ML20043F661
| ML20043F661 | |
| Person / Time | |
|---|---|
| Issue date: | 02/12/2020 |
| From: | Office of the Chief Human Capital Officer, Woodard Corp |
| To: | |
| Gary Callaway | |
| Shared Package | |
| ML20043F634 | List:
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| References | |
| Download: ML20043F661 (33) | |
Text
Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation 9.0 GENERATOR, EXCITER, AND through a magnetic field. 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 electrical generators, the cuts each of the lines of magnetic force (flux) exciter, and the voltage regulator and which causes a current to be "induced" in explains how they relate to the development the conductor. Because the conductor has of power by the diesel engine driven a resistance, it is known from 'ohms law' that generator unit. the voltage is equal to the current times the resistance. Therefore, a voltage is also Learning Objectives 'induced' between the two ends of the conductor. If the conductor is connected to As a result of this lesson, you will be able to: a closed electrical circuit, this voltage would cause a current to flow. The amount of
- 1. Describe the functions of the generator, current flow is a function of the voltage exciter, and voltage regulator. induced and the electrical resistance of the load in the circuit.
- 2. Identify the major components of the generator, exciter, and voltage regulator. 9.1.2 Induced Voltage
- 3. Explain the purpose of the Generator The actual voltage induced in the conductor Differential Fault Protection system. is determined by the number of lines of flux cut per unit of time. Two key factors affect
- 4. Describe key considerations for the magnitude of voltage induced.
connecting generator and engine, to protect bearings and engine crankshaft.
- The speed at which the conductor moves through the fixed magnetic field and the
- 5. Describe how diesel engine operation strength of the magnetic field determine relates to the power demand on the the output voltage. This speed is a generator. function of the rotational speed (RPM) of the generator /engine. As the speed of 9.1 Generator Principles the engine and generator increases, the voltage produced also increases.
The following is a brief discussion of generator operation and its relationship to Since the operating speed of the engine the mechanical load placed on the diesel and generator is constant in order to engine. maintain the desired frequency, another method of voltage control must be 9.1.1 Electromagnetic Induction employed.
Electromagnetic induction, the basic
- Generator output voltage is most often principle of generator operation, involves the controlled by regulating the strength of movement of an electrical conductor the magnetic field (its flux intensity). This Rev 3/16 9-1 of 33 USNRC HRTD
Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation is accomplished through the generator ends of the loop are acted upon in a manner excitation system. The excitation system wherein the voltages generated at each end monitors the generator output and of the loop are additive, as shown in the '1/4-regulates the magnetic field to maintain cycle' diagram. Peak output voltage is the desired voltage. As the load on the generated at each cycle point. As the generator is increased, an increase in armature continues to rotate, it again gets to current flow causes the voltage to drop. a position of no voltage generation, shown The excitation system senses this in the 1/2-cycle diagram. As the rotation decrease in voltage and increases the continues, a voltage is again generated. A strength of the magnetic field to return close examination of the wiring out of the the voltage to the desired level. armature reveals that the connections have become inverted. This results in the 9.1.3 How the generator works opposite polarity of voltage.
Figure 9-2 shows the principles discussed The diagram at the bottom of the figure above implemented into a machine to shows the resulting build-up and decay and produce a voltage. In this implementation, a opposite polarity build up and decay again 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 'Alternating Current' 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 alternator. Two other sets of coils offset by by brushes that connect the generator to the 120 degrees and connected to slip rings outside electric circuit. An engine or some would form a machine to generate 3-phase other prime mover is connected to the AC power. This is the basis for all AC 3-armature causing it to rotate inside the gap. phase generation.
When the field coil is energized to establish a magnetic field/flux in the gap and the If instead of using two slip rings we have a armature is then rotated, a voltage is single ring that is split into two segments, as generated in the armature. The slip rings the armature rotated there would again be a and brushes conduct this voltage out to buildup and decay of voltage but the split slip some load "A." ring would reverse the connection on each half revolution. Such a split slip ring Figure 9-3 shows a blowup of the armature configuration is commonly referred to as a in the gap. As the armature is rotated in its 'commutator.' This would result in a initial position, no voltage is created machine that puts out a pulsating DC because the magnetic flux is equal but current. By combining a great many poles opposite on both branches of the loop. This and the same number of segments on the is shown in the part of the diagram labeled armature commutator, an almost steady DC
'Start.' As the armature is turned to a output would be produced. This is the position 90 degrees from the first, the two principle of the DC generator, or motor.
