Main Generator Electrical
Draw and label a basic diagram of the static exciter excitation system and describe its two basic modes of voltage regulation
1) MANUAL MODE • Compares manual regulator Setpoint with field volts from the bridge output 2) AUTO MODE • Compares voltage setpoint to generator output voltage - UEL (under excitation limiter) limits the auto voltage regulator demand for under-excited current
What are all the locations from which the main generator breaker can be operated?
1. MCB 2. Local CP-168 3. Individual Pole Cabinets
How are the 25 kV iso-phase buses monitored for phase-to-ground faults?
46TG-1 relay gives operators ability to monitor negative sequence currents and will trip generator through generator backup protection lockout relay • Local meter next to 46TG-1 relay reads out in % I2 currents for generator (0-20% span) • Relay will trip generator at 9% I2 current with some time delay to prevent rotor overheating (if I2 % is higher, time delay is decreased)
How are the GSU transformers cooled and what are the automatic control features?
5 Banks of coolers on each GSU • Each cooler has: - 4 fans (all 4 operate together, all ON or all OFF) - One oil pump Each cooler has Test switch with three positions • TEST - Starts that cooler bank (fan and pump) • NORMAL - Aligns cooler for NORMAL control scheme - Normally 4 banks in normal • STANDBY - Aligns that cooler as standby cooler - Normally 1 in standby • Cooler 1 and 2 assigned as Group 1 • Cooler 3 and 4 assigned as Group 2 • Cooler 5 can be standby or serve as swing cooler for maintenance of a Group 1 or 2 cooler Manual transfer switch picks Group 1 or Group 2 to run in LEAD and run continuously • Opposite group is temperature controlled and cycles ON and OFF at winding temperature of 75ºC (167ºF) OR oil temperature >80ºC (176ºF) • Standby cooler (Bank 5) will start >100ºC winding temperature
What is the purpose of the Main Generator Backup Protection Lockout Relay?
86GB Isolates turbine generator during any of the following conditions with no blocking signal: • Reverse power to the generator (67TG-1) • Unbalanced current sharing between the 3 generator phases (negative sequence Overcurrent, 46TG-1) • Static exciter protective relay (94EX) • Generator stator ground (64TG-1) Uses Tripping System #2‼️
What is the purpose of the Main Generator Primary Protection Lockout Relay?
86GP Used to isolate turbine generator during any of the following conditions: • Differential current across generator (87GP) • Exciter PPT protective relay (SR745) • Loss of generator field (40TG-1) • Activation of the turbine generator distance relay (21G) • Trip of the steam turbine (32) Uses Tripping System #1‼️
What is the purpose of the GSU Primary and Backup Protection Relays And Main Generator/Transformer Overall Protection Lockout Relay?
86SP Differential phase current (87SP) 86SB 345 kV Neutral Ground Fault (51N) 86GT • Timed Differential current condition for specified time period within protective zone, which includes the generator, GSU, and isolated phase bus to UATs (87GT) • Differential current condition across the isolated phase bus alone (87B) • Isophase ground with generator breaker open (64B) Isolates: • Turbine generator (Breaker, Turbine, Exciter) • UAT supply breakers to Busses 1-6. • GSU Cooling Units • Breakers 11 and 12 • Generator Breaker and Isophase Bus duct cooling trip signals (86GT Only)
How does the main generator breaker cooling system automatically operate?
Compressed air surrounding the contacts is circulated through the tubes of the Single Pole Heat Exchangers and back to contact assemblies • Each Pole has: - Air to Air Hx - 6 Total Fans (3 per Bank) - 6 Total forced air circulators (3 per bank) FANS & CIRCULATORS • When Generator Breaker is closed, one bank will auto start to cool the pole units - Will trip when breaker opens) • Non-operating bank will start if associated operating bank component loses power OR air temp in pole unit cooler > 167ºF - No High temperature start for circulators • Continuous timer swaps banks every 12 hours when in AUTO
What is the purpose of the Main Generator shaft-grounding device?
Consists of 3 bristle brushes which have constant contact with turbine shaft adjacent to bearing #8. • Any voltage transmitted to shaft is conducted through the brushes to ground • Has lower resistance than rotor bearings to ensure current travels preferred path and does not damage bearings. • Monitored by Shaft Voltage Monitor
What is the function of the main generator exciter collector assemblies?
Excitation current is supplied to the generator through a pair of slip rings (collectors) and several sets of graphite brushes • Collector assembly is mounted on one end of the rotor shaft, outside of the generator outer end assembly • The two slip rings (1 ➕, 1➖) are connected through terminal studs on 2 connecting bars located in a hole bored through the center of the rotor shaft. • 2 connecting bars are connected to the field windings through terminal studs, thereby allowing conduction of the DC current from the collector to the windings.
What is the meaning of each of the indicating lights associated with the MCB control switch for the main generator breaker?
Green Light • Breaker Open Amber Light • Breaker Disagreement Red Light #1 & #2 • Trip Continuity on all 3 trip coils • Low pressure trip interlock satisfied, • Selector in Remote • All three poles closed Trip Position feeds Trip System #2‼️
How is the main generator protected from equipment damage resulting from full phase-to-ground faults?
