By application:
Transmission system
Distribution.
A single line-up may incorporate several different types of devices, for example, air-insulated bus, vacuum circuit breakers, and manually-operated switches may all exist in the same row of cubicles. Ratings, design, specifications and details of switchgear are set by a multitude of standards. In North America mostly IEEE and ANSI standards are used, much of the rest of the world uses IEC standards, sometimes with local national derivatives or variations.
Functions One of the basic functions of switchgear is protection, which is interruption of short-circuit and overload fault currents while maintaining service to unaffected circuits. Switchgear also provides isolation of circuits from power supplies. Switchgear also is used to enhance system availability by allowing more than one source to feed a load.
Safety To help ensure safe operation sequences of switchgear, trapped key interlocking provides predefined scenarios of operation. James Harry Castell invented this technique in 1922. For example, if only one of two sources of supply is permitted to be connected at a given time, the interlock scheme may require that the first switch must be opened to release a key that will allow closing the second switch. Complex schemes are possible.
HIGH TENSION SWITCHGEAR High voltage switchgear is any switchgear and switchgear assembly of rated voltage higher than 1000 volts. High voltage switchgear is any switchgear used to connect or to disconnect a part of a high voltage power system. These switchgears are essential elements for the protection and for a safety operating mode without interruption of a high voltage power system. This type of equipment is really important because it is directly linked to the quality of the electricity supply. The high voltage is a voltage above 1000 V for alternating current and above 1500 V for direct current.
High Tension Switchgear of a Thermal Power Plant
The high voltage switchgear was invented at the end of the 19th century for operating the motors and others electric machines. It has been improved and it can be used in the whole range of high voltage until 1100 kV.
Functional Classification Disconnectors and Earthing Switches They are above all safety devices used to open or to close a circuit when there is no current through them. They are used to isolate a part of a circuit, a machine, a part of an overhead-line or an underground line for the operating staff to access it without any danger.
The opening of the line isolator or busbar section isolator is necessary for the safety but it is not enough. Grounding must be done at the upstream sector and the downstream sector on the device which they want to intervene thanks to the earthing switches. In principle, disconnecting switches do not have to interrupt currents, but some of them can interrupt currents (up to 1600 A under 10 to 300V) and some earthing switches must interrupt induced currents which are generated in a non-current-carrying line by inductive and capacitive coupling with nearby lines (up to 160 A under 20 kV) ).
A Vacuum Circuit Breaker (High Tension Switchgear)
High-Current Switching Mechanism They can open or close a circuit in normal load. Some of them can be used as a disconnecting switch. But if they can create a short-circuit current, they can not interrupt it.
Contactor Their functions are similar to the high-current switching mechanism, but they can be used at higher rates. They have a high electrical endurance and a high mechanical endurance. Contactors are used to frequently operate device like electric furnaces, high voltage motors. They cannot be used as a disconnecting switch. They are used only in the band 30 kV to 100 kV.
Fuses The fuses can interrupt automatically a circuit with an overcurrent flowing in it for a fixed time. The current interrupting is got by the fusion of an electrical conductor which is graded.
They are mainly used ot protect against the short-circuits. They limit the peak value of the fault current. In three-phase electric power, they only eliminate the phases where the fault current is flowing, which is a risk for the devices and the people. Against this trouble, the fuses can be associated with high-current switches or contactors. They are used only in the band 30 kV to 100 kV.
Circuit Breaker A high voltage circuit breaker is capable of making, carrying and breaking currents under the rated voltage (the maximal voltage of the power system which it is protecting) : Under normal circuit conditions, for example to connect or disconnect a line in a power system; Under specified abnormal circuit conditions especially to eliminate a short circuit. From its characteristics, a circuit breaker is the protection device essential for a high voltage power system, because it is the only one able to interrupt a short circuit current and so to avoid the others devices to be damaged by this short circuit. The international standard IEC 62271-100 defines the demands linked to the characteristics of a high voltage circuit breaker. The circuit breaker can be equipped with electronic devices in order to know at any moment their states (wear, gaz pressure…) and possibly to detect faults from characteristics derivatives and it can permit to plan maintenance operations and to avoid failures. To operate on long lines, the circuit breakers are equipped with a closing resistor to limit the overvoltages. They can be equipped with devices to synchronize the closing and/or the opening to limit the overvoltages and the inrush currents from the lines, the unloaded transformers, the shunt reactances and the capacitor banks. Some devices are designed to have the characteristics of the circuit breaker and the disconnector. But their use is limited.
DIRECT ON LINE STARTER A direct on line starter, often abbreviated DOL starter, is a widely-used starting method of electric motors. The term is used in electrical engineering and associated with electric motors. There are many types of motor starters, the simplest of which is the DOL starter. A motor starter is an electrical/electronic circuit composed of electro-mechanical and electronic devices which are employed to start and stop an electric motor. Regardless of the motor type (AC or DC), the types of starters differ depending on the method of starting the motor. A DOL starter connects the motor terminals directly to the power supply. Hence, the motor is subjected to the full voltage of the power supply. Consequently, high starting current flows through the motor. This type of starting is suitable for small motors below 5 hp (3.75 kW). Reduced-voltage starters are employed with motors above 5 hp. Although DOL motor starters are available for motors less than 150 kW on 400 V and for motors less than 1 MW on 6.6 kV. Supply reliability and reserve power generation dictates the use of reduced voltage or not.
Internal View of a Direct On Line Starter
Major Components There are four major components of a Direct On Line Starter. They are given as follows: 1. Switch 2. Fuse 3. Conductor (Electromagnetic) 4. Thermal Overload Relay (Heat & Temperature)
Auxiliary Components According to our desire and use of work, we use auxiliary components in a DOL Starter. There are basically two types of Auxiliary Components given as follows: 1. Auxiliary Conductor 2. Timer (Range – 0.5s to 60s)
DOL Reversing Starter Most motors are reversible or, in other words, they can be run clockwise and anti-clockwise. A reversing starter is an electrical or electronic circuit that reverses the direction of a motor automatically. Logically, the circuit is composed of two DOL circuits; one for clockwise operation and the other for anti-clockwise operation.
External View of a Direct On Line Starter
Example of Motor Starters A very well-known motor starter is the DOL Starter of a 3-Phase Squirrel-Cage Motor. This starter is sometimes used to start water pumps, compressors, fans and conveyor belts. With a 400V, 50 Hz, 3-phase supply, the power circuit connects the motor to 400V. Consequently, the starting current may reach 3-8 times the normal current. The control circuit is typically run at 24V with the aid of a 400V/24V transformer.
Motor Direction Reversal Changing the direction of a 3-Phase Squirrel-Cage Motor requires swapping any two phases. This could be achieved by a contactor KM1 swapping phase L2 and L3 between the supply and the motor.
Module - II
EMD - II • Generator • Protection • Transformer
GENERATORS The basic function of the generator is to convert mechanical power, delivered from the shaft of the turbine, into electrical power. Therefore a generator is actually a rotating mechanical energy converter. The mechanical energy from the turbine is converted by means of a rotating magnetic field produced by direct current in the copper winding of the rotor or field, which generates three-phase alternating currents and voltages in the copper winding of the stator (armature). The stator winding is connected to terminals, which are in turn connected to the power system for delivery of the output power to the system.
A 210 MW Turbine Generator at Badarpur Thermal Power Station, ew Delhi
The class of generator under consideration is steam turbine-driven generators, commonly called turbo generators. These machines are generally used in nuclear and fossil fueled power plants, co-generation plants, and combustion turbine units. They range from relatively small machines of a few Megawatts (MW) to very large generators with ratings up to 1900 MW. The generators particular to this category are of the two- and four-pole design employing round-rotors, with rotational operating speeds of 3600 and 1800 rpm in North America, parts of Japan, and Asia (3000 and 1500 rpm in Europe, Africa, Australia, Asia, and South America). At Badarpur Thermal Power Station 3000 rpm, 50 Hz generators are used of capacities 210 MW and 95 MW. As the system load demands more active power from the generator, more steam (or fuel in a combustion turbine) needs to be admitted to the turbine to increase power output. Hence more energy is transmitted to the generator from the turbine, in the form of a torque. This torque is
mechanical in nature, but electromagnetically coupled to the power system through the generator. The higher the power output, the higher the torque between turbine and generator. The power output of the generator generally follows the load demand from the system. Therefore the voltages and currents in the generator are continually changing based on the load demand. The generator design must be able to cope with large and fast load changes, which show up inside the machine as changes in mechanical forces and temperatures. The design must therefore incorporate electrical current-carrying materials (i.e., copper), magnetic flux-carrying materials (i.e., highly permeable steels), insulating materials (i.e., organic), structural members (i.e., steel and organic), and cooling media (i.e., gases and liquids), all working together under the operating conditions of a turbo generator.