Rev 3/16 9-2 of 33 USNRC HRTD
Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation The AC alternator described above cannot 9.2 Generator Construction produce high output power because all the load current passes through the brushes Figure 9-5 shows a cutaway of a typical and slip rings, which have limited capacity. generator. The generator consists of a shaft To eliminate this problem a new design was on which is mounted a hub, more often developed, with the small excitation current called the spider. The spider may be going through the brushes and slip rings to attached to the shaft by a press fit, with or the field, now on the rotating armature. The without keys, or by a flange and bolting. The large AC current induced into the stationary spider has slots into which the field pole stator windings is transmitted to the loads by pieces are attached. Together, this makes solid connections. This same principle also up the rotor. The rotor assembly usually applies to AC synchronous motors as there also includes slip rings used to convey the is little difference between an AC generator field current into the field windings. These and an AC synchronous motor. It is a matter windings are wrapped around the pole of what is driving the system - an electric pieces. A view of a rotor and shaft assembly motor or an engine-driven generator. is shown in Figure 9.7.
This simplifies generator construction. They The stator core consists of a special steel are often called 'alternators' because the stampings, called laminations, with slots to voltage and current are alternating. A three- hold the stator windings. The stator core phase generator / alternator is simply three has spaces between some of the single phase machines interlaced with one laminations through which air is force by another, sharing the same rotor assembly, fans on the rotor assembly. This is to with wiring brought out to connect each provide cooling for the stator core and the phase into the electrical system. The result windings. A steel framework supports and of interlacing alternator windings is shown aligns the stator core assembly. The steel on the voltage trace at the bottom of Figure framework also usually supports the 9-4. bearings of the generator. Some generators are supplied with two dedicated bearings, The upper part of Figure 9-4 shows how the one at each end of the generator rotor current of each / any phase acts to produce assembly. Others are supplied with an the power. The left-most diagram shows the outboard bearing at the far end of the current in phase with the voltage. This is the generator; with the engines end bearing case when the power factor is 1.0 (unity). supporting the other end of the generator.
The real power (KW) in that case is equal to the apparent power (KVA). These terms will The wiring in the stator slots is grouped in be discussed later. When the current is not sections. The number for each phase in phase with the voltage, the power factor matches the number of poles on the rotor.
is less than 1.0 and the KW is less than the These various sections are wired together KVA. The KW is still in phase with the around the periphery of the stator. The end voltage, but the KVA has shifted slightly due of these groups are gathered together and to the shift in the current. This introduces brought out to the generator electrical KVAR, which will be explained later. connection box.
Rev 3/16 9-3 of 33 USNRC HRTD
Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation Figure 9-6 shows an assembled generator, As the electrical conductors move through ready for installation to an engine. This the magnetic field of the generator, an particular machine has a shaft driven exciter opposing magnetic field is created around unit, the smaller diameter cylinder shown on the conductors. This opposing field resists the end of the generator. Figures 9-6 the movement of the conductors through the through Figure 9-22 show various parts and generator magnetic field. The physical aspects of the assembly of the parts of the force, or power, provided by the diesel generator. engine must be sufficient to overcome the resistance of the opposing field, in order to 9.3 Generator Terminology achieve the desired voltage and frequency.
9.3.1 Generator Frequency As the load on the generator increases in the form of an increase in current demand, the Another key element in the output of the excitation system increases the density of generator is the frequency. Frequency is a the magnetic flux by increasing the current function of the rotational speed (RPM) of the in the generator field. This increases the engine and the number of poles (magnetic resistance to movement of the conductors fields) in the generator as shown in the through the field. As a result, the generator following equation. As the operating speed induced voltage decreases momentarily of the engine and generator is reduced, the until the excitation system compensates to number of poles must be increased to return the voltage to its previous level. Since achieve 60 hertz. An EDG operating at 450 the load was increased with relatively RPM would require twice as many poles as constant voltage, the output current (and a unit operating at 900 RPM. Once hence, output power) of the generator must designed, the number of poles in an increase. This power demand is also felt on alternating current (AC) machine is fixed. the diesel engine which is trying to maintain the speed constant as the load from the generator increases.
=
120 9.4 Generator Control F is frequency in hertz (Hz)
P is the number of poles Alternating current (AC) generators of this N is the generator speed (RPM) type are driven by high horsepower diesel engines in nuclear applications, which As a short cut, for 60 Hz power generators, require stable and accurate means of N = 7,200 / P and, therefore, P = 7,200 / N control. Licensee Technical Specifications stipulate voltage and frequency limits during All generators have an even number of fast start, sequential loading, load rejection poles. An engine running at 900 rpm would testing, and normal operation. For a generic have 8 poles on its generator to produce 60 EDG, typical voltage and frequency limits Hz power. are 4160 + 420 volts (10%) and 60 + 1.2 Hz (2%), respectively.
