Unit 2
Mechanical Science ; diesel engines
NUPT 113: diesel engines and the emergency diesel generator system
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Objectives: Basic components knowledge and power plant fundamentals: are industry required standards for BSC students in the NUPT program. The learning requirements of two directly related requirements have been combined in this course and are indicated below by acronyms. Desired outcome for Basic Component Knowledge (BCK): describe basic theory, construction, application, and operation of basic plant components. Desired outcome for power plant fundamentals (PPF): explain the principles and/or describe items as listed below. Enabling statements: * Describe and / or explain the theory, construction, principles, and application of diesel engines, including the following: * Accessories/support systems (BCK & PPF) * Failure mechanisms and symptoms (such as failure to start, failure to reach operating speed, failure to stop, rough idling) (BCK & PPF) * Main structural components (such as frame, block, pedestal, fuel distribution system) (BCK & PPF) * Main moving components (BCK & PPF) * Principles of operations (BCK & PPF) * Switch gear distribution, protective relays, engineered safeguard systems. separation / cable routing * Control circuits, relays, (time delay energize & de-energize to actuate), starting, protective, Indications, alarms, and monitored parameters. *Meters such as (such as voltage and current and how a change in meter indication could indicate circuit degradation of a change in process) EDG technical specifications NOTE: whenever there are contradictions between the lecture notes and the text book; go by the lecture notes for testing purposes and for understanding industry standards.
Diesel engines: this unit will cover information on diesel engines and will concentrate on the nuclear plant Safety related Emergency Diesel Generators (EDG). After the Fukushima nuclear accidents 1
the need to understand the importance of these systems cannot be overstated. It is important that NUPT students understand the objectives of this unit. General information and principles of diesel d iesel operation: A diesel engine (also known as a compression-ignition engine) is an internal combustion
engine that uses the heat of compression to initiate ignition. (to burn the fuel). fuel). The diesel engine was developed by Rudolf Diesel in 1893. The diesel engine has the highest thermal efficiency of any regular internal or external
combustion engine due to its very high compression ratio. Low-speed Diesel engines (as used in ships and other applications where overall engine weight is relatively unimportant) often have a thermal efficiency which exceeds 50 percent. Diesel engines are manufactured in two stroke and four stroke versions. They were originally used as
a more efficient replacement for stationary steam engines. Since the 1910s they have been used in submarines and ships, locomotives, emergency generators, large trucks, and power stations.
The compression ratio is a measure of how much the engine compresses the gasses in the engine's cylinder. In a gasoline engine the compression compression ratio (which controls t he compression temperature) temperature) is limited by the air-fuel mixture entering the cylinders. The lower ignition temperature temperature of gasoline will cause it to ignite (burn) at a compression ratio of less than 10:1. The average car has a 7:1 compression ratio. In a diesel engine, compression ratios ranging from 14:1 to as high as 24:1 are commonly used. The higher compression ratios are possible because only air is compressed, and then the fuel is injected. This is one of the factors that allow the diesel engine to be so efficient.
Unlike a gasoline engine, a diesel engine does not require an ignition system because in a diesel engine; the fuel is injected into the cylinder as the piston comes to the top of its compression stroke. When fuel is injected, it vaporizes and ignites due to the heat created by the compression of the air in the cylinder.
Working principles of the diesel engine: 4 stroke; (like those used at nuclear plants). plan ts).
Induction: Induction : air is drawn drawn into the cylinder.
Compress Compr ession: ion: the air is compressed to high temperatur temperature e and pressure. pressure.
P o w e r S t r o k e : around around cylind cylinder er Top Dead Dead Center Center (TDC) (TDC) (piston (piston is is at top of of travel travel in the the cylinder cylinder with with valves closed) fuel is injected, which self-ignites and burns, further increasing pressure and thereby forcing the piston down. Exhaust: the hot gas is released to the exhaust system.
On the next page is an illustration showing the process.
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Every nuclear power plant has at least 2 diesel generators that provide emergency electrical power in
the event that all offsite electrical power is lost. The diesel generators have turbochargers that supply combustion air. To assure that they start, emergency air storage tanks supply high pressure air to start the diesel. As with most rotating mechanical equipment, water cooling systems and oil lubrication systems are used. The diesel generators are typically tested 1-2 times per month when they are run for about 1-4 hours. Several times per year the diesels may be run for up to 24 hours to ensure that the equipment would function during a loss of offsite power.
Emergency Diesel generator components, accessories, and support systems: The EDG system consists of the following basic components, systems, criteria, technical
specifications and special requirements. Note: System designs vary from plant to plant. The information provided in this section is for general knowledge about diesel generators. Pedestal Frame Block and internal components Starting system Fuel distribution system Lube oil system Cooling water systems Turbo charger / air induction system. Generator. Seismic restraints and structure. Containing room / Building. Switch gear distribution, protective relays, engineered safeguard systems. Train separation / cable routing. routing. 3
Control circuits, relays, (time delay energize & de-energize to actuate), starting, protective, Indications and alarms. Monitored parameters. Types of meters used. Security Protection, (fire, flooding, etc.) Maintenance. Procedures. (technical specifications, and surveillances) Pedestal: is the base on which the diesel and generator frame is mounted. This in turn is seismically mounted to the EDG room floor which is also seismically constructed and qualified. Frame: supports the engine and auxiliary components of the EDG. Block and Internal components: below are drawings of the diesel engine showing component parts
Diesel block
Diesel internal components:
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Air systems: there are four air systems that support the EDGs; these are: Air start Engine intake Engine exhaust Crankcase ventilation Air start: each EDG has two independent air start systems, describe in the Final Safety Analysis Report (FSAR). Each system is capable of starting the EDG five times (three times automatically, and twice more manually) before outside power is required at the air compressors to recharge the group of air tanks. Alternately, each system can crank the diesel generator for sixty seconds without recharging. The drawing below is an example drawing of an air start system. If you have to, you can go back to the valve unit to determine which valves are used. The solenoid valve (the one with the imitation wire coming out of the little ball on the top) gave the industry starting problems because they were failing to open due to a factory defect. The NRC issued a part21 warning on them.
