MARK II High VCE Problem

We have a MS5001P gas turbine Controlled by MK II speedtronic system

- Due to 96FG failure, we replaced it with another one calibrated as per GE specs.

-started the turbine, but with higher P2 value (17.5) compared with old value (14.27)

-Re calibrated the SRV and GCV and P2 as per specs.

-P2 returned back to its initial value(14.27) OK.

-The turbine is stable but with a higher VCE value (14.5 &13.5) at a load of 12 mw compared with last values at the same load (10 &9.5)
(ON BOTH GAS AND LIQUID FUEL) resp. although liquid fuel loop wasn't changed.

+what is the cause and solution of this problem?
 
AMS,

If the P2 pressure transducer was not indicating the proper pressure before it was replaced (if the actual pressure was higher than indicated) then the VCE would have been lower. P2 pressure has an effect on Gas Contro Valve position and reference (VCE).

It was not necessary to recalibrate the SRV or GCV.

Other than liquid fuel supply pressure differences or a change in fuel characteristics I can offer no reason why the liquid fuel VCE is different--except differences in how the meter is being read (presuming the analog meter is still being used).
 
CSA,

Thank you very much for your quick response.

-First: before 96FG replacement, it was calibrated as per GE specs (0-17.5) bar gives (0-5) vdc.

-Started up the unit successfully but with two abnormal conditions:

- Unit startup was noticeably slower than before.

- P2 was at 17.5bar (4.9vdc) should be 14.27bar as per GE Spec. Also noticed that at any inlet gas pressure decreasing below 17.5 bar the VSR opens fully.

-Looking on our historical calibration sheet, found that the old 96FG was calibrated as follow (0-14.7) bar gives( 1.58-5) vdc. and surely the gas fuel control loop was calibrated according to this setting.

- So the gas control loop(SRV & GCV) stroke and P2 were calibrated again as per spec.

-The unit started up successfully up to FSNL with P2 equal 14.27 bar (fixed).

-By loading up to 11MW the VCE became 14.5 as i said before which is high value compared to a last value before 96FG replacement but the turbine was stable .

- The VCE is still high also at liquid fuel. although no any change occurred in their cards.

Few days ago and after changing the air inlet filters due to high Delta P alarm, the unit was started but the SRV opened a very small opening and the turbine couldn't fire before i returned the
turbine back to crank and fire several times until the firing started successfully but with very slow turbine acceleration.

-the turbine was synchronized to another turbine and was loaded by 12MW and the other turbine was stopped.

-The turbine was stable at droop with VCE (14.5) until i switched it to ISOCH. option for more stable frequency.

-Suddenly a huge swing occurred in the hydraulic pressure header (from 88) bar and the turbine annunciated (hydr.pressure low) alarm.

-switched it again to Droop, and the problem disappeared 100%.

-Noticed that GCV piston pulsating up-down quickly.

-Changed the GCV servo but with the same results.

-Measured the null bias of it found around (-0.3) and the turbine is in speed control.

Finally ..What is the first step to clear those two problems (High VCE & GCV instability)?

your usual cooperation is highly appreciated.
Regards
 
AMS,

The thing all of the issues you describe have in common is: the hydraulic system. Not the servos, the hydraulic system (except for the inlet air filter problem).

And, speed sensing. All of the loops--including the liquid fuel system--use speed sensing.

It's not clear what data you are comparing--if the conditions are similar or not, how the data was taken, and so on. Changing the calibrations of the gas valve regulators will have an effect. Mark II Speedtronic control systems with analog circuits do drift, and are sensitive to power supply issues and compartment temperature.

The VCE differences don't really concern me, because the loop is closed (speed control--Droop or Isochronous). The instability on Isochronous control, it's necessary to know if it's caused by the Mark II or something else to which the Mark II is reacting.

Wish I could be more help, but that's about all I can think of. Perhaps otised or Bob Johnston can offer more help with Mark II-specific suggestions.

Please write back to let us know how you fare in troubleshooting and resolving the issue(s).
 
