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Home , Alternator, Armature (electrical), Brush (electric), Commutator (electric), Diesel generator, Electrical load

Emergency The Generator, Exciter, and Regulation

9.0 GENERATOR, EXCITER, AND through a . Figure 9-1 shows VOLTAGE REGULATION the principles being discussed in this section. As the conductor passes through This chapter presents the major the magnetic field, in this case downward, it components of the electrical generator, the cuts each of the lines of magnetic exciter, and the and (flux) which causes a current to be explains how they relate to the "induced" in the conductor. Because the development of power by the diesel conductor has a resistance, it is known driven generator unit. from 'ohms law' that the voltage is equal to the current times the resistance. Learning Objectives Therefore, a voltage is also 'induced' between the two ends of the conductor. If As a result of this lesson, you will be able to: the conductor is connected to a closed electrical circuit, this voltage would cause a 1. Describe the functions of the generator, current to flow. The amount of current flow exciter, and voltage regulator. is a function of the voltage induced and the electrical resistance of the load in the 2. Identify the major components of the circuit. generator and how they inter-relate. 9.1.2 Induced Voltage 3. Describe how output power relates to the power demands of The actual voltage induced in the conductor the generator. is determined by the number of lines of flux cut per unit of time. Two key factors affect 4. Describe the function of the the magnitude of voltage induced. system and the associated voltage regulator. • The speed at which the conductor moves through the fixed magnetic field 5. Identity the major components of the and the strength of the magnetic field exciter and voltage regulator system. determine the output voltage. This speed is a function of the rotational 9.1 Generator Principles speed (RPM) of the generator/engine. As the speed of the engine the The following is a brief discussion of generator increases, the voltage generator operation and its relationship to produced also increases. the mechanical load placed on the diesel engine. Since the operating speed of the engine and generator is constant in order to 9.1.1 Electromagnetic Induction maintain the desired , another method of voltage control must be Electromagnetic induction, the basic employed. principle of generator operation, involves the movement of an • Generator output voltage is most often

Rev 1/11 9-1 of 34 USNRC HRTD Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation

controlled by regulating the strength This is shown in the part of the diagram (flux intensity) of the magnetic field. labeled 'Start.' As the is turned to This is accomplished by the generator a position 90 degrees from the first, the two excitation system. The excitation ends of the loop is acted upon in a manner system monitors the generator output wherein the generated at each and regulates the magnetic field to end of the loop are additive, as shown in maintain the desired voltage. As the the '1/4-cycle' diagram. Peak output load on the generator is increased, an voltage is generated at each cycle point. increase in current flow causes the As the armature continues to rotate, it voltage to drop. The excitation system again gets to a position of no voltage senses this decrease in voltage and generation, shown in the ‘½-cycle diagram’. increases the strength of the magnetic As the rotation continues, a voltage is again field to return the voltage to the desired generated. A close examination of the level. wiring out of the armature reveals that the connections have become inverted. This 9.1.3 How the generator works results in the opposite polarity of voltage.

Figure 9-2 shows the principles discussed The diagram at the bottom of the figure above implemented into a to shows the resulting build-up and decay and produce a voltage. In this implementation, opposite polarity build up and decay again a ‘U’ shaped form is provided with a ‘gap’ through two cycles (two rotations of the between the open ends of the ‘U’. A coil of armature). The resulting voltage build-up wire is wrapped about the legs of this form and decay forms a sinusoidal wave that is to produce a magnetic field across the gap. defined as '' or AC. In the gap, an armature is formed by a loop of wire. The loop exits the armature onto This is the basis for a single phase two slip rings. The slip rings are contacted . Two other sets of coils offset by by brushes that connect the generator to 120 degrees and connected to slip rings the outside electric circuit. An engine or would form a machine to generate 3-phase some other prime mover is connected to AC power. This is the basis for all AC 3- the armature causing it to rotate inside the phase generation. gap. When the ‘field’ coil is energized to establish a magnetic field/flux in the gap If instead of using two slip rings a single and the armature is then rotated, a voltage ring that is split into two segments were is generated in the armature. The slip rings used, as the armature rotated, there would and brushes conduct this voltage out to be a buildup and decay of voltage as some load "A." before; but the split would reverse the connection on each half revolution. Figure 9-3 shows a blowup of the armature The split slip ring configuration is commonly in the gap. As the armature is rotated in its referred to as a '.' This would initial position, no voltage is created result in a machine that puts out a pulsating because the is equal but DC current. By combining a great many opposite on both branches of the loop. poles and the same number of segments