9.3.2 Mechanical Loading Rev 3/16 9-4 of 33 USNRC HRTD
Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation 9.4.1 Frequency Control The nuclear plant FSAR specifies governor and voltage regulator compatibility testing Control of the generators frequency is (bench-marking) to demonstrate acceptable accomplished by the engine governor performance of both subsystems during sensing changes in engine speed from the EDG major load changes. Periodic EDG set point. The governor adjusts fuel flow to testing is incorporated into Technical the engine as required to keep the engine Specification surveillance to demonstrate operating at its intended speed. acceptable performance.
9.4.2 Voltage Control 9.4.3 Generator Rating Large AC generators commonly use a For discussion involving the EDG electrical combination Exciter-Voltage Regulation characteristics and electrical load, several system to maintain generator field current parameters associated with AC machines under varying electrical loads. The basic are defined. These include real power, voltage regulation system is designed to reactive power, apparent power, and power automatically regulate generator output factor.
terminal voltage within close tolerances from the specified value. The generic regulation Generators are normally rated by the KVA system is a 'closed loop' feedback system in theyre designed to produce, at a power which generator output voltage is factor of 0.8, in accordance with the NEMA automatically compared to a reference 1 Standard, Motors and Generators.
voltage. The error signal is used to change Some are rated in KW, in which case the generator excitation, discussed in 9.5. power factor must also be stated.
The generic EDG must be capable of 9.4.3.1 Real Power KW accepting emergency loads assigned by the auto-sequencing system while maintaining Real power in an alternating current (AC) voltage above a minimum of 75% of the system refers to the true electrical power nominal voltage (25% voltage dip). The that is converted to mechanical energy such EDG Voltage Regulator is essential to as a motor driving a pump. This power is correct the EDG output voltage. It is measured in watts or kilowatts (kW), or in powered from a class 1E power supply and mega-watts (MW) for large electrical loads is tested for satisfactory operation during or systems.
periodic EDG surveillance testing. If the Voltage Regulator is inoperable, the EDG is 9.4.3.2 Reactive Power KVAR inoperable.
Most electrical systems that undergo The generator exciter varies the strength of changes of voltage or current have either generator rotor field current automatically, capacitance or inductance. In a DC system, during either auto-sequence loading or these are only important during a significant manual loading from the control room or change of state. Because AC power is EDG local control panel. continually changing (voltage is sinusoidal),
Rev 3/16 9-5 of 33 USNRC HRTD
Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation the inductance and/or capacitance become For example, say a generic plant FSAR more important. Electrical power is needed specifies the standby EDG be rated at 4000 in AC machinery (motors, generators, and KW. To provide this real power input and transformers) to create the magnetic fields generate enough 'magnetizing' power for and voltage induction that enable these the generator rotor and stator, the generator machines to operate. This magnetizing must be rated at an apparent power of 5000 power, which is stored energy in the AC KVA, derived as follows:
system, is reactive power.
4000 Because reactive power does no work it is = = = 5000 0.8 sometimes called imaginary power. The current component of this power acts at 90 9.4.3.5 Relationship between KW, KVA, degrees from the real power (KW). Reactive and Power Factor power is measured in volt-amperes-reactive or VARs. For high voltage systems, reactive The relationships between KW, KVA, and power may be measured in kilo-VARs or Power Factor is the same as that for any mega-VARs (KVAR or MVAR). right triangle - The Pythagorean theorem.
The triangular relationship between the 9.4.3.3 Apparent Power KVA three factors can be solved if any two are known. So knowing KW or KVA and the As its name implies, apparent power is the Power Factor, one can get the other and power that is apparently required to be from that the third.
input or drawn from the AC system.
Apparent power is volt-amperes, kilovolt- KW is usually measured and displayed on a amperes, or megavolt-amperes (VA, KVA or meter. KVA can be obtained by taking the MVA). It is determined by multiplying the voltage (average for the 3 phases) and the voltage times the amperes times the square current (average for the 3 phases),
root of three for 3-phase electrical systems multiplying them together, and then and then dividing the result by 1000 to get multiplying that result by the square root of KVA, or by 1,000,000 for MVA. 3 (1.73) for a three-phase power system.
