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Flashcards in Main Generator Deck (49)

What is the basic design of the main generator?

  • Hydrogen Inner Cooled
  • 3 phase
  • 60 Hz
  • 1800 RPM
  • 4-Pole


What is the Stator?

Armature windings in which output voltage is developed.

Rated at 25,062 amps and 22,000 volts.


What is the Rotor?

Contains the field windings for voltage generation.

Rated at 7,432 amps and 500 volts.



What is the Exciter?

A three-phase AC generator whose output (500 VAC at 400 Hz) is rectified (converted to DC) by rotating rectifier wheels on the generator shaft.

Provides field excitation current to the rotor (field windings) to produce a magnetic field.


What is the Permanent Magnet Generator?

A series of permanent magnets mounted on rotating shaft.

Conductors wound on a stationary segment. As shaft rotates AC voltage is produced. This AC powers the Voltage Regulator and provides power to be amplified/rectified for generator field excitation.


What are the Rectifier Wheels?

Located on the shaft.

Rectify exciter AC current to DC current for Main Gen field windings (rotor).

Contain diodes, resistors, capacitors, fuses, and blown fuse indicators.


What is the field breaker?

Connects PMG output to exciter stator windings (via voltage regulator) and to Power Supply Unit for Voltage Regulator.

Breaker handswitch on C-01.

Control power from D-11-2.


What is the voltage regulator?

The voltage regulator controls the field strength of the exciter to maintain the generator's voltage output constant at its setpoint, if in automatic.

  • Inputs: Exciter current and Main Gen terminal voltage.
  • Outputs: Signals to Trinistat Power Unit to control amount of excitation.

Trinistat Firing Circuits send firing pulses to the Trinistat Power Amplifier, which varies the AC Exciter stator field current. Cabinet on South Mezzanine.

Handswitches on C-01


What is the Trinistat Power Unit?

Consists of Redundant Firing Circuits, Power Amplifiers, and 24V power supplies. Provides adjustable DC voltage to exciter field winding.

Control via input from DC Adjuster and AC Adjuster.

Range of adjustment is 0-330 of sine wave input.


What is the DC adjuster?

  • Provides direct vice automatic control of field excitation.
  • Control is by changing the reference signal. Senses exciter field current (DC) and compares to adjustable reference signal (controlled via handswitch on C-01).
  • Resultant error is applied to Trinistat circuits (via a motor operated transformer, i.e., potentiometer) to maintain exciter field current at a set reference value.
  • Terminal voltage will vary with turbine speed, load, and grid conditions.
  • Powered from D-11-2. Fails as is - exciter will still provide field to generator, but voltage cannot be adjusted.


What is the AC adjuster?

Provides automatic control of field excitation.

Senses Main Generator terminal voltage (AC) and compares it to an adjustable reference signal. Error signal sent to Trinistat Power Unit to control excitation such that a constant terminal voltage is maintained.

Note: There is some voltage droop built into the circuit. Prevents the voltage regulator from trying to maintain the entire grid voltage at a constant value.

When AC Regulator is in service, the DC Adjuster has no control over voltage, but its signal is still present if the auto circuit fails.


What is the Regulator Balance Meter?

Indicates the difference between DC and AC adjuster outputs.

Automatically maintained near zero (DC Adjuster tracks AC Adjuster output) via regulator "following" circuits for a bumpless transfer between AUTO and MANUAL control.


How is the Exciter cooled?

Utilizes a shaft mounted air blower and 2 coolers with each cooler having 2 cooling coils. Cooled by non-critical Service Water.

TCV-0852 provides automatic temperature control of the air space inside the exciter housing to 108º - 122ºF.

Temperature indication is via two RTDs (one at each end of the exciter). These two temperatures are balanced to within 3ºC (2º preferred) of each other.

10 seconds of no cooling results in Exciter Damage.


How is the Exciter ventilated?

An exhaust fan on top of the exciter housing with plug providing power is located on east side of Exciter housing.

Panel showing indicating lights for the fan is located at the Turbine end of the Main Generator on box JL-145. Local control switch, HAND/OFF/AUTO, inside JL-145.

  • Auto: Fan shuts off when turbine speed is above 600 rpm. During a turbine shutdown, fan comes on when shaft speed reaches 600 rpm.


What is the cycle of the hydrogen dryer?

One tower is in service while other is in regeneration.

The in-service tower removes moisture for 8 hours.

The tower in regeneration heats (driving out moisture) for 4 hours, and cools down for 4 hours.


What provides the motive force for hydrogen circulation through the driver?