Rev 1/11 9-2 of 34 USNRC HRTD Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation on the armature commutator, an almost voltage. This is the case when the power steady DC output would be produced. This factor is 1.0 (unity). The real power (KW) in is the principle of the DC generator or that case is equal to the apparent power motor. (KVA). These terms will be discussed later. When the current is not in phase with the The AC alternator described above has a voltage, there is a lesser , and number of problems. The armature and its the KW is less than the KVA. The KW is slip rings have to handle all the load current still in phase with the voltage, but the KVA that is produced by this generator. The has shifted slightly due to the shift in the brushes and slip rings restrict the amount current. This introduced KVAR, and will be of current that can be handled. To explained later. eliminate this problem, design and construction were changed such that the 9.2 Generator Construction small excitation current now goes through the brushes and slip rings to the rotating Figure 9-5 shows a cutaway of a typical armature field. The large AC current generator. The generator consists of a induced into the stationary windings shaft on which is mounted a hub, more is transmitted to the loads by solid often called the spider. The spider may be connections. This same principle also attached to the shaft by a press fit, with or applies to AC synchronous motors as there without keys, or by a flange and bolting. is little difference between an AC generator The spider has slots into which the field and an AC . It is a pole pieces are attached. Together, this matter of what is driving the system – an makes up the ‘’. The rotor assembly or an engine-driven usually also includes slip rings used to generator. convey the field current into the field windings. These windings are wrapped This also simplifies the construction of the around the pole pieces. A view of a rotor generator. These very often also and shaft assembly is shown in Figure 9.7. are called '' in as much as the voltage and current are alternating. A The stator core consists of a special steel three-phase generator/alternator is simply stampings, called laminations, with slots to three single phase machines interlaced hold the stator windings. The stator core with one another, sharing the same rotor has spaces between some of the assembly, with wiring brought out to laminations through which air is force by connect each phase into the electrical fans on the rotor assembly. This is to system. The result of the interlacing of the provide cooling for the stator core and the alternator windings is shown on the voltage windings. A steel framework supports and trace shown at the bottom of Figure 9-4. aligns the stator core assembly. The steel framework also usually supports the The upper part of Figure 9-4 shows how bearings of the generator. Some the current of each/any phase acts to generators are supplied with two dedicated produce the power. The left-most diagram bearings, one at each end of the generator shows the current in phase with the rotor assembly. Others are supplied with

Rev 1/11 9-3 of 34 USNRC HRTD Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation an outboard bearing at the far end of the F is frequency in hertz (Hz) generator; the engine end bearing supports P is the number of poles the other end of the generator. N is the generator speed (RPM)

The wiring in the stator slots is grouped in As a short cut, for 60 Hz power generators, sections. The number for each phase N = 7,200 / P and, therefore, P = 7,200 / N matches the number of poles on the rotor. These various sections are wired together All generators have an even number of around the periphery of the stator. The end poles. An engine running at 900 rpm would of these groups are gathered together and have 8 poles on its generator to produce 60 brought out to the generator electrical Hz power. connection box. 9.3.2 Mechanical Loading Figure 9-6 shows an assembled generator, ready for installation to an engine. This As the electrical conductors move through particular machine has a shaft driven the magnetic field of the generator, an exciter unit, the smaller diameter cylinder opposing magnetic field is created around shown on the end of the generator. the conductors. This opposing field resists Figures 9-6 through Figure 9-22 show the movement of the conductors through various parts and aspects of the assembly the generator magnetic field. The physical of the parts of the generator. force, or power, provided by the diesel engine must be sufficient to overcome the 9.3 Generator Terminology resistance of the opposing field in order to achieve the desired voltage and frequency. 9.3.1 Generator Frequency As the load on the generator increases in Another key element in the output of the the form of an increase in current demand, generator is the frequency. Frequency is a the excitation system increases the density function of the rotational speed (RPM) of of the magnetic flux by increasing the the engine and the number of poles current in the generator field. This (magnetic fields) in the generator as shown increases the resistance to movement of in the following equation. As the operating the conductors through the field. As a speed of the engine and generator is result, the generator induced voltage reduced, the number of poles must be decreases momentarily until the excitation increased to achieve 60 hertz. An EDG system compensates to return the voltage operating at 450 RPM would require twice to its previous level. Since the load was as many poles as a unit operating at 900 increased with relatively constant voltage, RPM. Once designed, the number of poles the output current (and hence, output in an alternating current (AC) machine is power) of the generator must increase. fixed. This power demand is also felt on the diesel engine which is trying to maintain the · speed constant as the load from the 120 generator increases.