Air Comp
Air P
Air
EDG air starting motor
Air
Air Comp
Air P
EDG air starting motor
Air
Air
Starting solenoids
The air compressors are driven with an electric motor and receive power from the associated EDG switchgear. Each compressor is equipped with an air intake filter and intercooler. The intercooler of each compressor is equipped with an automatic blowdown device that operates during compressor operation and a manual drain to remove any accumulated water. The compressors start/stop functions are automatically controlled by a pressure switch monitoring the starting air receiver pressure. The compressors are non-safety related. Each compressor has a non-safety non-safet y related air cooled aftercooler to cool the compressed air. The compressors are 5
designated as primary and backup. The backup compressor will start on a falling pressure in the receivers if the primary compressor fails to start or is unable to maintain system pressure. Both compressors start upon an engine start to minimize the time required to restore receiver pressure. The air compressors are reciprocating type compressors. Each compressor is capable of recharging the associated receiver tanks within 30 minutes following five EDG start attempts. Fuel distribution system: the fuel oil system is also described in the FSAR. No. 2 diesel fuel oil is supplied to each EDG by two independent motor-driven fuel pumps, drawing from the dedicated day tank. Each day tank, in turn, is supplied by two independent electric motor driven fuel transfer pumps from its corresponding storage tank. Each storage tank contains about 30,000 gallons of fuel oil, and each day tank contains over 500 gallons. The storage tanks are sufficient for three and a half days of continuous, rated output of both EDGs. The drawing below shows a simplified example of the diesel engine fuel system.
Fuel storage tank
Fuel day tank
EDG A
Cross tie
Fuel storage tank
Fuel Fuel da tank tank
EDG B
Fuel pump Level switch Valve Check valve
EDG lube oil system: each EDG has its own independent lubricating oil system, which is also
described in the FSAR. the lubricating oil system is pumped by both the engine driven pumps and by AC motor driven pumps. Cooling of the oil is discussed under cooling water later in the unit. Filters and strainers are also used to ensure oil quality. Instrumentation is included to monitor engine oil pressure; low lube oil pressure tends to indicate a leak, which would result in a trip of the diesel. Lube oil test programs can indicate the overall health of the diesel.
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Below is an example of a lube oil system drawing.
Diesel lube oil system
Cooling water system: The diesel generator cooling water system (DGCWS) provides the necessary cooling
for the engine and turbocharger for the EDGs. The DGCWS system dissipates heat from the crankcase, cylinder heads, turbochargers, governor oil, generator bearings, combustion charge air and lubricating oil resulting in reduced engine operating temperatures. Preheating bypass circuits are provided to establish quick starting times and short load acceptance times to minimize cold start wear. The system is composed of coolant preheater, preheating circulation pump, temperature regulating valves, cooling water pumps, heat exchangers, and an expansion tank. Each EDG has a separate, independent DGCWS, as shown in the Figure below.
TS
Water expansion
TI
tank
Heat exch xchan er
TI
Lube oil cooler
EDG Heater
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Turbo charger air induction system:
The emergency diesel engine combustion air intake and exhaust system supplies combustion air of reliable quality to the diesel engines, and exhausts the products of combustion from the diesel engines to the atmosphere.
The system design meets appropriate seismic design classification requirements and the components are designed, fabricated, erected, and tested to acceptable quality standards. The essential portions of the system are housed in or on a seismic Category I structure that is capable of protecting the system from extreme natural phenomena and external missiles. Each diesel engine has an independent combustion air intake and exhaust system. The consequences of a single active failure in an engine combustion air intake or exhaust system will not lead to the loss of function of more than one diesel generator. Below is a simplified drawing and pictures of the air induction system and exhaust for the EDG.
Diesel Exhaust Air intake
On the next page is a composite drawing of all the diesel support systems.
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A crankcase ventilation system is a way for gases to escape in a controlled manner
from the crankcase of an internal combustion engine PVC valves are used on your engine. car’s car’s engine. The protection system will be discussed in later sections
The generator will be discussed in the next section
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Generator section:
Each generator is a direct-shaft driven, air-cooled self-ventilated machine. The generator enclosure is open drip-proof type that facilitates free movement of ventilation air. The generator component design is in compliance with the NEMA MG-1(industry standards) requirements. Each generator produces its rated power at 6900 V, 60 Hz. Each generator continuous rating is based on supplying the electrical safety related and some non-safety related ac loads. The generators can also provide power for additional investment protection ac loads. The plant operator would normally provide power to these loads by de-energizing one of those system components that are redundantly supplied by both the diesel generators. The diesel generator design is compatible with the step loading requirements identified in the FSAR. The generator exciter and voltage regulator systems are capable of providing full voltage control during operating conditions including postulated fault conditions. Each generator has a set of potential and current transformers for protective relaying and metering purposes. The following generator protection functions are provided via relays that are mounted on the local generator control panel: Differential Overcurrent Reverse power Under-frequency Under/over voltage Loss of excitation Ground fault Negative sequence synchronization check Voltage balance
Seismic design and enclosure building: o
Definitions and Qualifications:
Seismic category I: all structures, systems, and components that are designed and built to withstand the maximum potential earthquake stresses for the particular region where a nuclear plant is sited. Seismic Category-2: shall include all structures, systems, and components which are required to:(i) prevent the escape of radioactivity beyond the limits prescribed for normal operation and not covered in category-1; or (ii) mitigate those accident conditions which last for such long periods that there is a reasonable likelihood of an earthquake of the defined severity occurring during this period and not covered in category-1. All seismic category-2 structures, systems, and components shall have demonstrated the capability to withstand the effects of the level ground motion corresponding to the Operating Basis Earthquake. Seismic Category-3: includes structures, systems, and components which are not important to safety and those not covered in category-1 or 2. Items under this category may follow 10
national practice; for example, the civil structures under this category can be designed and built as per IS-1893[5].
