Testing of Turbogenerator

TESTING OF TURBO GENERATOR

CHAPTER-1 INTRODUCTION 1.1. INTRODUCTION A Generator is a rotating Electromagnetic device producing electrical power taking mechanical input from prime mover (Gas Turbine / Steam Turbine) and magnetic energy from excitation. Generator Design will be conforming to International Standards like IEC & National standards like BS, VDE, IS etc. Generators driven by steam or gas turbines have cylindrical/ round rotors with slots into which distributed field windings are placed. These round rotor generators are usually referred to as turbo generators and they usually have 2 or 4 poles. Generators driven by hydraulic turbines have laminated salient pole rotors with concentrated field winding and a large number of poles. Testing is the most important process to be done on a machine after it is designed. The testing of machine is necessary primarily to establish that the machine performance complies with customer specifications. Tests ensure that the piece of equipment concerned is suitable for and capable for performing duty for which it is intended. Testing has to be done on a machine at every step in its manufacturing process for the company to certify it to be a ―deliverable good”. Test brings out the impact of process variations. Testing is done in simulations which tend to closely resemble the practical scenario under which the machine works. Testing provides the experimental data like the efficiency, losses, characteristics, temperature limits etc. for the use of design office, both as confirmation of design forecast and also as basic information for the production of future designs.

1.2 NECESSITY OF TESTING : To ensure that all functional requirements are fulfilled, and to estimate the performance of generator, the turbo generators are required to undergo some tests. For testing, the turbo generator was mechanically coupled to a drive motor-motor generator set

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with gearbox. The rotor was excited by thyristor converter system located in an independent test room and the operation was controlled from the test gallery. The following first two tests will be conducted on the stator and rotor before assembling and the third and final routine tests will be conducted a fter assembling the turbo generator. generato r. a. Tests conducted on Stator b. Tests conducted on Rotor 1.3. OBJECTIVE OF TESTING :

Testing is the most important process to be conducted on a machine after it is designed. The testing of machine is necessary primarily to establish that the machine performance complies with the customer specifications. Tests ensure that the piece of equipment concerned is suitable for and capable for performing duty for which it is intended. Testing is done under condition simulating closely as possible to those, which will apply when the set is finally installed with a view to demonstrate to purchaser‘s representative its satisfactory operation. Test provides the experimental data like efficiency, losses, characteristics, temperature limits, etc. for the use of design office, both as confirmation of design forecast and also as basic information for the production of future designs. With ever increasing rating of the modern turbo generators and reliability of service expected, testing at manufacturer‘s works has become become of paramount importance. The machine performance is evaluated from the results of the equivalent tests.

Advantages of testing

1. Provides data for optimization of design 2. Provides quality assurance 3. Meets the requirement of legal and contract requirements. 4. Reduction in rework cost. 5. Ensures process capability and develops checklist. 6. Increases confidence levels in manufacture. 7. Establishes control over raw materials. 8. Helps in building of safety and general operation and manual.

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with gearbox. The rotor was excited by thyristor converter system located in an independent test room and the operation was controlled from the test gallery. The following first two tests will be conducted on the stator and rotor before assembling and the third and final routine tests will be conducted a fter assembling the turbo generator. generato r. a. Tests conducted on Stator b. Tests conducted on Rotor 1.3. OBJECTIVE OF TESTING :

Testing is the most important process to be conducted on a machine after it is designed. The testing of machine is necessary primarily to establish that the machine performance complies with the customer specifications. Tests ensure that the piece of equipment concerned is suitable for and capable for performing duty for which it is intended. Testing is done under condition simulating closely as possible to those, which will apply when the set is finally installed with a view to demonstrate to purchaser‘s representative its satisfactory operation. Test provides the experimental data like efficiency, losses, characteristics, temperature limits, etc. for the use of design office, both as confirmation of design forecast and also as basic information for the production of future designs. With ever increasing rating of the modern turbo generators and reliability of service expected, testing at manufacturer‘s works has become become of paramount importance. The machine performance is evaluated from the results of the equivalent tests.

Advantages of testing

1. Provides data for optimization of design 2. Provides quality assurance 3. Meets the requirement of legal and contract requirements. 4. Reduction in rework cost. 5. Ensures process capability and develops checklist. 6. Increases confidence levels in manufacture. 7. Establishes control over raw materials. 8. Helps in building of safety and general operation and manual.

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1.4. THEME Testing is an activity, which basically evaluates a component, and or a product (built up of component assemblies) as to whether it has the technical capability that has been built into it by way of design, materials, and technological processes employed while manufacturing and workmanship. wor kmanship. As such, testing activities can broadly be classified in to a number of categories as follows: a. Type tests. b. Routine tests. c. Process tests

The characteristics of testing: 1) Provides quality assurance. 2) Meets the requirements of legal & contract requirements. 3) Ensures process capability & develops checklist. 4) Have an approved procedure. 5) Check the equipment before use. 6) Calibrate the test equipment & instruments. 7) Ensure interlocks of the equipment

1.5. ORGANISATION

The definition and objective of the project as well as the design of the project which is followed by the implementation and testing phases is studied in detail. Finally the project has been concluded successfully and also the future enhancements of the project were shown. The organization of the project is as follows by 1. Introduction 2. Literature survey 3. System development 4. Analysis of a turbo generator is studied and the precise results are shown in order to ensure that the turbo generator chosen is deliverable good.

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CHAPTER -2 LITERATURE SURVEY 2.1. INTRODUCTION Testing is the most important process to be conducted on a machine after it is designed. The testing of machine is necessary primarily to establish that the machine performance complies with the customer specifications. Tests ensure that the piece of equipment concerned is suitable for and capable for performing duty for which it is intended. Testing is done under condition simulating closely as possible to those, which will apply when the set is finally installed with a view to demonstrate to purchaser‘s representative its satisfactory operation. Test provides the experimental data like efficiency, losses, characteristics, temperature limits, etc. for the use of design office, both as confirmation of design forecast and also as basic information for the production of future designs. With ever increasing rating of the modern turbo generators and reliability of service expected, testing at manufacturer‘s works has become of paramount importance. The machine performance is evaluated from the results of the equivalent tests.

Advantages:

a. Provides data for optimization of design b. Provides quality assurance c. Meets the requirement of legal and contract requirements. d. Reduction in rework cost. e. Ensures process capability and develops checklist. f.

Increases confidence levels in manufacture.

g. Establishes control over raw materials. h. Helps in building of safety and general operation and manual.

2.2. EXISTING SYSTEM The existing system of a turbo generator and their inspection is as shown follows

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2.2.1. Generator Testing & Inspection Service

Generally generator testing and inspection services will be done for all types of fossil and nuclear generators working with nearly all types of equipment and can offer complete and accurate testing services. They will inspect and test all parts of your equipment during your generator testing service. This thorough inspection will allow to generate accurate reports and make complete recommendations in order to keep your equipment working properly and at maximum efficiency. Generator testing and inspection services are available for all types of turbine generators including: a)

Fossil Steam Turbine Generators

b) Nuclear Steam Turbine Generators c)

Gas Turbine Generators

d)

Industrial Turbine Generator

2.2.2. About Generator Testing Services: Generally experts offer generator testing services for all types, sizes, and brands of equipment and worked with a variety of customers and are familiar with nearly any type of generator including fossil steam turbine generators and nuclear steam turbine generator and have an complete selection of test equipment available for generator testing including equipment for routine low-voltage generator testing. Routine low-voltage testing services include: 1.

RTD resistance testing with temperature conversion and 500 volt megger

2.

500 volt megger of the field with Polarization Index (P.I.)

3.

Impedance testing of the field

4.

Copper resistance of the field with temperature conversion to factory test temperature

5.

Copper resistance of stator 3 phases converted to factory test temperature

6.

Megger of stator 3 phases with P.I. up to 5000 volts

7.

Visual inspection of all accessible areas

8.

Comprehensive report including photos, recommendations and data sheets

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We also offer a variety of optional tests as well. These optional generator testing services can be completed as required to meet your individual needs. Our optional tests include: A. Pressure / Vacuum Testing B. El Cid Testing C. DC Leakage and Hi Pot Testing D. Capacitance Mapping

2.2.3. The Importance of Generator Testing: Keeping your Systems Working

Both the electrical tests and the visual inspections, which are included in our generator testing services, are important for ensuring proper generator performance. These generator testing and inspection services will allow us to generate accurate recommendations, which can be used when planning and scheduling for outages and turbine or generator repairs. It will also help you evaluate the condition of your equipment in order to determine if replacement or modernization projects are necessary. The main goal of our generator testing and turbine generator service is to optimize your equipment so that it will run reliably and efficiently. We are familiar with all types of equipment and understand the intricate details of the inside of your machine. This allows us to provide thorough service to help you achieve the best results. i.

Complete Turbine Generator Testing & Inspection Services

We can inspect all parts and aspects of your equipment when performing our turbine generator testing and inspection services. This includes performing testing for generators, exciters, and other related equipment. These complete turbine and generator testing services will ensure that your entire system is working properly and efficie ntly. 2.2.4. Turbo Generator Testing Procedure and Manufacturing Process:

A sequential approach is followed here in implementing turbo generator assembly. Here the process of manufacturing closed circuit air cooled turbo generator is explained, the implementation is carried out in, Preparing a design layout – manufacturing parts of stator section like stator frame, stator core, stator windings, and end covers – providing insulation

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for stator elements – assembly of rotor with rotor windings and rotor retaining rings – positioning the shaft based on equivalent weigh concept – desired tests are performed under manufacturing time and after assembly. The stator assembly involves preparation of laminations – compounding operation – blanking and notching operations – varnishing – debugging – core assembly – slot discharges – stator windings assembly – tapping – stator end covers – fixing resistance temperature sensors – phase connections – bottom bar laying – top bar laying – connected rings – insulation. The insulation for lamination is carried out in Vacuum pressure impregnation The rotor assembly carried out by placing the rotor shaft – rotor windings – rotor slot wedges – end winding bracing – rotor retaining rings – rotor fan assembly – fixing bearings – bearing insulation – Lubrication system- Skewing – Scavenging. Ventilation for the turbo generator is basically three types

:

Radial ventilation system, Axial Ventilation system, and multiple inlet Ventilation system. 2.2.5. Recent technologies implemented at BHEL:

Vacuum press impregnated moralistic high voltage insulation, polyester fleece tape impregnation for outer corona protection are two latest technologies implementing in insulation section to provide high quality insulation for turbines with high standards and life time.

2.3. PROPOSED SYSTEM 2.3.1. TESTING METHODS 1. EL CID

To detect failures between laminations of stator cores 2. RSO Testing

To test both the turn and ground insulations of generator rotors

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3. IMCA(Induction Motor Current Analyzer)

To test for cracked rotor bars while machines are running in service 4. Insulation Resistance

Testing of all electrical equipment before any high voltage test ing is commenced 5. Partial Discharge Analyzer

To test the condition of a machines insulation by measuring the levels of partial discharges at operating voltages 6. Tan Delta Testing

Main cell wall insulation of all coils above 4.0V AC are tested using an inductively coupled capacitive bridge to measure tan delta 7. TVA Probe Testing

To locate areas of localized discharge within the stator slots of high voltage stators 8. Underwater Testing

It can be used after the VPI (Vacuum Pressure Impregnation) Process before being returned to site.

2.3.2. SCOPE OF WORK: The following are the broad scope of work (detailed scope of work enclosed), but not limited to:1. Decoupling, Opening of end covers and pulling out the rotor from the position and placing, it on the proper stand 2. Cleaning of stator winding portion, slots over hang portion, etc., using the appropriate Cleaning, Agent3. Replacement of damaged wedges 4. Cleaning of rotor portioned. 5. Inspection of bearings and measuring bearing clearances.

The above tests have to be conducted before assembling the machine. Necessary epoxy spray coating is to be applied, wherever required. CPCL scope is limited only to disconnection of all cables connected to the machine. All consumables, special tools and tackles required for the above jobs is to be brought by the contractor.

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The following tests have to be conducted:Stator: -

1. 2.

Stator core Stator windings

-

ELCID Test or Flux, Loop Test

-

IR & PI

-

DC hipot step voltage

-

Partial discharge

-

Capacitance

-

Winding DC resistance

Rotor:

1.

Rotor winding

-

IR

-

Winding DC resistance

-

Impedance

-

RSO (earth fault /inter- turn short)

Detailed scope of work-. Apart from the broad scope of work as detailed above, the following needs to be carr ied out: I.

Generator Rotor Removal

i. De-coupling the Generator / Turbine and Pilot exciter, Removal of Pilot exciter from the bed. Remove slip-ring brush holder assembly measure and record diameter of both positive and negative rings, check for any abnormal wear / pitting on the surface. ii. Replace the shaft seal at outer covers. iii. Disconnect and tag the slip ring terminals. iv. Measure air gap between the stator and rotor at 4 points diametrically opposite at right angle. This should be done for both turbine and exciter end. v. Open bearing cover check for clearances and abnormality, if any, on the bearing sur face.

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vi. Decouple the generator and record alignment readings. vii. Remove bearings after ensuring that stator is not jammed by threading out of rotor by inserting packing material (such as leatheroid, etc). viii. Remove and place the rotor on the stand specia lty provided for. ix. Check the rotor for any sign of overheating, mechanical abrasion, loose wedges, etc., and clean it with compressed air and cloth. x. Check the rotor end rings for any damage or check by ultrasonic inspection method. xi. Check fan blades and hubs for erosion and cracks. xii. Check that balancing weight are secured firmly. xiii. Measure field and insulation resistance of the rotor and compare it with design data. xiv. Clean the rotor and apply finish coat as recommended by the manufacturer. Dry up the rotor. II.