9.4.3.4 Power Factor PF The following core principles apply to all engine-driven generator systems.
Power factor is the ratio of Real Power (KW) to Apparent Power (KVA). It is a measure of
- THE GOVERNOR ON THE ENGINE the utilization of the input power of a system CONTROLS OR RESPONDS TO THE or equipment. A typical power factor for the KW LOADING ON THE GENERATOR.
rating of a generator, per NEMA 1, is 0.8 (80%). Power factor can be calculated by
- THE VOLTAGE REGULATOR AND the following formula:
EXCITER SYSTEM CONTROLS OR RESPOND TO KVA AND, THEREBY,
() =
THE KVAR LOADING.
Rev 3/16 9-6 of 33 USNRC HRTD
Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation These two control devices are quite Most modern excitation systems involve independent otherwise. Voltage dip is a electronics in voltage sensing - regulation, result of generator, exciter, and voltage and are relatively fast and accurate. Many regulator characteristics and response. The of these systems consist of an excitation frequency dip is a result of engine and generator driven by the engine, either governor characteristics and response. directly or by a system of pulleys and belts.
The excitation generator supplies the power KW loading on the generator is converted to and a voltage regulator controls the field.
the engine Horsepower required by using the following relationship: Many commercial systems use what is termed a brushless exciter. They have an alternator / generator mounted on the end of
() =
0.746 the generator shaft, driven along with the main generator. A permanent magnet field 9.5 Excitation and Voltage Regulation excites the alternator as it rotates. The alternator output goes through a diode Every power generation system requires bridge rectifier which converts it into DC that some means of controlling the voltage is fed into the generator field through wires and / or current produced by the generator. running along the shaft between the The output of a generator is normally alternators output section and the main controlled by manipulating the current in the generator field. This type system has no field of the generator, the speed being brushes, thus the term brushless exciter. A constant for a set frequency. Various control winding is included in the permanent excitation systems are possible and all magnet field to control the output of the usually include some means of sensing and exciter alternator and, thereby, the main controlling the generator output voltage. generators field. There are a few of these systems at nuclear stations, though they 9.5.1 Types of Excitation Systems lack the power capability of fully electronic systems (covered in 9.5.2). Response is Excitation systems range from the simplest slow because the system has to operate to rather complex designs. The simplest through two sets of magnetics: the exciter would consist of a battery to supply alternator, and the main generator field.
excitation power to the generator field along with a rheostat to control the amount of Most excitation systems used at nuclear excitation current, that being managed power stations are either Series Boost (SB) manually by an operator. This system is or the fully electronic Static Exciter Voltage generally not acceptable; therefore, some Regulator (SEVR) system. These are more means of automatic control is desired. It similar than different, and will be explained may consist of automating the rheostat such in our examination of Figure 9-23.
as the old Westinghouse rocking arm Silver Stat design. It was slow to react and had The exciter output DC voltage is fed through droop in the voltage regulation, but it worked brushes, to slip rings, and on into the main and was very simple. generator field windings.
Rev 3/16 9-7 of 33 USNRC HRTD
Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation 9.5.2 Explanation of Elements of the connection, there is another PT connection Electronic (Static) Type Excitation for sensing the generator output. This is fed System into the Voltage Regulator section as a sense of the generator output voltage.
Figure 9-23 shows the elements in the typical modern electronic excitation system. Beyond the sensing PT connection, there is While there are differences between the another Current Transformer (CT) for Series Boost (SB) and the Static Exciter- sensing the amount of load (via the current, Voltage Regulator (SEVR) systems, the assuming the voltage is constant). This is basic elements are the same in both used to adjust the voltage regulators output systems. Their differences will be explained when the unit is being paralleled to the grid as each section of the systems is explained. (infinite bus). In order to control the reactive power component of the load current when Figure 9-23 shows the generator on the far paralleled, the voltage regulator has to left with its load lines going to the right, until receive some sense of the load on the they finally pass through the generator system. This is equivalent to the droop output circuit breaker and on to the loads. function in the governing system. This As each load line exits the generator, it goes principle is variously called 'current through the primary side of a Power Current compensation,' 'load compensation,'
Transformer (PCT). The secondary side of paralleling load sensing or voltage droop.
these transformers feed power for excitation This function is usually switched out into the exciter package. Most modern automatically when the EDG is aligned tor exciters have Power Current Transformers 'emergency mode.' It is required only in to supply some of the excitation current parallel operation. NOTE: If this were left required by the generator field. active in emergency mode there would be a more drop in voltage as load is put on the Properly sized PCTs supply power for unit. This is not desirable in the isolated excitation in direct proportion to the load on 'emergency mode'. The switch controlling the generator. The generator exciter system this function is often labeled as follows:
typically requires about 1 to 2 percent of the 'Unit' (without current compensation), and generators output power. 'Parallel' (with current compensation).