General Design Criterion 17, “Electric Power Systems,” of Appendix A, “General “General Design Criteria for Nuclear Power Plants,” to Title 10, Part 50, of the Code of Federal Regulations (10 CFR Part 50),“Domestic Licensing of Production and Utilization Facilities” (Ref. 1), requires that onsite electric power systems have sufficient independence, capacity, capability, redundancy, and testability to ensure that (1) specified acceptable nuclear fuel design limits and design conditions of the reactor coolant pressure boundary are not exceeded as a result of anticipated operational occurrences, and (2) the core is cooled and containment integrity and other vital functions are maintained in the event of postulated accidents, assuming a single failure. The Emergency Power Generating Buildings (EPGB) are safety-related, Seismic Category I reinforced concrete structures supported by a reinforced concrete basemat. There are two essentially identical EPGBs. Each structure houses a diesel generator, fuel oil tanks, control boards, heating ventilation and air conditioning (HVAC) equipment, electrical equipment, and miscellaneous equipment associated with the operation of each generator. The two diesel generators are separated by a reinforced concrete wall to protect against internal hazards. The two fuel oil tanks are separated from the diesel generators by a reinforced concrete wall to protect against internal hazards. The EPGBs are Seismic Category I structures, which are capable of performing their safety-related function during and following a safe shutdown earthquake (SSE). These structures are designed for external hazards including rain and snow loads, flooding, and wind loads, tornado loads, missile impact loads, SSE loads, and site-proximity hazards. The buildings are also designed for structure and component dead loads, live loads, pipe reactions and thermal effects. There are no internally generated missile impact loads applicable to the design of these buildings. These building: • Supports the emergency diesel generators and associated mech anical, anical, electrical, and instrumentation and control equipment required to function during and after a design basis event. • Provides protection for safety-related safety-related equipment against external hazards. • Provides separation between the main diesel generators and and fuel oil tanks. Seismic forces are exerted ex erted on a building and its contents during du ring an earthquake. These forces act horizontally upon the structure itself, as well as the piping, cable trays, ductwork, and other building systems within. Typical supports for piping, trays and other equipment are designed for gravity, or vertical, loads but do not take into account the horizontal loading caused by earthquakes. Seismic restraints (i.e. braces) resist the horizontal forces and keep the systems in place and secure. The main purpose of seismic bracing is to prevent damage to equipment important to safety due to an earthquake
Electrical separation of safety related electrical equipment and instrumentation; diesel: In IEEE-279, Paragraph 4.6, and regulatory Guide 1.75; which endorses IEEE-384. Class 1E
circuits and Class 1E-associated circuits are identified and separated from redundant and non-Class 1E circuits. Isolation devices are provided in the design where an interface exists between redundant Class 1E divisions and between non-Class 1E and Class 1E or Class 1E-associated circuits. 11
Independence and separation of safety-related systems is discussed in Sub-sections 8.3.1.3 and 8.3.1.4. Compliance to fire codes for class 1E: (10CFR 50) various sections and appendices) The function of electrical cable is to provide a medium for transmitting electrical energy (power
control or signals) between two points in a common electrical circuit, while simultaneously maintaining the electrical isolation of the transmission path from other elements of the same circuit and from other co-located circuits. Cable failure, therefore, implies loss of continuity in the energy transmission path or diversion of a sufficient fraction of the available electrical energy to an unintended circuit destination such that proper function of the circuit is no longer assured. A typical boiling-water reactor (BWR) requires approximately 97 km (60 miles) of power cable, 80.5 km (50 miles) of control cable and 402 km (250 miles) of instrument cable. A pressurizedwater reactor (PWR) may require far more, as illustrated by the containment building of Waterford Steam Electric Generating Station, Unit 3 which required nearly 1,609 km (1,000 miles) of cable (NUREG/CR-6384). The majority of fire dynamics, fire risk evaluations will focus on electrical cables because of their thermal fragility. It is therefore necessary to have a fundamental understanding of electrical cables. Nuclear power plants: use three functional types of cables for power, control, and instrumentation.
Virtually every system in depends on the continued operation of one or more electrical cables. Power cables may be single-conductor, multi-conductor, or triplex. Control and instrumentation cables cables are are generally of a multi -conductor design.
They can run through fire walls in cable trays where special seals are required for fire barrier protection. Cables are generally routed through the plant in horizontally raceways (generally trays or conduits) with vertical runs as required between different elevations in the plant. The cables are generally segregated by type (power, control, and instrumentation) but cables of various voltages and functions can be found together in some plants (generally older plants). Highvoltage power cables are typically routed by themselves and may use maintained spacing to address ampacity concerns. Under maintained spacing, cables are not stacked and each cable is individually strapped to the electrical raceway. Gaps between cables ensure that they do not come into physical contact with each other. For most cables, random placement within the tray is common (that is, the cables are simply laid into the tray in a more or less random manner). Fire exposure of an electrical cable can cause a loss of insulation resistance, loss of insulation physical integrity (i.e., melting of the insulation), and electrical breakdown or short-circuiting. Fire-induced damage to a cable can result in one of the following electrical conductor failure modes:
An open circuit results in a loss of electrical continuity of an individual conductor (i.e., the conductor is broken and the signal or power does not reach its destination).