Rev 1/11 9-4 of 34 USNRC HRTD Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation

9.4 Generator Control sequencing system while maintaining voltage above a minimum of 75% of the Alternating current (AC) generators of this nominal voltage (25% voltage dip). The type are driven by high horsepower diesel EDG Voltage Regulator is essential to in nuclear applications, which correct the EDG output voltage. It is require stable and accurate means of powered from a class 1E and control. Licensee Technical Specifications is tested for satisfactory operation during stipulate voltage and frequency limits periodic EDG surveillance testing. If the during fast start, sequential loading, load Voltage Regulator is inoperable, the EDG is rejection testing, and normal operation. For inoperable. a generic EDG, typical voltage and frequency limits are 4160 + 420 volts (10%) The generator exciter varies the strength of and 60 + 1.2 Hz (2%) respectively. the generator rotor field current, either automatically during auto-sequencing or 9.4.1 Frequency Control manually from the control room or EDG control cabinet. Generator frequency control is accomplished by the engine The generic plant FSAR specifies governor sensing speed changes from the desired and voltage regulator comparison test-ing speed set point. The governor then adjusts (bench-marking) to demonstrate acceptable the fuel to the engine as required to keep performance of both subsystems when the engine operating at the desired (set experiencing major load changes. This point) speed. testing is incorporated into Technical Specification surveillance items. 9.4.2 Voltage Control 9.4.3 Generator Rating Large AC generators commonly use a combination Exciter-Voltage Regulation For discussion involving the EDG electrical system to maintain generator field current characteristics and electrical load, several under varying electrical loads. The basic parameters associated with AC machines voltage regulation system is designed to are defined. These include real power, automatically regulate generator output reactive power, apparent power, and power terminal voltage within close tolerances of a factor. specified value. The generic regulation system is a 'closed loop' feedback system Generators are normally rated by the KVA in which generator output voltage is they are designed to produce. This is automatically compared to a reference normally at a power factor of 0.8 per NEMA voltage. The error signal is used to change 1 standards. Sometimes, the generator is generator excitation. The excitation is rated in KW, but then the power factor must discussed in more detail in section 9.5. also be stated.

The generic EDG must be capable of 9.4.3.1 Real Power -- KW accepting loads assigned by the auto

Rev 1/11 9-5 of 34 USNRC HRTD Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation

Real power in an alternating current (AC) 1000 for KA or 1,000,000 for MVA. system refers to the true electrical power that is converted to 9.4.3.4 Power Factor such as a motor driving a pump. This power is measured in watts or kilowatts Power factor is the ratio of Real Power (kW) (or mega-watts - MW) for large (KW) to Apparent Power (KVA). It is a electrical systems. measure of the utilization of the input power of a system or equipment. A typical power 9.4.3.2 Reactive Power -- KVAR factor for the rating of the generator, per NEMA standards, is for a power factor of Most electrical systems that undergo 0.8 (80%). changes of voltage or current have either capacitance or inductance. In a DC For example, the generic plant FSAR system, these are only important during a specifies that the standby EDG be rated at significant change. Because AC power is 4000 KW. To provide this real power input continually changing (the voltage is and generate enough 'magnetizing' power sinusoidal), the inductance and/or for the generator rotor and stator, the capacitance become more important. generator is rated at an apparent power of Electrical power is needed in AC machinery 5000 KVA. (motors, generators, and ) to create the magnetic fields and voltage 9.4.3.5 Relationship between KW, KVA, induction that enable these machines to KVAR, and Power Factor operate. This ‘magnetizing’ power, which is ‘stored’ energy in the AC system, is There is a relationship between the KW, reactive power. The current component of KVA, KVAR, and Power Factor that allows this power acts at 90 degrees from the real us to calculate any one, knowing at least power (KW). Reactive power is measured two others. That relationship is described in volt-amperes-reactive or VARs. For high by the following formulae: voltage systems, reactive power may be measured in kilo-VARs or mega-VARs (KVAR or MVAR) 9.4.3.3 Apparent Power -- KVA

As its name implies, apparent power is the For power factor in percent, multiply the power that is ‘apparently’ required to be decimal value by 100. input or drawn from the AC system. Apparent power is volt-amperes, kilovolt- Note that the relationships between KW, amperes, or megavolt-amperes (VA, KVA KVAR, and KVA is the same as that for any or MVA). It is determined by multiplying the right triangle – The Pythagorean theorem. voltage times the amperes times the square root of three for 3-phase electrical Also, note that the triangular relationship systems and then dividing the result by between the three factors can be solved if

Rev 1/11 9-6 of 34 USNRC HRTD Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation any two are known. This also includes the result of generator, exciter, and voltage power factor in as much as the power regulator characteristics and response. factor gives the relationship (ratio) between The frequency dip is a result of engine and the KW and KVA. So knowing KW or KVA governor characteristics and response. and the Power Factor, one can get the The engine knows nothing of the KVAR other and from that the third. loading except as that may influence the efficiency of the generator and its KW is usually measured and displayed on relationship to the engine. a meter. KVA can be obtained by taking the voltage (average for the 3 phases) and The KW loading on the generator is the current (average for the 3 phases), converted to the engine Horsepower multiplying them together, and then required using the following relationship: multiplying that result by the square root of 3 (1.73) for a three-phase power system, · then dividing by 1000 to get the kilo-VA. KVAR may be displayed on a meter as well .% as Power Factor being displayed. 9.5 Excitation and Voltage Regulation

