After the 7EA Gas Turbine shutdown, Our first approach of course, was review the alarm list, the trip log, and the prevote data display. Here we confirmed the change of state of some digital signals associated with the e-cards managed for both processors that lead machine to trip condition.

We requested GE support but the recommendations were general such as check false contact, cables, voltage, etc, but we didn't find anything to get a conclusion.

Has somebody experienced similar failure with Mark V processor?
Appreciate any feedback.

 

It's not exactly clear what happened or the sequence of events. I'm presuming the unit was running (at what load we don't know; on what fuel we don't know; with what kind of combustors (conventions or DLN) we don't know) and tripped on what is being attributed to "loss" or re-boot of two processors (<S> & <T>). We also don't know what alarms (Process and Diagnostic) were present prior to the event, and what alarms you found after the event.

Is the Alarm Printer working and connected to the operator interface? Can you provide a chronological list of alarms from, say, 10 seconds before the event to 20 seconds after the event? If you have the Trip History saved or printed out, you can provide the information from 10 seconds prior to the trip and three seconds after the trip.

Whenever I've tried to troubleshoot problems like this (usually without an alarm printout because the printer is "broken" or has been out of service for some time), it turns out that a 125 VDC Battery Ground alarm has persisted for some time, and/or it has been toggling for some time (another reason the alarm printer has been disabled).

It is usually also learned that there are multiple Diagnostic Alarms which have persisted for some time, and some fairly important Process Alarms, as well.

It is a well-known fact that the pins and sockets of the multitude of ribbon cables used in the Mark V are prone to develop corrosion that causes communications problems. GE has recommended using conductive grease on these cable connectors. Has this been done at your site? (There is usually a small tube of conductive grease provided with each spare card purchased. With the Mark V powered down, remove each ribbon cable one at a time (this will take the better part of a day), and apply a small amount of conductive grease to the socket connector covering the holes of the sockets (don't apply too much) and then plug the connector in and remove it a couple of times before plugging it in firmly and moving to the next cable until all the cables are "greased".) This is very effective at reducing or eliminating intermittent problems. (And, it's not a one-time thing, either. Greasing the pins/sockets should be done regularly, say at least every other year, and regularly.)

When you post for help with a problem like this, you need to provide as much information as possible about the unit, what fuel was being used, what load the problem occurred at, and the results of any troubleshooting you did if you want a quick and concise response. We also need to know if there was anything else happening at the time that was out of the ordinary (a thundershower, or heavy rain, or electrical switching in the high yard, etc.). And provide a list of alarms (Process and Diagnostic) that were active prior to the event, as well as a chronological listing of alarms through the event.

 

I've had this happen once and it was caused by 125V DC ground faults. The machine had a "solid" -125 V ground and an intermittent + 125 ground dipped the supply enough to reboot a processor. Do you have any ground faults?

 

Thank you everybody for your reply.


Yes, the machine was running with 60MW, Isochronous Mode, 1 MWAR. This GTG # 4 was sharing load with other three GTGs running in Isochronous mode as well. This GTG eventually had figured out Battery 125 VDC Ground alarm, with +60 VDC -30 vdc.

We have reviewed several instruments and terminal connection boxes trying to find any ground path without success. Before the trip was not present any ground condition as shows the following alarms:

10-JAN-2012 21:49:31.468 T4 1* Q 0110 COMPRESSOR INLET THERMOCOUPLES DISAGREE.
10-JAN-2012 21:49:31.343 T4 0* Q 0271 TCEA 4 RELAY CIRCUIT FDBK (EXTRNL TRIP.
10-JAN-2012 21:49:31.343 T4 1* Q 0041 PROTECTIVE SPEED SIGNAL TROUBLE - HP.
10-JAN-2012 21:49:31.343 T4 0* Q 0040 PROTECTIVE MODULE OVERSPEED TRIP - HP.
10-JAN-2012 21:47.56.468 T1 1 Q 0380 IN BASE -PEAK EXHAUST TEMP REGION.
10-JAN-2012 21:47.30.343 T3 1 Q 0380 IN BASE -PEAK EXHAUST TEMP REGION.
10-JAN-2012 21:47.19.343 T4 0 Q 0380 IN BASE -PEAK EXHAUST TEMP REGION.
10-JAN-2012 14:14.33.718 T1 1 Q 0309 BENTLY NEVADA MONITOR FAULT.

