EFFECT OF MARK-VI POWER SUPPLY (125 VDC ) FAILURE WHILE GAS TURBINE GENERATOR RUNNING ON LOAD AT 60 MW & 42 MVAR. ALSO PLEASE EXPLAIN WHAT TO DO IN THIS TYPE OF EMERGENCY CONDITION?

 

Are your controllers redundant? RST?
I might be going out on a limb but should I shut down?

 

Well if the 125 VDC supply from the 125 VDC battery was truly lost, then the question would be: Why?

Was there a 125 VDC battery ground present before the loss of 125 VDC power?

Did you find any fuses blown in the <PD> core, and if so, which ones? Did you find any fuses blown on any TRLY cards, and if so, which ones?

Is your unit one of those with a <DACA> back-up? If so, did you lose power to/from the <DACA>(s) also?

What should happen when the control system loses 125 VDC power is that the turbine should trip. The 125 VDC fuel trip solenoids should lose power and shut, and, hopefully there is still 125 VDC power available for the Emer. L.O. Pump and the Emer. Seal Oil Pump (if the generator is hydrogen cooled) for the coastdown.

Because the fuel trip solenoids shut, the generator breaker was probably be opened by the reverse power relay. When there's a large, sudden change in MW, the MVAr reading usually goes askew as well. If the exciter regulator is powered by the same 125 VDC and the exciter regulator lost power as well, then the MVAr reading probably went really "south".

But, the question still remains: What cause the loss of 125 VDC? Was it a ground that was left unresolved for some time and then a ground on the other DC leg caused fuses or the circuit breaker to open and lose power? Or, did the DC breaker of the battery charger open?

There's not much to do in this situation except to try to make sure the Emer. Oil pumps (L.O. and Seal Oil, if so equipped) work during the coastdown to zero speed. Then, the Emer. L.O. pump should be cycled periodically to provide cooling oil flow to the bearings. If the unit has a hydrogen-cooled generator, once the unit reaches zero speed, it should be purged as quickly as possible. Since the Mark VI usually does this, it will have to be done manually, by opening the vent valve to vent the hydrogen, and then opening the CO2 purge valve to admit CO2 to push the hydrogen out. All of this needs to be done while the Emer. S.O. Pump is still running, once the hydrogen is purged and the pressure in the generator casing has dropped significantly, you should stop the Emer. S.O. Pump to preserve the battery for the Emer. L.O. Pump cycling.

So, the major thrust of your activities should be to cycle the Emer. L.O. Pump approximately every 3-5 minutes for a minute or so to keep L.O. flowing to the bearings for cooling while the unit is at zero speed, because a hot shaft will eventually cause the bearing material to soften and deform without cooling oil flow.

While you're finding out and resolving the cause for the loss of 125 VDC power. And don't try turning the shaft for approximately 24 hours until after the unit reaches zero speed, even if the Mark VI gets restored.

 

Mike brings up a point. I presumed the originator was talking about a complete loss of 125 VDC to the Mark VI, not a loss of 125 VDC to a single processor rack. (That impression might have been due to the use of all cap's for the posting!)

If the Mark VI is SIMPLEX and power is lost to the <R> processor rack while the turbine is running, then, TRIP. No power for the trip solenoid means the fuel stop valve will be closed, the generator breaker will open, and the unit will coast down to zero speed.

If the Mark VI is TMR and power is lost to one processor rack while the turbine is running, then the turbine will continue to run on the Designated Voter (RTFM; Read The Fine Manual, GEH-6421) until the power can be re-applied to the affected processor rack and the processor re-joins the two-out-of-three voting and TMR operation is restored. The unit can also be shut down with just two of three controllers in service. BUT, it cannot be re-started with only two of three controllers in service.

The paragraph above presumes the polarity of the servo currents for all three processors was properly verified for all servo-operated devices. It also presumes that critical protective inputs and outputs were properly connected to TMR inputs and not SIMPLEX inputs of the processor whose power supply was lost. (Yes; TMR panels can and usually do have "SIMPLEX" (non-redundant) inputs.) If not, then it's likely there will be problems, and the unit could even trip when if all parameters were properly verified and configured the unit should have been able to continue running on the designated voter of the two remaining processors.