If a phase to ground fault condition occurs close to generator electrical terminals and causes a severe phase-to-phase current, the negative sequence currents could destroy the rotor • Negative sequence current relay (46/TG-1) protects generator from severe unbalanced current conditions • Relay gives operators ability to monitor negative sequence currents and will trip generator through generator backup protection lockout relay.
How will a partial or complete loss of generator breaker pole unit cooling affect the main generator electrical system⁉️
Loss of generator breaker cooling initiates turbine setback to 55% power at 5%/min if: • Air Temp >185ºF at cooler inlet or outlet of any one pole OR • Cooling lost to any one pole unit - Defined as complete loss of fans OR the loss of both banks of circulators on any one pole unit (> 3 seconds)
What are the main generator breaker compressed air system low pressure closing and tripping interlocks⁉️
Low Air Pressure Disabling • Pole Pressure <470 psig - Closing circuit disabled - Rapid shutting of contacts no longer guaranteed • Pole Pressure <435 psig - Tripping circuit disabled‼️ - No longer have adequate arc quenching
What is the meaning of each of the indication lights associated with the MCB control Switches for the main generator voltage regulation system?
Low Limit Setting (22.5 kV) Normal Range (25 kV) - Normally lit High Limit Setting (27.5 kV)
What is required to Trip the Main Generator Breaker from the MCB?
MCB Tripping uses Trip System #2 Generator Breaker Switch will trip the Breaker provided: • 125 VDC Control Power Available • >435 psig air pressure available at all poles • Continuity through Trip Coils • Selector Switch in REMOTE
What conditions will automatically shift the Main Generator static exciter voltage regulator from automatic to manual?
Mode is normally controlled by operator by mains of regulator mode transfer switch • 2 Sets of PTs (potential transformers) provide transformed voltage upstream of MG Breaker for generator output voltage signal input to auto voltage control • If the generator output PT fuse has a fault condition (loss of PT feedback voltage), voltage control will auto swap from auto to manual - Will not automatically swap back Exciter software ramps the "non-active" regulator output such that it tracks the active regulator. - Allows for a smooth transfer between Auto and Manual regulators
What are the sources of motive power for operation of the main generator breaker, and how are they controlled?
Pole Unit contact assembly filled with compressed air at 490 psig • High pressure air surrounding movable contacts supplies motive force to operate the contacts. • When a ∆P is created across the piston, contact is moved to open or closed position - ∆P created by venting one of the sides to atmosphere while other side remains at normal pressure. - Air to OPEN, Air to CLOSE • High pressure air used for arc quenching as well
What is the purpose of the static exciter SCR bridge assemblies?
Rectifies main generator output into DC for the exciter voltage regulator
How does the main generator breaker compressed air system operate and what is its function?
Utilized to parallel and connect the main generator output through the GSUs and UATs with the power grid • Can isolate generator for maintenance or during fault conditions • Located such that opening of breaker does not prevent offsite power from powering important plant equipment. 2 two-stage compressors • LEAD (3400-3700 psig) • LAG (3300-3600 psig) - Alternated automatically every 12 hours - Air delivered to reducing cabinet that reduces air to 480-490 psig before going to pole cabinets • Each pole has - Single coiled pneumatic closing valve for closing - Dual Coil Tripping Valve (System #1 and #2)
What is required to close the Main Generator Breaker from the MCB?
• 125 VDC Control Power • Selector Switch in REMOTE • 345 kV Breaker 11 and 12 Breaker Failure Circuits Reset • GSU Primary and Backup Protection REset • Both UAT's Primary and Backup Protection Reset • 345 kV Bux 6 ∆I Protection Reset • Exciter ON • Anti-Pump Relay Not Energized • SYNCH Switch ON - Voltages Matched - SYNCH stop between -10º and 0º • Key interlock in place • Sufficient closing and tripping air pressure • Breaker Open on all 3 phases
What is the basic layout of the 25 kV system?
• Generator high voltage bushings, which are directly connected to the stator windings, act as transfer point between the generator and its distribution system. • The Isophase bus connects the high voltage bushings to the generator circuit breaker. This breaker is used to synchronize and connect the generator to the remainder of the distribution system
What are the conditions that will cause the main generator breaker to automatically trip open?
• Primary or Backup Protection on: - GSU - UAT - Generator • 345 kV Bus 3 System #1 or System #2 ∆I Protection • 345 kV Breaker 11 or 12 System #1 or 2 Breaker Failure • Generator/Transformer Overall Protection
What is the function of the static exciter and what is its relationship to the main generator?
• Produces magnetic field used to induce armature voltage in the Main Generator Stator • Static exciter uses main generator output to supply DC to the main generator rotor field • Voltage regulation system controls DC voltage applied to the main generator field
How will an electrical fault within the static exciter or main generator affect the electrical distribution system alignment?
• Trip Breakers 11 and 12 • Trip Generator Breaker • Trip Turbine • Trips 4.16 kV and 13.8 kV AC Bus UAT Supply Breakers • Interlock signals to the closing circuits of all breakers tripped by this relay to prevent re-closure until failure cleared and protective relay reset • Inputs to protective relay system #2 breaker failure circuits for 345 kV breakers 11 and 12