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A short to ground is experienced when an individual conductor comes into elect electric rical al con contac tactt with a grounded conducti ng medi um (s uch as a cab le tray, conduit, or a grounded conductor) resulting in a low-resistance path that diverts current from a circuit. The fault may be accompanied by a surge of excess current to ground (particularly in higher voltage circuits) that is often damaging to the conductor. A hot short is characterized by electrical faults that involve an energized conductor contacting another conductor of either the same cable (a conductor-to-conductor hot short) or an adjacent cable (a cableto-cable hot short). A hot short has the potential to energize the affected conductor or to complete an undesirable circuit path.
Fire detection and suppression: Because of the expanse of a nuclear power plant and the normally limited number of resident operating person per sonnel nel,, remot rem otee automa aut omati ticc fire fir e detection detection repres represents ents the the only viable viable method method of provid providing ing early early fire warning warning for for most plant areas. By installing throughout a power plant devices which are sensitive to fire and by electrically connecting these these devic devices es to centra centralize lized d alarm alarm panels, panels, opera operators tors can can receiv receivee fire warn warnings ings from anyw an ywhe here re in the th e plan pl ant. t. The The reliability of such a system, however, depends upon correct design and maintenance. For any automatic fire extinguishing system, automatic detection of fire conditions is required.
Detection: types of detectors: Area heat detectors Continuous line heat detectors Ionization type products of combustion detectors Photoelectric smoke detectors Ultraviolet/infrared flame detectors
Establishing Area Detection Requirements:
When automatic detection is required for a particular power plant area; from a plant safety viewpoint; a careful assessment of automatic detection detection must be made on the basis of many factors, including: Importance of the area to overall plant safety; for EDGs this is vital. Susceptibility of the area to surrounding fire hazards Degree of fire hazard within the area (No. 2 fuel oil and electrical fires) Potential of fire spreading to other areas (Fire barriers, walls, doors, etc.) Type of available fire suppression (e.g., manual or automatic; inert gas, powders, or water) Normal occupancy of the area Nureg/cr-0488 sand78-1373 lists the following for EDG rooms.
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Anticipate d Fire Developme nt
Predominant Combustibles
Lube Oil
Room Room Congestion Ceiling for Detection Height
Fast or Slow Low
Diesel Fuel Oil
High
Other Factors
Diesel Combustion
Heat - Rate of Rise or
Products
Ultraviolet or Infrared
combustion products
Cable Insulation
Suitable Detector Choice
Ultraviolet or infrared
Suppression: After detection; relays are activated which energize annunciators in the control room
that a fire has been detected.
Seismic restraints and hangers: All seismic snubbers / restraint assemblies shall meet the following minimum requirements:
Meet Ashrae SPC-171. qualification testing. The snubber/restrained isolator for isolated equipment shall include a resilient element that will ensure that no un-cushioned shock can occur (this does not include cable restraints). It shall be possible to visually inspect the resilient material for damage and allow for replacement, if necessary. All snubbers are to include a maximum air gap of 0.25 in (6 mm). Seismic restraint systems shall be designed to offer seismic restraint in all directions, unless otherwise noted. Skid units must be verified and qualified by the manufacturers through actual testing that they meet the required standards.
Seismic restraint capacities are to be verified by an independent test laboratory or certified by a registered Professional Engineer to ensure that the design intent of this specification is realized. Verification shall be by one of the following methods:
An NRTL (National Recognized Testing Laboratory), or laboratory recommended by
Vibration Isolation and Seismic Control Manufacturers Association (VISCMA).
Certified by a registered Professional Engineer with at least 5 years’ experience, using industry standard methods of analysis, which employ common engineering practices. 14
Adherence to the ratings standard within ASHRAE SPC171 and VISCMA 102-2007 is required. By a nationally recognized agency, such as VISCMA, that has reviewed and approved the restraint. All safety related components, cables, cable trays, piping; HVAC, skids etc. must be seismically qualified to its seismic category.
NRC Regulatory Guide 1.29, Revision 3, Seismic Design, regulatory guide 1.6 dampening values for seismic design of nuclear power plants provides guidance for seismic mount of systems and components.
Containing room / building:
Air temperature with in the diesel room must be maintained at less than 100 degrees F by the HVAC system. This prevents damage to the electronic controls and instruments in the room. The building shall have fire walls, dampers and fire doors as required to meet codes. The building shall meet the sites seismic requirements The building will be constructed such that missiles or wind from tornadoes or other events will not cause damage to any of the contained safety related systems. The building will provide appropriate separation of class 1E electrical cables, trays, and systems. (train separation)
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Switch gear distribution, protective relays, engineered safeguard systems. Train separation / cable routing:
B train emergency diesel generator On a loss of off-site power the breaker from the non-vital bus to the 4160 vital bus opens. The diesel starts and load shedding, load sequencing starts for vital loads; the EDG breaker is closed to the vital bus. The same thing is happening on the A train side.
Note: the symbology for electrical distribution may be found in the In the DOE hand book Vol 2 in docsharing
The following definitions will help the student understand the electrical distribution system:
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Load sheading: the process of disconnecting all non-vital connections and deliberately removing most (preselected) large loads from a vital bus in response to an under-voltage condition on the vital bus.
Load sequencing: the process of starting large emergency motor loads in a preselected order (rather than all at once) on a vital bus after load shedding has occurred on that bus. Load sequencing sequenc ing assure a ssuress that t hat essentia es sentiall equipment equipment is energized energized as quickly quickly as possibl possible e while overcurrent overcurre nt conditions conditions (i.e., overloading the emergency emergency diesel generator) due to starting a large number of motors at the same time are avoided.