An open Electric Generator at Power Plant
Since the turbo generator is a synchronous machine, it operates at one very specific speed to produce a constant system frequency of 50 Hz, depending on the frequency of the grid to which it is connected. As a synchronous machine, a turbine generator employs a steady magnetic flux passing radially across an air gap that exists between the rotor and the stator. (The term “air gap” is commonly used for air- and gas-cooled machines). For the machines in this discussion, this means a magnetic flux distribution of two or four poles on the rotor. This flux pattern rotates with the rotor, as it spins at its synchronous speed. The rotating magnetic field moves past a three-phase symmetrically distributed winding installed in the stator core, generating an alternating voltage in the stator winding. The voltage waveform created in each of the three phases of the stator winding is very nearly sinusoidal. The output of the stator winding is the three-phase power, delivered to the power system at the voltage generated in the stator winding. In addition to the normal flux distribution in the main body of the generator, there are stray fluxes at the extreme ends of the generator that create fringing flux patterns and induce stray losses in the generator. The stray fluxes must be accounted for in the overall design.
Generators are made up of two basic members, the stator and the rotor, but the stator and rotor are each constructed from numerous parts themselves. Rotors are the high-speed rotating member of the two, and they undergo severe dynamic mechanical loading as well as the electromagnetic and thermal loads. The most critical component in the generator are the retaining rings, mounted on the rotor. These components are very carefully designed for high-stress operation. The stator is stationary, as the term suggests, but it also sees significant dynamic forces in terms of vibration and torsional loads, as well as the electromagnetic, thermal, and high-voltage loading. The most critical component of the stator is arguably the stator winding because it is a very high cost item and it must be designed to handle all of the harsh effects described above. Most stator problems occur with the winding.
STATOR The stator winding is made up of insulated copper conductor bars that are distributed around the inside diameter of the stator core, commonly called the stator bore, in equally spaced slots in the core to ensure symmetrical flux linkage with the field produced by the rotor. Each slot contains two conductor bars, one on top of the other. These are generally referred to as top and bottom bars. Top bars are the ones nearest the slot opening (just under the wedge) and the bottom bars are the ones at the slot bottom. The core area between slots is generally called a core tooth.
Stator of a Turbo Generator
The stator winding is then divided into three phases, which are almost always wye connected. Wye connection is done to allow a neural grounding point and for relay protection of the winding. The three phases are connected to create symmetry between them in the 360 degree arc of the stator bore. The distribution of the winding is done in such a way as to produce a 120 degree difference in voltage peaks from one phase to the other, hence the term “three-phase voltage.” Each of the three phases may have one or more parallel circuits within the phase. The
parallels can be connected in series or parallel, or a combination of both if it is a four-pole generator. This will be discussed in the next section. The parallels in all of the phases are essentially equal on average, in their performance in the machine. Therefore, they each “see” equal voltage and current, magnitudes and phase angles, when averaged over one alternating cycle. The stator bars in any particular phase group are arranged such that there are parallel paths, which overlap between top and bottom bars. The overlap is staggered between top and bottom bars. The top bars on one side of the stator bore are connected to the bottom bars on the other side of the bore in one direction while the bottom bars are connected in the other direction on the opposite side of the stator. This connection with the bars on the other side of the stator creates a “reach” or “pitch” of a certain number of slots. The pitch is therefore the number slots that the stator bars have to reach in the stator bore arc, separating the two bars to be connected. This is always less than 180 degrees. Once connected, the stator bars form a single coil or turn. The total width of the overlapping parallels is called the “breadth.” The combination of the pitch and breadth create a “winding or distribution factor.” The distribution factor is used to minimize the harmonic content of the generated voltage. In the case of a two parallel path winding, these may be connected in series or parallel outside the stator bore, at the termination end of the generator. The connection type will depend on a number of other design issues regarding current-carrying ability of the copper in the winding. In a two-parallel path, three-phase winding, alternating voltage is created by the action of the rotor field as it moves past these windings. Since there is a plus and minus, or north and south, to the rotating magnetic field, opposite polarity currents flow on each side of the stator bore in the distributed winding. The currents normally flowing in large turbo generators can be in the order of thousands of amperes. Due to the very high currents, the conductor bars in a turbo generator have a large cross-sectional area. In addition they are usually one single turn per bar, as opposed to motors or small generators that have multiple turn bars or coils. These stator or conductor bars are also very rigid and do not bend unless significant force is exerted on them.
ROTOR The rotor winding is installed in the slots machined in the forging main body and is distributed symmetrically around the rotor between the poles. The winding itself is made up of many turns of copper to form the entire series connected winding. All of the turns associated with a single slot are generally called a coil. The coils are wound into the winding slots in the forging, concentrically in corresponding positions on opposite sides of a pole. The series connection essentially creates a single multi-turn coil overall, that develops the total ampere-turns of the rotor (which is the total current flowing in the rotor winding times the total number of turns).
There are numerous copper-winding designs employed in generator rotors, but all rotor windings function basically in the same way. They are configured differently for different methods of heat removal during operation. In addition almost all large turbo generators have directly cooled copper windings by air or hydrogen cooling gas.
Rotor of a Turbo Generator
Cooling passages are provided within the conductors themselves to eliminate the temperature drop across the ground insulation and preserve the life of the insulation material. In an “axially” cooled winding, the gas passes through axial passages in the conductors, being fed from both ends, and exhausted to the air gap at the axial center of the rotor. In other designs, “radial” passages in the stack of conductors are fed from sub slots machined along the length of the rotor at the bottom of each slot. In the “air gap pickup” method, the cooling gas is picked up from the air gap, and cooling is accomplished over a relatively short length of the rotor, and then discharged back to the air gap. The cooling of the end-regions of the winding varies from design to design, as much as that of the slot section. In smaller turbine generators the indirect cooling method is used (similar to indirectly cooled stator windings), where the heat is removed by conduction through the ground insulation to the rotor body.
The winding is held in place in the slots by wedges, in a similar manner as the stator windings. The difference is that the rotor winding loading on the wedges is far greater due to centrifugal forces at speed. The wedges therefore are subjected to a tremendous static load from these forces and bending stresses because of the rotation effects. The wedges in the rotor are not generally a tight fit in order to accommodate the axial thermal expansion of the rotor winding during operation. There are also many available designs and configurations for the end-winding construction and ventilation methods. As in the rotor slots, the copper turns in the end-winding must be isolated from one another so that they do not touch and create shorts between turns. Therefore packing and blocking are used to keep the coils separated, and in their relative position as the rotor winding expands from thermal effects during operation. To restrain the end winding portion of the rotor winding during high-speed operation, retaining-rings are employed to keep the copper coils in place.
BEARINGS All turbo generators require bearings to rotate freely with minimal friction and vibration. The main rotor body must be supported by a bearing at each end of the generator for this purpose. In some cases where the rotor shaft is very long at the excitation end of the machine to accommodate the slip/collector rings, a “steady” bearing is installed outboard of the slipcollector rings. This ensures that the excitation end of the rotor shaft does not create a wobble that transmits through the shaft and stimulates excessive vibration in the overall generator rotor or the turbo generator line. There are generally two common types of bearings employed in large generators, “journal” and “tilting pad” bearings. Journal bearings are the most common. Both require lubricating and jacking oil systems, which will be discussed later in the book, under auxiliary systems. When installing the bearings, they must be aligned in terms of height and angle to ensure that the rotor “sits” in the bearing correctly. Such things as shaft “catinery” must be considered and “pre-loading” or “shimming” of the bearings to account for the difference when the rotor is at standstill and at speed. Getting any of these things wrong in the assembly can cause the rotor to vibrate excessively and damage either the rotor shaft or the bearing itself. Generally, a “wipe” of the bearing running surface or “babbitt” results.
AUXILIARY SYSTEMS All large generators require auxiliary systems to handle such things as lubricating oil for the rotor bearings, hydrogen cooling apparatus, hydrogen sealing oil, de-mineralized water for stator winding cooling, and excitation systems for field-current application. Not all generators require all these systems and the requirement depends on the size and nature of the machine. For instance, air cooled turbo generators do not require hydrogen for cooling and therefore no sealing oil as well. On the other hand, large generators with high outputs, generally above 400
MVA, have water-cooled stator windings, hydrogen for cooling the stator core and rotor, seal oil to contain the hydrogen cooling gas under high pressure, lubricating oil for the bearings, and of course, an excitation system for field current. There are five major auxiliary systems that may be used in a generator. They are given as follows: 1. Lubricating Oil System 2. Hydrogen Cooling System 3. Seal Oil System 4. Stator Cooling Water System 5. Excitation System Each system has numerous variations to accommodate the hundreds of different generator configurations that may be found in operation. But regardless of the generator design and which variation of a system is in use, they all individually have the same basic function as described before.