B

Bob Johnston

I think CSA is probably correct in being suspicious of the Hydraulic system. When you went from Droop to Isoch I think that you had a big change in fuel demand which is what caused the instability of the hydraulic pressure, what caused the change in fuel demand is unknown but could be something in the Isoch. system or the Network frequency. How stable is your hydraulic pressure when the machine is stable? Do you have accumulator(s) in your hydraulic system, if yes, are they charged and working? When the GCV is pulsating, is the SRV moving and/or is the P2 moving?

For what you are calling high VCE, I think that CSA is correct as well. As long as VCE is operating within a controllable range it should be OK. Have you tried taking the machine to full load and what is the VCE then? For the change in VCE on Liquid fuel, has the LP Fuel Pressure been changed at all? This will cause a change in VCE, if the VCE is higher than before then maybe the LP Fuel pressure is lower than before, it doesn't take much change in pressure to cause this.

That's about all I can think of at the moment, please report back and good luck.
 
First, I'd like to thank everyone for help.

2nd:-

-After Re calibration of gas fuel loop,Both of VCE and the stroke of GCV increased compared with their values just before calibration at the same >>{operating conditions}<<.

-The speed loop wasn't altered during calibration of Gas Fuel loop. and its common in both liquid and gas fuel loops and the pulsating hydraulic pressure appears only on gas fuel.

-There are no accumulators in hydraulic system and it's constant at (88 bar) on both Liquid fuel or Droop mode on gas fuel.

-The turbine generator (27MW-50HZ) feeds an isolated load (12MW) i.e the two turbines are synchronized only to load one turbine and stop the other one for maintenance (no continuous load share and not connected to a national grid).

-After loading the turbine with our max available load (12Mw) and opening the other turbine breaker,the turbine still stable on droop mode but with lower frequency (49.5 Hz).

-As soon the Isoch. mode is selected,the frequency rises to 50Hz and the hydraulic swing problem appears.

- If the droop mode is returned back,the frequency drop to 49.5 again and the swing disappears immediately.

-During the problem,the GCV is moving up and down irregularly.

-As a result of this, The SRV correctly responds but with a very small motion to fix the pressure P2 around 14.27 OK.

-The gain Pot. was varied (as per GE spec) to increase the stability of GCV but with no success.

The question is: what raised the VCE and GCV stroke values after calibration while it was done as per GE spec? and what makes the GCV to pulsate during ISOCH. mode only?

Appreciate your patience.
Regards
 
Hi...

I have lots of experience with this type of system, and this problem seems difficult to troubleshoot remotely.

However, I'll make a suggestion. verify the voltage offsets for these cards that interact with VCE control.

I would start looking into that.
 
AMS,

Analog control systems are not nearly as reliable as their digital counterparts have proven to be. And, <b>based on the information provided,</b> it now seems as if Isoch control was working before the "calibrations" were performed. And, calibrating analog control systems is much more demanding and requires much more training and experience and a certain amount of judgement and skill that digital control systems don't. My best <b>guess</b>--and that's all it can be at this point--is that while the instrumentation & control department insists they did everything as per GE procedures, something wasn't. And I further submit that based on the P2 pressure transmitter calibration record previously submitted that something was amiss for a while and that was not known or understood before beginning the "calibrations."

In my personal experience, far too many people assume that it's necessary and harmless to "calibrate" the SRV or GCV to correct an issue that is usually incorrectly perceived or incorrectly attributed to "the SRV or GCV" that shouldn't be (this includes OEM field service personnel, who set a bad example for site personnel in so doing). And this rush to calibrate without understanding what is happening when "calibrating" the SRV or GCV usually leads to knock-on
effects/problems.

I doubt there are many people in the world, and not many people reading control.com, who can guide you through troubleshooting a Mark II turbine control system. Without any ability to record and trend data on a Mark II they require a lot of meters and skill and patience.

Best of luck with the problem(s)--please write back to let us know what you find and how it is resolved!
 
Hi AMS ,

With regard to your Mark II Control System, Do let us know where you procure your spares from. As We at Gas Turbine Controls corp. have an immense stock of Mark II Cards and are looking forward to providing our whole hearted support to Power plants which use Obsolete Technology.

Mail Id : [email protected]
 
Top