Selective tripping: the process of electrical fault isolation using protective devices in series with one another such that the protective device nearest to the fault operates first to protect the integrity of the overall system. A properly designed protective scheme will isolate a fault by removing the least number of components from service.
Train separation: the physical and electrical separation of redundant electrical trains, structures, and components components to prevent a fault in one electrical train from affecting the operation of the redundant train.
Diesel starting sequence:
If both sources of normal auxiliary power are lost (unit auxiliary transformer and system auxiliary transforme transformer), r), the equipmen equipmentt essential essential to safe shutdown will be supplied by the t he Seismic Clas C lasss I diesel die sel gener gen erato ators rs.. These The se diesel die selss will wil l automati automatically cally start and tie tie onto the 4160 4160 volt volt AC vital vital buses buses on a loss loss of voltag voltage e on the the bus. bus. E Each ach diesel diesel is designed to reach rated speed and be ready read y to accept load within ten seconds, and accept full load within thirty to sixty (depending on type) seconds after receiving a start signal. The diesel generators will start on an engineered safety features actuation signal or a loss of power on its associated vital bus. The generato generatorr tie breake breakerr to the bus bus will not not close unless the bus is de-energized.
To prevent overloading and tripping the diesel, the major loads are automatically stripped off the bus before the diesel generator output breaker closes. The loads will then be sequenced back onto the bus one at a time. The entire sequence takes about 60 seconds and is accomplished by an automatic sequencer. The sequencer also prevents overloading by loading only the equipment considered vital to a safe shutdown.
There are two sequencers that are used. The first is the blackout sequencer. It will sequence on loads that are needed after a loss of offsite power to maintain the reactor in a shutdown condition. This includes the auxiliary cooling water systems, the auxiliary feedwater systems, and the charging pumps. If an accident has ha s occur occ urre red d and an d the engineere engineered d safety safety feature featuress has been been actuate actuated, d, the loss-of-cool loss-of-coolant ant accident sequencer will sequence on the equipment needed to combat an accident. This will include the loads sequenced on for the loss of offsite power and the other emergency core cooling systems and containment spray if needed.
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The electrical power system is perhaps the most important engineered safety feature installed in the nuclear power unit. A major purpose of the system is to supply a reliable source of power, under all conditions, to systems that are required for plant safety. Examples of safety systems and components are: (1) engineered safety features valves and pump motors, (2) control power for engineered safety features equipment, and (3) instrumentation required to monitor plant status during normal and abnormal events.
Diesel protective relays: Prior to listing the various trip relays the student needs to understand the different types of relays and how they work. The term Relay generally refers to a device that provides an electrical connection between two or more points in response to the application of a control signal. The most common and widely used type of electrical relay is the electromechanical relay or EMR.
The most fundamental control of any equipment is the ability to turn it "ON" and "OFF". The easiest way to do this is using switches to interrupt the electrical supply or in our case the fuel supply and output breakers. Although switches can be used to control something, they have their disadvantages. The biggest one is that they have to be manually (physically) turned "ON" or "OFF". Also, they are relatively large, slow and only switch small electrical currents.
Electrical Relays however, are basically electrically operated switches that come in many shapes, sizes and power ratings suitable for all types of applications with the larger power relays being called "contactors". In this section about electrical relays we are just concerned with the fundamental operating principles of "light duty" electromechanical relays. Such relays are used in general electrical and electronic control or switching circuits either mounted directly onto PCB boards or connected free standing and in which the load currents are normally fractions of an ampere up to 20+ amperes.
As their name implies, electromechanical relays are electro-magnetic devices that convert a magnetic flux generated by the application of a low voltage electrical control signal either AC or DC across the relay terminals, into a pulling mechanical force which operates the electrical contacts within the relay. The most common form of electromechanical relay consists of an energizing coil called the "primary circuit" wound around a permeable iron core. It has both a fixed portion called the yoke, and a moveable spring loaded part called the armature, that completes the magnetic field circuit by closing the air gap between the fixed electrical coil and the moveable armature. This armature is hinged or pivoted and is free to move within the generated magnetic field closing the electrical contacts that are attached to it. Connected between the yoke and armature is normally a spring (or springs) for the return stroke to "reset" the contacts back to their initial rest position when the relay coil is in the "de-energized" condition, i.e. turned "OFF". Relay and time delays come in the following configurations Energize to actuate De-energize to actuate Time delay energize
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Time delay de-energized
In our simple relay above, we have two sets of electrically conductive contacts. Relays may be "Normally Open", or "Normally Closed". One pair of contacts is classed as Normally Open, (NO) or make contacts and another set which are classed as normally closed, (NC) or break contacts. In the normally open position, the contacts are closed only when the field current is "ON" and the switch contacts are pulled towards the inductive coil. In the normally closed position, the contacts are permanently closed when the field current is "OFF" as the switch contacts return to their normal position. These terms Normally Open, Normally Closed or Make and Break Contacts refer to the state of the electrical contacts when the relay coil is "de-energized", i.e., no supply voltage connected to the inductive coil. An example of this arrangement is given below.
The relays contacts are electrically conductive pieces of metal which touch together completing a circuit and allow the circuit current to flow, just like a switch. When the contacts are open the resistance between the contacts is very high in the Mega-Ohms, producing an open circuit condition and no circuit current flows. When the contacts are closed the contact resistance should be zero or very low.
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Relays can be either energized to actuate or de-energized to actuate depending on the systems function. For exam ple let’s look at the station blackout diesel start below which gives an example of de-energized to actuate signal. This is not an actual starting circuit but gives the principle.