1. Lubricating Oil System The lube-oil system provides oil for all of the turbine and generator bearings as well as being the source of seal oil for the seal-oil system. The lube-oil system is generally grouped in with the turbine components and is not usually looked after by the generator side during maintenance. It is mentioned primarily for completeness.
Lubricating Oil System Layout
The main components of the lube-oil system consists generally of the main lube-oil tank, pumps, heat exchangers, filters and strainers, centrifuge or purifier, vapor extractor, and various check valves and instrumentation. The main oil tank serves both the turbine and generator bearing and is often also the source of the sealing oil for the hydrogen seals. It is usually located under the turbines and holds thousands of gallons of oil.
Heat exchangers are provided for heat removal from the lube oil. Raw water from the local lake or river is circulated on one side of the cooler to remove the heat from the lube oil circulating on the other side of the heat exchanger. Full flow filters and/or strainers, or a combination of both, are employed for removal of debris from the lube oil. Strainers are generally sized to remove larger debris and filters for debris in the range of a few microns and larger. They can be mechanical or organic type filters and strainers. Debris removal is important to reduce the possibility of scoring the bearing Babbitt or plugging of the oil lines. A centrifuge or purifier is used to remove moisture from the oil. Moisture is also a contaminant to oil and can cause it to lose its lubricating properties.
2. Hydrogen Cooling System As the hydrogen cooling gas picks up heat from the various generator components within the machine, its temperature rises significantly. This can be as much as 46oC, and therefore the hydrogen must be cooled down prior to being re-circulated through the machine for continuous cooling. Hydrogen coolers or heat exchangers are employed for this purpose. Hydrogen coolers are basically heat exchangers mounted inside the generator in the enclosed atmosphere. Cooling tubes with “fins” are used to enlarge the surface area for cooling, as the hydrogen gas passes over the outside of the finned tubes. “Raw water” (filtered and treated) from the local river or lake is pumped through the tubes to take the heat away from the hydrogen gas and outside the generator. The tubes must be extremely leak-tight to ensure that hydrogen gas does not enter into the tubes, since the gas is at a higher pressure than the raw water.
3. Seal Oil System As most large generators use hydrogen under high pressure for cooling the various internal components. To keep the hydrogen inside the generator, various places in the generator are required to seal against hydrogen leakage to atmosphere. One of the most difficult seals made is the juncture between the stator and the rotating shaft of the rotor. This is done by a set of hydrogen seals at both ends of the machine. The seals may be of the journal (ring) type or the thrust-collar type. But one thing both arrangements have in common is the requirement of highpressure oil into the seal to make the actual “seal.” The system, which provides the oil to do this, is called the seal-oil system. In general, the most common type of seal is the journal type. This arrangement functions by pressurized oil fed between two floating segmented rings, usually made of bronze or Babbitt steel. At the ring outlet, against the shaft, oil flows in both directions from the seals along the rotating shaft. For the thrust-collar type, the oil is fed into a Babbitt running face via oil delivery ports, and makes the seal against the rotating thrust collar. Again, the oil flows in two directions, to the air side and the hydrogen side of the seals. The seal oil itself is actually a portion of the lube oil, diverted from the lubricating oil system. It is then fed to a separate system of its own with pumps, motors, hydrogen detraining or vacuum degassing equipment, and controls to regulate the pressure and flow.
Seal Oil System – Packaged Unit
The seal-oil pressure at the hydrogen seals is maintained generally about 15 psi above the hydrogen pressure to stop hydrogen from leaking past the seals. The differential pressure is maintained by a controller to ensure continuous and positive sealing at all times when there is hydrogen in the generator. One of the critical components of the seal oil system is the hydrogen degasifying plant. The most common method of removing entrained hydrogen and other gases is to vacuum-treat the seal oil before supplying it to the seals. This is generally done in the main seal oil supply tank. As the oil is pulled into the storage tank under vacuum, through a spray nozzle, the seal oil is broken up into a fine spray. This allows the removal of dissolved gases. In addition there is often a re-circulating pump to re-circulate oil back to the tank through a series of spray nozzles for continuous gas removal. After passing through the generator shaft seals, the oil goes through the detraining sections before it returns to the bearing oil drain. As a safety feature there is often a dc motor driven emergency seal-oil pump provided. This motor will start automatically on loss of oil pressure from the main seal-oil pump. This is to ensure that the generator can be shut down safely without risk to personnel or the equipment.
4. Stator Cooling Water System The stator cooling water system (SCW) is used to provide a source of de-mineralized water to the generator stator winding for direct cooling of the stator winding and associated components. SCW is generally used in machines rated at or above 300 MVA. Most SCW systems are provided as package units, mounted on a singular platform, which includes all of the SCW system components. All components of the system are generally made from stainless steel or copper materials.
Stator Water Cooling System
System Components Pumps: Generally, ac motor driven pumps are used to deliver the cooling water to the windings. In some instances a dc motor driven pump is used for emergency shutdown. Heat Exchangers: Heat exchangers are provided for heat removal from the SCW. Raw water from the local lake or river is circulated on one side of the cooler to remove the heat from the demineralized SCW circulating on the other side of the heat exchanger. Filters and/or Strainers: Full-flow filters and/or strainers, or a combination of both, are employed for removal of debris from the SCW. Strainers are generally sized to remove debris in the 20 to 50 µ range and larger and filters for debris in the range of 3 µ and larger. They can be mechanical or organic type filters and strainers. Debris removal is important to reduce the possibility of plugging in the stator conductor bar strands. De-Ionizing Subsystem: A de-ionizing subsystem is required to maintain low conductivity in the SCW, generally in the order of 0.1 µS/cm. High conductivity can cause a flashover to ground in the stator winding, particularly at the Teflon hoses where an internal tracking path to ground exists. The system generally maintains a continuous bleed-off of 5% from the main SCW flow to keep the conductivity in the operable range. Stator Cooling Water System Storage or Makeup Tank: In the event the SCW is lost, or the SCW system must be refilled after shutdown and draining, the system requires replenishing. Therefore a storage tank to hold sufficient makeup water is required. Some systems are open to
atmosphere while others maintain a hydrogen blanket on top of the water to keep the level of oxygen at a minimum. Gas Collection and Venting Arrangement: Since no SCW system is leak proof, there is some ingress of hydrogen and natural collection of other gases such as oxygen in the SCW system. A means for venting off these gases is required. Generally, the excess gases are vented to atmosphere. In some systems the venting process is monitored and/or quantified and in other systems there is none. This is manufacturer-specific.
5. Excitation System Rotating commutator exciters as a source of DC power for the AC generator field generally have been replaced by silicon diode power rectifier systems of the static or brushless type. •
A typical brushless system includes a rotating permanent magnet pilot exciter with the stator connected through the excitation switchgear to the stationary field of an AC exciter with rotating armature and a rotating silicon diode rectifier assembly, which in turn is connected to the rotating field of the generator. This arrangement eliminates both the commutator and the collector rings. Also, part of the system is a solid state automatic voltage regulator, a means of manual voltage regulation, and necessary control devices for mounting on a remote panel. The exciter rotating parts and the diodes are mounted on the generator shaft; viewing during operation must utilize a strobe light.
Schematic Diagram of Excitation System (Brushless)
•
A typical static system includes a three-phase excitation potential transformer, three singlephase current transformers, an excitation cubicle with field breaker and discharge resistor, one automatic and one manual static thyristor type voltage regulators, a full wave static
rectifier, necessary devices for mounting on a remote panel, and a collector assembly for connection to the generator field.
