Electrohydraulic Control System (EHC)

Question 1

What is the normal AC power supply for the Electrohydraulic Control (EHC) system?

Answer 1

• House Power (NNN-D15)
• Shaft driven PMG – supplies when turbine is operating (90%), set slightly higher than house power
  o Change over relay auto swaps AC power source.

Question 2

What does the “Close Valve” button do on the EHC Speed Control Unit?

Answer 2

• Speed compared to Speed Set value [Close valves / 100 / 800 / 1500 / 1800 / Overspeed]
  o Close Valve – closes all valves.
   Turbine trip will reset SCU to Close Valve
  o Overspeed – must be held to raise speed for trip test

Question 3

What are the “Starting Rates” in the EHC Speed Control Unit?

Answer 3

• Acceleration (Starting Rate RPM/MIN) buttons: [Slow 60 / Med 90 / Fast 180]
  o SLOW automatically selected after turbine reset

Question 4

What happens if the EHC Speed Control Unit loses both speed sensors?

Answer 4

• Loss of BOTH speed signals: Turbine trip
  o Trip is blocked if in Standby OR < 100 rpm

Question 5

What happens if the EHC Speed Control Unit loses one speed signal?

Answer 5

• Loss of one speed signal (primary or backup):
  o Causes the actual acceleration rate to be twice the selected rate (doubles) because acceleration error is calculated by differentiating each speed signal and adding them to the acceleration setpoint.
  o SCU will read ~1/2 the values of actual acceleration and attempt to double acceleration.

Question 6

During a turbine startup, is the speed error or acceleration error in control?

Answer 6

• Turbine startup - Acceleration error passes [Speed error is large / acceleration error is small]
• At set speed - Speed error passes [Speed error is small / acceleration error is large]

Question 7

What does the Power Load Unbalance circuitry monitor?

Answer 7

Power Load Unbalance circuitry monitors Turbine load and MW output to ensure any mismatch is
within acceptable limits. Protects against Partial Load rejection.

Question 8

How is the Reactor Power Cutback setback signal cleared?

Answer 8

The Setback seals in until it is cleared by the operator taking the action of turning the Load Limit potentiometer counter-clockwise (reducing setting) until Load Limit is in control as evidenced bythe Load Limit light illuminating. Clearing the seal in will have NO effect unless the incoming setback signal is first removed by resetting the RPCS.