GE MARK5 CONTROL SYSTEM

S

Thread Starter

shyam

We have frame-5 gas turbine machine for power generation with GE mark-5 control system. Now we are getting the alarms "BATTERY 125V DC EARTH GROUND."

We checked the floating voltage on the battery charger panel side found 110 V on positive (+ - 0) side and 15V on negative side (- - 0). Now the machine is in shutdown condition still we getting the alarms. can anybody suggest is this alarm related with battery charger circuitry or related with control system or any i/o circuitry and what are the check points have to carry out?
 
Shyam,

> We checked the floating voltage on the battery charger panel side found
> 110 V on positive (+ - 0) side and 15V on negative side (- - 0).

What you describe is referred to as a "soft" ground because neither battery leg is fully grounded (which would be indicated by a reading of 0 VDC with respect to ground). The reason the Speedtronic turbine control panels alarm before a ground becomes "hard" or "solid" (0 VDC with respect to ground) is to allow operators and technicians an opportunity to find and resolve the ground before it becomes a hard or solid ground.

Notice that the Process Alarm text message reads: "BATTERY 125 VDC GROUND." This indicates <t>two</b> very important things:

1) Only devices powered by the 125 VDC battery supply can cause this alarm; and,

2) ANY device powered by the 125 VDC battery supply can cause this alarm.

Let's start with the second condition first. The 125 VDC battery of a GE-design heavy duty gas turbine control system usually powers the following areas/devices on the unit:

a) D.C. Emergency L.O. Pump Motor

b) D.C. Hydraulic Ratchet Pump Motor (for a Frame 5 unit)

c) Generator Protection Panel (protective relays; indicating lights; switches; generator breaker trip/close circuit; etc.)

d) Emergency D.C. Compartment Lighting

e) Fire Detection and/or Discharge System

f) Speedtronic turbine control panel

A ground in <b>any</b> of these systems can cause the "BATTERY 125 VDC GROUND alarm to be annunciated by the Speedtronic turbine control system. So, while most operators, technicians and their supervisors believe that <b>ONLY</b> devices connected to the Speedtronic (the Mark V in this case) can cause this alarm, again, a ground in any of these systems can result in the Speedtronic annunciating the alarm. So, troubleshooting may need to include these areas as well, and in some cases--especially when the turbine is shut down for an outage and is off Cooldown (Ratchet)--it can be very easy to isolate some of these systems to check to see if the ground exists in any one of them.

Now, let's get to devices connected to the Mark V turbine control system. Again, <b>ONLY</b> devices that are directly powered by 125 VDC can cause this alarm, and for most Mark V turbine control panels that includes the following list of types of devices:

aa) Discrete (contact) inputs (temperature switches; pressure switches; limit switches; etc.);

bb) Solenoid outputs (20CB-1; 20VG-1; 20AA-1; in other words, any 125 VDC-powered solenoid-operated device connected to the Speedtronic);

cc) Fuel trip solenoids (20FG-1; 20FL-1--these are connected to the PTBA terminal board on the <P> core)

And, that's it! Pressure transmitters cannot cause the BATTERY 125 VDC GROUND alarm because they are not directly powered by the 125 VDC battery. Thermocouples are self-powered--so they can't cause the BATTERY 125 VDC GROUND. Servo-valves are not powered directly by the 125 VDC battery so they can't cause this alarm. LVDTs are not powered directly by the 125 VDC battery; RTDs are not powered directly by the 125 VDC battery; speed pick-ups are self-powered; motor starter signals are not powered by the 125 VDC battery (with the exception of the D.C. Emer. L.O. Pump and the D.C. Hydraulic Ratchet Pump--which were covered above). So, it's a complete waste of time to check any terminals or circuits other than those which are directly powered by the 125 VDC battery for grounds--in fact, Speedtronic turbine control panels are designed to be capable of operating with grounded thermocouples and RTDs, so finding a ground on any of those circuits is meaningless to solving the BATTERY 125VDC GROUND alarm.

Therefore, it's best to concentrate on only those devices which are powered by 125 VDC--in the list above. And, if the turbine is shut down and off Cooldown (Ratchet) that's the best time to look for grounds. It's also a good idea--when using this approach below--to "rack out" (open the Mains breakers) of all of the auxiliary motors for the turbine auxiliaries. This is because as we progress through this procedure many of them may start and run--some at the same time--and we don't need to do that. So, make a list of all the motors whose starter Mains breakers are opened (so you can go back and close the Mains breakers when finished!).

Next, in the <PD> core there are some plugs which can be useful for isolating the 125 VDC-powered devices. The plugs labeled J8x (where 'x' can be A, B, C or D) provide the power to most the 125 VDC solenoids connected to the DTBC and DTBD terminal boards of <CD>, <QD1> (and <QD2>, if so equipped). The plugs labeled J12x (where 'x' can be A, B, or C) provide the power to the discrete (contact) inputs connected to the DTBA and DTBB terminal boards of <CD>, <QD1> (and <QD2>, if so equipped). The plugs labeled J19 and J20 provide the power to the special solenoid output of <QD1> for 20CB-1 and 20FD-1 (if so equipped). And finally, the plugs labeled J7X, J7Y and J7Z provide the power to the <P> core for the fuel trip solenoids.