 

ONLY MARK-VI POWER SUPPLY (125 VDC) LOST IN THIS CASE DUE TO LOOSE CONNECTION AT TERMINAL BLOCK .

IN THIS TYPE OF CASE SOME CRITICAL DRIVES LIKE 88QA-1, 88TG-1, 88BT, 88BAT, 88QB, 88QV, 88VG WILL BE REMAINED ON OR WILL BE STOPPED. I AM ASKING THIS BECAUSE THESE CRITICAL DRIVES HAVING NORMALLY CLOSED CONTACT FOR MASTER PROTECTIVE (START COMMAND FROM MARK-VI BY ELECTRO-MAGNETIC CONTACTERS). SO PLEASE REQUESTED TO EXPLAIN THIS TOPIC IN BRIEF. LIKE EFFECTS OF THIS & WHAT SHOULD ELECTRICAL OR MECHANICAL ENGG. SHOULD DO IN THIS EMERGENCY CONDITION.

 

GE deems some pumps and fans as critical to turbine operation. So, as part of their operational reliability scheme they use normally-closed contacts of the Speedtronic relays that start & stop these devices to start & stop them. So, to start the motors the Speedtronic relays must be de-energized, or "dropped out", to close the normally-closed contacts in the motor starter control circuits. GE refer to this as "drop-out to run"; the Speedtronic relays for the starting and stopping of these devices deemed critical are de-energized (dropped out) to start and run the pumps and fans deemed critical to unit operation.

This is done because the predominant failure mode of electromechanical relays used in Speedtronic control systems for these devices (as opposed to solid-state relays) is to fail in the de-energized state. This means that to start these devices the logic driving them is false, de-energize, dropped out, to run the devices. The typical signal name for the "Master" relay that controls the Aux. L.O. Pump is L4QAZ. When this signal is a logic <b>Z</b>ero (false; de-energized; dropped out) the Aux. L.O. Pump motor should be running. The last character in the signal name means that when the logic signal is a zero the device should be running. [NOTE: This generally applies to signals which begin with L4 and which are used as the "Master Control" relay driving signals for motor starters. A suffix of Z means something else for other signals (isn't this Speedtronic stuff fun!!!).]

Also, if normally closed contacts are used to start the critical devices and the relays are energized to stop the critical devices, then if the relay fails in the predominant mode while energized to stop the device, the normally closed contacts will close and the motor will start and run when it should not be running. This condition is typically alarmed (motor running when it should not be). So, if a motor starts for no apparent reason one of the troubleshooting steps would be to check the Speedtronic relay to see if it was working or not. So, 'drop-out-to-run' has several advantages.

If normally open contacts were used to start a critical motor, and the relay had to be energized (picked up) to close the contacts to start the motor, and the relay failed in the predominant mode (the de-energized state) the relay could not pick up (be energized) to close the normally open contacts to start the critical motor, and the results could be catastrophic.

Of course, most people are surprised (and confused) by this when they encounter it (usually just like you, when a failure or emergency condition occurs) because they are only accustomed to seeing normally open contacts used to start and stop motors and devices. But, it's hoped that the explanation above shows that there is merit to this approach, and, in fact, it could be used to improve reliability in many other industries and areas of the plant as well. It's not just because GE tries to be different. They are being different for a well thought-out and reasoned purpose. (Unfortunately, the reasons and thought processes are not well documented, but, that might be changing in the not-to-distant future.)

Now, in your case, when all DC power is lost to the Speedtronic and all of the drop-out-to-run relays drop out, the critical motors will start and run. Even if they don't need to be running. The problem with this is that many times the in-rush current drawn by all these motors starting at the same time can trip the Aux. Transformer protective relay and cause a loss of AC, which is even worse if the unit is coasting down after a trip. If this didn't occur at your site, consider yourself lucky. Very lucky. (And consider checking the Aux. Transformer protective relay settings to ensure they could withstand all the motors controlled by drop-out-to-run relays starting at the same time. Because if they can't, you're going to be faced with a worse scenario than just loss of 125 VDC to the Speedtronic. Consider this an opportunity to improve the reliability of the plant.)