PROTECTION The protection system of any modern electric power grid is the most crucial function in the system. Protection is a system because it comprises discrete devices (relays, communication means, etc.) and an algorithm that establishes a coordinated method of operation among the protective devices. This is termed coordination. Thus, for a protective system to operate correctly, both the settings of the individual relays and the coordination among them must be right. Wrong settings might result in no protection to the protected equipment and systems, and improper coordination might result in unwarranted loss of production. The key function of any protective system is to minimize the possibility of physical damage to equipment due to a fault anywhere in the system or from abnormal operation of the equipment (over speed, under voltage, etc.). However, the most critical function of any protective scheme is to safeguard those persons who operate the equipment that produces, transmits, and utilizes electricity. Protective systems are inherently different from other systems in a power plant (or for that matter any other place where electric power is present). They are called to operate seldom, and when they are, it is crucial they do so flawlessly. One problem that arises from protective systems being activated not often is that they are sometimes overlooked. This is a recipe for disaster. The most common reason for catastrophic failure of equipment in power systems is failure to operate or miss-operation of protective systems. Purchasing, installing, setting/coordinating, and properly maintaining protective systems are not an insignificant expense. Therefore the extent any device or electric circuit is protected depends on the potential cost of not doing so adequately. Electric power generators are most often the most critical electrical apparatus in any power plant. In fact, given the electrical proximity between the generator and the main step-up transformer (SUT), those two most important apparatuses share some of the protective functions. Given the prohibited cost of replacing any of these two, in particular, the generator, significant expense goes in providing the most comprehensive protection coverage. Protection is considered by many an art as much as a science. Although the basic protective components are well known, and the commonly used settings for those devices are spelled out in a number of standards and other widely available literature, the particular combination of protective relays, settings, and coordination schemes are particular to every site. Therefore it is impossible to describe or prescribe a single protective system for generators. The description we attempt here is on the most commonly encountered protection arrangements and functions. Protection systems can be divided into systems monitoring current, voltage (at the machine’s main terminals and excitation system), windings, and/or cooling media temperature and pressure, and systems monitoring internal activity, such as partial discharge, decomposition of organic insulation materials, water content, hydrogen impurities, and flux probes. Protective functions acting on the current, voltage, temperature, and pressure parameters are commonly referred to as primary protection. The others are referred to as secondary protection or monitoring devices. Secondary functions tend to be monitored real time, or on demand. For
instance, hydrogen purity is monitored on-line real time, while water content (for water leaks) is not. Temperature detectors (RTDs or thermocouples) on bearings (and sometimes in on windings) may be monitored on-line real time, or they may not. Furthermore these functions may more often than not result in an alarm, rather than directly trip the unit (e.g., core monitors). The discussion of where and when to use these monitoring devices and how to set them is provided in. To the primary protective functions monitoring currents, voltages, temperatures and pressures, there can be added the mechanical protective function of vibration. Typically it will alarm, but it can also be set to trip the unit. Protections function can also be divided into shortcircuit protection functions. The short-circuit protection comprises impedance, distance, and current differential protection.
Multi-function Generator Protection Device
GENERATOR PROTECTIVE FUNCTION Protection devices are designed to monitor certain conditions, and subsequently, to alarm or trip if a specified condition is detected. The condition is represented by a function or protective function code. Thus there is a relay for every protective function. If a relay only monitors and thus protects against a single set of conditions, it is said that the relay is a “single-function device”. In the past most relays were single-function devices. With the advent of solid-state electronics, manufacturers have combined several functions in one unit or device. These “multi-function” relays or protective devices offer specific protective functions designed for certain types of apparatus. Some multi-function relays are dedicated to transformers, others to motors, and others to generators. Advances in solid-state electronics have led to less costly devices. Today a multi-function solid-state device with, for instance, five protective functions, is less expensive than five separate relays for five protective functions. The number of functions covered by different relays and the number of multifunction devices are decided, among other things, by the expected losses of all the protective functions covered by the multi-functional relay, if that particular device becomes faulty. A multi-functional relay
containing all the protective functions required for the protection of a generator can be combined with a few discrete relays providing backup protection for critical functions. Alternatively, two or more multi-functional relays can be applied, providing partial or comprehensive redundancy. There are many combinations of these discrete and multi-functional relays that can be adopted, depending on when the power plant was build, the size of the units, system conditions, the idiosyncrasy of the designer, and many other factors. Relays or protection devices are divided into two categories according to how they process data. The first category is that of analog relays; the second is that of numerical (also called digital) relays. Bear in mind that a relay can be electronic but still process the data in an analog manner. The advantages of numerical processing are various. Accuracy is enhanced. So is flexibility in use. For instance, a numerical relay offers user-shaped protection widows such that the user can change the shape of the operation/non-operation areas for a specific function of the relay. Furthermore the shape of the region of operation may change according to system conditions (adaptive function). Finally, there is rather a new—still evolving—approach (from the early 1990s) for protecting large generating units by the so-called expert protection systems. The idea is to protect the unit based not only on the basic protective functions (given below), but also as a combination of protective and monitoring data and built-in expertise in the form of diagnostic prescriptions. Invariably, building the expertise base of these systems consists in expressing probable causes for a particular combination of symptoms, expressed as a probabilistic tree. A number, according to a worldwide-accepted nomenclature, identifies protective functions. The functions shown in table are typical of generation protection. A number of the functions included in table are so important that they will always find their way into the protection scheme of any generator (e.g., 25, 59, and 87). Others may be omitted in some applications (e.g., 49). The larger and more expensive the generator and the more critical the application, the more intense is the protection applied to protect it from abnormal operating conditions or faults. As explained before, for most large machines, some of the applied protective functions are covered by more than one relay or protective device. Table: Generator Protection Device Function Numbers Synchronizer 15 Distance protection; backup for system generator zone phase faults 21 Volts/Hertz protection for the generator 24 Sync-check protection 25 Under voltage 27 Reverse power protection; anti-motoring protection for generator (and associated prime mover) 32 Loss-of-field protection 40 Stator unbalanced current protection 46 Stator thermal protection 49 Instantaneous over current protection used as current detector in a breakerfailure scheme 50B
51GN 51TN 51V 59 59BG 59GN 60 61 62B 64F 78 81 86 87B 87GN 87T 87U 94
Time over current protection; backup for generator ground faults Time over current protection; backup for ground faults Voltage-controlled or voltage-restrained time over current protection; backup for system and generator zone phase faults Overvoltage protection Zero-sequence voltage protection; ground fault protection for an ungrounded bus Voltage protection; primary ground fault protection for a generator Voltage balance protection; detection of blown potential transformer fuses or otherwise open circuits Time over current protection; detection of turn-to-turn faults in generator windings Breaker failure protection Voltage protection; primary protection for rotor ground faults Loss-of-synchronism protection; not commonly used as part of the generator protection package Over- and under frequency protection Hand-reset lockout auxiliary relay Differential protection. Primary phase-fault protection for the generator Sensitive ground fault protection for the generator Differential protection for the transformer; may include the generator in some protective schemes Differential protection for overall unit protection of generator and transformers Self-reset auxiliary tripping relay
It is beyond the scope and purpose of this report to go into a detailed description of each protective function and the various schemes that incorporate them into a generator’s protection package. Instead, a basic description of the protective functions and their application will follow. For the same reason no specific values are recommended for setting protective relays. These values oftentimes depend in the particular machine and system to which it is connected. There are numerous sources for information on the setting of protective relays. The vendors’ manuals are one good place to start. Various methods of Generator Protection are explained further.
Synchronizer and Sync-Check Relays (Functions 15 and 25) The combination of function (15) with function (25) provides the means by which the unit can be brought up to speed automatically and synchronized to the system. Before doing so, the amplitude of the voltages of the system and generator terminal must be within a narrow margin so that the breaker can be closed. So must be the angle of the terminal and system voltages. The slip, which is the frequency difference between the machine and the system, must be lower than a given value. Almost always two relays are provided: the synchronizer and the sync-check. This division of labor is based on the need to avoiding the destructive results of synchronizing a unit out of step due to the failure of a single protective device. In older installations, mainly with steam-driven units, it is customary to start and bring the unit up to speed under manual control. Closing the breaker is done manually while the sync-check
relay monitors all voltages, vector angles, and frequencies, making sure they are within their prescribed values. Although seldom encountered, some operators close the breaker by keeping the “close” button depressed when the unit is brought to the right speed and voltages, letting the angle be taken care by the sync-check relay. This practice has resulted in more than one unit synchronizing out of step due to a failure of the relay (function 25). The failure can be catastrophic. Thus it is imperative that during manual operation the actual breaker-closing signal be sent when the conditions for synchronization are met; leaving the sync-check system as a backup device, as it is supposed to be.
Short-Circuit Protection (Functions 21, 50, 51, 51V, and 87) These functions are designed to protect the unit against short-circuits in or outside the windings of the alternator. Outside faults can be in the system close to the station’s busses, on the main unit transformer or auxiliary transformer(s), on the cable, segregated busses, or insulated phase busses (IPB), between the alternator and the transformers, or on the alternator’s windings. In large units the IPB is designed to reduce any short-circuit between the generator and main and auxiliary transformers to a single phase-to-ground fault. This is possible because of the highimpedance grounding of the machine, and the fact that all transformers connected to the generator are connected delta on the generator’s side, which results in ground faults of very low currents. However, a “benign” single-ground fault inside the generator can develop into a highly destructive phase-to-phase short-circuit, and this is the main reason why ground faults inside the generator ought to trip to unit promptly.