The generator blower and is assisted by a small muffin fan mounted on bottom of tower.

The fan allows the Hydrogen Dryer to be in service when the Main Generator is on the turning gear and pressurized.


What are the hydrogen dryer performance indicators?

HI-2244, Generator Hydrogen Dewpoint indicator should be reading < 4.5º C and liquid detectors should indicate no condensation.


When regenerating, what is the target temperature of the reactivator *hydrogen" gas.

The reactivator gas flow is adjusted to achieve a target temperature of 200 - 230º F at the end of the heating cycle.


What results from the failure of the hydrogen dryer moisture separator drain trap?

Potential source of hydrogen escaping to the turbine bldg.

A malfunctioning moisture separator, or loss of the reactivator gas could mean inadequate moisture removal from the generator hydrogen when the tower goes back into service.


What could be the result of a hydrogen dryer failure?

A loss or malfunction of the Hydrogen Dryer would result in reduced hydrogen purity and lower heat removal capability and could lead to overheating of the Main Generator.


What could be the issue if hydrogen purity high/low alarm comes in?

IF Purity meter indication is greater than 100%, THEN the Meter pointer could be stuck, or Purity Blower has stopped.

IF Purity meter indication is less than or equal to 90%, THEN Purge


What is the effect of inductive loads on current, voltage and the plant?

Current will lag voltage.

Creates high voltage on grid.

Plant takes in VARS by reducing excitation - lowering voltage.


What is the effect of capacitive loads on current, voltage and the plant?

Current will lead voltage.

Creates low voltage on grid.

Plant puts VARS out by increasing excitation - increasing voltage.


What is the limiting component for VARS out?

The rotor. Rotor current will be higher when supplying VARs out because excitation has been raised.


What is the limiting component for VARS in?

The stator core due to an uneven magnetic flux distribution.

The rotor has minimal current flow due to being underexcited.


What is the 386P relay?

Primary Turbine Generator Lockout Relay. 86 is a lockout relay.


What happens when 386P trips?

  1. Turbine Trip
  2. Fast transfer of electrical power to off-site power
  3. Generator Breakers Trip
  4. Backup Transfer Trip
  5. Exciter trip (field breaker opens)


What actuates the 386P relay?

Generator Differential (387)

Generator Neutral Overvoltage (359N)

Volts per Hertz (395)

Generator Loss of Field (340)

FF-K3 (Field Forcing)

Generator Overexcitation (376-K3)

Station Power 1-2 Breaker 152-402 failure

Generator Breaker 25F7 OR 25H9 Failure (486BF)

Breaker 25 F7 CT failure (486CF)

Switchyard to Plant Primary Differential Auxiliary Relay (494P)

Switchyard to Plant Backup Phase & Ground Auxiliary Relay (494B)

Other protective relays associated with Station Power Transformers and pilot wire


What is the 386B relay?

Backup Turbine Generator Lockout Relay. 86 is a lockout relay.


What actuates the 386B relay?

Actuated by Main and Station Transformer protective relays and Switchyard protective relays:

Main Transformer Differential (487U) or Overcurrent (451MN)2)

Generator Breaker Failure (486BF)

Breaker 25 F7 CT failure (486CF)

Exciter trip (field breaker opens)


What happens when 386B trips?

  1. Turbine Trip
  2. Fast transfer of electrical power to off-site power
  3. Generator Breakers Trip
  4. Backup Transfer Trip
  5. Exciter trip (field breaker opens)


What is the 386C relay?

Indirect also known as Coastdown Relay.

Coastdown uses kinetic energy of turbine generator to maintain primary coolant flow for 10 seconds after a plant trip with a loss of all offsite power.

Provides coolant flow until initial high decay heat is past (2-3 seconds after a trip).


What actuates the 386C relay?

Actuated by various Main Generator protective relays

  1. Distance Backup (321)
  2. Reverse Power (332)
  3. Negative Sequence (346)
  4. Unit synch with turbine trip
  5. Generator offline and power >15% from NI-5 and NI-6


What happens when 386C trips?

  1. Turbine Trip
  2. Fast transfer of electrical power to off-site power
  3. Generator Breakers Trip
  4. Backup Transfer Trip
  5. Exciter trip

Portions delayed for 10 sec if loss of offsite power


What does the Generator Neutral Overvoltage (359N) relay do?

Limits current caused by ground faults and detects faults for 96% of the winding in the generator.

Trips the 386P lockout relay.


What is the Voltage Balance (360) relay?

