EHC: Electro-Hydraulic Control

Question 1

What valves are controlled by the EHC System?

Answer 1

Main Stop Valves
Control Valves
Control Intercept Valves

Question 2

How many Main Turbine speed signals are used by the EHC System?

Answer 2

2

Question 3

What happens if one of the speed signals used by the EHC System is lost?

Answer 3

Acceleration will double.

EHC uses both speed signals to get the acceleration indication. Losing one will cut indicated acceleration in half so the EHC System will double actual acceleration in an attempt to return indicated acceleration to its original value.

Question 4

What happens if both speed signals to the EHC System are lost?

Answer 4

The Main Turbine will trip unless its < 100 RPM or in Standby

Question 5

If the Main Turbine Output Breakers are closed, what is the only speed reference signal that can be selected on the EHC Control Panel on B06?

Answer 5

1800 RPM

Question 6

How are the speed signals generated for the EHC System? How do they work?

Answer 6

• Magnetic Speed Sensors
• Sensors are located over a toothed wheel and generate a sinusoidal frequency proportional to Turbine speed. At 1800 RPM they output 4800Hz which is then converted to -10VDC for use by the EHC System.

Question 7

What is the trip logic for Aux Speed Control in EHC?

Answer 7

2/3

Question 8

What functions do the Aux Speed Control Units provide in the EHC System?

Answer 8

Backup Overspeed Trip at 111.5%
Permissive for MSLO Pressure Trip (100 psig) at 75% rated speed (1350 RPM)
Permissive for Loss of Both Speed Signals Trip at 5.5% rated speed (100 RPM)

Question 9

When in Standby, what are the overspeed trip set points?

Answer 9

Normal - 110%

Backup - 105% - Backup lowered from its original value.

Question 10

When a Load Rate (1/2, 1, 3, 10) is selected by an operator on the EHC Control Panel, when is this rate applicable: Increase, Decrease, or Both?

Answer 10

Increase ONLY

Decrease is always 130%/min

Question 11

When is the INCREASE button on the EHC Control Panel inhibited?

Answer 11

• PLUB
• RPCB
• Anytime the Load Set Motor is adjusted any more than about 2% greater than the Load Limit Potentiometer. (i.e. you can’t completely move the Load Set Motor out of the way. It’ll stop at 2% above and follow the Pot down remaining 3.5%-4% above the Pot)

Question 12

In addition to the INCREASE/DECREASE pushbuttons, what else can control the Load Set Motor?

Answer 12

• RPCB System - “Runback” on RPCB
• Lowering the Load Limit Pot
• Speed Matcher - not used
• PLUB - “Runback” generated but transparent because Turbine is tripped.

Question 13

What trip of the Main Turbine is deactivated when in Standby?

Answer 13

Loss of Both Speed Signals

Question 14

What has to be done with the controlling valve signals prior to entering Standby mode in EHC?

Answer 14

They should be matched. There’s a pushbutton to bypass this requirement that works going both ways.

Question 15

What two types of tripping protection are available for the Main Turbine?

Answer 15

Mechanical (MTSV)

Electrical (ETSV)

Question 16

Do Main Turbine Setback and Runback work while in Standby?

Answer 16

No

Question 17

What happens when the DECREASE button for the Load Set Motor is depressed?

Answer 17

Load Set Motor output lowers 130%/min and Load Reference Signal goes with it (assuming it is below the Load Limit Pot)

Question 18

What Load Rate is automatically selected when the first Main Generator Output Breaker is closed?

Answer 18

10%

Question 19

How do you clear a Main Turbine Setback?

Answer 19

Lower the Load Limit Potentiometer until the Load Limiting Light comes on.

Question 20

What is performed first: Shell Warming or Chest Warming?

Answer 20

Shell Warming

Question 21

Chest Warming refers to what?

Answer 21

The warming of the valve chests

Question 22

What do the INCREASE/DECREASE pushbuttons on the Chest/Shell Warming section of the EHC Control Panel operate?

Answer 22

MSV-2

Question 23

When performing Chest Warming, which valves are open?

Answer 23

MSV-2 only.

Question 24

Which Starting Rate is automatically selected when the Main Turbine is reset?

Answer 24

Slow

Question 25

What must be done to the Standby Potentiometer before a reset of the Standby Circuit can be performed?

Answer 25

It has to be taken to zero.

Question 26

When do the Intercept Stop Valves open?

Answer 26

When the Main Turbine is reset.

Question 27

When is Stage Pressure Feedback used?

Answer 27

Control Valve Testing

Question 28

What are the power supplies for the EHC System?

Answer 28

House Power - NNN-D15

PMG while > 90% speed

Question 29

How is a PLUB generated?

Answer 29

Looks at MW vs. Hot Reheat Pressure. If MW go down rapidly in relation to Hot Reheat Pressure a PLUB is generated which will trip the turbine.