There is no means of directly measuring Every power generation system requires KVAR or PF other than special meters for some means of controlling the voltage that purpose or by calculation using the and/or current produced by the machine. above formulas. The Power factor meter is The output of a generator is normally the least accurate of any of the electrical controlled by controlling the current in the parameter meters. It is far more accurate to field of the generator, the speed being solve the triangular relationship between constant for a set frequency. Various KW and KVA as shown in paragraph excitation systems are possible and all 9.4.3.4. usually include some system of sensing

and controlling the generator output It is convenient at this point to emphasize a voltage. principle that applies to all engine driven generator systems. 9.5.1 Types of Excitation Systems

• THE GOVERNOR ON THE ENGINE Excitation systems vary from the very CONTROLS OR RESPONDS TO THE simplest to rather complex systems. The KW LOADING ON THE GENERATOR. simplest would consist of a battery to

supply excitation power to the generator • THE VOLTAGE REGULATOR AND field along with a rheostat to control the EXCITER SYSTEM CONTROLS OR amount of excitation current, that being RESPONDS TO THE KVA and managed manually by an operator. This thereby the KVAR loading. system is generally not acceptable;

therefore, some means of automatic The two control devices are quite control is desired. It may consist of independent otherwise. Voltage dip is a automating the rheostat such as the old

Rev 1/11 9-7 of 34 USNRC HRTD Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation

Westinghouse rocking arm Silver Stat Most excitation systems used at nuclear design. It worked; it wasn’t fast and had power stations are either the Series Boost droop in the voltage regulation, but it was (SB) or the fully electronic Static Exciter simple. Voltage Regulator (SEVR) system. These are more similar than they are different in Most modern excitation systems involve most cases, and they will be explained by in the voltage sensing and explaining the excitation system shown in regulation and are relatively fast and Figure 9-23. accurate. Many exciter systems consist of an excitation generator driven by the The exciter output DC voltage type is fed engine either directly or by a system of through bundles of slip rings into the main pulleys and belts. The excitation generator generator field windings. puts out the power, and a voltage regulator controls that generator’s field in such a way that the main generator’s field is under 9.5.2 Explanation of Elements of the control. Electronic (Static) Type Excitation System Many commercial systems use what is termed a brushless exciter. With a Figure 9-23 shows the elements in the brushless excitation system, an typical modern electronic excitation system. alternator/generator is mounted on the end While there are differences between the of the generator shaft and is driven along Series Boost (SB) and the Static Exciter- with the main generator. A permanent Voltage Regulator (SEVR) systems, the field excites the alternator as it basic elements are the same in both rotates. The alternator output goes through systems. Their differences will be a bridge which converts it into DC explained as each section of the systems is that is fed into the generator field through explained. wires running along the shaft between the alternator’s output section and the main Figure 9-23 shows the generator on the far generator field. There are no brushes in left with its load lines going to the right, until this type system, thus the term ‘brushless they finally pass through the generator exciter’. A control winding is included in the output and on to the loads. permanent magnet field to control the As each load line exits the generator, it output of the exciter alternator, and thereby goes through the primary side of a Power the main generator’s field. There are a few Current (PCT). The of these systems at nuclear stations. They secondary side of these transformers feed are not as fast and responsive as the fully power for excitation into the exciter electronic system that is discussed below. package. Most of the modern exciter They are limited in power capability, and systems have these Power Current their overall response is slow because the Transformers to supply some of the system has to operate through two sets of excitation current required by the generator magnetics the exciter alternator and the field. main generator field.

Rev 1/11 9-8 of 34 USNRC HRTD Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation

