Runing Gas Turbine with Battery Ground Fault

A

Thread Starter

Abdollah

Hi everybody
GE 9E gas turbine with MARK6e control system.

I need to know if it's allowed from GE to run a turbine while there is a battery ground fault.

-If it's allowed, can't this be a risk for the equipment?
-If it's not allowed why?

Best regards
Abdollah
 
Abdollah,

I presume you are asking if a turbine should be allowed by operators and supervisors and management to run with a battery ground, not if the turbine control system will "allow" the turbine to run with a battery ground.

The unqualified answer is: Yes. The turbine can and will continue to run with a ground on either the positive or the negative leg of the battery supply, and it will even continue to run with multiple grounds on either the positive or negative leg of the battery supply. But, arcs and sparks and blown fuses and failed Speedtronic cards and tripped circuit breakers will occur if a ground exists on either leg and another develops or occurs on the other leg.

The qualified answer is, No. The unit should not be allowed to run for very long after a battery ground is annunciated. The reason is that left for more than a couple of days in many cases, one ground becomes two, and two becomes three, and then a ground develops on the other leg and that's when the fun begins (NOT!).

Speedtronic turbine control systems are designed to be as reliable as possible, by not grounding either leg of the battery and then monitoring both legs for the existence of a ground and annunciating the ground with a Process Alarm. That's a clue to find and resolve the ground, because left unresolved if a ground develops on the other leg of the battery that is, effectively, a short across the battery. And that, my friend, as a very bad thing. And, recovering from such a condition can be very time-consuming and costly, in addition to frustrating.

Typically, there is no Start Check permissive preventing a unit from being started if a battery ground exists (though in my opinion there should be!).

Keep the Mark VIe free and clear of grounds. Usually, when a ground is located it leads to a review of other locations (junction boxes; conduit; wiring) where similar grounds might also develop. Most grounds are the result of moisture getting into junction boxes because of poor construction practices when connecting conduits to junction boxes, or when connecting conduits together, etc. So, a prudent technician and his supervisor, will when finding such a problem in the course of resolving a ground go to other similar locations and ensure they are properly weather-tight. (Moisture can include lube oil!)

Grounds can occur when steam packing leaks cause condensate to drip onto junction boxes and conduits and switch enclosures.

Another source of battery grounds is melted wire insulation because conduits were run too close to hot locations, or because wire insulation with too low a melting point was used in a high-temperature location.

Lastly, left unresolved, grounds usually precede "strange" occurrences such as nuisance Process- and Diagnostic Alarms; turbine-generator instability; odd data appearing on the displays; and general confusion and frustration.

Hope this helps!
 
>I presume you are asking if a turbine should be allowed by operators and supervisors and management to run
> with a battery ground, not if the turbine control system will "allow" the turbine to run with a battery ground.

In fact I meant by my question if GE recommend to not run the turbine with a ground fault if grid needs power. (in its documentation or TIL..; If you have idea about doc reference to consult it will be great)

> Typically, there is no Start Check permissive preventing a unit from being started if a battery ground exists.

That makes me think that it's possible to start and synchronize the turbine, because if not, GE should take this in consideration and add a logic preventing the starting with battery ground

Thank you CSA and thank you again for your helpful answer.
 
Abdollah... Followng is a scenario that illustrates why the unit should not be kept on line!

Consider a ground on circuit X! Then, if a second circuit Y is suddenly grounded the protective fuse/breaker for both circuits, will respond, rendering both X and Y inoperable!

In addition, for the duration it takes either rotective device to clear the double-fault, other circuits powered by the bus in question may de-eneregize!

Regards, Phil Corso
 
Abdollah,

You are welcome.

Please write back to let us know where you found the battery ground. Remember: The Speedtronic is monitoring EVERY device connected directly to the battery--not just the devices connected to the Speedtronic. A ground can occur in the Emergency L.O. Pump motor circuit, or the Emergency D.C. Lighting circuit, or the fire protection circuits of some units (depending on the age and design of the fire protection system). Just because the Speedtronic annunciates a battery ground <b>DOES NOT</b> the ground is on a device connected to the Speedtronic!

Also, grounds can ONLY occur on devices directly powered by the battery (and that can be through switches and/or fuses). That means grounded T/Cs can't cause a battery ground alarm to be annunciated. Grounded transmitters can't cause a battery ground to be annunciated. Grounded speed pick-ups can't cause a battery ground to be annunciated. Grounded LVDTs can't cause a battery ground to be annunciated. Grounded servo-valve coils can't cause a battery ground to be annunciated. Grounded RTDs can't cause a battery ground to be annunciated.

Only devices which are connected directly to battery voltage (and many devices connected to the Speedtronic ARE directly connected to the battery, sometimes through switches and fuses) can cause battery grounds, and for most Speedtronic panels that means discrete (contact) inputs, solenoid outputs, and fuel trip solenoids (20FG-1; 20FL-1). So don't waste any time testing T/Cs or RTDs or transmitters or LVDTs or servo-valves for grounds when troubleshooting a battery ground alarm. Have a look at the drawings for the DC power distribution and only look at those devices which are powered by battery power--when troubleshooting Speedtronic field devices.

But, remember: It's the BATTERY that's being monitored by the Speedtronic for grounds, NOT JUST devices connected to the Speedtronic panel. So, ANY device or circuit that's powered by the battery which develops a ground can cause a battery ground.

But, most low-level analog devices are either self-powered or powered by power supplies that convert battery power to some other voltage, and grounds in those devices or their circuits can't cause battery grounds because they are isolated from the battery by the power supply internals--not to mention that many of the Speedtronic low-voltage power supplies are grounded! In fact, GE documentation specifically states the Speedtronic can operate with grounded T/Cs and RTDs, so finding a ground in those circuits is <b>NOT</b> going to resolve a battery ground alarm.

As for documentation, well, that's kind of GE's Achilles Heel. They're not very good at that, and I don't know of any GE documentation specifically related to operating with or without battery grounds. And, you are correct--if they'd been highly concerned about a problem they would have made it a start-check permissive or a shutdown or a trip. But, if they'd done that SOMEBODY, SOMEWHERE would complain very loudly that GE was obstructing their production and generation, feeling that a battery ground alarm should not be cause for preventing a start or causing a shutdown or trip.

So, the decision to start and/or run with a battery ground alarm is really left up to the site; again, based on their needs and their business practices. The thing GE should do--but they don't--is provide the pros and cons of starting and/or operating with a battery ground alarm. Which I hope was explained appropriately here.
 
Phil Corso,

Thank you for you explanation, so it's more better to avoid starting turbine

CSA,

What I understood is that Battery Ground fault annunciated by speedtronic can be caused only by devices using 125 VDC, and not something else.

I look for something preventing the starting, and I found only in an alarm interpretation document about L64D, they say (Try to stop turbine as soon as possible and check.....), ONLY THIS but not in others like documents explaining what to check and do before starting (Hand valves position...)

When we found the problem I will write back

Regards
 
"Another source of battery grounds is melted wire insulation because conduits were run too close to hot locations, or because wire insulation with too low a melting point was used in a high-temperature location."

In our Frame6, MarkV system a similar case lead to ground fault!! We searched and searched which lead us atop the exhaust duct. The tray carrying cables for pressure switches had fallen onto the exhaust duct. Cable insulation slowly wore off leading to supply being grounded.
 
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