Generator Stator:

1 .Clean stator windings, ventilating ducts with dry compressed air (compressed air will be supplied by CPCL). 2. Inspect for defects like i. ii.

Discoloration of winding (for hot spots) Loose missing slot wedges

iii.

Inter coil spacers on overhangs

iv.

Broken overhaul coil bindings for end supports

v.

Protective coatings on the core steps at slot ends

3.

Replace any broken wedges as required.

III.

Generator Assembly:

1.

Insert rotor inside the stator carefully. Put packing material (such as leatheroid etc.,)

in the air gap between stator and rotor for protection and assemble all removed parts. Fix the pilot exciter in bed. Assemble bearing pedestal.

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2.

Ensure that the bearing has been cleaned, necessary scrapping has been done to

remove any uneven surface. Bearing insulation should be taken care, wherever provided assemble the bearings. 3.

Alignment and coupling of the generator with Turbine and pilot exciter with

Generator. Check air gap and ensure it matches with origina l gap. 4.

Check the pedestal pipe flange insulation and also the same for pipe connection and

bolt. Replace if necessary. Box up the bearing. 5.

Fix inner and outer end covers. Generator details

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Make

Ercole marelli,Italy

Year of Commissioning

1969

Apparent power output

14MVA

Voltage

6.6KV

Rated current

1225A

Power factor

0.8 lag

Speed

3000 RPM, Directly coupled

Class of insulation

B

Type of cooling

Air cooled

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CHAPTER - 3 SYSTEM DEVELOPMENT

3.1. INTRODUCTION 3.1.1. DEVELOPMENT IN TURBO GENERATOR TECHNOLOGIES Since the 1901 invention of the cylindrical rotor of Charles Brown for a high-speed generator, the turbo generator has been the unique solution for converting steam turbine power into electrical power. The continuously transposed stator bar, invented by Ludwig Roebel in 1912, opened the door for large scale winding application. Up to the 1930ies the generators were designed in 2-, 4- and even 6- pole, in accordance with the speed optimums of the steam turbines in those days. The 1920 ended with impressive power generation plants, having generator units in the 100 MVA range. The stator winding insulation consisted in the beginning of plied-on mica-paper, compounded by Shellac varnish, later substituted by asphalt. Voltages were up to 12 kV. In the early 1930s two European manufacturers were introducing 36 kV stator windings, thus eliminating the machine transformer. All such designs were suffering of continuous heavy electrical discharges, and were soon discontinued. After a 60-year time-out, a manufacturer surprised the world in 1998 with a cable-based high-voltage generator up to 400 kV. However again, the cable technology was not ready for turbo generator requirements, and a breakthrough for commercial application was not achieved. In the 1930 US manufacturers were introducing hydrogen as coolant. When combined with direct conductor hydrogen cooling in the rotor, and later in the stator, this allowed a considerable increase in specific utilization and efficiency. By early 1960s the unit ratings were achieving 500 MVA. At that time deionized water cooling in the stator winding was introduced. Around 1960 all major manufacturers changed their insulation system to mica tape with synthetic resin impregnation, a technology for thermal qualification at 155°C, and which has been lasting into these days. By end of the 1960, with the power semiconductors becoming mature, the dc machine excitation was superseded by the static excitation, and by an ac exc iter machine with rotating diodes.

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The 1970 brought again a tremendous growth in unit ratings, going along with the introduction of nuclear power. Units of 1200 MVA at 3000 rpm and 1600 MVA at 1500 rpm at up to 27 kV were designed and put in operation. The rotor diameters were arriving at their physical limits. Water-cooling of the rotor winding was introduced. Along with plans for 2000 MVA and beyond, superconducting rotor windings and stator air-gap windings were studied. However, in early 1980 the market focus was shifting to gas turbine technology, with some 100 MW beginning to grow into the area of large power plants, and initiating a new round of up rating the simple and robust air-cooling technology in the 300 MVA range by 1996. The generator has for a long time been developed by repeating the cycle: design – test – adjust design tools – extrapolate design. A tremendous breakthrough came with the large computers in the 1960ies, immediately being used for the ke y competences, such as magnetic field calculations, nonlinear coolant flow networks and mechanical turbine generator shaft calculations. Some programs of that area are even in use in the today‘s PC environment. As an example, magnetic equivalent circuits were established to determine excitation currents. Once these programs were calibrated on measured data, they have been proven very accurate and still today, for most applications make obsolete any FEM method.

3.1.2. TODAY’S TURBOGENERATOR TECHNOLOGIES A. SMALL UNITS UPTO 150MVA The size of these small air cooled units has evolved quite quickly. These machines are mainly devoted for gas turbines and steam turbines accepting cycling expansion. The gas turbines market has led to a very standardized range of machine based on the evolution of the turbine technologies and on the market requests. The models developed in 1980 for 40 MW 50 Hz/60 Hz; same generator for 50 Hz and 60 Hz with a gear box wheel and pinion adaptation; are nowadays joined by models in the 130-150 MW range. These generators are always designed using the simplest solution in order to reach low costs using modular solutions. For example the stator is cooled using one chamber and the excitation system does not need a third bearing and no pilot exciter. By this way, the models used for gas turbines are easily adapted for steam turbine or double drive solutions. All these machines are easy to transport and to mount on site and are very often mounted and coupled to the turbine by the turbine manufacturer. They are delivered in a short time and a

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lot of engineering is done to improve the through put time of these models. The maintenance of these groups is quite simple requiring a small sto rage of spare parts.

A recent trend is the increase of the power of the electrical drives used in the oil and gas industry, mainly for liquid natural gas pumps. Such drive motors require options similar to those developed for the generators, however having a variable speed drives controlled by static frequency converters. The performance is evolving quite strongly: a world record for this kind of motor at 21 MW 5900 rpm in 1985, seems modest in view of today‘s 100 MW. The speed values are close to generation with values between 3600 and 4200 rpm.

B. MEDIUM RANGE UPTO 500MVA Since the introduction of the 300 MVA class ten years ago, subsequent development has extended the rating up to the 400 MVA range. One of the main technology drivers has been the improvement of the rotor axial cooling and winding indirect cooling using a modular stator multi-chamber airflow. These generators are characterized by their simplicity and ease of operation and maintenance. They have also proven their maturity in GT24/GT26 gas turbine applications as well as on numerous steam turbines and turbines of other manufacturers. The new ratings of the aircooled generator series allow for the application of air-cooled technology in power ranges where hydrogen cooled generators were used previously. As a result of electrical and cooling optimization the present air-cooled turbo generators achieve efficiency up to 98.8 % and are used with a maximum voltage of 21 kV. Air cooled turbo generators technology with highest ratings has now accumulated more than 1.8 million of successful operating hours with more than 100 units in operat ion. In two decades the power output of air-cooled generators has been increased from 200 MVA to 400 MVA. Fig.4 shows this exceptional increase in generator power as a function of the time. It is clear that this strong increase in power that has occurred in the last decade was a direct response to the market demands. Recently, the increase of air-pressure inside the generator was realized. This measure allows a better cooling and consequently enhances the capability of the air-cooled turbo generators.

The hydrogen-cooled types have hydrogen filling up to 5.5 bar. They are

designed for single-shaft and combined-cycle applications and are increasingly used with steam turbines. The main features of the gas-cooled design are the same as the air-cooled.

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The cooling principle, end winding support system, the retightening system and the aluminum press plate are excellent examples of the design similarities. The hydrogen-cooled types are setting the benchmark for efficiency, large units commonly achieving 99.0 %. Since 1996, ALSTOM has supplied more than 50 unit‘s hydrogen-cooled turbo generators of the 500 MVA range. However, the achievable power is much higher and will be soon at 600 MVA

Fig 3.1 Evolution of the air-cooled turbo generators in the last decades.

C. LARGE UNITS UP TO 2000MVA These generators are driven by steam turbines in large coal fired power plants and nuclear power plants. They are all equipped with hydrogen-cooling with up to 6 bar overpressure, and with direct water cooling in the stator winding bars. The two-pole generator series begins at 500 MVA, and units up to 1300 MVA are in commercial operation. They are of highest specific utilization and therefore need complete direct cooling. Depending on the size the rotor, cooling is performed by axial flow of hydrogen through all conductors of a slot, either in one path over half-length of the rotor, or in two paths, supported by a sub slot. The stator core is axially flown by hydrogen, symmetrically fed from both ends driven by a radial fan, arranged on the non-driving end of the rotor shaft. The stator winding is cooled by water-flown stainless steel tubes embedded in the Roebel bars. Thanks to the water cooling the stator winding has ever been open factor for up ratings. The rotor

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winding has revealed to be the limiting part for up ratings. At 1.25 m for 50 Hz, the rotor diameter is at the limits of mechanical stress. Any extension in active length beyond 8m needs careful consideration of the shaft line dynamics. Potential lies in multi-zone cooling concepts for the rotor winding, in an increase of hydrogen absolute pressure and fan pressure. All the described measures will lead to a consolidation at 1400 MVA unit rating.

Any higher unit rating must go along with a break in rotor winding cooling, and the parasitic effects due to stray flux will remain a challenge as such. The four poles machines are running at 1500 rpm up to 1700 MVA. This is a key advantage for nuclear units, where the temperature of the steam is relatively low and its flow in the low pressure parts of the turbines huge. This allows the turbine to have very large diameter by using very long blades. The hydrogen/water-cooled generators coupled to these turbines are the largest electric turbo machines both in term of size and performance. This type of machine is ensuring 80% of the electrical production in France, which is a country with a very high electrical nuclear production. Some 50 machines in operation of this type have shown a ver y good reliability in operation and have a potential of improvement in performance.

Based on this situation, the solutions preferred in the nuclear market are not based on new technologies, but, more safely, they tend to still improve the existing validated wellrunning units. The 2000 MVA limit for turbo generators for the 3 rd generation of reactors is now close to be reached with improved life time and reliability. In order to reach this level of power, following choices have been done:

a. Use the basic solutions validated by years of operation on running nuclear units b. Analyze those parts which have led to the faults on exist ing machines c. Implement improvements validated on full-speed hydrogen and water-cooled machines in the last decades. d. Adapt the cantilever type of excitation technology and adapt it to be even less sensitive to diode aging. e. Implement an improved type of cooling in the rotor copper ducts.

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The maintenance of such a machine has to be done very carefully in order to reach the guaranteed lifetime. The periodic stops to refuel the reactor are to be used for optimum maintenance. The trend on the modern reactors is also to reduce the time between refueling and the maintenance has to be adapted accordingly. A wide experience has been accumulated on the existing machines.

III. AN OUTLOOK INTO THE NEXT 10 YEARS A. MARKETTRENDS

As a part of the energy chain, the turbo generator requires present and future developments that have to comply with the market requirements as following: a. Higher efficiency b. Higher reliability c. Low cost energy production d. Grid stability enhancement To fulfill continuously these requirements huge developments are in progress as presented in the following sections.

B. SUBSTITUTION OF HYDROGEN-COOLED UNITS BY AIR COOLED UNITS BY FOR HIGHER RELIABILITY AND LOW COST ENERGY PRODUCTION .

The substitution of hydrogen-cooled units by air-cooled and of hydrogen/watercooled by hydrogen-cooled will be continuing to shift the ratings upwards. The limits are given by transport dimensions, by the established temperature classes, and by the degree of complexity of design. The engineering will further exploit these limits involving mainly cooling and insulation materials developments. Air-cooled turbo generators offer many benefits to the operator. Some of which are listed below:

a) Excellent reliability b) Less civil work, simpler foundation c) No hydrogen treatment system d) No seal oil system and less sealing e) Less piping f) Simple engineering work due to its advanced technology

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These advantages are the consequences of not using hydrogen gas as a cooling medium. This results in much simpler and shorter maintenance periods as well as a shorter delivery time and an increased reliability. The good experience with large air-cooled turbo generators demonstrates the high potential of these generators. The largest air-cooled generator was designed for 500 MVA. This design has been proven by tests and represents the maximum achievable capability of air-cooled generator s.

C. EFFICIENCY ENHANCEMENT The improvement of the efficiency is of first importance for the turbo generator of all kind in particular in air-cooled 60 Hz units for closing the gap to the benchmark values of hydrogen-cooled units. Actually, it is one of the first issues considered in any new turbo generator development. In this section, some examples of new design solutions and new technologies implementation to increase the efficiency will be described.

3.2. TESTING METHODS A 3-phase, 4 pole micro-alternator system was used for practical tests. The microalternator field is driven through a time constant regulator; a setting of 6 seconds was used in these tests. The DC motor drive to the micro-alternator can also be electronically controlled to represent the turbine and its governor, if needed. All the major system variables are accessible for testing. Initial tests probed controller performance during normal operation, these were later extended to cover behavior with power system faults. A specially written ‗C‘ code standard two-axis theory flux linkage based state space simulation allowed tests beyond the capability of the micro-alternator system, including wide ranging fault simulation studies. A 10th order model with constant reactance values was used for much of the work, with single damper coils on each d-q axis, and lumped rotor inertia. Other model complexities are possible.