As the load lines continue beyond the PCT The voltage regulator receives a reference connections, there are connections to a signal from the operator or system which Power Potential Transformer (PPT). Since tells it the voltage to maintain. This voltage there is no generator output current when may be modified by the current there is no load on the generator, there compensation circuit if the exciter is set up must be some other source of power for for parallel operation. In this case, the generating excitation when there is no load. reference input of the regulator is compared The PPTs supply the power for generator to the sensed voltage input. The voltage excitation when the generator is not loaded. regulators output is changed to restore the generator output voltage back to the As the load lines continue beyond the PPT reference voltage input.
Rev 3/16 9-8 of 33 USNRC HRTD
Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation The output signal from the voltage regulator conditions can be monitored. The generator controls elements in the Power Section of field can accept only a direct current supply.
the exciter system. This section is attached The generator acts as a fixed resistance, to the Power Potential Transformers. changed only slightly by its temperature.
Properly sized PTs control the output Therefore, the field voltage and current are voltage without the need for current proportional. This helps in troubleshooting compensation. some types of exciter / generator problems such as detecting a short or ground in the Generally there are two methods of generator field.
controlling the power section. In the case of the Series Boost (SB) exciter, the There are a number of differences in some transformers (PPTs) have a third (tertiary) excitation techniques that are worth winding. The PPTs are like normal mentioning. In the Westinghouse and GE transformers if the tertiary winding is not exciters, the power section output, and the turned on. However, if there is current in the PCT, contributions are rectified separately tertiary winding, it causes the transformers and joined together in the DC field circuit. In to become saturated to the point current the Basler and Portec systems, the power transfer from the primary of the PPT to the section and PCT contributions are joined on secondary is (can be) restricted; thus, the the AC side and rectified after being joined.
output current can be controlled. These transformers are therefore often called The Portec exciter is a 'shunt' type regulator.
'saturable reactors.' That is, all of the power generated in the exciter goes to the generator field circuit.
In the case of the SEVR type exciter, the There are SCRs that are connected in amount of current out of the power section parallel with the generator field that shunt is controlled by turning on or holding off the the proper amount of current around the firing of silicone controlled rectifiers (SCRs) field in order to control the current that goes or similar electronic elements. If a large through the field.
amount of power is required, the SCRs are fired early in the voltage cycle. If less power Other than these differences, most exciter is required, the SCRs are fired later. If no systems are more alike than different, and power is required (in the voltage overshoot all contain the basic elements shown in situation), the SCRs are not turned on. This Figure 9-24.
is the means of controlling the no-load excitation power in such a system. There is generally some feedback from the exciter output section to the voltage The power from the power section is sent regulator section so that the voltage over to the rectifier section where it is joined regulator can anticipate what it needs to do with the power from the PCTs. This is next or if it needs to help stabilize the rectified (converted) from AC to DC (direct regulator. In most regulation systems current) and sent on to the generator field as without feedback, the systems tend to turn shown. There is usually a voltage meter and all the way 'on' or all the way 'off' and this is an amperage meter in the field circuit so field not desirable. Therefore, feedback helps Rev 3/16 9-9 of 33 USNRC HRTD
Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation the regulator know how much excitation is transient and sub-transient reactances).
being put out so it can gauge that against However, the rate of recovery and a few the error and anticipate that the problem is percent of the voltage dip can be influenced nearly resolved. by the response and the field forcing capability of the excitation system.
These exciters are self-sustaining. That is, they take the power they need from the 9.5.3 Brushless Excitation Systems generator output. They require no external power for normal operation. But that also A brushless excitation system has the means that they will probably not start advantage of not having brushes to transmit themselves up when required. Therefore, a the current to the generator field through slip means of starting the excitation process has rings and brushes. Figure 9-24 shows a to be provided. This is done with the 'Field schematic diagram of the typical brushless Flashing' circuit, also shown on Figure 9-23. exciter system.