Short Circuit and Volts/Hertz Protection Device
The (51V) is a voltage-controlled over current relay, where the voltage control is provided to differentiate between a low-current fault and a normal or abnormally high load condition. To some extent most of these functions back each other up. Thus occasionally some are omitted. Additionally current-based relays are backed in the detection of short-circuit events by some voltage-based relays. A typical case is on the ground-fault detection scheme of the generator with high-impedance grounding via a transformer. The differential protection function (87) is the most critical as it provides protection against the very serious phase-to-phase short circuits. Normally there are at least three protected areas, each one covered by its own 87 relay. One is
the generator itself. The other covers the auxiliary transformer, and the third covers the main transformer, generator, and low-voltage side of the auxiliary transformer. Each 87 scheme utilizes a dedicated set of current transformers. The ground protection schemes in use today often incorporate a third-harmonic function. This addition to the standard overvoltage and/or over current relays is based on the fact that during normal operation of the generator a given amount of third-harmonic voltages are present, and during a ground fault these third harmonic voltages are highly reduced. This fact is used for protection of the third of the generator’s winding close to the neutral, where ground-faults tend to generate very small neutral currents (and hence may not be detected by the neutral overvoltage or over current protection). Third-harmonic protective devices must be tested periodically, the same as any other protective functions. In some instances, no overload protection is provided, other than alarming and expected operator intervention. In others, function (51) relays are provided that will alarm, and then trip the unit under overload conditions. The overload can be extremely onerous if allowed to continue beyond the withstand capabilities of the windings.
Volts/Hertz Protection (Function 24) Core damage due to over excitation is a rare event. However, when a severe over excitation occurs, the most probable result is partial or complete destruction of the core’s insulation, with the consequential need to replace it. Therefore it is critical that V/Hz protection be applied and properly set. Almost invariably, the cases of severe over excitation occur during run-up, prior to synchronization. One vital component in all V/Hz schemes for any turbo generator is double feed from two independent potential transformers (PTs). Otherwise, loss of a single PT connection may give the excitation system wrong information about the terminal voltage, forcing the field current (and terminal voltage) beyond the V/Hz capability of the machine.
Over and Under Voltage Protection (Functions 59 and 27) Some voltage relays are used for short-circuit protection (on the neutral of the generator— 59GN). Overvoltage relays are also used as backup to the (24) (over excitation) during normal operation of the machine. During start-up, the (59) will not provide backup to the (24), because a V/Hz condition can readily develop during run-up, even while the terminal voltage is below its rated value. The under voltage relays are mainly installed for the purpose of identifying loss of PT voltage, or to identify dead-bus condition for certain alignments.
Reverse Power Protection (Function 32) This protective function trips the unit when power flows from the system to the generator. In this situation, depending on the generator’s field condition, the alternator is driven as a synchronous or induction motor. If it is driven as an induction motor, negative-sequence currents will be established in the rotor, potentially damaging damper windings, wedges, retaining-rings, and forging. This phenomenon is discussed elsewhere in this book. However, in either case, reverse power condition may adversely affect the integrity of the prime mover.
Of all the prime movers, steam turbines are the most sensitive to motoring. They also happen to operate on less power input (only a few percent of rated load, compared to combustion turbines requiring up to 50% of rated power). For these reasons steam-driven generators require sensitive settings for the reverse power relays (32), plus some additional protection that may be indicated.
Loss-of-Field Protection (Function 40) There are a number of events that may result in an accidental removal of the source of excitation to the generator. This can happen for both brushless and externally excited units. For instance, a unplanned opening of the field breaker, a failure of the exciter, a flashover in the brush-rigging, a failure of the automatic voltage regulator (AVR), and a short-circuit in the field winding, can all result in a loss-of-excitation condition. When a generator loses its excitation during normal operation, its speed increases by some amount of up to 3 to 5% of normal. The amount of speed increase depends on the generator’s load prior to losing its excitation. A lightly loaded unit will experience a much smaller increase in speed than one fully loaded. Additionally the stator current will normally increase because the generator without its field will operate as an induction machine, receiving its excitation VARs from the network. Accordingly the stator current may increase by up to 100% of its nominal value. The increase in line current will be aggravated by the overheating of rotor components, by the currents induced in the forging and damping winding if present, and by the overheating of the stator core-end regions. A fully loaded unit that loses its field may experience serious damage very quickly under these conditions. Therefore the protection against loss-of-field occurrences is set to alarm and trip the unit relatively quickly. The most widely utilized method of protecting against loss-of-field conditions is that relying on impedance elements. They are based on the fact that the impedance seen from the terminals of the machine follows a distinctive pattern when the field is lost. Sometimes two relays are used, each looking at the impedance within a different region of operation, so that a loss-of-field condition is captured regardless of the level of pre-fault loading. Sensing the field current directly or sensing the VAR power flowing into the generator is sometimes used for alarm and trip, but mainly for alarm and rarely as primary protection.
Stator Unbalanced Current Protection (Function 46) There are a number of incidents that may result in unbalanced three-phase currents at the terminals of an alternator: for instance, unbalance loads, single-pole opening of a breaker, asymmetrical transmission systems (without or with insufficient transposition) and open circuits. Unbalanced currents will result in negative-sequence current components flowing on the rotor forging surfaces, retaining-rings, rotor wedges, and to some extent in the field windings, in particular, the amortisseur. These rotor negative-sequence currents have the potential of generating high temperatures within seconds, with severe detrimental effects to specific areas of the forging and other rotor components. However, rotors with spindle-mounted retaining-rings are also susceptible to damage by negative-sequence currents.
Generators must meet minimal requirements for sustaining unbalance currents without damage. The protection against unbalanced currents is implemented by using over current relays that measure negative-sequence components. Electromechanical relays provide basic protection against most negative-sequence current conditions. However, digital relays allow setting the protected region of operation in such a way that closely matches the withstand capability of the protected generator. This allows a more sensitive and discriminatory approach.
Stator and Rotor Thermal Protection (Function 49) There are a number of conditions that may result in elevated temperature inside the generator. Presently available techniques allow directly monitoring temperatures of the stator winding, core, and cooling media. Rotor winding temperature, when monitored, is done by measuring field voltage and current, then calculating the rotor-field resistance, and comparing the obtained resistance with a known value of ohms at a known temperature. Conditions that may result in higher than normal temperatures are overload, core hot spots, bent laminations swelling into vent-ducts, winding failures, and cooling failure (clogged filters in air-cooled machines, lack of hydrogen pressure in hydrogen-cooled generators or failure of the hydrogen cooling system, and water blockage or other failure of the water cooling system in water-cooled units). There are other conditions that may result in higher temperatures such as unbalanced currents; however, these are detected and protected by other protective functions, so they are discussed elsewhere. In addition to the design limits of each machine (based on such things as temperature rise class and class of insulation), there are ANSI guidelines regarding minimum withstand capability requirements under overload conditions. For instance, at 130% overload, the machine should be able to operate without damage for a minimum of 60 seconds. These numbers show that once in an overload condition, the time available to remove the dangerous situation gets very short very fast. Typically generators have a number of RTDs (resistance temperature detectors) embedded in their stator windings, with a minimum of two per phase. In some designs these RTDs are wired to the control room via SCADA or DCS. In manned stations (all large turbo generators fall under this category, with exception of some “peaking” units), the winding RTDs are used for alarming (over current protection is used for high and sudden overload conditions). In unattended stations (mainly smaller machines) the output from the RTDs may be used to remotely alarm and to control and/or trip the unit. In the United States the standard RTD has a resistance of 25 ohms at 25◦C. When the RTDs are installed during original manufacture, the OEM will place the proper RTD. However, if for any reasons RTDs are installed by the operator (e.g., during a partial rewind or any other overhaul), the RTDs must match the operating temperature of the winding. This temperature will most likely be related to the temperature class of the unit and the insulation class. Some vendors of directly cooled stators-by-hydrogen generators omit the embedded RTDs. In lieu of them, they install a number of RTDs monitoring the hydrogen paths in the stator bars (in addition to other RTDs monitoring other areas along the machine’s gas flow path). The RTDs monitoring the flow of gas in the stator bars are normally installed in the exit boxes at the endwindings. If any overheating occurs in a bar or section of the winding, some RTDs will pick up the excess temperature in the gas flowing in that region. During troubleshooting activities
carried out on a number of hydrogen-cooled units with directly cooled stators, the authors ascertained that the existence of embedded RTDs, in addition to the gas-flow RTDs made it easier to determine the location of the faults in the coils. Therefore it is not a bad idea to specify generators with embedded RTDs (a minimum of two per phase), for those machines with gaspath-flow RTDs. All RTDs should be monitored, including the embedded ones. Another way for monitoring onerous temperatures in the stator is by applying tagging compounds. This technique can also be used to alarm for developing core problems. These tagging components may be used in hydrogen-cooled machines where a core monitor is installed. Core monitors have the ability to monitor and alarm against deterioration of the field winding due to overheating. Recall that for rotor winding, the only method of estimating the temperature rise of the winding is by measuring the field voltage and current at the collector, and comparing the resistance calculated thereof, with known value at known temperature. This technique is restricted to those machines with collector rings (external excitation). One method commonly used is to monitor the excitation current. As the excitation current exceeds certain nominal value, relays time the duration of the occurrence and trip the unit as soon as a certain setting is reached. The time-current characteristics followed by the protection try to match the withstand curves for short-time field overloading as contained in ANSI guidelines.