De-e De-ene nerr ize ize to to actu actuat atee rel rela a exam exam le
Normally closed contact opens when diesel comes up to rated speed. This cuts off the air to the diesel start motors
Normally closed contact
Star Starti tin n air air
Air to EDG starting motors
From uninterruptable power supply From Vital AC bus When power to the vital bus is lost, the coil is de-energized causing the coil to return to its normally closed position. this powers up the solenoid from the battery bus allowing starting air to the diesel. When power is returned to the bus or the diesel speed comes up to rated speed. Air is cut off to the starting motor by de -energizing the solenoid valve. (See unit on Valve actuators)
Note to students: As an astute student, that I know you are, you may be asking yourself what all this electrical stuff is doing in this mechanical science course. Let me just say that sometimes; life is not fair. Each BSC student must be able to meet industry standards, and sometimes we have to add these standards where we can. This is the case here. If you are Having problems with the electrical aspects of this and other units in NUPT 113, contact your instructor for places to get additional information about electrical theory and components.
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Below is an actual starting circuit for a motor showing relay interaction:
In this system, resistors limit the motor’s line current during the initial start -up period, and then are bypassed after the time delay relay times out. When the start button is pressed power goes through Coil CR1 which closes contact CR1and timer TD1 starts to time out. The m1 coil contacts are now closed and the motor starts with the resisters in the line. when Time delay TD1 has timed out It cl oses contact TD1 and energizes M2 . This causes contact M2 to open and the M1 contacts drop out at the same time Contacts M2 close and the resisters are bypassed.
TD1 is a time delay to energize/actuate CR1 is a energize to actuate M1 is a energize to actuate M2 is a energize to actuate EDG protective relays: Generator differential: Differential protection is a very reliable method
of protecting generators, transformers, buses, and transmission lines from the effects of internal faults. The principle of the differential d ifferential protection of a station bus is the same as for generators. The sum of all currents entering and leaving the bus must be equal to zero under 21
normal conditions or if the fault is outside of the protected zone. If there is a fault on the bus, there will be a net flow of current to the bus and the differential d ifferential relay will operate.
Lube oil pressure low: protects moving components from excessive friction because of low lube oil flow. Engine overspeed: Prevents damage to moving parts due to high centripetal forces. Jacket coolant pressure low: protects the diesel from overheating. Jacket coolant temperature high: protects t he diesel from overheating. Lube oil temperature high: prevents damage to moving parts due to insufficient lubrication. Ground overcurrent: prevents over current condition of phase to ground Loss of field: detectors simply look at a minimum level of excitation voltage or current and if the excitation drops below that level (sometimes there's a time delay; sometimes not) then the "Loss of Field" indication and trip is initiated.
Reverse power: is when the generator starts to act as a motor when connected to a grid or bus and is tripped. Voltage restrained time over current: this relay adjusts the pickup setting and characteristic curve of the overcurrent function based on the system voltage and time at current level. Sensitivity increases as the voltage drops. Bus differential: Relay trips by the difference between the current (measured by CTs) going into and out of the zone of protection. (the generator) Load center phase overcurrent: directional overcurrent relaying refers to relaying that can use the phase relationship of voltage and current to determine direction to a fault. Generator under frequency: if the diesel drops below it required output frequency the relay will trip the unit. Generator overvoltage: This relay should have a time-delay unit with pickup at about 110% of rated voltage, and an instantaneous unit with pickup at about130% to 150% of rated voltage. Parameters monitored during operation:
MONITORING DEVICE
LIMIT
22
0
water jacket temperature
200
Oil pressure at 1800 rpm
50 PSI minimum
Oil temperature temperature
230
Oil filter differential pressure
15 PSI maximum
0
F maximum
F maximum
The following protective devices will cause a fault shutdown and a
corresponding alarm:
Fuel filter differential pressure
5 PSI maximum
Air filter differential pressure
30 In. In. maximum maxi mum
Engine Engi ne speed spee d
1850 rpm maximum maximum
The following warning devices alarm to warn operator of an impending serious condition:
Warning device
Pre oil pressure
disengages at 600 550 to 650 r m 30 psi
Pre water temperature
200
Low fuel level
1/4 tank tan k
Over current
@2000
Engine crank
set point
0
F
amps
The following protective devices will cause a fault shutdown and a corresponding alarm: PROTECTIVE PROTECTIVE DEVICE
Start failure
SET POINT POIN T
maximum of 4 automatic start attempts of 15 seconds each
Low oil pressure
20 psi
High water temperature
210
Engine overspeed
1980 198 0 rpm rp m
0
F
Emergency Stop:
Diesel Generators 1 and 2 local/remote emergency stop pushbuttons shut off fuel to the engine, and trip output circuit breakers
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Meters and functions used for operating parameters of the EDG: Meters and devices that may be found on the EDG control board: (depending on type and
plant)
Engine lubricating oil temperature gauge Jacket water temperature gauge Charge air temperature gauge Fuel day tank level gauge Diesel generator stop/start control switch Governor control switch Raise/Lower Voltage Adjuster (excitation control) Voltmeter (with Voltmeter selector switch) Ammeters (1 in each phase) Wattmeter Syncro scope meter with indicator light Power factor meter Frequency meter kWh meter Control panel supply voltmeter and ammeter Engine running time meter Engine RPM indicator Starting air pressure Engine lubricating oil pressure gauge
EDG control panel
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General Theory, construction, and application of meters: Volt meters: are used all over a nuclear power plant and the EDG control panel is no exception.
A voltmeter is an instrument used for measuring electrical potential difference between two points in an electric circuit. Analog voltmeters move a pointer across a scale in proportion to the voltage of the circuit; digital voltmeters give a numerical display of voltage by use of an analog to digital converter.