That's the reason that the motors started, because of the drop-out-to-run philosophy. And that's done to improve the reliability and availability of the turbine. Everything about GE heavy duty gas turbine controls philosophy is about improving and enhancing reliability and availability.

The problem with GE heavy duty gas turbine control philosophy is that it's not very well documented.

(But, again, something tells me that's about to change.)

If you are worried about motors running when the unit reaches zero speed, you can shut them down manually from the motor starters. You don't need compartment vent fans or exhaust frame blowers running at zero speed (in fact, it's worse to be running them!).

Just remember, it's better to leave the Aux. L.O. Pump running to have cooling oil flowing to the bearings after a trip. If the Emer. L.O. Pump is running (and it is usually also driven by drop-out-to-run logic!) then you can manually shut that off, as well, to reduce the load on the 125 VDC battery and charger.

Hope this helps! (And, personally, I find it very difficult to read something when it's written all in capital letters, and I know many others do as well. If you want to add emphasis to your post, there are other methods.)

 

It's more than GE documents. Great CSA.

Thanks
G.Rajesh

 

<b>Moderator's note:</b> original post corrected, but leaving this in.

I made another mistake in proofreading....

The fourth paragraph in my previous post to this thread reads:

>If normally closed contacts were used
>to start a critical motor, and the relay
>had to be energized (picked up) to close
>the contacts to start the motor, and the
>relay failed in the predominant mode
>(the de-energized state) the relay could
>not close the contacts to start the
>critical motor, and the results could be
>catastrophic.

It <b>should have</b> read:

"If normally <b>open</b> contacts were used to start a critical motor, and the relay had to be energized (picked up) to close the contacts to start the motor, and the relay failed in the predominant mode (the de-energized state) the relay <b>could not pick up (be energized)</b> to close the normally open contacts to start the critical motor, and the results could be catastrophic."

I apologize profusely for any confusion this may have caused! A picture is worth a thousand words, but that's not an option sometimes.

 

I too noticed this mistake, and read twice and understood the real meaning.

G.Rajesh

 

DEAR SIR,

AT THE TIME OF MARK-6 SUPPLY RESUMPTION IF ALL GT MOTORS ARE NOT ISOLATED. THEN WHICH MOTORS WILL START INSTANTLY WITHOUT DOING ANYTHING, MEANS WITHOUT GIVING ANY COMMAND. MY OTHER QUERY IS AT THE TIME OF RESUMPTION OF MARK-6 SUPPLY WHICH GT MOTOR MODULES REQUIRED TO BE ISOLATED. PLEASE EXPLAIN.

THANKS & REGARDS,
GIRISH.

 

You need to examine the relay outputs of your Mark VI to see how they are configured. As previously described, GE typically uses NC contacts of relay outputs in the AUTO circuit of many, but not all, motor starters. Some examples of motors that are typically not driven with NC contacts are Liq. Fuel Forwarding Pump motors and an electric starting motor, and a Hydraulic Ratchet Pump motor. Examples of motor that are typically driven by NC contacts are Auxiliary L.O. Pump motor, many vent fan motors (Accessory- and Turbine Compartment; Load Compartment; Gas Fuel Module Vent Fan motor(s); etc.

But, we can't tell you how your relay outputs and motor starters at your site are configured. You need to look at your unit's configuration and determine that.

The answer to your second query is that when power is applied or restored to a Speedtronic turbine control panel, the default "state" is Cool down, so the Speedtronic will try to operate the motors required for Cool down, and we don't know what kind of Cool down your unit has (Hydraulic Ratchet; Turning Gear; Slow-roll; or ?). Most other auxiliary motors will shut down, but in general, the motors required for Cool down will run. Technically, you don't have to do anything to the motor starters before re-applying power.