Voltage Balance Protection (Function 60) The main function of the voltage balance relay is to avoid false tripping of other protection relays due to a loss of secondary voltage feed—for instance, by a blown potential transformer (PT) fuse. Voltage balance schemes are possible in most modern and/or large generators because such units have at least two PTs feeding the protection and monitoring systems. The voltage balance relays senses and compares the secondary voltage of different PTs, and when it determines that a “blown-fuse” situation arises, it blocks the operation of certain voltage controlled relays and alarms. In those older alternators (or small units) where only one PT feeds the protective and excitation systems, it is still possible to sense and alarm for a blown-fuse condition. This is attained by using a scheme that compares negative-sequence voltages in the secondary of the PT (that will arise as consequence of a primary fault or a blown-fuse condition), with negative currents in the secondary of the current transformer (CT). If negative-sequence currents are not present, it indicates that a fault in the primary system does not occur, and thus it must be a blown fuse condition. This voltage/current negative-sequence comparative function can be found in certain modern digital protective packages.
Time over Current Protection for Detection of Turn-to-Turn Faults (Function 61) The most common stator winding design for large turbine generators is based on a single-turn arrangement. Smaller machines may sport multi-turn windings. For such machines with at least two parallel circuits, the “split-phase” protective scheme can be used for the protection against turn-to-turn short circuits. In this scheme the circuits in each phase are split into two equal
groups and the currents in each group are compared. Any significant difference would indicate an inter turn failure. The relays used are normally very inverse over current relays and instantaneous trip combination.
Breaker Failure Protection (Function 62B) Most faults involving the generator require tripping the line breakers. Failure of any such breaker to operate properly results in loss of protection and other abnormal conditions, such as motoring. Adverse conditions arise also if only one or two poles of a line breaker operates, for instance, resulting in energization from one line, or single-phase operation with the accompanying negative-sequence currents. Activation of a breaker failure scheme is carried out by a combination of triggering signals from the generator protective relays, over current relays and breaker auxiliary switches, via a timer. Some modified schemes also included in their triggering circuit the trip signal from the neutral of the main step-up transformer’s over current relay. This change is to protect against breaker head flashover, which is when arcing occurs across the breaker contacts due to high voltages. The protection is designed to operate against the flashover of two poles.
Rotor Ground Fault Protection (Function 64F) Rotor field-windings are designed to operate ungrounded. As a result a single short to ground, in theory, should not be reason for concern, because it will not interfere with the normal operation of the machine. However, the appearance of a second ground can be very detrimental to the operation of the generator, as well as to its integrity. In fact the existence of one ground fault will make a second more probable, due to induced field voltages resulting from stator transients. Two coetaneous grounds may result in the following: •
Unbalanced air/gas gap fluxes with increased rotor vibrations
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Unbalanced thermal heating of the rotor with increased vibrations
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Fluctuating VARs and output voltage
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Major damage to the forging by dc currents (dc currents are known to be able to produce arcs several inches long in the forging of a turbo generator’s rotor, during a double ground occurrence). There are enough cases documented of large rotors having to be replaced because of such events.
There are a number of methods in existence for the detection, alarming, and/or tripping of generators due to field ground faults. Some methods use a voltage source, and others use a passive unbalanced bridge. One can still found an older unit protected by an ac-source field-ground detection method. Such methods can damage the non insulated bearing of the generator if the grounding brush somehow becomes ineffective. To avoid this from happening, the newer designs have the ac source replaced with a dc, or any one of the nonlinear resistor-based bridge methods. Generators with brushless excitation cannot be directly protected with the type of schemes shown above, due to the lack of collector rings. Many designs of such units sport a set of small rings that periodically
can be temporarily connected to brushes, via which the tests described above can be performed. For large generators additional circuits can be found that complement the basic schemes. For instance, a circuit is added to verify that the brushes are sitting on the rotor.
Over/Under Frequency Protection (Function 81) Over and under frequency operation generally results from full or partial load rejection or overloading conditions. Load rejection can be caused by a fault in the system or load shedding. Overload conditions may arise from tripping a large generator or a transmission line. What frequency the machine will attain following load rejection or overload is a function of how much load has changed and the governor droop characteristics. For instance, a governor with a 5% droop characteristic will cause a 1.5% speed increase for a 30% load rejection. The manufacturers provide withstand curves that should be used in setting the Function (81) relay.
Over/Under Frequency Protection Device
Out-of-Step Operation (Loss of Synchronism) (Function 78) There are a number of reasons why a generator may lose synchronization to the system during operation. Regardless of the reason, loss of synchronization (out of step) can have serious detrimental effects to the generator. The end-windings and end-windings’ support are prone to damage and dislocation during such an event. Rotor and coupling damage is also possible. This condition is not too unlike the out-of-step synchronization. To minimize any harmful effects, the protection should separate the generator from the system as soon as possible, preferably during the first half-slip cycle. Protection against out-of-step condition is based on the fact that the apparent impedance, as seen at the generator’s terminals, changes in a predicted manner during an unstable condition. This is similar to the loss-of-excitation condition. However, the loss-of-excitation relay will not pick up an out-of-step condition in every occurrence because the apparent-impedance behavior is different for both conditions. Therefore, to fully protect against out-of-step condition, a dedicated relay (or protective function within a multi-functional device) must be included in the protection package. Tripping the unit within the first slip cycle has major advantages in the case of out-of-step events. This fast protective action tends to reduce considerably the very large oscillating shaft torque that can otherwise occur.
TRANSFORMER A transformer is a static device consisting of a winding, or two or more coupled windings, with or without a magnetic core, for inducing mutual coupling between circuits. When an alternating current flows in a conductor, a magnetic field exists around the conductor. If another conductor is placed in the field created by the first conductor such that the flux lines link the second conductor, then a voltage is induced into the second conductor. The use of a magnetic field from one coil to induce a voltage into a second coil is the principle on which transformer theory and application is based.
A 220 kV Transformer at Power Plant
ANSI/IEEE defines a transformer as a static electrical device, involving no continuously moving parts, used in electric power systems to transfer power between circuits through the use of electromagnetic induction. The transformer is one of the most reliable pieces of electrical distribution equipment. It has no moving parts, requires minimal maintenance, and is capable of withstanding overloads, surges, faults, and physical abuse that may damage or destroy other items in the circuit. Often, the electrical event that burns up a motor, opens a circuit breaker, or blows a fuse has a subtle effect on the transformer. Although the transformer may continue to operate as before, repeat occurrences of such damaging electrical events, or lack of even minimal maintenance can greatly accelerate the eventual failure of the transformer. The fact that a transformer continues to operate satisfactorily in spite of neglect and abuse is a testament to its durability. However, this durability is no excuse for not providing the proper care. Most of the effects of aging, faults, or abuse can be detected and corrected by a comprehensive maintenance, inspection, and testing program.
Transformers are exclusively used in electric power systems to transfer power by electromagnetic induction between circuits at the same frequency, usually with changed values of voltage and current. There are numerous types of transformers used in various applications including audio, radio, instrument, and power. In Badarpur Thermal Power Station, we deal exclusively with power transformer applications involving the transmission and distribution of electrical power. Power transformers are used extensively by traditional electric utility companies, power plants, and industrial plants. The term power transformer is used to refer to those transformers used between the generator and the distribution circuits, and these are usually rated at 220 kVA and above. Power systems typically consist of a large number of generation locations, distribution points, and interconnections within the system or with nearby systems, such as a neighboring utility. The complexity of the system leads to a variety of transmission and distribution voltages. Power transformers must be used at each of these points where there is a transition between voltage levels. Power transformers are selected based on the application, with the emphasis toward custom design being more apparent the larger the unit. Power transformers are available for step-up operation, primarily used at the generator and referred to as generator step-up (GSU) transformers, and for step-down operation, mainly used to feed distribution circuits. Power transformers are available as single-phase or three-phase apparatus.
A Power Transformer at a Thermal Power Plant
CONSTRUCTION A power transformer is a device that changes (transforms) an alternating voltage and current from one level to another. Power transformers are used to “step up” (transform) the voltages that are produced at generation to levels that are suitable for transmission (higher voltage, lower current). Conversely, a transformer is used to “step down” (transform) the higher transmission voltages to levels that are suitable for use at various facilities (lower voltage, higher current). Electric power can undergo numerous transformations between the source and the final end use point. •
Voltages must be stepped-up for transmission. Every conductor, no matter how large, will lose an appreciable amount of power (watts) to its resistance (R) when a current (T) passes through it. This loss is expressed as a function of the applied current (P=I2R). Because this loss is dependent on the current, and since the power to be transmitted is a function of the applied volts (E) times the amps (P=IE), significant savings can be obtained by stepping the voltage up to a higher voltage level, with the corresponding reduction of the current value. Whether 100 amps is to be transmitted at 100 volts (P=IE, 100 amps X 100 volts = 10,000 watts) or 10 amps is to be transmitted at 1,000 volts (P=IE, 10 amps X 1,000 volts = 10,000 watts) the same 10,000 watts will be applied to the beginning of the transmission line.