Properly sized PCTs, supply power for the unit. This is not desirable in the excitation in direct proportion to the load on isolated 'emergency mode'. The terms the generator. The generator exciter used on the switching this control function system requires about 1 to 2 percent are usually referred to as 'unit' (without generator’s output power. current compensation) and 'parallel' (with current compensation). As the load lines continue beyond the PCT connections, there are connections to a The voltage regulator receives a reference Power Potential Transformer (PPT). Since signal from the operator or system which there is no generator output current when tells it the voltage to maintain. This voltage there is ‘no load’ on the generator, there may be modified by the current must be some other source of power for compensation circuit if the exciter is set up generating excitation when there is no load. for ‘parallel’ operation. In this case, the The PPTs supply the power for generator reference input of the regulator is excitation when the generator is not loaded. compared to the sensed voltage input. The voltage regulator’s output is changed to As the load lines continue beyond the PPT restore the generator output voltage back connection, there is another PT connection to the reference voltage input. for sensing the generator output. This is fed into the Voltage Regulator section as a The output signal from the voltage regulator sense of the generator output voltage. controls elements in the Power Section of the exciter system. This section is attached Beyond the sensing PT connection, there is to the Power Potential Transformers. another Current Transformer (CT) for Properly sized PTs control the output sensing the amount of load (via the current, voltage without the need for current assuming the voltage is constant). This is compensation. used to adjust the voltage regulator’s output when the unit is being paralleled to Generally there are two methods of the grid (infinite bus). In order to control controlling the power section. In the case the reactive power component of the load of the Series Boost (SB) exciter, the current when paralleled, the voltage transformers (PPTs) have a third (tertiary) regulator has to receive some sense of the winding. The PPTs are like normal load on the system. This is equivalent to transformers if the tertiary winding is not the droop function in the governing system. turned on. However, if there is current in This principle is variously called 'current the tertiary winding, it causes the compensation,' 'load compensation,' transformers to become saturated to the ‘paralleling load sensing’ or ‘voltage droop’. point that the current transfer from the The function is usually switched out primary of the PPT to the secondary is (can automatically when the diesel generator is be) restricted; thus, the output current can in the 'emergency mode.' It is required only be controlled. These transformers are in parallel operation. If the circuit were left therefore often called 'saturable reactors'. active in ‘emergency mode’, there would be a slight change in voltage as load is put on In the case of the SEVR type exciter, the

Rev 1/11 9-9 of 34 USNRC HRTD Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation amount of current out of the power section regulator. That is, all of the power is controlled by turning on or holding off the generated in the exciter goes to the firing of silicone controlled (SCRs) generator field circuit. There are SCRs that or similar electronic elements. If a large are connected in parallel with the generator amount of power is required, the SCRs are field that shunt the proper amount of fired early in the voltage cycle. If less current around the field in order to control power is required, the SCRs are fired later. the current that goes through the field. If no power is required (in the voltage overshoot situation), the SCRs are not Other than these differences, most exciter turned on. This is the means of controlling systems are more alike than different, and the no-load excitation power in such a all contain the basic elements shown in system. Figure 9-24.

The power from the power section is sent There is generally some feedback from the over to the section where it is exciter output section to the voltage joined with the power from the PCTs. This regulator section so that the voltage is rectified (converted) from AC to DC regulator can anticipate what it needs to do () and sent on to the generator next or if it needs to help stabilize the field as shown. There is usually a voltage regulator. In most regulation systems meter and an amperage meter in the field without feedback, the systems tend to turn circuit so that the field conditions can be all the way 'on' or all the way 'off' and this is monitored. The generator field can accept not desirable. Therefore, feedback helps only a direct current supply. The generator the regulator know how much excitation is acts as a fixed resistance, changed only being put out so it can gauge that against slightly by its temperature. Therefore, the the error and anticipate that the problem is field voltage and current are proportional. nearly resolved, et.al. This is helpful in troubleshooting some types of exciter/generator problems such These exciters are self-sustaining. That is, as detecting a short or ground in the they take the power they need from the generator field. generator output. They require no external power for normal operation. But that also There are a number of differences in some means that they will probably not start excitation techniques that are worth themselves up when required. Therefore, a mentioning. In the Westinghouse and GE means of starting the excitation process exciters, the power section output, and the has to be provided. This is done with the PCT, contributions are rectified separately 'Field Flashing' circuit, also shown on and joined together in the DC field circuit. Figure 9-23. That circuit consists of a In the Basler and Portec systems, the battery (maybe the station batteries) and a power section and PCT contributions are . The switch is closed when the joined on the AC side and rectified after system is signaled to start up (usually upon they are joined. the engine achieving a certain speed), and the switch is opened when the voltage The Portec exciter is a 'shunt' type regulator senses that the generator voltage

Rev 1/11 9-10 of 34 USNRC HRTD Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation has built up to a certain value (usually side of the exciter rotor. It serves to about 65 to 75% of rated voltage). Inside change the AC exciter output to DC. The the rectifier section this circuit usually output is fed through a hole in the exciter includes a diode to block reverse current shaft to the main generator. It exits after from the exciter back into the batteries. passing through the rear of the generator Usually, some resistors are included to limit housing and connects to the rotating main the current available from the batteries generator field windings. There are no slip because only a small current is needed into rings or brushes. the generator field to get generation started. The field of the exciter generator is stationary. Its housing is mounted onto the Most of the modern excitation systems rear end of the main generator. have the capability of putting out an immense amount of power if needed. To Another alternator may be mounted sustain rated load may not take much outboard of the exciter generator. This unit power, but to start a large motor load may would have a permanent magnet rotor, tax the generator and exciter system. Most usually mounted on the back of the of the modern exciters are capable of generator shaft. The stator winding of this putting out about 3 to 5 times (300 to alternator puts out an AC current/voltage to 500%) of the generator’s normal full load a voltage regulator. The voltage regulator excitation current. This is called ‘field turns that AC input into a DC output which forcing’. The voltage dip caused by starting is controlled so as to ultimately control the a large motor is primarily a function of the current into the main generator field. generator characteristics (specifically the transient and sub-transient reactances). If the unit does not have this permanent However, the rate of recovery and a few magnet alternator (PMA), another source of percent of the voltage dip can be influenced voltage for the voltage regulator must be by the response and the field forcing provided. In a number of cases, that may capability of the excitation system. be from a battery. In other cases, it is from the main generator output (reduced in 9.5.3 Brushless Excitation Systems voltage by a transformer). In this last case, since the is not available A brushless excitation system has the when the EDG is being started, a battery is advantage of not having brushes to used to ‘flash’ the system until the transmit the current to the generator field generator voltage has built up to a near through slip rings and brushes. Figure 9-24 rated voltage condition, similar to flashing shows a schematic diagram of the typical the field on the static exciter systems brushless exciter system. discussed above.