I have the prevote data display but how can i post it?

NOTE: When the GTG4 trip at 21:47:31 the other three GTGs lost the Isochronous mode, but could be different topic or there is any relationship?

 

The second-, third- and fourth alarms from the top of the list you provided indicate that one of the TCEA (<P> Core protective processors) was already trying to trip the turbine because it thought there was an HP overspeed.

And, if the total DC supply voltage is only 90 VDC then that is at the very bottom of the limit of the Mark V power supplies (the allowable range of operation is 90-140 VDC). When the DC supply exceeds either limit there is crowbar circuitry on each power supply to protect the power supply which shuts down the processor.

30 VDC with respect to ground on one leg should have resulted in a 125 VDC battery ground alarm, presuming the normal configuration is present on the panel. Perhaps because the supply was so low it may not have triggered the alarm.

Such a "high" voltage (30 VDC) is typically considered to be a "soft" ground, meaning that a ground condition is developing somewhere, and usually it's the result of moisture in some junction box, or some wire insulation is slowly degrading (melting; drying and/or cracking; etc.) and this is allowing some leakage to ground.

Most 125 VDC battery grounds are in JBs exposed to ambient weather conditions (outside of compartments or enclosures) and are the result of poor construction practices that resulted in conduits penetrating the tops of JBs (rather than the bottoms only, or the sides), and/or poor or non-existent conduit penetration sealing. Common problem areas are the cooling water fan vibration switches (many times flexible conduit was used and it cracks or gets damaged over time; or the reset switch covers are damaged or missing; or the switch covers are not replaced properly; etc.). As was said before, look for exterior JBs that have conduit penetrations through the top edge of the JB, or the sides.

Remember: ANY device or circuit that is powered by the same battery as is powering the Speedtronic turbine control panel can be the source of a ground or multiple grounds. That includes Emergency DC Oil Pumps, and some fire detection/protection circuits, and Generator Protective/Control Panel devices. Some sites used the 125 VDC battery for powering devices on the main/step-up transformer and circuits/breakers in the switch yard (both of which are usually outside in ambient weather conditions).

There is a FALSE belief that because the Speedtronic annunciates a battery ground alarm that the ground MUST be on some device connected to the Speedtronic. The Speedtronic monitors any and all circuits connected to the battery that powers the Speedtronic.

Also, the ONLY circuits and devices that can cause 125 VDC battery grounds are those that are powered by the 125 VDC battery, such as discrete (contact) inputs, and 125 VDC solenoids (including 20FG-1 and 20FL-1, and any other solenoid connected to the <P> core). Grounded thermocouples cannot cause 125 VDC battery grounds; grounded mA transmitters cannot cause 125 VDC battery grounds; grounded electro-hydraulic servo-valves or LVDTs cannot cause 125 VDC battery grounds; grounded RTDs cannot cause 125 VDC battery grounds--in short, analog field devices cannot cause 125 VDC battery grounds. Only devices powered by the 125 VDC battery--including those not powered by the battery through the Speedtronic turbine control panel!--CAN cause 125 VDC battery grounds.

But, from the new information provided, it appears that (1), the DC supply voltage is extremely low, and, (2), that one of the TCEAs had previously detected an HP overspeed ("emergency" electrical overspeed) and was trying to trip the turbine via it's ETR (Emergency Trip Relay).

Hope this helps!

 

Of course CSA your explanation is well appreciated and lead to change the mind about the importance of not to run for long period and/or fix ASAP the 125 VDC grounding alarm. The floating reference (+65/-65 VDC) sometime let us to run the Mark V under partial grounding but we shouldn´t abuse.

 

You are exactly, positively, 1000% correct. The purpose of allowing the turbine to continue to run when there is a ground is to improve the reliability and availability of the turbine. However, leaving grounds unresolved is just asking for more, and worse, trouble.

The Speedtronic can easily withstand a ground on one leg or the other. It can easily withstand multiple grounds on one leg or the other. BUT, when there is/are grounds on one leg and a ground develops on the other leg, well, then interesting things can happen--and they're not usually pleasantly interesting.

I'm still worried about the 60/30 magnitude; it only adds up to 90 VDC, which is dangerously low. Dangerously.

 

As pointed out by CSA, the grounding problem is pretty severe. Regarding detection of Overspeed by one processor, we had a similar problem which was traced to grounding in one of the speed probe due to a cut cable.