That circuit consists of a battery (maybe the station batteries) and a switch. The switch The brushless exciter system consists of an is closed when the system is signaled to exciter generator mounted on the generator start up (usually upon the engine achieving shaft, usually outboard of the generator a certain speed), and the switch is opened bearing. A diode plate is attached to one when the voltage regulator senses that the side of the exciter rotor. It serves to change generator voltage has built up to a certain the AC exciter output to DC. The output is value (usually about 65 to 75% of rated fed through a hole in the exciter shaft to the voltage). Inside the rectifier section this main generator. It exits after passing circuit usually includes a diode to block through the rear of the generator housing reverse current from the exciter back into and connects to the rotating main generator the batteries. Usually, some resistors are field windings. There are no slip rings or included to limit the current available from brushes.
the batteries because only a small current is needed into the generator field to get The field of the exciter generator is generation started. stationary. Its housing is mounted onto the rear end of the main generator.
Many excitation systems have the capability of putting out an immense amount of power Another alternator may be mounted if needed. To sustain rated load may not outboard of the exciter generator. This unit take much power, but to start a large motor would have a permanent magnet rotor, load may tax the generator and exciter usually mounted on the back of the system. Most of the modern exciters are generator shaft. The stator winding of this capable of putting out about 3 to 5 times alternator puts out an AC current / voltage to (300 to 500%) of the generators normal full a voltage regulator. The voltage regulator load excitation current. This is called field turns that AC input into a DC output which is forcing. The voltage dip caused by starting 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 Rev 3/16 9-10 of 33 USNRC HRTD
Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation 9.5.4 Digital Voltage Regulation When the EDG is started for surveillance testing, the output breaker is closed The only section of the excitation system manually by the plant operators, after that lends itself to digital control is the bringing the unit into sync with the offsite voltage regulator. While there are some power system. The operator must closely advantages to digitizing the voltage match the frequency of the generator to that regulator for the purpose of better voltage of the off-site power, with the generator stability during isolated operation (powering frequency slightly higher than the offsite the emergency bus), and of controlling (grid) frequency, so the unit will assume reactive power (power factor) a little tighter some load when the output breaker is when in parallel with the offsite power closed. The generator voltage must also be system, the effect would be small when closely matched but slightly higher, so the compared to existing analog systems. The KVAR loading will be outbound rather than voltage regulator operates basically from inbound. A synchroscope is typically used analog inputs and in the case of the Basler to get the EDG in sync with the grid and the Series Boost systems (used at most plants), system may also include sync-check relays the output is also analog, so there is little to protect the generator output breaker from advantage to inserting digital components in being closed with the unit not synchronized.
between the input and output. Regarding the voltage dip caused by starting large 9.7 Generator Differential Protection motors, that is primarily influenced by the characteristics of the generator and would The generator requires minimal monitoring not be appreciably influenced by having a of its normal operation. Frequency, output faster, more complicated voltage regulator. voltage and currents (each phase), field The more components in a system, the less voltage and current are essential. Stator and reliable the system may become. bearing temperatures are recommended, plus bearing oil level (before-after each run).
9.6 Generator Output Breaker However, there is one item required by NRC regulations that must not only be monitored Figure 9-23 also shows the main generator but, if such a fault is detected, the engine output beaker at the right end of the and generator immediately shut down.
diagram. The generator output breaker connects the generator to its Class 1E A system is set up to monitor the currents emergency bus. When an emergency start into the generator on the neutral side and signal is received, the EDG starts and also the currents in each phase on the comes up to rated speed and voltage. The output (load) side of the generator. If one of output circuit breaker is closed the phases develops an electrical fault, automatically. An auxiliary contact on the whether phase to ground or phase-to-circuit breaker provides a signal to the load phase, the input and output currents would sequencer to connect the emergency load in not match. That indicates a problem exists proper sequence within approximately 30 within the generator because In normal seconds. operation the currents in each phase are balanced.
Rev 3/16 9-11 of 33 USNRC HRTD
Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation To determine that the current going to the mechanical damage to the generator and / or loads on the generator and the currents the engine in the event that the generator within the generator loop are the same, as components expand or separate and cause they normally would be, a detection system rubbing or seizure of the generator rotor is set up as shown in Figure 9-25. Note a within the stator.
set of current transformers (CTs) is installed in the lines into the generator on the neutral It is important to note that each phase of the side of the generator windings. There is a generator is monitored by an 87 relay and like set of CTs installed beyond the units that operation of any of these relays will circuit breaker which monitor the currents cause the 86 device to trip, which shuts going to the generator load. The diagram down the generator and engine. Inclusion of shows inter-connection of these CTs for just the Generator Differential protection is the C phase of this system. However, this required by the licensees FSAR and is done for each phase, A, B and C. technical specifications, in accordance with NRC Reg Guide 1.9 and IEEE 387. This is The circuit includes a device called the 87 one of the two conditions that will shut down trip relay (differential relay). If the currents the EDG under all circumstances. The other become unbalanced due to abnormal single mandatory shutdown is for engine over-phase loading, or mismatched because of speed. Other shutdowns may be included some generator output current is shorted to only if provided with coincident logic, means ground or to another phase, it would cause to test each sensor, and individual alarm for a current through the coil of the 87 relay. If each sensor.