3.2.1. STEP RESPONSE TESTS

Often the specification on desired behavior includes TG open circuit response, this was certainly the case here. Frequently an AVR is site tuned on open circuit. Consequently the first tests used the micro-alternator in this condition, at rated voltage and speed. Each controller design was evaluated by standard tests, including applying a 3% positive step. A

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small number of specifications were set as controller design goals. The 'fast' design specification is typical where rapid action is required for dynamic response control. This set aims of: overshoot 4.3%, rise time 130ms, settling time 230ms, and closed-loop system bandwidth 4.0Hz. A conventional digital AVR which attempts the design specification for the 'fast' excitation control system was produced for comparisons. Such AVRs are the digital equivalents of the sort of controller in use for many years, and offer a good standard of performance. An approximate design using simulation studies fine-t uned by trial and error on the micro-alternator gave the parameters of this digital AVR as: loop gain with generator on open circuit = 325; lag time constants = 9.0 and 0.025 seconds; lead time constant = 3.0 seconds. For future reference, this design is ter med DGAVRF. The frequency response of the micro-alternator system was obtained by a Dynamic Signal Analyzer using Fast Fourier Transforms. These tests used a small (3%) set point change in output; the eventual field demand settles to a new steady state value also close to a 3% change showing operation is close to magnetically linear here. The terminal voltage response given had overshoot 4%,, rise time 175ms, settling time 350ms and bandwidth 2.8Hz, considered acceptably close to the design aims. GPC has various parameters or 'tuning knobs' which can be chosen to vary the behavior. One such is the control horizon Nu which specifies the number of steps over which the demand increments are varied. Initial trials used values of 1-3, with large values causing a faster response. Nu = 2 is a good compromise giving a terminal voltage step response similar but slightly improved over the previous test under identical conditions. Values given were: overshoot 1%, rise time 180ms, settling time 300ms and bandwidth 3.0Hz. This and the previous DGAVR-F result are shown. As Nu approaches the prediction horizon Ny(the number of steps over which the output directly influences the controller, typically set to 10), the step response gets closer to the design values. The chosen value yields a reasonably 'fast' response which is not very different from the design values, without the possible reduction in the controller robustness and additional computational burden imposed by higher values. For future reference, this 'fast' design using Nu = 2 is termed STAVR-F.

3.2.2. STEP RESPONSE: GENERATOR ON LOAD

As mentioned earlier, it is the response of the excitation control system when the turbine generator is on load that is really important since the system operates in this mode

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most of its life. The responses of the different types of AVR obtained on open circuit in the previous section cannot normally be achieved when the generator is on load. This is due to the significant changes that the generator characteristics undergo when the operating mode is changed from open circuit to the loaded state. It was observed during an evaluation of the ST parameter estimator that the steady state gain and dominant time constant with load are considerably lower than their open circuit values and the system can exhibit some degree of oscillatory behavior at high load conditions. The step response obtained when using the different AVRs on the TG simulator, representing a typical 660MW set, has also been investigated. A positive step of 3% was used with an operating point of P = 0.8pu; Q = 0pu. These tests confirmed that performance similar to that obtained with the micro alternator can be repeated with the TG simulator. The terminal voltage step response with the STAVR-F, values given were: overshoot 3.5%, rise time 500ms, settling time 500ms; also shown is the variation in real power. The rise time differs markedly from OC conditions, since the alternator system‘s steady state gain has changed by about 5. The bandwidth of 3.6Hz is similar to the OC case. The corresponding step response with the conventional DGAVR-F gave overshoot 3%, rise time 840ms, settling time 840ms, bandwidth 1.7Hz, showing considerable changes from the OC values. These results clearly indicate that the STAVR is able to maintain its response characteristics under changing system conditions, while a fixed AVR fails to do so. These responses are comparable to those on the simulator, a useful confirmation.

3.2.3 RESPONSE TO POWER SYSTEM FAULTS

Major disturbances that occur in the power system from time to time can seriously affect the smooth operation of the excitation control system. These disturbances which are transient in nature are classed as abnormal operating conditions of the generator. Although the occurrence of these abnormal operating conditions is very infrequent, the performance of an AVR during these events should be evaluated to assess whether the controller is able to cope with such situations satisfactorily. In the case of the STAVR, the GPC cost function considers only the deviations of the terminal voltage from its set point and the liveliness of the control signal. However, during major disturbances the rotor angle of the generator with respect to the infinite bus bar of the power system is disturbed significantly and can take some time to settle down following the event.

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It is generally well known that a fast acting AVR such as the ST controller can reduce the damping torque of the generator if it uses only the terminal voltage as its feedback signal. The consequence of this is the reduction in the damping of rotor oscillations following a major disturbance. This aspect should therefore be examined in detail to ensure that sufficient damping of rotor oscillations is provided. The response of a turbine generator to severe disturbances depends very much on its severity as well as the conditions of the power system at which the disturbance occurs. A severe disturbance, regarded as a standard test, is a 3-phase short circuit. The performance of the new AVR is now examined under these conditions using the simulator.

3.2.4. THREE PHASE SHORT CIRCUIT

During this test, a sudden short circuit is applied to the stator terminals of the generator and is removed after a period of 100 ms The operating point of the generator has been chosen as P = 0.8 pu, Q = 0 pu to obtain a large rotor load angle. The greater the rotor angle the more severe is the test since the stability margin of the rotor is lesser in that case. For comparisons on the damping available to the rotor during the disturbance, a factor called the 'Effective Damping Ratio' (EDR) has been used. This factor is widely used in the industry and is defined as the ratio of the peak-to peak amplitude between the first undershoot of a signal following a disturbance and the second over-shoot to the peak-to-peak amplitude between the first undershoot and the first overshoot. A lower value of the EDR indicates higher damping. gives the response with STAVR-F. The EDR of the rotor angle signal is 0.64 and its settling time to within 2% is found to be 1.25 seconds; the terminal voltage settles down in 0.39 seconds, a satisfactory performance. The test was repeated with the conventional AVR, DGAVR-F, and a rather similar response was obtained. The EDR and the settling time of the rotor angle found were 0.73 and 2.14 seconds respectively and the settling time of the terminal voltage is 0.6 seconds. This performance indicates that the STAVR has improved the rotor damping in this case

3.3. SUCCESSFUL PROTOTYPE TEST RUNS

A first generator from the new series was set up in the test bay in the summer of 1995 and put through exhaustive development and type tests. These tests concentrated mainly on the following:

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i. ii. iii.

Measurement of the open-circuit and short-circuit characteristics Determination of the temperature rise in the windings and of the losse s Determination of reactance‘s and time constants (including those for the quadrature axis) and verification of the short-circuit strength, each by means of sudden short circuits starting from

iv. v.

No-load and preload conditions Determination of negative-sequence reactance and resistance by means of a sustained two-phase short circuit.

vi.

Standstill tests in order to determine the sub transient reactance‘s in the direct and quadrature axis.

vii.

Additional temperature measurements on the press plate and press fingers, on clamps and end connections of the stator winding as well as measurements of the cooling-air temperature at different locations in the generator

viii.

Pressure measurements to verify the distribution of the cooling air

ix.

Measurement of the mechanical vibration in the shaft and bearing pedestals, stator core and housing, and the winding overhangs

x.

Noise measurements to determine the sound level.

A total of approximately 70 vibration pick-ups, 80 pressure and 200 temperature measuring probes were used for the tests. The results of the test runs will be looked at in detail in a future article. The prototype fulfilled the requirements covering the running quality and vibration as well as temperature rises and losses – in every respect, even exceeding the high expectations in certain areas. The rated data of the prototype together with some of the more important measured values. Special mention has to be made of the excellent effic iency, which lies only marginally below that of the hydrogen-cooled generators. Based on the partial temperature rises measured under open-circuit and short-circuit conditions, the temperature rise during full-load operation at 300 MVA will lie below the limit for temperature class B by a sufficient margin of safety. Given the information available today and looking to the future and further innovations (eg, in the stator winding insulation), it is evident that air-cooled generators are potentially capable of another increase in output.

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Fig 3.2 Layout of Turbo generator 3.4.TEST METHOD

1. INSTRUMENT

IR is measured with a ‗mega-ohmmeter‘. Sometimes this is called Megger Tester after the name of the instrument first developed for this purpose (Megger is a trade name of AVO). Mega-ohmmeter generates and applies a regulated DC supply. It measures the flow of current and IR is directly read on its dial. Hand driven and motorized mega-ohmmeters are available. But for constant rotation and steady DC voltage, a motorized meter is preferred. Modern mega-ohmmeter can apply voltages exceeding 10 KV, and measure resistance higher than 100GΩ

2. TEST VOLTAGE

Test voltage should be well below the rated peak line-to-ground voltage of the winding as it is not a ‗high potential‘ test. But the voltage should be high enough to find defects such as cuts though the insulation in the windings.

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Table 3.1: Guidelines for DC Voltage to be applied for IR test Guidelines for DC voltages to be applied for the test are given in Table-1 Rated voltage (line-to-line) of the winding in volts

Test voltage (DC) in volt

<600

500

1000-2500

500-1000

2500-5000

1000-2500

>5000 2500-5000 . 3. TEST PROCEDURE

IR & PI tests shall be done simultaneously. If the winding temperature is below the dew point, the winding may be heated to dry off the moisture that has condensed on the winding. If the temperature is below the dew point, there is no way to correct the IR & PI for humidity Otherwise, the test is very simple. The procedure is as follows: Remove all external connections to the machine and completely discharge t he windings to the grounded machine frame Apply required DC voltage between the winding and ground using a direct indicating, motor driven mega-ohmmeter. Continue to apply the voltage for 10 minutes. Measure the insulation resistance after 1 minute and 10 minutes. Switch off supply to the meter and completely discharge the windings to the grounded machine frame. Calculate the polarization index by dividing the 10-minute insulation resistance by the 1-minute insulation resistance. Note the winding temperature. If test is carried out only on one winding of three phase equipment, then other windings should be grounded during the test. If IR is below the above recommended value, the winding should not be subjected to high potential test or be taken to service, since failure may occur. However, if historical record indicates that a low IR value is always obtained on a particular winding, then the machine can probably be returned to service with little risk of failure. If IR or PI is below the minimum value in a modern stator winding, it is an indication that the winding is contaminated or soaked with water. Interpretation of PI value.

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For modern form wound stators, if a very high value of IR is measured (say greater than 5GΩ) then PI is not likely to indicate anything about the winding. Thus the test may be stopped after one minute to save time. If a high PI result is obtained on an older stato r winding, then there is a possibility that the insulation has suffered thermal deterioration. This occurs because thermal deterioration fundamentally changes the nature of insulation and thus the polarization currents that flow. In general IR & PI test are an excellent means of ascertaining winding conditions that are contaminated or soaked with moisture. The tests are also good detecting major flaws where the insulation is cracked or has been cut through. The test can also detect thermal deterioration for form wound stators using thermoplastic insulation system.

Figure3.3.Layout of turbo generator foundation

NISA/McFdn , customized software from Cranes Software International Limited, offers CAD

based solutions to different power house structures such as Turbo generator foundations with or without VIS and Block foundations. Backed by powerful NISA II Analysis and DISPLAY III/IV – the graphical Pre and Post processor of NISA suite of programs, NISA/McFdn provides seamless interface for modeling, Static, Eigen, Shock & Forced Vibration analysis and design of TG Foundation.

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2.PROPORTIONING CRITERION:

Proportioning of the components of TG foundation is carried out based on the following criteria: a. Shape of top deck, number of level of beams & their sizing based on TG configuration and its auxiliary units b. To separate the frequency of machine with natural frequency of foundation, c. To limit maximum amplitude of structure as per codal provisions and functional specifications of equipments. d. To carry Dead loads, thermal loads, equipment loads, operating loads, erection loads, unbalanced loads, loss of blade, short circuit and seismic load s.

3. TYPES OF TG FOUNDATIONS:

Different types of TG foundations are considered based on power generation capacity & supported with or without vibration isolators. Types of TG foundations are: 1) Top Deck with Vibration Isolation System with power generation capacity 210MW, 250MW, 500MW and support Frame. In this type top deck and supporting structure are modeled separately. The VIS is modeled using spring elements. 2) Top Deck without VIS with power generation capacity 210MW, 250MW, 500MW. In this an integrated model of Top deck, supporting structure and foundation is generated.

4. GUI DESIGN:

NISA/McFdn as a tool gives an end to end solution with a user friendly Interface for input of Geometric, Loading and other important connection details such as Insert plates and Embedded Parts as per TG manufactures data without sacrificing the flexibility for possible variations in data. Friendly features such as import of data in the Excel format are also provided. Soil parameters as per site conditions are also considered for computation of spring stiffness. User interface also provides direct specification of input details like geometry and elevations. Figure 2 & Figure 3 shows a typical UI text input for McFdn.

5. Finite Element Modeling:

Based on the geometry and loadings input, FE models are generated automatically. Two types of FE models are generated i.e. Beam/shell model and a detailed 3D solid model.

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Beam/Shell integrated model is generated for the design of RC structural components conforming to Indian standards. This model is also used for arriving at the required foundation size. Beam elements are modeled with a two noded NKTP 12, 3D beam element having six degrees of freedom with 3 translations and 3 rotations at each node. Shell elements are modeled with a four noded NKTP 20, 3D General Shell element having six degrees of freedom with 3 translations and 3 rotations at each node. All these elements belong to NISA element Library. A detailed 3D solid model is used to evaluate the dynamic behavior of the structure. Solid model uses an eight node NKTP4, 3D Solid element having three degrees of freedom with 3 translations at each node. This model is used to evaluate the natural frequency of the structure and perform the frequency response analysis due to harmonic loads on the structure. Vibration isolators (VIS) used to isolate top deck with rest of the supporting structure which eliminates dependency on approximate soil properties, to avoid disturbance on to the surrounding structure. In this case of Top deck supported on VIS, the vibration isolators are modeled using spring elements using a two node NKTP-38, 3-D general spring element with six independent spring rates and six degrees of freedom (UX, UY, UZ, ROTX, ROTY, ROTZ) per node. The spring constants are computed based on Standard specifications. McFdn has an extensive database of the Isolators from which relevant spring data are extracted and applied on the FE Model for analysis. A facility of automatic selection of Isolation springs is also available. The soil base below foundation is also modeled using the spring elements and corresponding constants are modeled using the soil data. Loads and boundary conditions are applied on the FE models and a typical FE model auto generated by McFdn are given in Figure-4 through Figure 9. Equipment loads are modeled using 3D mass elements @ the c.g locations and connected to the anchoring points in the FE model by rigid links. Application of the loadings are based on user specified input data and provisions of IS 2974 – Parts III are also considered.