The brushless exciter system consists of an 9.5.4 Digital Voltage Regulation exciter generator mounted on the generator shaft, usually outboard of the generator The only section of the excitation system bearing. A diode plate is attached to one that lends itself to digital control is the

Rev 1/11 9-11 of 34 USNRC HRTD Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation voltage regulator. While there are some and output. When it comes to response to advantages to digitizing the voltage the voltage dip caused by starting large regulator for the purpose of better voltage motors, that is primarily influenced by the stability during isolated operation (powering characteristics of the generator and would the emergency bus), and of controlling not be appreciably influenced by having a reactive power (power factor) a little tighter faster and more complicated voltage when in parallel with the offsite power regulator. The more components in a system, the effect would be small in a system, the less reliable the system comparison to the existing analog systems. becomes. The voltage regulator operates basically from analog inputs and in the case of the 9.6 Generator Output Breaker Basler Series Boost systems (used at most plants), the output is also analog, there is Figure 9-23 also shows the main generator little advantage to inserting digital output beaker at the right end of the components in between the input and diagram. The generator output breaker output. When it comes to response to the connects the generator to its Class 1E voltage dip caused by starting large motors, emergency bus. When an emergency that is primarily influenced by the start signal is received, the EDG starts and characteristics of the generator and would comes up to rated speed and voltage. The not be appreciably influenced by having a output circuit breaker is closed faster and more complicated voltage automatically. An auxiliary contact on the regulator. The more components in a circuit breaker provides a signal to the load system, the less reliable the system sequencer to connect the emergency load becomes. in proper sequence within a time period of approximately 30 seconds. 9.5.4 Digital Voltage Regulation In the event of the unit being started for The only section of the excitation system surveillance testing, the output breaker is that lends itself to digital control is the closed manually by the plant operators voltage regulator. While there are some after bringing the unit into synchronizism advantages to digitizing the voltage with the offsite power system. The regulator for the purpose of better voltage operator must closely match the frequency stability during isolated operation (powering of the generator to that of the off- site the emergency bus), and of controlling power, with the generator slightly higher reactive power (power factor) a little tighter than the offsite frequency so that the unit when in parallel with the offsite power will assume some load when the output system, the effect would be small in a breaker is closed. The voltage must also comparison to the existing analog systems. be closely matched but slightly high so that The voltage regulator operates basically the KVAR loading will be outbound rather from analog inputs and in the case of the than inbound. A synchroscope is typically Series Boost systems, the output is also used to bring the unit into synchronizism, analog, there is little advantage to inserting and the system may also include sync- digital components in between the input check relays to protect the generator output

Rev 1/11 9-12 of 34 USNRC HRTD Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation breaker from being closed with the unit not phase of this system. There is a like device synchronized properly. and circuit for each phase, A, B and C.

