Digital EHC Flashcards
What are the power supplies for the EHC oil pumps? Control Logic power?
A - 600 VAC Bus “A” (R23-S001)
B - 600 VAC Bus “B” (R23-S002)
Logic - 125 VDC Cabinet “C” (R25-S003)
What is the normal operating pressure in the EHC system? When does the standby pump start? Low pressure alarm? Turbine Trip?
Normal - 1600 psig
Standby pump - 1500 psig (or trip of running pump if standby is in Pull to Autostart)
Alarm - 1300 psig
Turbine Trip - 1100 psig
What valves are closed when the “Close Valves” button is selected on the Mark VI HMI? When does this button not work?
Speed Reference signal is negative, forcing TSVs, TCVs and IVs closed. ISVs and BPVs remain open.
When generator is tied to the grid (either PCB closed), all speed reference push buttons are disabled
How many accumulators are in the EHC system? What specific components are the associated with and what purpose do they serve?
13 accumulators throughout the system
6 located at the Hydraulic Power Unit - dampen pressure pulses at pump discharge
3 located at the BPVs
- Stores enough force to operate BPVs for ~ 1 min in case all hydraulic press is lost & turbine trips.
(4) x1 for each TCV
Dampens pressure spikes at the valve, to prevent the TCVs from oscillating.
What is the relationship between Rx pressure, Throttle pressure and pressure setpoint?
Pressure set error = 30# @ 100%: (30 x % Pwr + pressure set = throttle pressure)
Can be used to determine throttle pressure if given pressure setpoint, add the error to the pressure setpoint to get throttle pressure
What is Max Combined Flow Limit? What happens if this value is lowered when at 100% load?
MCFL = Allowed max for TCV and BPV steam flow
Normally set to 115% (allows 15% steam through BPVs for pressure oscillations if TCVs full flow or if one TCV fails closed)
If MCFL is lowered with TCVs at 100%, then as MCFL goes below 100%, TCVs will go closed, BPVs will not be allowed to open.
How does EHC choose which pressure transmitter to use as the reference value?
Median pressure transmitter value is the one selected. If one of the other transmitters fails upscale or downscale, the median will shift if needed.
This can cause TCVs to open or close, which will in turn cause RX pressure to rise or lower.
Example, if failure cause median value to go up 5 psig, then TCVs will open (trying to maintain pressure setpoint), this will in turn cause Rx pressure to lower ~5psig
What happens to TCVs on a loss of Feedwater Heating?
Due to higher power in the Rx, throttle pressure will rise. This will cause TCVs to open more.
How is turbine load raised after initial turbine roll and turbine tied to the grid?
Load setpoint is raised. But since pressure control is not sensing higher throttle pressure, TCVs will remain as is.
In order to get TCVs to open more, have to raise Rx pressure by pulling control rods or raising recirc flow. This will cause throttle pressure to rise and TCVs to modulate open. Thus the turbine is slaved to the reactor.
What is the purpose of the Wobbulator? How does it function?
Purpose is to avoid operating the turbine for an extended time period at or near a bucket critical speed.
When turbine speed is within the range of 1400 - 1600 rpm and it has been detected that it is within 0.5% of the speed command or that it is being held in that range with a zero acceleration rate, the Wobbulator function is activated to bias the speed reference to raise or lower speed by 50 rpm (1450-1550 rpm) → Keeps it from being constant
What is the turbine response to a Rx SCRAM?
- After Rx scram, turbine continues to operate, acting as a heat sink for the energy remaining in the Rx (decay and stored heat)
- As stored energy decreases, steam production decreases, causing throttle pressure to decrease
- As throttle pressure decreases, the pressure regulators will throttle down on the TCVs to maintain pressure
- Turbine load will continue to decrease as the TCVs close to maintain pressure
- @ turbine load of 80 MWe, operator should trip main turbine & verify BPVs open to maintain pressure
What is the turbine response to a trip at 100% load?
- Hydraulic Trip system will depressurize the ETS header which will activate the hydraulic lock-in signal, close all Extraction Non-Return valves, and depressurize the FASTC header → dump valves open bleeding off FAS pressure in Pacs.
- All TSVs, TCVs, IVs & ISVs immediately go closed & turbine coasts down
- Rx scram signal is generated on both TSV & TCV fast closure (>27.6%)
- BPVs will open fully in an attempt to control pressure
What is the turbine response to a Loss of Stator water cooling?
- Once a Loss of Stator cooling condition exists, the Load Set motor will start to runback the Load Set signal to 25.6%
- When the speed Error/Load Set signal becomes the smallest flow signal input to Pressure/Load Low Value Gate (LVG), the TCVs will start stroking shut to follow the Load Set signal
- As TCVs throttle closed, BPVs will open in an attempt to maintain pressure control
- Once all BPVs are open, as TCVs continue to stroke closed, pressure control is lost & Rx pressure starts to increase
- ↑ in pressure adds positive reactivity to the Rx, forcing both Rx power and pressure up
- Rx will scram on either high thermal flux (APRM) or high pressure, depending on initial plant conditions
- Load Set signal will runback to 25.6% then stop
- As stored energy from Rx ↓ & pressure ↓, BPVs will throttle shut & pressure regulators will again maintain pressure using TCVs until the main turbine must be tripped (< 80 MWe)
What is the heatup rate limitation during shell warming?
150F/hr
What is the T.S. limitations for Main Turbine Bypass?
3.7.7 Mode Thermal Power ≥ 24% LCO MT Byp system shall be OPER OR MCPR adjusted limit apply Conditions LCO Not Met - Satisfy LCO 2 hrs A. Not Met - Reduce Therm Pwr < 24%