Voltmeters are made in a wide range of styles. Instruments permanently mounted in a panel are used to monitor generators or other fixed apparatus.
A moving coil galvanometer can be used as a voltmeter by inserting a resistor in series with the instrument. It employs a small coil of fine wire suspended in a strong magnetic field. When an electric current is applied, the galvanometer's indicator rotates and compresses a small spring. The angular rotation is proportional to the current through the coil. For use as a voltmeter, a series resistance is added so that the angular rotation becomes proportional to the applied voltage.
A moving coil galvanometer of the d'Arsonval type The red wire carries the current to be measured . The restoring spring is shown in green. N and S are the north and south poles of the magnet.
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Ammeter: is a measuring instrument used to measure the electric current in a circuit. Electric
currents are measured in amperes (A), hence the name.
Types of ammeters, construction, theory, and application:
The D'Arsonval galvanometer is a moving coil ammeter. It uses magnetic deflection, where current passing through a coil causes the coil to move in a magnetic field. The modern form of this instrument was developed by Edward, and uses two spiral springs to provide the restoring force. By maintaining a uniform air gap between the iron core of the instrument and the poles of its permanent magnet, the instrument has good linearity and accuracy. Basic meter movements can have full-scale deflection for currents from about 25 microamperes to [2]
10 milliampere and have linear scales .
of iron which moves when acted upon by the Moving iron ammeters use a piece of iron electromagnetic force of a fixed coil of wire. This type of meter responds to both direct and alternating currents (as opposed to the moving coil ammeter, which works on direct current only). The iron element consists of a moving vane attached to a pointer, and a fixed vane, surrounded by a coil. As alternating or direct current flows through the coil and induces a magnetic field in both vanes, the vanes repel each other and the moving vane [2]
deflects against the restoring force provided by fine helical springs .
The non-linear scale of
these meters makes them unpopular.
An electrodynamic movement uses an electromagnet instead of the permanent magnet of the d'Arsonval movement. This instrument can respond to both alternating and direct current.
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In a hot-wire ammeter, a current passes through a wire which expands as it heats. Although these instruments have slow response time and low accuracy, they were sometimes used in measuring radio-frequency current.
Digital ammeter designs use an analog to digital converter (ADC) to measure the voltage
across the shunt resistor; the digital display is calibrated to read the current through the shunt.
There is also a whole range of devices referred to as integrating ammeters. In these ammeters, the amount of current is summed over time giving as a result the product of current and time, which is proportional to the energy transferred with that current. These can be used for energy meters (watt-hour meters) or for estimating the charge of battery or capacitor.
Synchroscope meter:
For two electrical systems to be considered synchronized, both systems must be operating at the same frequency, and the phase angle between the systems must be zero. Synchroscopes measure and display the frequency difference and phase angle between two power systems. Only when these two quantities are zero is it safe to connect the two systems together. Connecting two unsynchronized AC power systems together is likely to severely damage any equipment not protected by fuses or circuit breakers.
For proper operation of these synchroscope, the difference between the frequency of the "incoming" machine and the frequency of the "running" machine, or bus, should be within approximately 3 cycles per second. Within this range of frequency difference, the instrument will operate smoothly.
The words FAST and SLOW on the scale indicate that the frequency of the incoming machine is respectively higher or lower than that of the bus or running machine. Clockwise rotation of the pointer signifies that the incoming machine is operating faster than the running machine.
Synchronism is indicated when the instrument pointer is over the index at the top of the scale. If the frequency of the incoming machine differs by more than 3 cycles per second from the frequency of the running machine, or bus, the synchroscope pointer may oscillate at a fixed position on the scale; or the pointer may oscillate and simultaneously rotate at an irregular velocity. This condition should not be confused with the smooth movement of the pointer which occurs when the frequency difference is within 3 cycles per second.
If you are tying the diesel to a live bus you want to have the synchroscope needle turning slow in the fast direction. When the needle hits the 12 o’clock position; close the output breaker and then increase the speed of the diesel to increase the load on the EDG.
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There are two basic types of synchroscope. The first is the electro-mechanical type which indicates phase and frequency relationships with a pointer and dial indicator. This instrument is similar in construction to an electric motor. It has a set of static stator windings opposed at 90 degrees, a polarizing coil, and a rotor element which drives the indicator needle. One supply is connected to the stator windings and the other to the polarizing coil. If the phase and frequency of the two supplies vary, the rotor will turn and give an indication of the difference.
The second type of instrument is the electronic synchroscope. This type utilizes a microprocessor designed to sample and compare two AC power supplies for phase and frequency characteristics. Any differences between the two are then calculated and indicated on either a digital liquid crystal display (LCD) or by means of LEDs. The latter type synchroscope typically has a circle of LEDs on its front panel which illuminate in a set pattern to indicate the presence and magnitude of phase and frequency differences.
Watt meter:
The wattmeter is an instrument for measuring the electric power (or the supply rate of electrical energy) in watts of any given circuit. The device consists of a pair of fixed coils, known as current coils, and a movable coil known as the potential coil.
An instrument which measures electrical energy in watt hours (electricity meter or energy analyzer) is essentially a wattmeter which accumulates or averages readings; many such instruments measure and can display many parameters and can be used where a wattmeter is needed.
The current coils connected in series with the circuit, while the potential coil is connected in parallel. Also, on analog watt meters, the potential coil carries a needle that moves over a scale to indicate the measurement. A current flowing through the current coil generates an electromagnetic field around the coil. The strength of this field is proportional to the line current and in phase with it. The potential coil has, as a general rule, a highvalue resistor connected in series with it to reduce the current that flows through it.
The result of this arrangement is that on a dc circuit, the deflection of the needle is proportional to both the current and the voltage, thus conforming to the equation W=VA or P=VI.