Prevents unnecessary trips due to blown fuse on voltage sensing network.

It is looking at the input to the voltage regulator, and to the metering/relaying.

A blown fuse on this sensing network would be seen by 321 GEN DISTANCE BACKUP and the 340 LOSS OF FIELD relays and initiate a generator trip.

Relay 360 blocks 321 and 340 operation.


If the 360A side of the Votage Balance (360) relay is dropped, what does this mean?

The input to metering / relaying has blown a fuse.

360A is the device that blocks the 340 (Loss of Field) and 321 (Gen Distance Backup) relays from tripping the unit.


If the 360B side of the Voltage Balance (360) relay is dropped, what does this mean?

Normal power to regulator is lost.

The loss of sensing module will swap regulator input to relay side.

Control swaps to DC Adjuster (AC Adjuster trips).


What happens if BOTH sides (360A and B) of the Voltage Balance (360) relay have dropped?

Control swaps to DC Adjuster (AC Adjuster trips).


What is the Loss of Field Relay (340)?

Alarms and trips the plant (386P) on abnormally low or loss of field.

Loss of field caused by

  • Regulator malfunction
  • Short circuit in the Generator field
  • Accidental exciter field breaker trip


What is the Volts/Hertz relay (395)?

VH Limiter is Designed to limit terminal voltage to 23.1KV by limiting output of AC Adjuster.

Does not work with DC Adjuster.

  • The VHL will initiate an alarm that AC Adjuster is being limited.
  • If the VHL fails, this will pick up the volts/Hertz relay (395).
  • Assuming 60Hz: Alarm at 23.54 KV; 50 minutes at 23.6 KV; 30 seconds at 26.6 KV.
  • Trips the plant via 386P lockout relay on an inverse time basis


What is the Maximum Excitation Limiter (MXL)?

Limits maximum field current to 273 amps to protect exciter field windings. (114% of rated 236 amps).

Functional only with AC Adjuster.

Initiates alarm that Voltage Regulator output is being limited.

A timer (MXT) allows momentary exceeding of setpoint.


What is the Regulator Overexcitation Module (OXP)

If field current continues to rise above MXL set points, at 283 amps the OXP actuates.

  • It forces voltage regulator output to a preset value to lower excitation.
  • Persists for 33 seconds, trips AC Reg to DC Reg.
  • If Overexcitation persists, relay 376-K3 actuates after 3 seconds (36 seconds total time) and 386P relay actuates plant trip.
  • If the overexcitation is corrected at any time during the transient, OXP resets.


What is the Field Ground Detector Panel Rely (364)?

Applies a DC test voltage from the generator field to ground and monitors the resultant current flow every 24 hours. The current flow is inversely proportional to the resistance.

Can manually be initiated at Exciter control cabinet (South Mezz)

Generator Field Ground/Brush Failure Alarm

  • Alarm clears after 20 seconds if brush failure.
  • Alarm locks in if a ground is detected on field windings.
  • If exciter field current or voltage is fluctuating ± 10%, THEN remove unit from service.


What are the Voltage Reg Controls?

Voltage Reg Controls - no power if field breaker open

Voltage reg switch in OFF

  • DC control because there is no power to AC adjuster

Voltage reg switch in TEST

  • DC and AC adjusters have power. The AC adjuster won’t adjust terminal voltage but will affect Balance Meter

Voltage reg switch in ON

  • AC adjuster has power and control
  • Rise in machine reactive loading lowers terminal voltage.
  • Auto control raises excitation to bring back to setpoint


What are the impacts from high Hot/Cold Gas temperatures?

Hot/Cold Gas Impacts

  1. High cold gas temp can lead to high hot spot temps in stator and reduce life of gaskets and seals
  2. High hot gas temp can lead to high hot spot temps as well


What should the generator pick of up in terms of load during synching? What are the issues?

Synching Issues

  1. If generator has not picked up at least 20 MW, then use DEH system to adjust load as needed to prevent reverse power trip
  2. If “Turbine No Load Pretrip” comes in during Synching, Turbine will trip if persists for 60 seconds à if Generator didn’t pick up load, do it manually.


What happens on Loss of D-11-1?

Loss of D-11-1

  1. Lose 25F7/25H9 indication
  2. Lose some protective relays
  3. Can’t trip from C-01 or via 386P/C relays
  4. 386B will still trip the unit


What happens on Loss of D11-2?

Loss of D11-2

  • Lose Voltage reg controls, still have limiting functions
  • Lose MOD 389 indication
  • Field bkr will not auto open on turb trip