3.5.

DIAGNOSTIC

TEST,

GENERATOR,

PARTIAL

DISCHARGE,

SLOT

DISCHARGE, STATOR INSULATION. 3.5.1. INTRODUCTION

The rate of occurrence and the consequences of service failures in high-voltage generator stator insulation systems can be reduced by the use of sensitive diagnostic tests

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designed to detect the early stages of insulation deterioration. Degradation processes include insulation delamination, shrinkage of wedges and/or side packing permitting vibration and abrasion, and loss of function of gradient control coatings. All these processes are almost invariably accompanied by partial discharges which increase in severity as the deterioration progresses, usually making an additional contribution to the insulation damage rate. Especially important with modem insulation systems are discharges occurring in the slot between the electrical shield of the stator bar and the core, usually referred to as slot discharge, which can attain levels of energy ( > 5000 pC) sufficient to cause damage in times as short as several months. Detection of discharges at the earliest practical stage, and proper interpretation of test results can permit corrective action to be taken before a winding deteriorates beyond the point of economic salvage, and particularly before the risk of a failure in service becomes unacceptably high. Reliable early warning from a suitable diagnostic test may permit relatively inexpensive repairs, such as re-establishing ground connections, side-packing, rewedging or touch-up of stress grading paint, to be accomplished during a scheduled outage. In many power systems with mixed generation, hydraulic machines are reluctantly removed from service for discharge tests because of the relatively high cost of replacement fossil fuel energy. Thus, a diagnostic test that, at least for screening purposes, can be performed without service interruption, presents a distinct advantage. For large thermal machines, such a test also offers advantages in that the long time restraints for shedding and picking up load may be avoided. The new test methods described later in this Paper have been designed to respond to partial discharges originate in the stator insulation system, using signal- coupling techniques inherently insensitive to system noise. The signal to-noise ratio is further improved by electronic processing of the detected signals. To put into perspective the diagnostic tests described in this Paper, a review follows of the most widely employed tests to ascert ain stator insulation condition.

3.5.2. REVIEW OF DIAGNOSTIC TEST TECHNIQUES 3.5.2.1. MEASURABLE QUANTITIES WHICH CORRELATE WITH DAMAGE

Mechanical vibration, gaseous products and partial discharges are three quantities which can be monitored readily with negligible service interruption, while providing information with respect to the total stator insulation condition. The first two quantities have

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only relatively recently been under study and have yet to demonstrate their sensitivity and resolution, since data correlating measurements with visual inspections of stator condition are sparse.

3.5.2.2. VIBRATION

If a bar or coil side loosens within a stator slot, vibration can cause ground wall erosion and wear, contributing to ultimate insulation failure. Hence, the presence of a vibrating bar indicates that the winding is loose and may eventually fail. By measuring the magnitude and rate of increase of these vibrations by means of accelerometers attached to the stator frame, the expected remaining useful life might be estimated.

3.5.3.

DIAGNOSTIC

TESTING

OF

GENERATOR

INSULATION

WITHOUT

SERVICE INTERRUPTION

3.5.3.1 GASEOUS BY PRODUCTS

Certain dielectrics, exposed to partial discharges or to heat, evolve various gaseous products. For insulations commonly used in stator windings of hydrogen-cooled units, some of these products can be readily distinguished against the background by gas chromatography or spectroscopy. The quantity of evolved gas car indicates t he degree of degradation.

3.5.4. PARTIAL DISCHARGES a. TIP UP TEST

The ―tip-up‖ test provides a measure of the void content and partial discharge activity in a dielectric by measuring the change in dissipation factor between two discrete voltage stress levels, usually 50 percent and 100 percent of operating voltage. Unfortunately, this test tends to be insensitive to localized partial discharges because the loss component is averaged throughout the entire test sample, unless individual coils are isolated for test — an expensive procedure. Also, the end-grading material will distort results for in-situ measurements. The tip-up test requires an external supply to energize the winding, thus applying maximum voltage stresses to the entire winding which is not representative of operating conditions.

b. DIELECTRIC LOSS ANALYZER

The dielectric Loss Analyzer reacts to the power loss in an insulation system as a function of voltage per cycle, thus indirectly measuring the presence and effects of partial

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discharge. This test method, through more sensitive than the ―tip-up‖ test, can identify a number of weaknesses, but cannot detect the presence of a small number of intense discharge events in a background of many more moderate discharges. This test also requires an external supply, though the duration of the test outage may be comparatively short.

c. INDUCTIVE PROBE

Inductively coupled radio frequency probes have been employed to detect local discharges. This test requires a lengthy service interruption and an external high-voltage supply, though it does have the capability of pin-pointing those bars or coil sides suffering t he most intense internal or slot discharges.

d. ULTRASONIC DETECTOR

Signals from an ultrasonic probe have been introduced into a conventional partial discharge measuring circuit with some success, especially for locating specific discharge sites. This procedure does not provide any advantage over the Inductive Probe technique and is probably less quantitative.

e. PULSE DETECTION

Detection of individual partial-discharge pulses by direct capacitive or inductive coupling to a machine winding, with the generator self-excited and thus supplying its own high voltage with normal voltage distribution has been implemented in various measurement systems. In this class of tests, the pattern of individual pulses can be displayed on an oscilloscope or quantified by a pulse-height analyzer. In the early days of this type of measurement, the high partial-discharge repetition rate from the many sites in a generator could result in the superposition of pulses since tests were often performed with ―pulse shaping‖ circuitry to lengthen the duration of the individual pulses for easier observation. However, with modem wideband storage oscilloscopes and flatresponse filters, it is observed that actual superposition of pulses rarely occurs. The rise times of partial- discharge pulses measured with such equipment are about 10 ns or less. Ringing frequencies, which depend only on generator winding parameters and the measuring system, vary from about 1 to over 50 MHz and are ―second order‖ effects initiated by the original partial discharge event.

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Pulse durations, including ringing, are typically less than 1 psec ., and consecutive partial discharges are rarely observed at intervals less than 10 psec. An inexpensive version of this test has been in routine use within Ontario Hydro for more than 20 years, employing HV capacitors temporarily connected to the generator, a high-pass filter and an oscilloscope for display. This test has demonstrated that the condition of the stator ground wall insulation is correlated with the magnitude of the highest discharge pulse observed on the oscilloscope. However, distinguishing between generator insulation partial discharges and external noise is sometimes difficult, requiring an experienced operator. Additional difficulties arise because of the nature of the partial-discharge pulses. Since these pulses are extremely rapid, the peak magnitude is difficult to determine at the slow oscilloscope sweep speeds required to recognize partial discharges by their phase position in the power frequency cycle, making the test highly subjective. A further drawback to the test is that, in practice, only the magnitude of the highest pulses is recorded. Information such as the number of pulses and the distribution of pulse magnitudes, that is, the relative abundance of large pulses compared to small pulses, can only be noted qualitatively. Yet, significant information about the nature and extent of insulation degradation must be present in the total pulse patt ern.

3.6. IMPROVED GENERATOR TESTS

Although the partial discharge test is successful in quickly predicting stator insulation condition, the above limitations have restricted use of the test outside Ontario Hydro. As a consequence, CEA and one manufacturer began separately the development of more sophisticated procedures for observing and quantifying partial discharge activity in generator stator insulation systems. The test improvements described below comprise better methods of acquiring and treating partial discharge data with permanently installed coupling devices. The coupler or ―antenna‖ is mounted on the rotor in one system, while couplers are installed on the stator in the system developed for CEA. Both coupling techniques respond to the high frequency energy in an actual discharge. Means for reducing the influence of electrical noise are incorporated into both coupling techniques, thus permitting diagnostic testing while the generator is operating normally. Methods for quantifying the signals from either coupling system differ, although in principle both are based on pulse magnitude analysis.

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3.6.1. STATOR-MOUNTED COUPLING SYSTEM

The partial discharge signals are acquired using rugged high-voltage capacitors of 50 to 100 pF which are solidly connected to the stator winding. The low-voltage sides of these couplers are connected to a convenient location external to the generator housing by terminated 50 ^l coaxial signal cable. The couplers are sensitive only to the high frequency components of a discharge pulse. The placing and functioning of the couplers depends on whether the stator winding is in a hydraulic or a t urbine generator.

3.7. HYDRAULIC GENERATORS

In hydraulic generators, the couplers are often placed at or close to the connection point of the circuit ring bus to each split or parallel of each phase in the winding. Since noise pulses entering the generator from the power system are first attenuated by surge capacitors and transformers and maybe further reduced by impedance mismatches as the pulse travels along the circuit ring buses, a measure of external noise immunity is inherently present. Additional attenuation of noise, including power frequency and solid-state dc exciter noise, is afforded by connecting pairs of capacitive couplers to a differential amplifier in such a way as to cancel common-mode signals, taking into account the pulse travel time from the machine terminal to each coupler. For example, it shows two capacitors installed on a hydraulic generator with asymmetrical winding. When a noise pulse enters the winding, voltage pulses travel along the ring bus and reach both couplers about 25 ns, say, after signal injection. Since the response is the same at each coupler, if these two signals are combined in a differential amplifier there will be no output, at least not until pulse reflections within the generator winding start to build up. For partial discharges, which usually occur near the high-voltage end of each parallel, a net response is obtained since the signal reaches one coupler almost 50 ns before it reaches the other coupler in the pair. This system works because the partial discharge pulse rise times are typically only 10 ns, much less than the pulse travel times along the transmission-line-like path of the circuit ring bus. Practical hydraulic-machine windings are rar ely symmetrical about the terminals. However, by the careful placement of the couplers and the use of delay lines, such permanent ―differential‖ couplers can be installed on the majority of windings currently in use. More than twenty installations have been made in a number of Canadian utilities. These

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installations are usually on generators which either are difficult to obtain for test purposes because of outage restraints or are subject to a high degree of external noise interference.

3.8. TURBO-ALTERNATORS

Because of the much smaller rotor radius involved, the type of differential coupling system described above is not possible for most turbo-alternators since the electrical length of circuit ring bus is often shorter than the discharge-pulse rise time. Sensitivity to external noise can be reduced, however, by the use of two permanently installed ―directional‖ couplers per phase on the output bus of the generator. External noise can be greatly attenuated by differential sensitivity to the direction of pulse travel on the bus, that is, either from the generator (assumed to be partial discharges) or from the power system (assumed to be noise).Retrofitting of the ―directional coupler‖ can often be readily implemented on Isolated Phase Bus, since the inspection covers, which are regularly placed in the bus sheath, can provide sufficient capacitive coupling when grounded through a suitable impedance.

3.9. ELECTRONIC ANALYSIS

Partial Discharge Analyzers (PDA) have been constructed to process the voltage pulses from pairs of couplers into information about the repetition rate and magnitude of the discharges. An analyzer consists of an 80-MHz bandwidth differential amplifier driving a single channel, dual polarity, pulse-height analyzer fabricated w ith ECL integrated circuits. The pulse-height analyzer is designed to handle generator partial discharges. Specifically, it responds to pulse rise times of less than 10 ns, ignores ringing, inhibits the counting of pulse overshoots such as the negative overshoot of a positive pulse which can cause a false indication in the negative channel, ignores reflections in noise signals, and accepts consecutive discharge pulses more than 3 psec. apart. This single-channel pulse-height analyzer provides multichannel operation by sequential variation of the threshold levels. Fifteen 100-mV-wide channels with lower thresholds ranging from 100 mV to 1500 MV have been found satisfactory for completely determining pulse magnitude spectra. The PDA is controlled by a microprocessor which automatically st eps the pulse height analyzer through the 15 voltage channels. The microprocessor also controls the counters which total the number of positive and negative pulses per second which occur in each channel and at the same time supervises a digital printer which provides the pulse magnitude spectra.

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Also produced is information on the generator‘s operating voltage at the time of test, which is obtained from the 10-mV power frequency signal appearing on the couplers‘ output. The source coupler of the partial discharges is identified automatically by comparing the polarity of the discharges with the phase of the power frequency voltage. Facilities are also included in the PDA for analyzing data from temporary couplers. These include requisite filters and a circuit which removes the very strong interference caused by thyristor excitation systems. Various versions of the PDA have been in use for more than 4 years and improvements are constantly being made. Several of the PDA‘S described above have now been commercially manufactured and are in routine use by a number of Canadian utilities.

3.10. TEST RESULTS

Test data on many operating hydraulic generators have shown that external noise and interference caused by thyristor excitation systems are reduced by more than 20 dB when the permanent couplers and the PDA are employed, whereas sensitivity to generator insulation partial discharges is maintained. Results are consistent with those obtained by skilled personnel using the ―conventional‖ test. Particularly the magnitude of pulses corresponding to a partial discharge repetition rate of about 10 Hz was found to correlate well with the magnitude of the peak discharge pulse observed from the oscilloscope trace in the conventional test. It indicates pulse magnitude spectra observed on two of the parallels of a modern 200MVA hydraulic generator. The stator winding in this machine has been visually examined and the parallel corresponding to the line on the right side was found to be damaged by slot discharge deterioration.