the current is sustained at a level above the 87 relay trip set point, the relay will actuate 9.7.1 Generator Monitoring and its contact. When that occurs it puts a Protection current into the latch coil on the 86 trip device shown in Figure 9-25. When the 86 A local EDG control panel for controlling and trip device coil is energized, the latch monitoring is usually found in nuclear plants.
holding the 86 trip device handle in the Various meters and relays and trip
'Reset' position is pulled by solenoid action, mechanisms are mounted on this a panel to moving the 86 Trip device to the Tripped give the operator a means of monitoring the position. Contacts in the 86 trip device close generators operation and performance.
circuits in the engine and generator control systems to shut down the engine and shut Typically the following meters would be off the generator excitation system. The 86 present:
trip device must be manually reset in order to restore the EDG unit to operating status.
- Phase Current and Phase Volt meter with phase selection switch.
Prompt shutdown of the generator will
- Field Volt and Amp meters minimize damage to it and could prevent
- KW and KVA meters generator destruction from prolonged (optionally, a Power Factor Meter) operation with high fault current. Prompt
- Synchroscope and Synchronizing Lights shutdown of the engine may prevent
- Frequency Meter (Engine Tachometer)
Rev 3/16 9-12 of 33 USNRC HRTD
Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation Typically, the relays and trips would include: 9-8 Generator Connection and Alignment to Engine (Fig. 9-26)
- Differential Fault and 86 trip switch
- Loss of Field and Field Ground One of the exercises that students will
- Over-current and Over Voltage participate in while in this school, is to check
- Under-Voltage for crankshaft deflection in the last crank
- Reverse Power throw of the engine. Figure 9-26 shows how
- Under Frequency a dial indicator is mounted between the cheeks of the crankshaft at the throws to 9.7.2 Generator Stresses determine that the crankshaft is straight.
This procedure for crank alignment is The generator is subject to stresses due to commonly known as crank web deflection or normal currents running through the crank alignment.
windings. To avoid undue stresses, there are some operations and conditions which The crank pin journal can be considered to should be guarded against if at all possible. be of fixed length. If the crankshaft is not These add extra stress to the generator and straight through the throw, as the crankshaft may cause generator damage or failure. rotates, the crankshaft webs go through a These fall into two basic categories, with bending to accommodate the bend in the some items appearing in both categories. crank. This causes the crank cheeks to go from being more open to being more closed, 9.7.2.1 Electrical Stresses: as shown in the lower left portion of the (primarily for the Stator) diagram. The crankshaft is attempting to bend in that section shown with as 'AREA OF
- High Sustained Currents HIGHEST STRESS'. If the crank continued to
- High Motor Starting Loading currents bend or deflect during each rotation, it could
- Paralleling Out of Phase induce high enough stress to fail over time.
- High operating temperatures Therefore, crank alignment is critical to prevent failure from metal fatigue, especially 9.7.2.2 Mechanical Stresses: in the last throw.
(primarily for the Rotor)
The lower right diagram shows a situation
- Dynamic Loading (engine problems) where the some of the weight of the
- Suddenly applied Fault Current Loading generator shaft and rotor are being carried
- Centrifugal rotational loading by the last bearing in the engine. This
- High operating temperatures bearing is next to the last throw on the crank.
- Paralleling out of phase The upper right lower diagram shows the
- Armature not concentric with stator bending of the last throw as a result of the
- Vibration (out of balance) generator rotor weight. To alleviate this
- Out of Alignment. problem, usually the generator rear bearing is raised so the generator-engine shaft is The above mechanical stresses may involve straight through the last bearing, which the engine as well as the generator. straightens the last crank throw. Of course Rev 3/16 9-13 of 33 USNRC HRTD
Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation this diagram visually exaggerates the actual situation. It usually is necessary to raise the generator bearing by just a few thousandths of an inch.
It may also be necessary to shift the generator bearing sideways in order to have the crankshaft straight in the horizontal plane.