For AC power, current and voltage may not be in step, owing to the delaying effects of circuit inductance or capacitance. On an ac circuit the deflection is proportional to the average instantaneous product of voltage and current, thus measuring true power, P=VI cos φ. Here, cos φ represents the power factor which shows that the power transmitted may be less than the 28
apparent power obtained by multiplying the readings of a voltmeter and ammeter in the same circuit.
Below is an example of a kilowatt meter you might see on the EDG control panel.
Power factor meter:
The power factor of an AC electric power system is defined as the ratio of the real power flowing to the load to the apparent power in the circuit and is a dimensionless number between 0 and 1 (frequently expressed as a percentage, e.g. 0.5 pf = 50% pf). Real power is the capacity of the circuit for performing work in a particular time. Apparent power is the product of the current and voltage of the circuit. Due to energy stored in the load and returned to the source, or due to a non-linear load that distorts the wave shape of the current drawn from the source, the apparent power will be greater than the real power.
In an electric power system, a load with a low power factor draws more current than a load with a high power factor for the same amount of useful power transferred. The higher currents increase the energy lost in the distribution system, and require larger wires and other equipment. Because of the costs of larger equipment and wasted energy, electrical utilities will usually charge a higher cost to industrial or commercial customers where there is a low power factor.
For EDGs the power factor should be about .8; this because of the reactance load that is normally encountered for bus loads.
Linear loads with low power factor (such as induction motors) can be corrected with a passive network of capacitors of capacitors or inductors. Non-linear loads, such as rectifiers, distort the current drawn from the system. In such cases, active or passive power factor correction may be used to counteract the distortion and raise the power factor.
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Power factor meter construction and operation
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Frequency meters:
Moving-disk frequency meters are most commonly out-of-circuit meters. They can be used to spot check the frequency of power sources or equipment signals. A moving-disk frequency meter is shown in the figure below One coil tends to turn the disk clockwise, and the other, counterclockwise. Magnetizing coil A is connected in series with a large value of resistance. Coil B is connected in series with a large inductance and the two circuits are supplied in parallel by the source.
For a given voltage, the current through coil A is practically constant. However, the current through coil B varies with the frequency. At a higher frequency the inductive reactance is greater and the current through coil B is less; the reverse is true at a lower frequency. The disk turns in the direction determined by the stronger coil
The analog meter show below, and describe above, is used in older systems and most power plants today.
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Digital frequency meters:
These meters use IC IC chips to do the work. An oscillator that is calibrated and a comparison circuit actually count pulses and supply an output for display. Power plants are beginning to switch over to this newer technology but you will see the analogs for years to come; unless you get lucky and go to work at one of the new plants.
Hour meters:
Hour meters are of prime importance on EDGs. This is because of required maintenance at specific run times and tracking the overall degradation of the system over time. Hour meters are installed on almost all generators over 10,000 Kw.
There are many types of hour meters but essentially it is a device that counts the hours of operation. They may be of a vibration activated type or counted pulse from electrical output; such as tachometer driven or even a clock type which is powered up when the diesel is running. most EDG hour run time are driven by the tachometer device.
Studies are under way to reduce the need of testing the diesels as degradation occurs every time it is started. The new concept is reliability and not testability. The results of this study may reduce surveillance requirements in the future. Industry failure analysis is also being considered in the mix as the impact of the Fukushima accident is being analyzed.
Below is an example of a control panel mounted hour meter.
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Temperature, pressure, and other meters: Temperature meters are an important part of all mechanical systems. The temperature meter indication
can be driven by resistive thermal device, mechanical thermal expansion etc. Pressure meters can be driven by pressure transducer (electromechanical) direct pressure bellows or
bourdon type. Other EDG meters may be encountered and would be more of the plant specific type. As a new hire at
the plant, an operator in training, should make physical walk downs of the diesel system with a qualified operator and is the best way to pick up needed information. Use of P&IDs, technical manuals, Plant procedures and training programs are all must do’s by the operator trainee.
Emergency diesel generator technical specification: Because the EDGs are safety related and important to plant nuclear safety, plant technical specifications apply. Although these specifications are plant specific, general guidelines can be given which will help the student understand requirements generally encountered. Limiting conditions of operation for the EDGs: On the next page is a copy of technical specifications from an NRC study on the EDGs at a
nuclear plant. The comments are not part of the technical specification.
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Plant operating modes: below is a listing of plant modes so you understand what the technical specifications are requiring.
General summary of the emergency diesel generator: Your text book reading assignment has an excellent section on diesel engines. My attempt here was
to introduce new information not in the text book and to introduce additional information regarding industry standards required of BSC students; all information is required and testable.
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An Emergency Diesel Generator (EDG) is the primary power source, supplying AC power to the
Class 1E power systems and equipment when the main turbine generator and offsite power source are not available in nuclear power stations. The EDG reduces the probability of a station blackout (SBO) due to AC power failure, and finally it reduces the core damage frequency. Thus, the performance of an EDG will be very important to sustain the long-term safety of a nuclear power plant. Within the industry, diesel engines are categorized by their rotational speeds as high, medium, and low speed. Medium speed diesel engines used in electrical generators operate between 500 to 1000 rpm, and are optimized to provide rapid response to load changes. Medium speed diesel engines provide much greater durability, reliability, and decreased down time in comparison to higher speed engines ; all essential requirements for nuclear service. The diesel is coupled to an electrical generator which produces the required electrical power needed for the class 1E electrical buses. The diesel sets are required to meet seismic category 1 standard and is enclosed a building that is also seismic cat 1. The diesel systems are redundant, separated and protected power trains. All fire protection standards apply to the diesel trains and all cabling, cable trays and penetrations must meet the class 1E electrical requirements.
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