3.11. MICRO TURBINES LATEST AND PAST TECHNOLOGY

High-speed micro-turbines and mini-turbines play a significant role in the Distributed Power Systems that provide dependable electric power close to the user. Several high-speed turbo-generators manufactured by various corporations are now available in the 30 kW to 90 KW range. These systems operate at speeds from 50000 RPM to 120000 RPM. The generator is directly coupled to the turbine shaft. This obviates the need for a gearbox, helps reduce the size of the generator, and lowers the cost of the overall system. The output power is

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electronically processed and conditioned to provide constant voltage dc or multi-phase ac power at constant frequency. Technology of micro-turbines is moving forward to address ratings above 100 kW due to the growing demand for larger units. There is a tendency to use multiple units of the existing 30to 90 kW packages to satisfy this demand for higher power capacit y. However, use of turbo generators of higher ratings is likely to be beneficial to the user for the following reasons: a) Lower cost of investment per kW for purchase and installation b) Lower cost of maintenance because of reduced parts count c) Higher efficiency d) Safer operation. At the present time most generators used with micro-turbines are based on permanent magnet technology. It is the objective of this paper to compare alternatives to the PM generator technology, and introduce induction generator technology as a more viable alternative in the power range exceeding 100 kW. The approach in this paper is to present the concept in all its dimensions including the issues of generator and controller design. The authors are currently engaged in the development of the high-speed induction generator systems. Their experience in the field of the technology forms the basis supporting the discussions in this paper.

1) SYSTEM DEFINITION AND CONSTRAINTS

It is realized that one specific technology does not necessarily provide the best answer under all situations. We must therefore limit our discussions to applications within certain constraints. At this time the following broad limits are applicable for the technology under consideration: i) The micro or mini turbine systems considered here are in the 100 kW to 500 kW power range. The system comprises mainly of high-speed turbine, generator, controller, protection, and instrumentation. ii) The prime mover operates at speeds between 30000 to 80000 RPM depending upon the rated output. Typically, the operating speed of the prime mover varies inversely with the rated output. iii) Constant speed of operation is considered. However, certain narrowly defined operating speed range may be required in specific applicat ions.

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iv) The generator must be designed for a cooling system that is compatible with the system requirements. Typically either air, or lubricant oil, or water glycol mixture is used. v) The integrated power system is located close to the user such as in a factory building, hospital, department store, and office complex. Alternatively, vehicle mounted applications in airborne, land based or marine situations are also considered. These mobile applications are valuable particularly for military requirements. vi) The electrical power output is typically 3-phase ac with single or multiple voltages. Alternatively, DC output may be required. In case of AC power systems, 50/60 Hz frequency is common for commercial applications, and 400 Hz. frequency is used in military / aerospace applications. vii) Compatibility with utility power systems may or may not be required. In most situations stand- alone capability in isolation from a utility system is required. In some other situations, power transfer from utility to the turbo-generator and vice versa may be necessary. viii) The generator must also provide electric start capability during the initial start up of the turbine. ix) The system must provide protection against hazards. Safety of operation is an important consideration. In approaching various issues, we have considered the following issues to define relative merits: i) Cost: Investment and Operational ii) Reliability and safety iii) Size, Power Density. The issues listed above are not necessarily listed in the order of their importance.

2) GENERATOR TECHNOLOGIES

We plan to review three different generator technologies for comparison: permanent magnet (PM), induction, and switched reluctance (SR). All these three are suitable for high speed operation in the speed range considered here. There are other technologies such as synchronous reluctance and homo polar that are suitable for high-speed operation but are not considered in this paper. We also limit our discussion to radial geometric configurations for the three technologies. Axial gap geometric configurations are not considered.

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A. PERMANENT MAGNET (PM)

Micro power systems currently in the market use the generator designs based on the PM technology. The generator itself has two electromagnetic components: the rotating magnetic field constructed using permanent magnets; and the stationary armature constructed using electrical windings located in a slotted iron core. The PM‘s are made using high-energy rare earth materials such as Neodymium Iron Boron or Samarium Cobalt. Retention of the PM‖S on the shaft is provided by high strength metallic or composite containment ring. The stationary iron core is made of laminated electrical grade steel. Electrical windings are made from high purity copper conductors insulated from one another and from the iron core. The entire armature assembly is impregnated using high temperature resin or epoxy. The voltage output from the generator is unregulated, multiple phase ac. This voltage varies as a function of the speed and load. This voltage output is connected to a solid state power conditioning system. Typically, the solid state power conditioning system uses buck/boost techniques and regulates the entire power output.

B. INDUCTION

The technology of induction generator is based on the relatively mature elect ric motor technology. Induction motors are perhaps the most common types of electric motors used throughout the industry. Early developments in induction generators were made using fixed capacitors for excitation, since suitable active power devices were not available. This resulted in unstable power output since the excitation could not be adjusted as the load or speed deviated from the nominal values. This approach became possible only where a large power system with infinite bus was available, such as in a utility power system. In this case the excitation was provided from the infinite bus. With the availability of high power switching devices, induction generator can be provided with adjustable excitation and operate in isolation in a stable manner with appropriate controls. Induction generator also has two electromagnetic components: the rotating magnetic field constructed using high conductivity, high strength bars located in a slotted iron core to form a squirrel cage; and the stationary armature similar to the one described in the previous paragraph for PM technology. The voltage output from the generator is regulated, multiple phase ac. The control of the voltage is accomplished in a closed loop operation where the excitat ion current is adjusted to generate constant output voltage regardless of the variations of speed and load current.

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The excitation current, its magnitude and frequency is determined by the control system. The excitation current is supplied to the stationary armature winding from which it is induced into the short circuited squirrel cage secondar y winding in the rotor.

C. SWITCHED RELUCTANCE (SR)

The technology of SR generator is based on the concepts that magnetically charged opposite poles attracts. Typically, there are unequal number of salient poles on the stator and rotor. Both are constructed of laminated electrical grade steel. A cross sectional view of the construction of the SR generator. The number of poles shown on the stator is 6. The number of poles shown the rotor is 4. Other pole combinations such as 8/6, 10/8 are possible. There is no winding on the rotor. Armature coils located on stator poles are concentric and are isolated from one another. When the coils on opposite poles such as 1 and 1 shown in are excited the corresponding stator poles are magnetized. The rotor poles A-A are closest to the stator poles 1 and 1. These are magnetized to opposite polarity by induction and are attracted to the stator poles. If the prime mover drives the rotor in the opposite direction, voltage is generated in the stator coil to produce power. The voltage output from the SR generator is DC and has high ripple content. The voltage output can be filtered, and is regulated by adjusting the duration of the excitation current. The commutation of the stator coil is accomplished by the controller.

D. INDUCTION GENERATOR OPERATION

The speed torque characteristics of an induction motor operating from a constant frequency power source. Most readers are familiar with this characteristic of the induction motor operation. The operation of the induction motor occurs in a stable manner in the region of the speed torque curve. The torque output as well as the power delivered by the motor varies as the motor speed changes. At synchronous speed no power is delivered at all. The difference between the synchronous speed and the operating speed is called the slip. The output torque and power vary linearly with the slip. If the induction motor is driven to a speed higher than the synchronous speed, the speed torque curve reverses. In the stable region of this curve, electric power is generated utilizing the mechanical input power from the prime mover. Once again the generated power is a function of the slip, and varies with the s lip itself.

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In the generator mode, if the slip is controlled in accordance with the load requirements, the induction generator will deliver the necessary power. It must be remembered that the synchronous speed is a function of the electrical frequency applied to the generator terminals. On the other hand, the operating shaft speed is determined by the prime mover. Therefore to generate power, the electrical frequency must be adjusted as the changes in the load and the prime mover speed occur. In addition to the requirement stated above, the excitation current must be provided to the generator stator windings for induction into the rotor. The magnitude of the excitation current will determine the voltage at the bus. Thus the excitation current must be regulated at specific levels to obtain a constant bus voltage. The controller for the induction generator has the dual function as follows: i) Adjust the electrical frequency to produce the slip corresponding to the load requirement. ii) Adjust the magnitude of the excitation current to provide the desirable bus voltage.

A number of torque speed characteristic curves in the stable region of operation are shown to explain the operation. As an example, consider the situation when the prime mover is at the nominal or 100% speed. The electrical frequency must be adjusted to cater for load changes from 0 to 100% of the load. If a vertical line is drawn along the speed of 100%, it can be observed that the electrical frequency must be changed from 100% at no load to about 95% at full load if the prime mover speed is held at 100%.

3) BENEFITS OF INDUCTION GENERATOR TECHNOLOGY

Induction generator has several benefits to offer for t he micro, mini power systems under consideration. These benefits relate to the generator design as follows: i) Cost of Materials: Use of electromagnets rather than permanent magnets means lower cost of materials for the induction generator. Rare earth permanent magnets are substantially more expensive than the electrical steel used in electromagnets. They also must be contained using additional supporting rings. ii) Cost of Labor: PM‘s require special machining operations and must be retained on the rotor structure by installation of the containment structure. Handling of permanent magnets that are pre-charged is generally difficult in production shops. These requirements increase the cost of labor for the PM generator. iii) Generator Power Quality: The PM generator produces raw ac power with unregulated

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voltage. Depending upon the changes in load and speed, the voltage variation can be wide. This is all the more true for generators exceeding about 75 kW power rating. On the other hand with SR generator, the output waveforms are non-sinusoidal and peaky. These waveforms must be filtered in order to get reasonably constant voltage output. The induction generator produces ac voltage that is reasonably sinusoidal as shown in the example from an actual test .This voltage can be rectified easily to produce a constant dc voltage. Additionally, the ac voltage can be stepped up or down using a transformer to provide multiple levels of voltages if required. iv) Fault Conditions: When an internal failure occurs in a PM generator, the failed w inding will continue to draw energy until the generator is stopped. For high-speed generators, this may mean a long enough duration during which further damage to electrical and mechanical components would occur. It could also mean a safety hazard for the individuals working in the vicinity. The induction generator on the other hand is safely shut down by de-excitation within a few milliseconds, preventing the hazardous situations.

4) INDUCTION GENERATOR CONTROLLER TECHNOLOGY

The controller may be broadly divided into three sections, namely, the power section, sensing circuits, and the control section. Power transistors using IGBT‘s or MOSFET‘s are used in the power section of the generator controller in a conventional multi-phase configuration, the number of phases being the same as the number of phases in the generator winding. Anti parallel diodes are connected across eac h of the transistor. The DC rail is connected to a power capacitor. An additional power inverter is used when an AC output at a constant frequency such as 60 or 50 Hz. is required. Sensing of currents and voltages is provided at the load as well as in the power section of the controller. In addition, the speed of the shaft is measured. All the parameters sensed by the sensing circuits are conditioned by filtering and digitizing as required. The control section receives the information provided by the sensors. The parametric model of the generator is incorporated in the control section. In conjunction with a PID control algorithm, appropriate switching commands for the power transistors are generated in the control section. This creates the necessary frequency and amplitude of the excitation currents that flow in the induction generator windings and are induced into the squirrel cage rotor. The control section also includes protective functions such as over-current, overvoltage, and over-temperature protection circuitry.

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5) INDUCTION GENERATOR CONTROLLER BENEFITS

When compared to PM and SR generator controllers, induction generator controller offers the following benefits: i) Sensing: The control of induction generator slip requires precise measurement of speed. On the other hand, the control of SR generator requires precise measurement of the rotor position. This is a much more more difficult task to accomplish than the measurement of speed. ii) Switching and control speed: For the SR generator, the operating frequency is extremely high, in the range of 6 kHz. at 60000 RPM. This requires high speed switching of power transistors. The switching commands also must be provided at a high rate. For the induction generator, the operating frequency is in the 1 kHz. to 2 kHz. range at 60000 RPM depending upon whether 2 pole or 4 pole generator design is selected. The switching rate for the power transistors can be lowered in a reasonable range. iii) Power Section Sizing: In the case of PM generators, due to the wide variation in the voltage output, complexities are introduced in the controller requiring voltage boost mechanisms. The power electronic components must function at high stress levels. In the SR generator controller, high rates of change of currents and voltages result in high stress levels for the power electronic devices. The induction generator has a well regulated sinusoidal output that can be conditioned without using highly stressed electronic components. Overall it is believed that the controller for the induction generator is more robust, smaller in size and cost less than the controller for PM or SR generators in the power range under consideration

6) STATUS OF CURRENT TECHNOLOGY

Electrodynamics Associates, Inc. is currently developing a 125-200 kW induction generator to operate at 62000 RPM on an SBIR Phase II contract from AFRL/WPAFB, Dayton, OH. The generator is an air-cooled design. An identical machine has built to operate as a motor. The generator and the motor are mounted on a base plate and coupled together. An optical speed counter is attached att ached at one shaft extension. It shows the photograph of this assembly Controllers for both the generator and the motor have been developed for the test purposes. The control functions are embedded in a software model and the PC in the loop system using Math Works TM software packages is used.

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At the time of writing this paper, the motor generator test set up is operational and tests have been completed to 67 kW power output from the generator at 24000 RPM. The generated power on the dc bus is fed back into the motor, so that only the losses in the motor generator set are provided from the utility bus. Tests are continuing at higher speeds to demonstrate the rated power by the end of the current ca lendar year. During the next phase of this project, improvements in the controller are planned. Use will be made of current technology DSP‘S or ASIC‘S along with more compact power electronic components to reduce the controller size.

3.12. SUMMARY

The two new systems described in this Paper illustrate the possibility of low-cost online monitoring of partial discharge activity in the stator windings of large generators. These tests may be performed on machines in their normal operating condition without an interruption to service, because they incorporate noise rejection. Test results are based on the direct measurement of partial discharge quantities, with none of the ―dilution‖ effects inherent in other techniques. Both techniques. Both systems will identify the part of the winding, and one system the exact slot where the most significant discharge activity is located. Future developments must strive for the establishment of clear definitions of acceptable limits for damaging discharge in different machines. In particular: A. It must be possible to positively identify high-intensity slot discharge so that it can be eliminated without delay. B. The identification and measurement of surface or internal insulation discharges will permit the monitor monitoring ing of o f any progressive increase in intensity throughout the life of the winding, as an indicator of insulation aging. Probably little can be done to reduce or eliminate internal discharge activity so that the usefulness of monitoring this mechanism lies mainly in making possible efficient scheduling of rewinding.

