Systems Exam 4 - Main Generator Flashcards
(19 cards)
Primary Water Turbine Trips
Turbine Speed ≥1710 rpm AND one of the following:
→Stator Flow Lo <650 gpm (2/2)
→Rotor Flow Lo <580 gpm (2/2)
→Phase Flow Lo <23.3 gpm (2/2 on any phase)
→Inlet Hi Temp >140°F (2/2)
→Head Tank Level Lo <78% (2/3)
Primary Water Important Head Tank Levels
→Normal Band 90-94%
→Hi Level alarm 97%
→Lo Level alarm 85%
→Turbine Trip 78%
Makeup is from Demin Water
How should Primary Water temperature compare to H2 gas temp?
Gas temp should always be colder than Primary Water temp to prevent condensation on the stator bars/current carrying components within the generator.
→Primary Water temp should not fall below 77°F
→Primary Water temp maintained ≥9°F above H2 temp
→alarm when PW and H2 temps get within 5.4°F to alert that condensation is possible
→alarm when temps get within 1.8°F to manually unload and de-excite the generator
What components in the Main Generator are cooled by Primary Water?
→rotor
→stator
→terminal bushings
→phase connectors
In the event of a Turbine trip, how long is the Main Generator trip delayed? Why?
→delayed for 30 seconds
→extends RCP coast down time
→minimizes chance of Main Turbine overspeed
What are the Primary Water Coolers cooled by?
→TPCW
→6 total, 5 normally in service (20% capacity each)
How do we circulate hydrogen through the Main Generator?
Fans are provided on each end of the rotor to provide motive force for moving the hydrogen.
Generator Core Monitor Function
→detects aerosols in hydrogen that are indicative of insulation breakdown
→provides early detection of abnormal heating by
→monitoring H2 environment for thermally produced particulates
→interlocked with turbine speed
→provides local alarms only at >1690 rpm
When activated either generator lockout relay (86-1/1G or 86-2/1G) will initiate the following:
→Turbine trip
→opens Generator output breakers 8000/8010 (U1) or 8020/8030 (U2)
→Exciter Field Breaker opens
→trips non-safeguards normal supply breakers to 6.9 kV from UTs
→stops MT cooling
→stops UT cooling
→stops Isophase Bus Duct cooling
→enables transformer fire protection deluge valves
The following events will initiate a generator lockout: (17)
The following events will initiate a generator lockout:
1. Total loss of field
2. 345 KV system voltage less than 85% in conjunction with a total loss of field
3. Field ground
4. Pilot exciter short
5. Generator phase differential
6. Stator ground
7. Distance protection/ Main transformer ground: the distance protection or a ground on either main transformer has a 1.4 second time delay; trips make up step 2 in the ground fault protection scheme
8. Main transformer sudden pressure
9. Unit auxiliary transformer phase differential
10. Primary trip signal for generator output breaker 8000 or 8010 (8020 or 8030)
11. Volts per hertz
12. Generator-transformer phase differential
13. Unit auxiliary transformer sudden pressure
14. Unit auxiliary transformer over current
15. Backup trip signal for generator output breaker 8000 or 8010 (8020 or 8030)
16. Generator negative sequence 12%
17. Generator terminal box water level high or PW Head Tank Level <78%; coincident with a reverse power
AC Seal Oil Pumps
→1 in service, 1 in standby
→79 gpm PDP
→Pump A powered from uB3-1
→Pump B powered from uB4-1
→normal seal oil pressure maintained at 12-15 psid
→standby pump auto starts on low system pressure of 10 psid
DC Seal Oil Pump
→powered by uD2-2
→auto starts on low system pressure of 5 psid
→if no seal oil pumps can be started within an hour, shut down the unit and reduce H2 pressure to 2 psig (per ABN-402)
→recirc solenoid valves auto close on loss of seal oil pressure (<1.5 psid) to prevent H2 leakage
Exciter Enclosure Emergency Ventilation Purpose and Setpoint
→provided to permit continued operation in the event of a cooler failure
→louvers in the hot (lower compartment) and cold (exciter enclosure) air compartments are automatically opened by actuators admitting air from outside the exciter enclosure and discharging the hot air through openings below the coolers.
→Setpoint: 122°F
Excitation System
→consists of the pilot exciter, the voltage regulator, the main exciter and the rectifier wheels
→overall, we control DC amperage to the Main Rotor in order to control Main Generator output voltage
Sequence:
→AC - Pilot Exciter induces 3-phase AC in the Pilot Stator via permanent magnets mounted on the exciter shaft
→DC - Voltage Regulator (TVR) provides variable DC current to the stator of the main exciter by rectifying/controlling 3-phase AC current from the Pilot Exciter using thyristor sets
→AC - variable DC is then applied to the Main Exciter Stator, which generates 3-phase AC
→DC - 3-phase AC from the Main Exciter is then sent to the Rectifier Wheels, which convert it to DC
→AC - Rectifier Wheels then send DC to the Main Rotor to generate a magnetic field which in turn causes 3-phase AC to be generated in the Main Generator Stator
Note: Rectifier Wheels, housed in their own enclosure, act as fans to draw air in at the ends and expel the warmed air to a compartment beneath the exciter.
Excitation Current Flowpath
→Pilot Exciter: permanent magnets send AC to TVR
→TVR sends variable DC to Main Exciter stator
→Main Exciter Stator sends AC to Main Exciter Rotor
→Main Exciter Rotor sends AC to to Rectifier Wheels
→Rectifier Wheels send DC to Main Generator Rotor (a.k.a. field winding)
→Rotor rotation feeds AC to Main Generator Stator
→Main Generator Stator feeds AC to electrical system grid
During turbine roll up, depressing the HOLD Setpoint will…
…take speed to 500 rpm until operator resumes startup.
If Upper TSE Margin stops the Main Turbine prior to 1765 rpm, the Main Turbine should immediately be lowered to 500 rpm.
When Main Turbine speed increases above 1765 rpm, then…
…stop ALL running Auxiliary Oil Pumps and place in Auto.
What is generator “negative sequence” current?
→unbalanced generator loading = unequal phase currents
→can induce negative phase sequence current component
→can result in localized heating of the rotor
→Main Generator is capable of continuous operation with a negative sequence current of 5%
→alarm comes in at 4%
→Turbine Trip at 12%
How do we ensure that Instrument Air doesn’t mix with hydrogen inside the Main Generator?
When Instrument Air is not in use, it is physically disconnected, so there is no flow path into the generator.