The circuit includes a device called the 87 9.7 Generator Differential Protection trip (differential relay). If the currents become unbalanced due to significant The generator requires very little monitoring single phase loading or mismatched its normal operation. The output voltage, because of some of the generator output currents, field voltage and current, stator current going to ground or shorted to temperatures and perhaps bearing another phase, it would cause a current temperature and oil level would be about through the coil of the 87 relay. If the the extent of the required monitoring. current is sustained at a level above the 87 However, there is one item that is required relay trip set point, the relay will actuate its by NRC regulations that must not only be contact. When that contact operates, it monitored but if such a fault is detected, the puts a current into the latch coil on the 86 unit must be immediately shutdown, not trip device shown in Figure 9-25. When the only the generator but the engine as well. 86 trip device coil in energized, the latch holding the 86 trip device handle in the A system is set up to monitor the currents 'Reset' position is pulled by action into the generator on the neutral side and moving the 86 Trip device to the Tripped also the currents in each phase on the position. Contacts in the 86 trip device output (load) side of the generator. If one close circuits in the engine and generator of the phases were to go to ground or if a control systems shut down the engine and phase-to-phase fault developed, the input shut off the generator excitation system. and output currents would not match, The 86 trip device must be manually reset indicating a problem exists within the in order to restore the EDG unit to protected area of the generator. In normal operating status. service, the currents in each phase of the generator are balanced. Prompt shutdown of the generator will minimize damage to the generator and To determine that the current going to the could prevent generator destruction from loads on the generator and the currents prolonged operation with high fault current. within the generator loop are the same as Prompt shutdown of the engine may they normally would be, a detection system prevent mechanical damage to the is set up as shown in Figure 9-25. Note generator and to the engine in the event that there is a set of current transformers that the generator components expand or (CTs) installed in the lines into the separate and cause rubbing or seizure of generator on the neutral side of the the generator rotor within the stator. generator windings. There is a like set of CTs installed beyond the unit’s circuit It is important to note that each phase of breaker which monitor the currents going to the generator is monitored by an 87 relay the generator load. The diagram shows the and that operation of any of these relays inter-connection of these CTs for the ‘C’ will cause the 86 device to trip, which shuts

Rev 1/11 9-13 of 34 USNRC HRTD Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation down the generator and engine. Inclusion of the Generator Differential protection is The generator is subject to stresses due to required by the licensee’s FSAR and normal currents running through the technical specifications as required by NRC windings. To avoid undue stresses, there Reg Guide 1.9 and IEEE 387. This is one are some operations and conditions which of the two conditions that will shut down the should be guarded against if at all possible. EDG under all circumstances. The other These add extra stress to the generator mandatory shutdown is for the engine and may cause the generator damaged or going into an over-speed condition. Other failure. These fall into two basic shutdowns may be included only if provided categories, with some items appearing in with coincident logic. both categories.

9.7.1 Generator Monitoring and 9.7.2.1 Electrical Stresses: (primarily for Protection the Stator)

A panel for control and monitoring is • High Sustained Currents usually found in nuclear plants. Various • High Motor Starting Loading currents meters and relays and trip mechanisms are • Paralleling Out of Phase mounted on this a panel to give the • High operating temperatures operator a means of monitoring the generator’s operation and performance. 9.7.2.2 Mechanical Stresses: (primarily for the Rotor) Typically the following meters would be present: • Dynamic Loading (engine problems) • Suddenly applied Fault Current Loading • Phase Current and Phase Volt meter • Centrifugal rotational loading with phase selection switch. • High operating temperatures • Field Volt and Amp meters • Paralleling out of phase • KW and KVA meters • Armature not concentric with stator (optionally, a Power Factor Meter) • Vibration (out of balance) • Synchroscope and Synchronizing Lights • Out of Alignment. • Frequency Meter (Engine Tachometer) The above mechanical stresses may Typically, the relays and trips would include: involve the engine as well as the generator.

• Differential Fault and 86 trip switch 9-8 Generator Connection and • Loss of Field and Field Ground Alignment to the Engine (Fig. 9-26) • Over-current and Over Voltage • Under-Voltage One of the exercises that students will • Reverse Power participate in while in this school, is to • Under Frequency check for crank shaft deflection in the last crank throw of the engine. Figure 9-26 9.7.2 Generator Stresses shows how a dial indicator is mounted

Rev 1/11 9-14 of 34 USNRC HRTD Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation between the cheeks of the crankshaft at the have the crankshaft straight in the throws to determine that the crankshaft is horizontal plane. straight. This procedure for crank alignment is commonly known as crank Once the generator bearing and the rotor web deflection or crank alignment. are properly set up to relieve the crank stress/strain, then the generator stator must The crank pin journal can be considered to be positioned with respect to the rotor be of fixed length. If the crankshaft is not position so as to equalize the air gap straight through the throw, as the between the stator and the rotor. This is crankshaft rotates, the crankshaft webs go usually done by shifting the generator on through a bending to accommodate the jack screws and then installing shims under bend in the crank. This causes the crank the generator feet before tightening the cheeks to go from being more open to generator to the foundation or the skid. being more closed, as shown in the lower left portion of the diagram. The crankshaft The details of how to perform a crankshaft is attempting to bend in that section shown alignment will be covered in the hands-on with as 'AREA OF HIGHEST STRESS'. If the exercise. crank continued to bend or deflect during each rotation, it could induce high enough stress to fail over time. It is necessary to change this situation to minimize the bending and flexing of the crankshaft particularly through the last throw.