Surface discharges may be reduced, when their intensity warrants, by suitable maintenance procedures. The greatest value from these diagnostic techniques will be obtained only after data from many different generators have been reviewed in relation to their design characteristics and their operating and maintenance histories, and correlated with the result of other more traditional diagnostic quality assessments of the bar insulation itself. To achieve this, continued close cooperation between manufacturers and users is essential.

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CHAPTER-4 ANALYSIS 4.1. INTRODUCTION To ensure that all functional requirements are fulfilled, and to estimate the performance of generator, the TURBO GENERATORS GENERATORS are required to undergo some tests. For testing, the TURBO GENERATOR was mechanically coupled to a drive motormotor generator set with gearbox. The rotor was excited by thyristor converter system located in an independent test room and the operation was controlled from the test gallery

4.2. CONTENT OF PROJECT The Testing of a machine is not only tested for Electrical machines like Turbo generators, but also for large motors and transformers etc. So as we are ar e about to explain about testing of a turbo generator, firstly it is mandatory to explain about the basics and introduction of Turbo Generator and also about Insulation systems of Turbo generator. This is the basic and required introduction about Turbo Generator and others.

4.2.1. Introduction to Turbo Generator: Machine acts as a generator converts the mechanical energy into electrical energy. The basic principle of rotating machine remains the same i.e. “FARADAY’S LAWS OF ELECTRO MAGNETIC INDUCTION” .

Faraday‘s first law states that whenever conductor cuts magnetic flux, dynamically induced EMF is produced. This EMF causes a current flow if the circuit is closed. Faraday‘s second law states that EMF induced induced in it, is proportional to rate rat e of change of flux.

…(4.1) e = EMF in volts N = No: of turns

= Change in flux linkage

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EMF induced will oppose both the flux and the rate of change of flux. In the case of AC generators the armature winding is acts as stator and the field winding acts as rotor. Efficiency of a machine is equal to the ratio of output to input

…(4.2)

To increase the efficiency of any machine we must decrease the losses, but losses are inevitable. There are different types of losses that occur in a generator. They are broadly divided into 2 types (1)

Constant losses (a) Iron losses (b) Friction and windage losses (air friction losses).

(2)

Variable losses (a) Copper losses 3 Phase all machines are of two types AC machines & DC machines. AC machines

are divided into single-phase AC machines and poly phase AC machines. Poly phase AC machines divided into 1. Synchronous machines: Synchronous generators or Alternators are those in which the speed of the rotor and flux are in synchronism (or) The machine which rotates with its synchronous speed. Synchronous speed ( Ns) =

rpm.

Inherently all the machines are AC machines. AC or DC depends

…(4.3) upon the flow of

current in the external circuit. Synchronous generators can be classified into various types based on the medium used for generation.

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1. Turbo-Alternators Steam (or) Gas 2. Hydro generators 3. Engine driven generators In every machine they are two parts (1) Flux carrying parts (2) Load carrying parts In large synchronous machines the stator have the load carrying parts, i.e. armature and the rotor has the flux carrying parts i.e.; field w inding. Iron losses are also called as magnetic losses and core losses. They are

broadly divided

into (1) Hysteresis losses (2) Eddy current losses These losses occur in the stator core. Copper losses occur in both stator and rotor winding. The general efficiency of a synchronous generator is 95-98 . A synchronous generator is the core of any generating power plant. A synchronous generator is a rotating electromagnetic device that converts mechanical energy into electrical energy by taking the mechanical input from a prime mover (Gas turbine or Steam turbine) and magnetic energy from excitation. Generators driven by steam or gas turbines have cylindrical/ round rotors with slots into which distributed field windings are placed. These round rotor generators are usually referred to as turbo generators and they usually have 2 or 4 poles. Generators driven by hydraulic turbines have laminated salient pole rotors with concentrated field winding and a large number of poles. Testing has to be done on a machine at every step in its manufacturing process for the company to certify it to be a “deliverable good”. Test brings out the impact of process variations. Testing is done in simulations which tend to closely resemble the practical scenario under which the machine works. Testing provides the experimental data like the efficiency, losses, characteristics, temperature limits etc. In this report of mine we shall look

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into the different types of tests, which are currently in use at BHEL, which are being conducted on stator, rotor and exciter.

4.2.2. TYPES OF TESTING:

Testing, in a generic sense can be classified into two different types, they are.

1. Process Testing (Tests conducted whilst manufacturing a machine).

2. Performance Testing (Tests conducted on a completed machine).

Process and performance tests have been listed below in a generic sense. As we proceed further with this report, we shall discuss the various tests carried out on stator, rotor and exciter individually and in a better detail.

4.2.3. LIST OF PROCESS TESTS:

1. Ring flux test 2. HV test. 3. Inter half test. 4. RTD test. 5. Resistance measurement. 6. Impedance test. 7. Tan δ test. 8. Led to Lead and Lead to Body test. 9. Megger Test. 10. Measurement of shaft voltage. 11. Lamp test. 12. Diode testing.

4.2.4. LIST OF PERFORMANCE TESTS:

1. Mechanical run test. 2. Short circuit test. 3. Open circuit test.

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4. Running impedance test. 5. Sudden short circuit test.

These are the type of process tests and performance tests of a turbo generator. The following tests shown are the type tests which include leakage reactance of stator winding &heat run.

4.2.5. LIST OF TYPE TESTS 1. Measurement of Leakage reactance of stator winding. 2. Heat run tests: a. Mechanical heat run test. b. Short circuit heat run test. c. Open circuit heat run test. 3. Voltage waveform analysis and determination of Telephone harmonic factor (THF). 4. Measurement of residual voltage of stator windings at rated speed. 5. Line to Line sustained short circuit test and determination of Negative sequence 6. Reactance (X2). 7. Line to Line and to Neutral sustained short circuit test and determination of Zero a. Sequence reactance (X0). 8. Retardation test for determination of GD2. 9. Three phase sudden short circuit test at 30%En and determination of reactance‘s and time constants.

4.2.6. STATOR TESTING

A stator serves as the armature for the synchronous generator. A stator is fundamentally tested for the detection of shorts in the coils (caused due to failure of insulation) and the insulation capacity along with the resistance, capacitance and ambient temperature of the stator are calculated.

I. RING FLUX TEST A cable is wound to the stator terminals; current is passed and increase d to the stator‘s rated value. Increase in temperature will be noted. An increase in temperature till 10˚c is

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allowed and if the temperature increases beyond this limit, it is an indication of an impurity at the core.

Fig 4.1 Stator before impregnation This test is conducted to check for any impurities in the core. The stator core is then treated with H2 SO4 and a voltage of 25-50 V is applied across the terminals, impurities will burst out

II. INTER HALF TEST:

When the coils are wound, they are so done in such a fashion that the end connections of the coil have a pair of copper sheets projecting out. In this test, the insulation between the two layers is checked. One layer is wounded by an aluminum strip and is supplied by phase; the other end will be connected to a bulb. If the insulation is good, then the bulb s hould not glow, if it does, the same test is carried out at further distance, if the problem still persists, the coil will be replaced. The same procedure is followed for all coils, once the bottom layer is laid, again when the top layer is laid, finally on the complete w inding.

III. RESISTANCE MEASUREMENT:

A 4-wire micro ohm meter is connected to different phases of the stator to obtain the total resistance of the coils in that phase.

a) RTD MEASUREMENT:

RTD wires are connected to all the coils in the stator; they are used to calculate the ambient temperature of the stator. Using a digital multimeter in resistance mode, all the

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RTD coils are checked and from the obtained resistance the temperature is calculated from the following law. Until 100Ώ the temperature will be 0˚c, but, from 100 Ώ, every 1 Ώ rise in resistance will represent 2.6˚c rise in temperature. b) “Tan δ” TEST:

This test is conducted on a stator to obtain the unknown capacitance of the stator and its loss angle factor. The capacitance and Tan δ measurement of stator winding is done using Schering Bridge and a standard capacitor. For a 3 phase synchronous machine, the windings are symmetrical and the value of capacitance is obtained by the following method. The stator body is isolated from ground by placing insulation packing between the body and the base. Connections to the Schering Bridge, Standard capacitor, Null indicator and transformer are done for the selected arrangement. Measurements are taken at Voltages 0.2Un to 1.0Un in steps of 0.2Un. The H.T. supply is switched on and raised to the required value. The bridge is balanced with the proper selection of variable resistances (R3+S) & the capacitance C4 and readings are recorded. After all measurements, Voltage is reduced to zero, supply is switched off and windings are discharged to earth.

Formulae used: i) Capacitance:

Cx =

μF

…(4.4)

ii ) Per cent tan δ :

% Tan δ = ({π*R 4*C4 ) * 10 -4 *100)

Where

…(4.5)

Cn and C4 are in μF & R 3 R 4,N &S are in Ω.

c) MEGGER TEST: (measurement of IR value)

In case of stator, the megger in use is a 1000/2500V megger. Megger is a high resistance bridge with a DC source. This test is usually done prior and after the HV test. The basic objective of this test is to check if the insulation resistance values of the insulations of the coils are up to the mark and obtain their values. This test is conducted on every phase. The expected resistance should be a minimum of (V+1) MΏ (practically the value turns out to be much more than the prescribed value). V has been defined in the forth coming section of HV test.

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The process is that the phase that is under consideration is connected to the positive probe of the megger and the other two phases are shorted and connected to the negative probe of megger. When the voltage is applied across the terminals, the insulation resistance of that particular phase appears in the screen. The test is conducted on all three phases.

d) HV TEST:

This test is conducted to check if the insulation of the winding is capable of withstanding the high voltage supplied to it. The HV supplied will be calculated by the following formula. …(4.6) Where

V = Final test voltage v = Rated voltage of the Generator The process remains same as that of the megger test except for the fact that the phase

to be measured will be supplied with a HV and the other two phases will be grounded. The same procedure will continue for all three phases. While testing prior to manufacturing, before and after impregnation in the absence of the customer, the voltage applied will be 1.5 times of V. In the presence of the customer though only V will be applied. One of the notable points here is that, before impregnation the HV applied will be DC and after impregnation the HV applied will be AC. The below graph shows the process of HV testing. For the first 20 sec the voltage is gradually increased to the desired HV. The HV is maintained for 1 minute an then drastically decreased

Fig 4.2. Representation of the application of HV to the stator

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The following is the STANDARD OPERATING PROCEDURE of STATOR TESTING employed at BHEL.

APPARATUS:

MEGGER, high potential tester, with adjustable power supply, compensation reactance if required, HV transformer, Earthing rod, and compensator rods for making connections.

Fig 4.3.Stator after impregnation

PROCEDURE:

1. Calculate V, where V = (2v + 1) KV where V is the final test voltage after test run. v is the rated voltage of the generator. 2. Connecting rings are tested with 1.2* V for 1 minute. a. If there is sparking due to less distance between live points, the same can be done with 1.1*V. if connecting rings are assembled after laying the bars, they are to be tested along with the bars. b. The HV is given to the copper and the core is earthed. The o/p voltage of the HV tester is continuously increased to test the voltage level, held at this level for 1 min and subsequently decreased to initial level. 3. After laying the bottom bars, 1.15* V HV is applied for 1 minute.

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4. Individual strips of each bar are tested with 110V AC for inter half shorts and inter bundle shorts with same voltage. 5. After laying the top bars HV testing with 1.1* V for 1 min. 6. Individual strips of each bar are tested with 110V AC for inter half shorts and inter bundle shorts with same voltage for top bars only. 7. After connecting the top and bottom bars but without connecting to the connecting rings, conduct inter half short test with 110V AC 8. After completion of the winding and baking, a. Conduct HV test of individual phases with 1.05* V for 1 min when one phase is under earthing the other phases are earthed. b. Measurement of R of individual phases. c. Measurement of the individual phases. 9. After test run, HV testing for individual phases with 1.0 * V for 1 minute. a) Where ever bunch brazing is employed for connecting top and bottom layers, only inter half test is to be carried out. b) During HV test, the phases except the measuring phase have to be earthed. c) IR value of the insulation should be 25 MΏ {KV+1} at least and is to be calculated by a 1000/2500 V megger.

4.2.7. ROTOR TESTING

The block representation of the construction of rotor. A rotor is tested for mechanical losses, winding short circuits, inter turn shorts, total insulation resistance of the rotor. Rotor in a synchronous generator serves the purpose of providing the field. Before going any further let‘s briefly discuss the various insulations required to be placed between the conductor and core. For the given machine (11KV), the following insulation is employed .

i. ii.

Mica – 1 layer – 0.25mm. Internal corona tape – 1 layer – 0.30mm.

iii.

Copper strip – 0.02mm – 1layer.

iv.

Mica – 8 layers – 0.25mm.

v. vi. vii.

Outer corona tape- -0.25mm – 1 layer. Split mica – 1mm – 1 layer. Outer corona tape – 0.25mm – 1mm.

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The above given list is the scheme in which the layers of insulations are arranged.

a. LEAD TO LEAD & LEAD TO BODY TEST:

There exists a DC lead inserted in the rotor. This lead is a couple of semi non-hollowcylinders separated by a thick layer of insulation and bounded by another layer of thick insulation. In this test the insulations mentioned above are t ested. In the LEAD TO LEAD test, one lead is grounded while the other is supplied by a HV; this is maintained for 1 minute. Any insulation failure will results in penetration of holes. In the LEAD TO BODY test, the leads are shorted and are supplied by a HV, while the rotor body is grounded, any short will results in, penetrations.