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If the transmission distance is long enough to produce 0.1 ohm of resistance across the transmission cable, P=I2R, (100 amp)2 X 0.1 ohm = 1,000 watts will be lost across the transmission line at the 100 volt transmission level. The 1000 volts transmission level will create a loss of P=I2R, (10 amp)2 X 0.1 ohm = 10 watts. This is where transformers play an important role.
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Although power can be transmitted more efficiently at higher voltage levels, sometimes as high as 500 or 750 thousand volts (kV), the devices and networks at the point of utilization are rarely capable of handling voltages above 32,000 volts. Voltage must be “stepped down” to be utilized by the various devices available. By adjusting the voltages to the levels necessary for the various end use and distribution levels, electric power can be used both efficiently and safely.
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All power transformers have three basic parts, a primary winding, secondary winding, and a core. Even though little more than an air space is necessary to insulate an “ideal” transformer, when higher voltages and larger amounts of power are involved, the insulating material becomes an integral part of the transformer’s operation. Because of this, the insulation system is often considered the fourth basic part of the transformer. It is important to note that, although the windings and core deteriorate very little with age, the insulation can be subjected to severe stresses and chemical deterioration. The insulation deteriorates at a relatively rapid rate, and its condition ultimately determines the service life of the transformer.
Core The core, which provides the magnetic path to channel the flux, consists of thin strips of highgrade steel, called laminations, which are electrically separated by a thin coating of insulating material. The strips can be stacked or wound, with the windings either built integrally around the core or built separately and assembled around the core sections. Core steel can be hot- or coldrolled, grain-oriented or non grain oriented, and even laser-scribed for additional performance. Thickness ranges from 0.23 mm to upwards of 0.36 mm. The core cross section can be circular or rectangular, with circular cores commonly referred to as cruciform construction. Rectangular cores are used for smaller ratings and as auxiliary transformers used within a power transformer. Rectangular cores use a single width of strip steel, while circular cores use a combination of different strip widths to approximate a circular cross-section. The type of steel and arrangement depends on the transformer rating as related to cost factors such as labor and performance. Just like other components in the transformer, the heat generated by the core must be adequately dissipated. While the steel and coating may be capable of withstanding higher temperatures, it will come in contact with insulating materials with limited temperature capabilities. In larger units, cooling ducts are used inside the core for additional convective surface area, and sections of laminations may be split to reduce localized losses. The core is held together by, but insulated from, mechanical structures and is grounded to a single point in order to dissipate electrostatic buildup. The core ground location is usually some readily accessible point inside the tank, but it can also be brought through a bushing on the tank wall or top for external access. This grounding point should be removable for testing purposes, such as checking for unintentional core grounds. Multiple core grounds, such as a case whereby the core is inadvertently making contact with otherwise grounded internal metallic mechanical structures, can provide a path for circulating currents induced by the main flux as well as a leakage flux, thus creating concentrations of losses that can result in localized heating. The maximum flux density of the core steel is normally designed as close to the knee of the saturation curve as practical, accounting for required over excitations and tolerances that exist due to materials and manufacturing processes. For power transformers the flux density is typically between 1.3 T and 1.8 T, with the saturation point for magnetic steel being around 2.03 T to 2.05 T. There are two basic types of core construction used in power transformers: core form and shell form. •
In core-form construction, there is a single path for the magnetic circuit. For single-phase applications, the windings are typically divided on both core legs as shown. In three-phase applications, the windings of a particular phase are typically on the same core leg. Windings are constructed separate of the core and placed on their respective core legs during core assembly.
Schematic Diagram of Shell-form Construction
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In shell-form construction, the core provides multiple paths for the magnetic circuit. The core is typically stacked directly around the windings, which are usually “pancake”-type windings, although some applications are such that the core and windings are assembled similar to core form. Due to advantages in short-circuit and transient-voltage performance, shell forms tend to be used more frequently in the largest transformers, where conditions can be more severe. Variations of three-phase shell-form construction include five and sevenlegged cores, depending on size and application.
Schematic Diagram of Shell-form Construction
Windings The windings consist of the current-carrying conductors wound around the sections of the core, and these must be properly insulated, supported, and cooled to withstand operational and test conditions. Copper and aluminum are the primary materials used as conductors in power-transformer windings. While aluminum is lighter and generally less expensive than copper, a larger cross section of aluminum conductor must be used to carry a current with similar performance as copper. Copper has higher mechanical strength and is used almost exclusively in all but the smaller size ranges, where aluminum conductors may be perfectly acceptable. In cases where extreme forces are encountered, materials such as silver-bearing copper can be used for even greater strength. The conductors used in power transformers are typically stranded with a rectangular cross section, although some transformers at the lowest ratings may use sheet or foil conductors. Multiple strands can be wound in parallel and joined together at the ends of the winding, in which case it is necessary to transpose the strands at various points throughout the winding to prevent circulating currents around the loop(s) created by joining the strands at the ends. Individual strands may be subjected to differences in the flux field due to their respective positions within the winding, which create differences in voltages between the strands and drive circulating currents through the conductor loops. Proper transposition of the strands cancels out these voltage differences and eliminates or greatly reduces the circulating currents. A variation of this technique, involving many rectangular conductor strands combined into a cable, is called continuously transposed cable (CTC).
A view of Pancake Winding
In core-form transformers, the windings are usually arranged concentrically around the core leg, which shows a winding being lowered over another winding already on the core leg of a threephase transformer. Shell-form transformers use a similar concentric arrangement or an interleaved arrangement.
With an interleaved arrangement, individual coils are stacked, separated by insulating barriers and cooling ducts. The coils are typically connected with the inside of one coil connected to the inside of an adjacent coil and, similarly, the outside of one coil connected to the outside of an adjacent coil. Sets of coils are assembled into groups, which then form the primary or secondary winding. When considering concentric windings, it is generally understood that circular windings have inherently higher mechanical strength than rectangular windings, whereas rectangular coils can have lower associated material and labor costs. Rectangular windings permit a more efficient use of space, but their use is limited to small power transformers and the lower range of mediumpower transformers, where the internal forces are not extremely high. As the rating increases, the forces significantly increase, and there is need for added strength in the windings, so circular coils, or shell-form construction, is used. In some special cases, elliptically shaped windings are used. Concentric coils are typically wound over cylinders with spacers attached so as to form a duct between the conductors and the cylinder. As previously mentioned, the flow of liquid through the windings can be based solely on natural convection, or the flow can be somewhat controlled through the use of strategically placed barriers within the winding. This concept is sometimes referred to as guided liquid flow. A variety of different types of windings have been used in power transformers through the years. Coils can be wound in an upright, vertical orientation, as is necessary with larger, heavier coils; or they can be wound horizontally and placed upright upon completion. As mentioned previously, the type of winding depends on the transformer rating as well as the core construction. Several of the more common winding types are discussed further.
1. Pancake Windings Several types of windings are commonly referred to as “pancake” windings due to the arrangement of conductors into discs. However, the term most often refers to a coil type that is used almost exclusively in shell-form transformers. The conductors are wound around a rectangular form, with the widest face of the conductor oriented either horizontally or vertically. This type of winding lends itself to the interleaved arrangement previously discussed.
2. Disc Windings A disc winding can involve a single strand or several strands of insulated conductors wound in a series of parallel discs of horizontal orientation, with the discs connected at either the inside or outside as a crossover point. Each disc comprises multiple turns wound over other turns, with the crossovers alternating between inside and outside. Most windings of 25-kV class and above used in core form transformers are disc type. Given the high voltages involved in test and operation, particular attention is required to avoid high stresses between discs and turns near the end of the winding when subjected to transient voltage surges. Numerous techniques have been developed to ensure an acceptable voltage distribution along the winding under these conditions.
3. Helical Windings Helical windings are also referred to as screw or spiral windings, with each term accurately characterizing the coil’s construction. A helical winding consists of a few to more than 100 insulated strands wound in parallel continuously along the length of the cylinder, with spacers inserted between adjacent turns or discs and suitable transpositions included to minimize circulating currents between parallel strands. The manner of construction is such that the coil resembles a corkscrew. Helical windings are used for the higher-current applications frequently encountered in the lower-voltage classes.
A View of Helical Winding
4. Layer (Barrel) Windings Layer (barrel) windings are among the simplest of windings in that the insulated conductors are wound directly next to each other around the cylinder and spacers. Several layers can be wound on top of one another, with the layers separated by solid insulation, ducts, or a combination. Several strands can be wound in parallel if the current magnitude so dictates. Variations of this winding are often used for applications such as tap windings used in load-tap-changing (LTC) transformers and for tertiary windings used for, among other things, third-harmonic suppression.