Once the generator bearing and the rotor are properly set up to relieve the crank stress / strain, then the generator stator must be positioned with respect to the rotor position, to equalize the air gap between the stator and the rotor. Magnetic fields in the generator are greatly affected by air gaps and if the rotor is not concentric within the stator those strong magnetic fields and their forces can become asymmetric, resulting in pronounced vibration when rotating under load. This can be prevented by shifting the generator on jack screws and then installing shims under the generator feet before tightening the generator to the foundation or the skid.
The details of how to perform a crankshaft alignment will be covered in the walkaround exercise.
Rev 3/16 9-14 of 33 USNRC HRTD
Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation Figure 9-1 Conductor in a Magnetic Field Rev 3/16 9-15 of 33 USNRC HRTD
Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation Figure 9-2 Simple Generator (Alternating Current Rev 3/16 9-16 of 33 USNRC HRTD
Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation Figure 9-3 Single Phase Sine Wave Rev 3/16 9-17 of 33 USNRC HRTD
Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation Figure 9-4 Three-Phase Generation Rev 3/16 9-18 of 33 USNRC HRTD
Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation Figure 9-5 Cutaway of Generator - Identification of Parts Rev 3/16 9-19 of 33 USNRC HRTD
Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation Figure 9-6 OP Engine Generator Assembly (under test)
Rev 3/16 9-20 of 33 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 3/16 9-21 of 33 USNRC HRTD
Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation Figure 9-9 Rotor Spider (hub)
Figure 9-10 Field Pole Wedges Rev 3/16 9-22 of 33 USNRC HRTD
Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation Figure 9-11 Field Pole Blocking Figure 9-12 Field Pole with Amortiser Bars Rev 3/16 9-23 of 33 USNRC HRTD
Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation Figure 9-13 Field Wiring to Slip Rings Figure 9-14 Slip Ring Assembly Rev 3/16 9-24 of 33 USNRC HRTD
Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation Figure 9-15 Winding Coil Form Figure 9-15 Windings Installed in Stator Slots Rev 3/16 9-25 of 33 USNRC HRTD
Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation Figure 9-17 Coil Tying and Bracing Figure 9-18 Lead Cable Terminations Rev 3/16 9-26 of 33 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 assembly)
Rev 3/16 9-27 of 33 USNRC HRTD
Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation Figure 9-21 Bearing & Housing Assembled in Rotor Shaft Figure 9-22 Brush Rigging and Slip Rings Rev 3/16 9-28 of 33 USNRC HRTD
Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation Figure 9-23 Exciter System Block Diagram (SEVR/SB Types)
Rev 3/16 9-29 of 33 USNRC HRTD
Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation Figure 9-24 Brushless Exciter System Diagram Rev 3/16 9-30 of 33 USNRC HRTD
Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation Figure 9-25 Generator Differential Fault Rev 3/16 9-31 of 33 USNRC HRTD
Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation Figure 9-26 Engine-Generator Alignment Rev 3/16 9-32 of 33 USNRC HRTD
Emergency Diesel Generator The Generator, Exciter, and Voltage Regulation WALKAROUND SESSION 9
- Show how the generator stator is mounted within the frame with its power 9.0 GENERATOR, EXCITER, AND leads brought out.
VOLTAGE REGULATION
- Show how the generator field pole pieces are mounted on the rotor and its Purpose mounting to the generator power input shaft.
This session is to complement Chapter 9.
- Identify and discuss the generator stator Learning Objectives to rotor air gap.
- Discuss air gap functions and Upon completion of this lesson you will:
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 generator rotor within generator stator
- Show the brush holders, spring clips, and brushes and discuss their
- 3. Understand the need for proper alignment, curvature, condition, and connection from external exciter through how to measure brush pressure.
generator brushes and slip rings to
- Show where the input from the external generator field windings exciter is connected into the brush holders.
- 4. Understand the need for insulation of generator end bearing from ground
- Show the generator outboard (inboard) bearings and the need for alignment,
- 5. Understand the need for periodic lubrication, and isolation from ground.
inspections and tests of slip rings,
- Show the generator shaft input coupling brushes, windings, bearing, and to the diesel engine. Discuss the need alignment for proper alignment and mounting to the engine.
9.1 The Generator
- Discuss periodic inspections and tests of the generator including slip rings, Using the EDG generator on the test floor, brushes, windings, bearings, and the instructor will conduct the following alignment.
training:
- Discuss potential problem areas,
- Identify the generator frame including its indications of problems, and tests.
mounting / provisions for an on-engine mounting.
Rev 3/16 9-33 of 33 USNRC HRTD