Fig.4.4.Rotor getting wounded and insulated

b. IMPEDANCE TEST:

This test is to check the existence of inter turn shorts. Two different windings in the same slot are connected to the phase and neutral of a power supply with 230V and 500Hz, for 1 minute. Any inter turn shorts are detected by this test. Shorts, if any are detected by connecting a Digital multi meter (voltage will be nullified) and that particular slot is sent for re-winding and insulation. This test did not demand any importance till the early 1980‘s, since late 1980‘s this test was identified to be of utmost importance and since then this test has been of utmost importance in the electrical testing department at BHEL.

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c. HV TEST:

This test is conducted to measure the total insulation of the rotor. A voltage which is equal to ten times the rotor‘s rated voltage is applied across the rotor leads; the applied voltage is applied in an exactly same fashion as that of the STATOR. …(4.7)

From the above equation, U = standard test voltage, v = rated voltage of the machine.

d. MEGGER TEST:

The test process remains same as that of the HV test except for the fact that only a small DC voltage replaces a high voltage AC. A 500/1000V megger is used here and the insulation capacity is checked for the execution of HV test. Both the leads of rotor are connected to the two ends of megger (none are grounded).This test is conducted before and after HV test.

e. RESISTANCE MEASUREMENT:

This test is to measure the resistance of the conductors of the rotor. The test process is similar to that of the stator except for the fact that the meter will be connected to the leads.

f. MECHANICAL RUN TEST:

This test is carried out in the BALANCING TUNNEL, just adjacent to testing area, where the rotor is made to run at 20%, 30%, 60%, 90% and 100% of its rated speed and is checked for any mechanical imbalances, which can be identified by the vibrations while the rotation take place. Now that the context has come, we shall have a glance at balancing tunnel too. Balancing tunnel is used to detect any mechanical imbalances in the rotor as they will cause heavy damage. The rotor is kept on a pedestal with wheels and is sent into the tunnel where it will be mechanically coupled to a DC Drive with whose support the rotor rotates. Lubricating oil will be introduced at the bearings so as to avoid any friction, at the surface of contact of the rotor ad bearings. The rotor, bearings, oil flow, temperature detector (at the ceiling), cameras (attached to the ceiling of the tunnel), vacuum motor (sucks all the air out of the tunnel and maintains I torr of vacuum pressure), cooling systems and the thyristor

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systems with the circuit breaker systems are connected to the control cum monitoring panel and the door is closed. The capacity of the balancing tunnel at BHEL is 50 tones and 20000 rpm. At rated speed, the rotor takes a lot of shapes due to imbalances an the allowed range of vibrations is 50 microns, while practically it can be reduced to 1.5 microns. Generally imbalances are identified horizontally, axially and vertically and a weight which is of the same value as that of the imbalance is added to the other side in the exact opposite side to compensate the imbalance

Fig .4.5.Rotor operating inside a balancing tunnel Generally, the best way of minimizing vibrations is ―weight manipulation‖ and ―weight addition” is universally accepted procedure rather than weight reduction. Whilst

manufacturing, a rotor is provided with certain holes called balancing holes exactly for this purpose. 7) SHORT CIRCUIT CHARACTERISTICS: A couple of current transformers are connected between one ends of the 3 phases while the other ends are shorted. The machine is run at rated speed and a current equivalent to 20% 40% 60% 80% 100% of the rated current is made to pass through the rotor and the reading on the current transformers are noted.

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8) OPEN CIRCUIT CHARACTERISTICS: The procedure remains same as short circuit characteristics except for some changes like potential transformers replace current transformers and a voltage equivalent to 20% 40% 60% 80% 100% of the rated voltage is applied across the terminals of the rotor and the corresponding readings are obtained at the potential transformers.

9) RUNNING IMPEDANCE TEST: This test checks the inter turn insulation of the rotor windings when it is running, as the rotor running at rated speed might displace the insulation. While the rotor is running at rated speed a 230V 500Hz supply is given through a couple of temporary slip rings at the rotor shaft and the voltage readings are taken at the same with a digital multimeter. Any inter turn shorts as such will be detected The Following is the STANDARD OPERATING PROCEDURE of ROTOR TESTING employed at BHEL:

APPARATUS:

Megger,high potential tester with adjustable power supply, HV transformer, voltmeter, earthing rod and copper wires for making connections. PROCEDURE

1. From the above equation, U = standard test voltage, v = rated voltage of the machine. 2. Before assembly in the rotor bore. i. ii.

Pole against HV testing with

V AC for 1 min.

Both poles against earth HV testing with

V AC for 1 min.

3. After assembly in the rotor bore. i. ii.

Pole against HV testing with

V AC for 1 min.

With both poles against earth, conduct HV testing with

V AC for 1 min.

4. After assembly of 2 coils on each pole. i. ii.

HV test with

V AC for 1 min.

Inter turn shorts with 75V AC per coil at 500 HZ.

5. Winding before wedging. i.

HV test with

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ii.

Inter turn shorts with 50 AC per coil at 500HZ.

6. Winding after wedging. i. ii.

HV test with

V AC for 1 min.

50 V 500 Hz Ac inter turn shorts.

7. After pressing and curing the insulation under retaining ring. i. ii. iii.

HV

V AC for 1 min.

50 V 500 Hz Ac inter turn shorts DC winding resistance measurement.

8. After mounting slip rings. i.

HV

V AC for 1 min.

9. After mounting retaining ring. i. ii. iii.

HV against shaft with

V AC for 1 min.

Inter turn shorts with 50 AC per coil at 500HZ DC winding resistance measurement.

1. Over speeding and balancing at stand still.

i.

HV

V AC for 1 min.

ii. Impedance measurement of rotor at 50 Hz. iii. DC winding resistance measurement iv. HV

V AC for 1 min at 3000 rpm.

2. After test run. i.

HV (U) V AC of the winding against shaft for 1 min.

12. Record test results. Measure IR values of the winding insulation before and after each HV t esting. i.

Minimum IR value should approximately be 4MΏ, use a 500/1000 megger.

4.2.8. EXCITER TESTING: The exciter in the discussion is a 583 KW, 550V, 1060A, 750rpm KWU type exciter. In a practical scenario, exciter serves the purpose of providing the excitation current to the rotor for the turbo alternator / synchronous generator to excite and produce electricity and is

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driven by the rotor of the synchronous generator to which it is mechanically coupled. In the testing area, however, we make use of a DC DRIVE to serve the purpose of the rotor. Before an exciter is tested it should first be magnetized.

Fig.4.6. Representation of an Exciter

1. MAGNETIZATION OF PMG: A cable is wound onto the Permanent magnet Generator (PMG) poles and a DC voltage is applied to it, this will trigger a magnetic field at the poles. This is not only a routine activity but also a test that verifies the nature of the metal used to construct a PMG.

a) LAMP TEST: This test is specifically for the yoke and is conducted to verify that there is no individual coil breakage. A lamp is placed to one end of a wire which is connected to neutral and all the coil terminals are shorted, the other end of the wire is connected to the last terminal of all the PMG coils and is supplied with voltage. If the bulb glows, it means that there is no breakage in the coils, else, the yoke should undergo a thorough retrospection and should be reworked

b) IMPEDANCE TEST:

Testing procedure of this test remains the same as that of its counterparts i.e. stator and rotor. This test is conducted on the rotor (armature) and the stator (field). Inter turn shorts of the armature and the stator are detected (if any) and rectified.

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Fig.4.7. permanent magnet generator poles

Fig.4.8. Yoke c) MEGGER TEST:

The megger used here is a 2.5KV megger. Intention of conducting this test is obviously same as that which were done on the rotor and stator, the ac lead and the stator are

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connected to the megger and the insulation capacity is verified. This test, too, is conducted before and after HV test. d) HV TEST:

This test is conducted to obtain overall insulation resistance of the armature. The HV is applied to the AC lead observed between rotor and the rectifier wheel, and the stator is grounded. The voltage applied is obtained by the following formula, KV) where V is the final test voltage and v is the rated voltage of the exciter. The remaining process remains same as that of the rotor of generator. This test is conducted before and after the armature is impregnated.

e) RESISTANCE MEASUREMENT:

This is conducted to the stator and the rotor of the exciter and the process remains same as that of the process applied for the ROTOR of the generator.

f) DIODE TEST:

The Diodes used here are of 400A, 750V rating and 28 diodes are connected in a wheel with 7 bridge connections. All diodes are individually tested using a multi met er, the criterion is that one terminal should indicate high and the other should indicate zero.

The following is the STANDARD OPERATING PROCEDURE employed for EXCITER TESTING at BHEL

APPARATUS:

HV tester(ac&dc) with adjustable power supply, HV transformer, voltmeter, micro ohm meter, impulse generator and 2.5 KV megger. PROCEDURE

1.

KV where V is the final test voltage and v is the rated voltage of the exciter.

2. Core tension balls vs. earth after assembly to be tested with 1000 V AC for 1 minute. 3. All connecting rings to be tested against each other and against earth with 1.2 V AC for 1 minute.

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Fig.4.9. Rectifier Wheel

4. DC HV test on armature. i.

After laying the coils and wedging with 2.5 KV Dc for 1 minute. (winding vs. earth)

ii.

After brazing and before connecting rings with 2KV DC for 1 min. (phase vs. phase & phase vs. earth)

5. Measurement of ohmic R before impregnation. 6. Measurement of R after impregnation & before bandage. i.

HV testing of winding with 1.2* V for 1 minute. (connecting rings to be provisionally connected for testing purposes, phase vs. phase & phase vs. earth)

ii.

Measurement of ohmic R.

7. After bandage and after brazing the connecting rings to the winding. i.

Winding to be tested with 1.2 * V AC for 1 minute against earth on the non magnetic steel with bandage.

ii.

Measurement of ohmic R.

8. After shrink fitting the wound armature on the shaft.

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i.

1.15*V AC for 1 minute against earth.

9. Measuring slip rings (against each other) and the corresponding leads (against earth) to be tested with 1.2*V for 1 minute. 10. HV for AC lead assembly. i. ii.

After assembly before bandage

V AC for 1 min.

After bandage HV with 1.2* V AC for 1 minute.

11. After mounting, rectifier wheel is to be tested with 1.2* V AC for 1 minute against earth. 12. Heat sinks (without diodes) to be tested again rectifier wheel with 1.15* V AC for 1 minute. 13. Total inverse current of rectifier wheel A and B to measured with 1000V dc. 14. Before balancing and over speeding all electrical act ive components to be tested with 1.1* V for 1 min. ( AC to DC leads shorted during this HV testing)

15. After balancing and over speeding, all electrical active components to be tested against earth with 1.05* V for 1 minute. (AC to DC leads shorted during this HV testing) 16. Total inverse current of rectifier wheel A and B t o measured with 1000V dc. 17. After magnetizing the PMG poles, the polarities are to be checked. 18. After painting and drying, resistances of the fuses are to be measured and total inverse current of rectifier wheel A and B to be measured with 1000V dc. 19. After test run all electrical active components to be tested with V for 1 min. ( AC to DC leads shorted during this HV testing), resistances of the fuses are to be measured and Total inverse current of rectifier wheel A and B to measured with 1000V dc. i. ii.

HV on Dc lead assembly is to be carried out as per rotor. IR value before and after each HV is to be 50MΏ at least and is to be so done using a 2.5 KV megger.

4.2.9. SUDDEN SHORT CIRCUIT TEST Apart from the above mentioned tests for rotor, stator and exciter, there are two type tests that have gained importance amongst the clients; one of them is the sudden short circuit test, the other being mechanical heat run test. A. LINE TO LINE SUSTAINED SHORT CIRCUIT TEST AND DETERMINATION OF NEGATIVE SEQUENCE REACTANCE (X2):

The line to line sustained short circuit test is carried out by short circuiting two line terminals of stator, using CT. The machine is driven at rated speed and excited. The

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following parameters are recorded at three values of short circuit current (limited to 30% of rated current). a) Short circuit current (Ik 2) through current transformer. b) Voltage between the open line terminal and one of the short-circuited terminals (Uk 2) through potential transformer (not used in calculation). c) Active power P.

B. EVALUATION OF NEGATIVE SEQUENCE REACTANCE (X2):

Negative sequence reactance is calculated as follows i) X2 =

…(4.8)

Where, P: Power measured during the test. Ik 2: Line to line short circuit current measured during the test. ii) %X2 =

*100

…(4.9)

Minimum time is taken for tests because serious rotor surface heating may result if current is sustained for a longer time or raised to too high a value.

C. LINE TO LINE AND TO NEUTRAL SUSTAINED SHORT CIRCUIT TEST AND DETERMINATION OF ZERO SEQUENCE REACTANCE (X0):

The line to line and to neutral sustained short circuit test is carried out by short circuiting two line terminals and shorted to neutral, using C.T. The machine is driven at rated speed and excited. The following parameters are recorded at several values of short circuit current limited to 30% of rated current. a) The neutral current (Io) through current transformer. b) Voltage between the open line terminal and neutral (V o) through potential transformer. a) EVALUATION OF ZERO SEQUENCE REACTANCE (X0):

Zero sequence reactance is calculated as follow…(4.10) Where, Vo: Line (which is in open condition) to neutral voltage measured during the test. Io: Line to neutral current measured during the test.

