we have two GE frame V gas turbine for power generation at our site.During planned shutdown we checked the moog servo valves of GCV of both the turbines.On one turbine we found following results for moog valve when turbine was tripped (zero speed)<pre>
TMR System
Resistance Current
COilA 1.07K 4.42mA
COilB 1.07K 4.2mA
COilC 1.07K 0 mA</pre>
Same checks were done on moog servo of GCV of other turbine and following results were found when turbine was tripped (zero speed)<pre>
TMR System
Resistance Current
COilA 1.07K 0 mA
COilB 1.07K 0 mA
COilC 1.07K 0 mA</pre>
why is there difference between currents of servo coils (same model) of turbines for same conditions?

 

The current through the moog coils depends on 3 factors:

1. Regulator null bias - the current to compensate for the elastic force in the valve's spring, if there is one.

2. The difference between the reference for the valve position and the actual position (Command-Feedback = Position Error)

3. Regulator gain - conversion factor from Position Error to mAmps (how fast the regulator will open/close to compensate)

So, if you want to know why you have a difference between the two cases, have a look at the valve command while tripped (probably -25%), the feedback you get (should be 0%), the NullBias and Gain settings. It might be the case that one of your valves needs a re-calibration.

Try to find somewhere the MarkVI manuals. In GEH 6421 Vol. II, the chapter for VSVO you have a quite good description of all these. There is probably something similar existing for MarkV but I can not give you any reference because I don't have those documents with me right now.

 

When the turbine is shut down and there are no active trips in the turbine control panel, the current flowing through the servo coils should be a relatively large positive value.

Positive servo current is used to shut off the flow of fuel or air or steam through the servo-operated device; negative servo current increases the flow of fuel or air or steam through the servo-operated device.

Some trip conditions result in a shorting of the Speedtronic servo-valve outputs, meaning that zero current will be applied to the coil. Some Speedtronic servo-valve outputs will sense a loss of current in the circuit and generate a Diagnostic Alarm that also results in the application of a short across the servo-valve outputs. When you are disconnecting the servo leads to measure resistance there would be no current flowing through the circuit(s), and there would likely be Diagnostic Alarms being annunciated to this effect. You may need to reset the Diagnostic Alarms to be able to see current flowing in the circuit. You would probably also need to have all three circuits "made up" in order to be able to remove the shorts.

When the turbine is running and the feedback from the servo-operated device is equal to the reference for the servo-operated device then the Speedtronic servo-valve output current should be approximately equal to the null bias current setting. The servo-valves used by GE have an internal spring that pushes the spool piece that ports the flow of hydraulic oil to the position that shuts off the flow of fuel or air or steam through the servo-operated device.

When the flow or position of the servo-operated device is equal to the reference, the spool piece in the servo-valve is in a middle position and that shuts off the flow of hydraulic oil to the actuator and keeps the flow or position equal to the reference. However, the null bias spring is continually applying force to the spool piece to try to move it to allow hydraulic oil to flow to the actuator to shut off the flow of fuel or air or steam.

So, the Speedtronic has to apply a small amount of current to the output to overcome the force being applied by the null bias spring. And, since negative current is required to increase the flow of fuel or air or steam the null bias is a negative current.

You can see the current being applied by the Speedtronic using Toolbox (for a Mark VI). The current values are displayed in percent, where 100% equals +10 mA, and -100% equal -10 mA.

 

I want to add that when the turbine is shut down it's common for the current through the IGV servo to be a large negative number. This is because the position reference is usually greater than the actual position when the turbine is shut down, so the Speedtronic will be putting out a large negative value to try to move the IGVs to make the actual position equal to the reference.

So, measuring or analyzing the servo current of the IGV servo when the unit is shut down may produce a different result than measuring or analyzing the servo current through fuel control valves when the unit is shut down.

 

thank you all for your explainations. Looking forward to learn from you in future also

 

Hi Raza,

Do you have any problem in the GCV operation?

OR

Are you checking the servo current for experiment?
If yes, then CSA already explained lot, please follow it and get it done.

Good luck.

Take care
G.Rajesh