The lower right diagram shows a situation where the some of the weight of the generator shaft and rotor are being carried by the last bearing in the engine. This bearing is next to the last throw on the crank. The upper right lower diagram shows the bending of the last throw as a result of the generator rotor weight. To alleviate this problem, usually the rear bearing of the generator is raised so that the generator-engine shaft is straight through the last bearing and straightens the last crank throw. While this diagram exaggerates the actual situation, it is usually only necessary to raise the generator bearing by a few thousandths of an inch. It may also be necessary to shift the generator bearing sideways in order to

Rev 1/11 9-15 of 34 USNRC HRTD Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation

netic Field Field netic g ure 9-1 Conductor in a Ma ure 9-1 g Fi

Rev 1/11 9-16 of 34 USNRC HRTD Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation

LOAD

Figure 9-2 Simple Generator (Alternating Current

Rev 1/11 9-17 of 34 USNRC HRTD Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation

Figure 9-3 Single Phase Sine Wave

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Figure 9-4 Three-Phase Generation

Rev 1/11 9-19 of 34 USNRC HRTD Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation

Figure 9-5 Cutaway of Generator – Identification of Parts

Rev 1/11 9-20 of 34 USNRC HRTD Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation

Figure 9-6 OP Engine Generator Assembly (under test)

Rev 1/11 9-21 of 34 USNRC HRTD Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation

Figure 9-7 Complete Rotor Assembly (coupling end-of-shaft view)

Figure 9-8 Generator Shaft (coupling end view)

Rev 1/11 9-22 of 34 USNRC HRTD Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation

Figure 9-9 Rotor Spider (hub)

Figure 9-10 Field Pole Wedges

Rev 1/11 9-23 of 34 USNRC HRTD Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation

Figure 9-11 Field Pole Blocking

Figure 9-12 Field Pole with Amortiser

Rev 1/11 9-24 of 34 USNRC HRTD Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation

Figure 9-13 Field Wiring to Slip

Figure 9-14 Slip Ring Assembly

Rev 1/11 9-25 of 34 USNRC HRTD Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation

Figure 9-15 Winding Coil

Figure 9-15 Windings Installed in Stator

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Figure 9-17 Coil Tying and Bracing

Figure 9-18 Lead Cable Terminations

Rev 1/11 9-27 of 34 USNRC HRTD Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation

Figure 9-19 Complete Stator Assembly

Figure 9-20 Rotor Installed within Stator (note fan assembly & slip ring

Rev 1/11 9-28 of 34 USNRC HRTD Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation

Figure 9-21 Bearing & Housing Assembled in Rotor

Figure 9-22 Rigging and Slip Rings

Rev 1/11 9-29 of 34 USNRC HRTD Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation

SEVR/SB SEVR/SB ( ram g stem Block Dia Block stem y ure 9-23 Exciter S Exciter ure 9-23 g Fi

Rev 1/11 9-30 of 34 USNRC HRTD Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation

ram ram g stem Dia stem y ure 9-24 Brushless Exciter S Exciter Brushless ure 9-24 g Fi

Rev 1/11 9-31 of 34 USNRC HRTD Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation

Figure 9-25 Generator Differential Fault

Rev 1/11 9-32 of 34 USNRC HRTD Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation

Figure 9-26 Engine-Generator Alignment

Rev 1/11 9-33 of 34 USNRC HRTD Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation

HANDS-ON SESSION 10 • Identify the generator frame including its mounting/ provisions for an on-engine 10.0 GENERATOR, EXCITER, AND mounting. VOLTAGE REGULATION • Show how the generator stator is mounted within the frame with its power Purpose leads brought out. • Show how the generator field pole The purpose of this session is to pieces are mounted on the rotor and its complement Chapter 9. mounting to the generator power input shaft. Learning Objectives • Identify and discuss the generator stator to rotor air gap. Upon completion of this lesson you will: • Discuss air gap functions and importance of its concentricity. 1. Become familiar with the EDG • Show how the slip rings are mounted to generator configuration, components the generator shaft and are connected locations and their functions. to the generator field. • Show how the brush rigging is mounted 2. Understand the alignment of generator above the slip rings. shaft with crankshaft and alignment of • Show the brush holders, spring clips, the generator rotor within the generator and brushes and discuss their stator alignment, curvature, condition, and

how to measure brush pressure. 3. Understand the need for proper • Show where the input from the external connection from external exciter through exciter is connected into the brush generator brushes and slip rings to holders. generator field windings • Show the generator outboard (inboard)

bearings and the need for alignment, 4. Understand the need for insulation of lubrication, and isolation from ground. generator end bearing from ground • Show the generator shaft input coupling

to the diesel engine. Discuss the need 5. Understand the need for periodic for proper alignment and mounting to inspections and tests of slip rings, the engine. brushes, windings, bearing, and • alignment Discuss periodic inspections and tests of the generator including slip rings, brushes, windings, bearings, and 10.1 The Generator alignment. • Discuss potential problem areas, Using the EDG generator on the test floor, indications of problems, and tests. the instructor will conduct the following training:

Rev 1/11 9-34 of 34 USNRC HRTD