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%Xo = (

) x 100

…(4.11)

4.2.10. MECHANICAL HEAT RUN TEST:

This test too, is a type test which has gained enormous importance amongst the clients. The machine is rolled and run at rated speed after ensuring the bearing oil (seal oil, cooling water to coolers - for hydrogen cooled m/cs only) and kept at rated speed for stabilization of temperatures. Temperatures of stator inbuilt RTDs, bearing oil, bearing metal, cold air and hot air are monitored normally through Data Acquisition System. Periodically, the readings are taken at interval of 15 minutes. The temperatures are confirmed to be stabilized, if the temperature rise over cold air in one hour is not more than 2 °C.

a. MEASUREMENT OF ROTOR TEMPERATURE:

As it is a rotating part of the machine, the measurement of rotor winding temperature is done by measuring the rotor resistance using voltage divided by current method. For voltage measurement, to minimize error, one set of brushes on slip rings are kept isolated (insulated from remaining current carrying brushes) and voltage is measured across these brushes i.e. directly across the slip rings. Before start of run test, in cold state giving excitation to the rotor, current and voltage are noted. The ohmic resistance of rotor is computed along with the cold air temperature. R20 (i.e. rotor resistance at 20 °C) is calculated and this becomes the base for temperature rise calculation during the heat run. Periodically, the excitation is injected to rotor winding and rotor resistance . Periodically, the excitation is injected to rotor winding and rotor resistance is calculated by noting down voltage and current.

b. EVALUATION OF STATOR TEMPERATURE RISE:

Temperature rise = Stabilized temperature - cold air temperature (average).

c. EVALUATION OF ROTOR TEMPERATURE RISE:

Rotor temperature rise ={(

)*255 - 235 }-Cold air temperature (average)

…(4.12)

where,

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R t: Stabilized rotor resistance in Ω. R 20: Rotor resistance at 20 °C in Ω. d. SHORT CIRCUIT HEAT RUN TEST:

After mechanical heat run test, preparation for short circuit heat run test is made. The machine is run at rated speed and excited to 100%I n. The machine is kept under this state for temperature stabilization.

e. EVALUATION OF STATOR TEMPERATURE RISE:

Temperature rise = Stabilized temperature - cold air temperature (average). f. EVALUATION OF ROTOR TEMPERATURE RISE:

As excitation is available continuously, measurement of rotor voltage and current are done periodically. Evaluation of temperature rise is done in same way as above.

g. OPEN CIRCUIT HEAT RUN TEST:

After short circuit heat run test, preparation for open circuit heat run is made the machine is run at rated speed and excited to 110%En. Machine is kept under this state for temperature stabilization. These are the different type of tests and the most important type of test is the vacuum pressure impregnation process is studied below

4.2.11. VACUUM PRESSURE IMPREGNATION (VPI) BHEL INSULATION SYSTEM FOR TURBO GENERATORS:

BHEL had Bitumen insulation system for low & medium rating TGS and switched over to resin rich Thermo setting type as a step towards increasing reliability and upgrading technology. Micalastic system has been adopted for high rating machinery.

A. BITUMEN SYSTEM & LIFE EXTENSION:

1. The experience with Bitumen system has been generally satisfactory & practically negligible service failure has been reported on these sets.

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2. Mechanical damage most commonly associated with this system i.e., tape separation, due to thermal expansion of the winding during normal or abnormal temperature eyeing is not met any of sets. 3. Though outage due to insulation failures has been considerably low, yet these machinery would need to be attended to have life extension above their estimated life of 25 years. 4. Major inspection of the machine condition is by checking the healthiness of windings & life of bar insulation. 5. Rehabilitation, if needed, requires restoration of varnish, removal of bitumen & cleaning, tightening of fasteners/supports, modification of bus bars, use of new wedges & other winding components. 6. The replacements are required because of vibration / external damage et c.

B. VARIOUS INSULATION SYSTEMS & PRACTICES:

Large & medium range motors are provided with following insulation system. 1. Resin flux Insulation System: This system is used on earlier designs & where duplicate or spare motors to suit the customer requirements are required. In the coming years this system may become absolute. 2. Resin Rich micalastic Insulation System: The system provides use of Resin rich polyester backed epoxy mica folium on straight portion & resin rich polyester backed epoxy mica paper tape on overhang with a final layer of polyester shrink tape. The system is highly productive during coil manufacture and housing. 3. Resin poor Micalastic Insulation System: Resin poor micalastic system is adopted for large range Ac Induction and synchronous machines. These are designated to meet specific customer requirement hence for unique in nature to each other. The main insulation consists of resin poor epoxy mica paper tape all over the oil periphery with varying number of layers on straight and overhang portions.

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RESIN POOR

RESIN RICH

1. Epoxy resin content is about 8%.

1.

2. This method follows Thermo Setting Process. 3. There is a need for addition of resin from outside. 4. Time required for this cycle is less. 5. Repairing is very difficult. 6. Overall cost is less compared to resin rich.

Epoxy resin content is about 40%.

2. This method also follows Thermo Setting Process. 3. Further addition of resin is not required from outside. 4. it‘s a very long process and time consuming. 5. Repairing is easy. 6. Overall cost is more

Comparison between resin poor and resin rich

C. INTRODUCTION TO VPI PROCESS:

DR. MEYER brought the VPI system with t he collaboration of WESTING HOUSE in the year

1956. Vacuum Pressure Impregnation has been used for many years as a basic

process for thorough filling of all interstices in insulated components, especially high voltage stator coils and bars. 1. VPI is a process, which is a step above the conventional vacuum system. VPI includes pressure in addition to vacuum, thus assuring good penetration of the varnish in the coil. 2. The result is improved mechanical strength and electrical properties. With the improved penetration, a void free coil is achieved as well as giving greater mechanical strength. 3. With the superior varnish distribution, the temperature gradient is also reduced. 4. In order to minimize the overall cost of the machine & to reduce the time cycle of the insulation system vacuum pressure Impregnated System is used. 5. The stator coils are taped with porous resin poor mica tapes before inserting in the slots of cage stator, subsequently wounded stator is subjected to VPI process, in

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which first the stator is vacuum dried and then impregnated in resin bath under pressure of Nitrogen gas.

D. FEATURES AND BENEFITS:

a. State-of-the-art process for completely penetrating air pockets in winding insulation. b. Increases voltage breakdown level. (Even under water!) c. Proven submergence duty system d. Improved heat transfer- windings are cooler, efficiency is improved. e. Improves resistance to moisture and chemicals. f.

Increases mechanical resistance to winding surges.

E.PROCESS: GENERAL:

a. The jobs that are entering tank for Vacuum Pressurized Impregnation shall not have any oil based coatings. Any such, rust preventive/ corrosion preventive viz., red oxide etc., shall be eliminated into the tank. b. Resin in the storage tank shall be stored at 10 to 12 C and measured for its viscosity, viscosity rise. c. Proper functioning of the impregnation plant and curing oven are to be checked by production and cleared for taking up of job for impregnation

F. PREHEATING:

a) The job is to be loaded in the curing oven and heated. The temperature is to be monitored by the RTD elements placed on the job and the readings are logged by production. The time of entry into the oven, time of taking out and the temperature maintained are to be noted. b) Depending on convenience of production the jobs can be preheated in impregnation tank by placing them in t ubs. c) The impregnation tubs used for impregnation of jobs are to be heated in the impregnated tank itself, when the jobs are preheated in the cur ing oven.

G. IMPREGNATION:

a) Job insertion into preheated tub and insertion into tank by the time, the preheating of job is completed, it is to be planned in such a way that the heating of tub and tank

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heating matches with the job. This is applicable when the job is heated in the curing oven separately. The preheated job is to be transferred into the tub by crane handling the job safely and carefully without damage to the green hot insulation.

The warm tub with job is inserted into impregnation tank by sliding on railing, in case of horizontal tank. The thermometer elements are to be placed at different places on the job. The connection for inlet resin is to be made for collection of resin into tub. After ensuring all these lid of the impregnation tank is closed. In case of vertical tank the job along with tub is slinged and inserted carefully into impregnation tank without damage to insulation.

H. DRYING THE JOB IN VACUUM

The job is to be dried under vacuum. Drain out the condensed moisture/ water at the exhausts of vacuum pumps for efficient and fast vacuum creation. Also check for oil replacement at pumps in case of delay in achieving desired vacuum.

I. HEATING THE RESIN IN THE STORAGE TANK

The completion of operations of drying and the heating of the resin in the storage tank are to be synchronized. The heating of resin in the tank and pipeline is to be maintained as at preheating temperature.

J. ADMISSION OF RESIN INTO IMPREGNATION TANK

The resin is allowed into the impregnation tank tub if required from various storage tanks one after the other up to a level of 100mm above the job generally, after which the res in admission is stopped. After 10mins of resin settling the tank is to be pressurized by nitrogen. While admitting resin from storage tanks pressurize to minimum so that nitrogen will not affect resin to spill over in tank.

K.WITHDRAWAL OF RESIN FROM OMPREGNATION TANK TO STORAGE TANK

The resin that is pressurized as per pressure cycle by which the opening of relevant valves will allow the resin to come back to the storage tank. The job also shall be allowed for dripping of residue of resin for about 10min. After dripping, withdrawal of resin in various storage tanks is to be carried out.

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L. TAKING OUT THE TUB WITH JOB FROM IMPREGNATION TANK

The lid is then opened after taking precautions of wearing mask and gloves for the operating personnel as a protection from fumes. The job is withdrawn from impregnation tank by sliding on railing for horizontal and slinging on to crane for vertical impregnation tanks.

M.POST CURING:

The job is post heated. The time for raising from job temperature to this temperature as per relevant annexure. The time at which the heating is started, achieved and maintained is to be logged.

N.ELECTRICAL TESTING:

All jobs that are impregnated till above process are to be tested for electrical tests. After ensuring that all the temperature/vacuum conditions stipulated for drying, impregnation and curing operations have been properly followed, the job is to be released for this operat ion

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CHAPTER-5 RESULTS Test result of one of the 115 MW turbo alternator have been listed below, the client‘s name, date of testing, rating of the material, name of the test, graphical representation and other. The ratings of the said machine are given below: TURBO ALTERNATOR / SYNCHRONOUS GENERATOR

115 MW, 143.75 MVA, 11 KV, 7545 Amps, 0.80 pf, 3000 rpm, 2-pole EXCITER : 583 KW, 550V, 1060A, 750rpm KWU type.

The results of insulation resistance, high voltage test, measurement of ohmic resistance and LR values of resistance temperature detectors are analyzed. The IR values at 2500V with static megger at each phase are analyzed by keeping each phase active and remaining two are earthed. The average value of IR before HV test is 2230 Mega ohm whereas the average value of IR after HV test is 2073 Mega ohm with reference to fig 5.1(a) The Resistance value measured at one minute is 3070M ohm and the resistance value measured at 10 minutes is 148000M ohm are analyzed with reference to fig 5.1(a) and fig 5.1(b) The sudden short circuit test results are analyzed and as the time in seconds is increasing, the current in amperes is deteriorating with reference to fig 5.2(a) and fig 5.2(b) The open and sustained short circuit characteristics and losses during OCC and SCC are analyzed and as the field current increases, the efficiency is increasing and the losses in kilowatts are the losses are at 750.11KW with reference to fig 5.3(a) and fig 5.3(b)

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Fig no 5.1(a)

Fig no 5.1(b)

Insulation resistance measurement &

Measurement of Ohmic Resistance &

High voltage test

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Fig no 5.2(a)

Fig no 5.2(b)

Sudden short circuit test results

sudden short circuit test results

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Fig no 5.3(a)

Fig no 5.3(b)

Open circuit and sustained short circuit

Losses measured during OCC and SC

characteristics

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CONCLUSION 1. CONCLUSION

Hence testing has to be done in order to ensure that a machine has to be deliverable good and should not possess any losses while working and shall run efficiently. By performing different types of testing for the turbo generator in the project ensures that the turbo generator will work effectively and reliably. With efficient testing and ensuring less losses, the machine works effectively. The Vocational training at BHEL Hyderabad helped us in improving our practical knowledge and awareness regarding Turbo Generator to a large extent. Here we came to know about the technology and material used in manufacturing of turbo generators. Besides this, we also visualized the parts involved or equipments used in the power generation. Here we learnt about how the electrical equipments are being manufactured and how they tackle the various problems under different circumstances. At least we could say that the training at BHEL Hyderabad is great experience for us and it really helped us in making or developing our knowledge about turbo generator and other equipment used in power generation. Since more than 100 years turbo generators have been in use for steam turbine and gas turbine applications of any size. The technical evolution has not stopped, new market requirements and new material technologies ask for adaptations in design. The future market will be characterized by a revitalized need for very large turbo generators, both 2-pole and 4 pole. The future will also be characterized by an exciting competition between wellestablished conventional solutions and new ―high tech‖ solutions. In any case highly skilled engineers paired with the best available design tools will be required.

2. FUTURE SCOPE:

The technology research and investigations division of BHEL is currently investigating the technical and logistical merit of performing offline quadratic-rate partial discharge tests on the stator winding insulation of its hydro & turbo generators. A series of laboratory based insulation research studies on stator bars have been conducted to gain a better understanding of the various partial discharge phenomena involved.

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REFERENCES REFERENCES (Url’s) 1. http://www.eriks.co.uk/NewSearch.aspx?Query=testing+introduction 2. http://www.eriks.co.uk/Testing-Methods/704 3. http://www.tgtsllc.com/ 4. eceprojects.net/ece-project-reports/turbo-generator-testing-procedure-projectreport/444/ 5. http://www.labplan.ufsc.br/congressos/cigre06/DATA/A1_103.PDF , Tests on a 500 MVA hydrogen cooled turbo generator in single shaft technology 6. http://www.energy.siemens.com/hq/pool/hq/energy-topics/technical papers/Paper_PowerGenEurope_500Hz_Richter_EN.pdf 7. http://www.labplan.ufsc.br/congressos/PowerTech07/papers/664.pdf 8. http://www.energymanagertraining.com/Journal/09032007/InsulationResistanceandPo larizationIndexTestofGenerator&Motor.pdf

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Testing of Turbogenerator

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