A View of Layer Winding
Taps-Turns Ratio Adjustment The ability to adjust the turn’s ratio of a transformer is often desirable to compensate for variations in voltage that occur due to the regulation of the transformer and loading cycles. This task can be accomplished by several means. There is a significant difference between a transformer that is capable of changing the ratio while the unit is on-line (a load tap changing [LTC] transformer) and one that must be taken off-line, or de-energized, to perform a tap change. Most transformers are provided with a means of changing the number of turns in the highvoltage circuit, whereby a part of the winding is tapped out of the circuit. In many transformers, this is done using one of the main windings and tapping out a section or sections. With larger units, a dedicated tap winding may be necessary to avoid the ampere-turn voids that occur along the length of the winding. Use and placement of tap windings vary with the application and among manufacturers. A manually operated switching mechanism, a DETC (deenergized tap changer), is normally provided for convenient access external to the transformer to change the tap position. When LTC capabilities are desired, additional windings and equipment are required, which significantly increase the size and cost of the transformer. This option is specified on about 60% of new medium and large power transformers. It should be recognized that there would be slight differences in this schematic based on the specific LTC being used. It is also possible for a transformer to have dual voltage ratings, as is popular in spare and mobile transformers. While there is no physical limit to the ratio between the dual ratings, even
ratios (for example 24.94 X 12.47 kV or 138 X 69 kV) are easier for manufacturers to accommodate.
MAINTENANCE AND TESTING Heat and contamination are the two greatest enemies to the transformer’s operation. Heat will break down the solid insulation and accelerate the chemical reactions that take place when the oil is contaminated. All transformers require a cooling method and it is important to ensure that the transformer has proper cooling. Proper cooling usually involves cleaning the cooling surfaces, maximizing ventilation, and monitoring loads to ensure the transformer is not producing excess heat. •
Contamination is detrimental to the transformer, both inside and out. The importance of basic cleanliness and general housekeeping becomes evident when long term service life is considered. Dirt builds up and grease deposits severely limit the cooling abilities of radiators and tank surfaces. Terminal and insulation surfaces are especially susceptible to dirt and grease build up. Such buildup will usually affect test results. The transformer’s general condition should be noted during any activity, and every effort should be made to maintain its integrity during all operations.
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The oil in the transformer should be kept as pure as possible. Dirt and moisture will start chemical reactions in the oil that lower both its electrical strength and its cooling capability. Contamination should be the primary concern any time the transformer must be opened. Most transformer oil is contaminated to some degree before it leaves the refinery. It is important to determine how contaminated the oil is and how fast it is degenerating. Determining the degree of contamination is accomplished by sampling and analyzing the oil on a regular basis.
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Although maintenance and work practices are designed to extend the transformer’s life, it is inevitable that the transformer will eventually deteriorate to the point that it fails or must be replaced. Transformer testing allows this aging process to be quantified and tracked, to help predict replacement intervals and avoid failures. Historical test data is valuable for determining damage to the transformer after a fault or failure has occurred elsewhere in the circuit. By comparing test data taken after the fault to previous test data, damage to the transformer can be determined.
SAFETY Safety is of primary concern when working around a transformer. The substation transformer is usually the highest voltage item in a facility’s electrical distribution system. The higher voltages found at the transformer deserve the respect and complete attention of anyone working in the area. A 6.6 kV system will arc to ground over 1.5 to 2.5 in. However, to extinguish that same arc will require a separation of 15 in. Therefore, working around energized conductors is not
recommended for anyone but the qualified professional. The best way to ensure safety when working around high voltage apparatus is to make absolutely certain that it is de-energized. •
Although inspections and sampling can usually be performed while the transformer is in service, all other service and testing functions will require that the transformer is deenergized and locked out. This means that a thorough understanding of the transformer’s circuit and the disconnecting methods should be reviewed before any work is performed.
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A properly installed transformer will usually have a means for disconnecting both the primary and the secondary sides; ensure that they are opened before any work is performed. Both disconnects should be opened because it is possible for generator or induced power to back feed into the secondary and step up into the primary. After verifying that the circuit is de-energized at the source, the area where the work is to be performed should be checked for voltage with a “hot stick” or some other voltage indicating device.
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It is also important to ensure that the circuit stays de-energized until the work is completed. This is especially important when the work area is not in plain view of the disconnect. Red or orange lock-out tags should be applied to all breakers and disconnects that will be opened for a service procedure. The tags should be highly visible, and as many people as possible should be made aware of their presence before the work begins.
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Some switches are equipped with physical locking devices (a hasp or latch). This is the best method for locking out a switch. The person performing the work should keep the key at all times, and tags should still be applied in case other keys exist.
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After verifying that all circuits are de-energized, grounds should be connected between all items that could have a different potential. This means that all conductors, hoses, ladders and other equipment should be grounded to the tank, and that the tank’s connection to ground should be verified before beginning any work on the transformer. Static charges can be created by many maintenance activities, including cleaning and filtering. The transformer’s inherent ability to step up voltages and currents can create lethal quantities of electricity.
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The inductive capabilities of the transformer should also be considered when working on a de-energized unit that is close to other conductors or devices that are energized. A deenergized transformer can be affected by these energized items, and dangerous currents or voltages can be induced in the adjacent windings.
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Most electrical measurements require the application of a potential, and these potentials can be stored, multiplied, and discharged at the wrong time if the proper precautions are not taken. Care should be taken during the tests to ensure that no one comes in contact with the transformer while it is being tested. Set up safety barriers, or appoint safety personnel to secure remote test areas. After a test is completed, grounds should be left on the tested item for twice the duration of the test, preferably longer.
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Once the operation of the transformer is understood, especially its inherent ability to multiply voltages and currents, then safety practices can be applied and modified for the type
of operation or test that is being performed. It is also recommended that anyone working on transformers receive regular training in basic first aid, CPR, and resuscitation.
NAMEPLATE DATA The transformer nameplate contains most of the important information that will be needed in the field. The nameplate should never be removed from the transformer and should always be kept clean and legible.
A Wye Delta Transformer Nameplate
Although other information can be provided, industry standards require that the following information be displayed on the nameplate of all power transformers: a. Serial Number: The serial number is required any time the manufacturer must be contacted for information or parts. It should be recorded on all transformer inspections and tests.
b. Class: The class will indicate the transformer’s cooling requirements and increased load capability. c. kVA Rating: The kVA rating, as opposed to the power output, is a true indication of the current carrying capacity of the transformer. kVA ratings for the vaious cooling classes should be displayed. For three phase transformers, the kVA rating is the sum of the power in all three legs. d. Voltage Rating: The voltage rating should be given for the primary and secondary, and for all tap positions. e. Temperature Rise: The temperature rise is the allowable temperature change from ambient that the transformer can undergo without incurring damage. f. Polarity (single phase): The polarity is important when the transformer is to be paralleled or used in conjunction with other transformers. g. Phasor Diagrams: Phasor Diagrams will be provided for both the primary and the secondary coils. Phasor diagrams indicate the order in which the three phases will reach their peak voltages, and also the angular displacement (rotation) between the primary and secondary h. Connection Diagram: The connection diagram will indicate the connections of the various windings, and the winding connections necessary for the various tap voltages. i. Percent Impedance: The impedance percent is the vector sum of the transformer’s resistance and reactance expressed in percent. It is the ratio of the voltage required to circulate rated current in the corresponding winding, to the rated voltage of that winding. With the secondary terminals shorted, a very small voltage is required on the primary to circulate rated current on the secondary. The impedance is defined by the ratio of the applied voltage to the rated voltage of the winding. If, with the secondary terminals shorted, 138 volts are required on the primary to produce rated current flow in the secondary, and if the primary is rated at 13,800 volts, then the impedance is 1 percent. The impedance affects the amount of current flowing through the transformer during short circuit or fault conditions. j. Impulse Level (BIL): The impulse level is the crest value of the impulse voltage the transformer is required to withstand without failure. The impulse level is designed to simulate a lightning strike or voltage surge condition. The impulse level is a withstand rating for extremely short duration surge voltages. Liquid-filled transformers have an inherently higher BIL rating than dry-type transformers of the same kVA rating. k. Weight: The weight should be expressed for the various parts and the total. Knowledge of the weight is important when moving or untanking the transformer.
l. Insulating Fluid: The type of insulating fl.uid is important when additional fluid must be added or when unserviceable fluid must be disposed of. Different insulating fluids should never be mixed. The number of gallons, both for the main tank, and for the various compartments should also be noted. m. Instruction Reference: This reference will indicate the manufacturer’s publication number for the transformer instruction manual.