Troubleshooting grounds is a lot of planning and a little bit of luck. Most grounds occur in exterior junction boxes where water can get in and cause a soft ground first, and then a hard ground as more and more corrosion builds up, or the water levels rises and remains. So, it's best to examine all outdoor or exposed junction boxes first when looking for grounds--especially soft grounds. Any junction boxes with conduits that penetrate through the top of the junction box should be suspect and continually checked for water tightness.

It's pretty useless to just use a voltmeter with respect to ground and go and touch every terminal with a wire on in on the DTBA, DTBB, DTBC and DTBD terminal boards of <CD>, <QD1> (and <QD2> if so equipped). So, when the turbine is shut down and the motor starter Mains breakers are all open it's best to start removing the plugs in the <PD> core. Start with the J12x plugs, while continually monitoring the leg with the lowest voltage reading with respect to ground. When you remove a plug and the voltage reading increases to approximately 65 VDC--then you can work to find the specific terminal on the terminal board which is disconnected that is grounded. And, if you lift a plug and the voltage doesn't increase very much if at all, leave that plug disconnected and lift the next one in the group--and if that doesn't cause the voltage to rise by very much disconnect the next plug in the group. Leave all disconnected plugs disconnected until the voltage reading rises. And continue with all groups. <b>There will be a LOT of alarms--that's to be expected.</b> Be patient and continue. When you remove a plug and the voltage reading rises--whatever terminal board that plug feeds is likely to have the grounded field wire connected to it.

If you disconnect all of the plugs and the voltage reading doesn't increase, then it's pretty likely the grounded wire is <b><i>NOT</i></b> connected to the Mark V, but is connected to one of the other systems powered by the 125 VDC battery. Again, just because the alarm is annunciated by the Speedtronic <b>DOES NOT MEAN</b> the grounded device is connected to the Speedtronic. The Speedtronic monitors the 125 VDC battery--and all of the devices powered by the battery. So a ground can be on any device which is powered by the 125 VDC battery.

The J8x cables that power most of the solenoids connected to the DTBC/DTBC terminal boards connect to the DTBC first, and there is a jumper to connect the DTBC to the DTBD. And, the J12x cables that power the discrete inputs connected to the DTBA/DTBB terminal boards connect first to the DTBA and there is a jumper that connected the DTBA to the DTBB. This can be useful in further isolating individual terminal boards from each other.

Just remember to write down every plug that is disconnected, so that list can be used to reconnect every plug before the unit is put back into service.

Again, most grounds--especially soft grounds--occur in junction boxes that get wet. So, start with those. L.O. in a junction box can also cause a soft ground.

Please write back to let us know what you find!
 
glenmorangie,

I almost like troubleshooting grounds--under decent circumstances, of course. Troubleshooting grounds isn't difficult--once one understands what can and can't be grounded. The two most difficult grounds I've ever troubleshot were grounds that were not on devices connected to the Speedtronic. One was in a plant fire protection system that was also powered by the same 125 VDC battery that powered the Speedtronic, and I had to fight tooth and nail to convince the Customer that the ground was NOT in the Speedtronic just because the Speedtronic annunciated the ground alarm. Had they followed my advice, we would have found and resolved the alarm at least 48 hours sooner than we did, and we would only have had to take the unit off-line once--instead of three times (which were all blamed on me!).

The other ground was in a cooling fan vibration switch that was connected to a breaker in the Generator Control Panel. The unit was a black-start unit and one of the cooling water fans was powered by a special transformer, through a breaker with a shunt trip mechanism in the Generator Control Panel. <b>THAT</b> was really difficult to find, because it was still on the 125 VDC breaker that feeds both the GCP and the Speedtronic--but it wasn't on a device that was connected to the Speedtronic. It took about 14 hours to find that problem--and only because the unit wasn't running.

That's the other thing that most people don't want to believe--if a ground has existed for days or weeks or months then most likely it's going to be necessary to shut the unit down, wait for it to cooldown, and then start troubleshooting. In fact, any ground troubleshooting is easier when the unit isn't running. But, as was the case in both the examples above the problem was moisture (rain water) in a local junction box. And both times it was because good construction practices hadn't been followed and conduit penetrations were made into the top of a weatherproof junction box and weren't subsequently properly sealed. And, again, most grounds can be attributed to water/moisture in outside junction boxes, either not properly closed, or with leaky conduit penetrations. One ground was traced to operators using the conduit as a ladder rung/step to take gauge readings, and the conduit was loose and pulling out of the junction box and monsoon rain water was pouring into the junction box. And I flew 11 hours and drove 4 hours to get to the site to point out the problem--which was obvious in about 25 minutes of just walking around the outside of the unit (in a monsoon rainstorm, of course).

And, of course, it was an OEM problem, too--the plant manager was absolutely certain of it!

Happy Holidays, to you, also--and congratulations on making MVP!!! It's about time!
 
Thanks CSA for your great support. electrical team agreed to start their side first, as per our superiors instruction. after their checking if the problem is still existing we will start our side. once again thanks a lot for your support and will come back to you with troubleshooting report.

"HAPPY X MAS TO ALL"
 
Dear CSA,

We removed each power cable which is going to the digital i/o modules like DTBC, DTBA, etc one by one from <PD> core and checked but we didn't get any result. After that we completely switched off MARK 5 then powered up. after that 125v dc earth ground problem solved, we are confused how it gone. The problem was persisting for one month but after this checking the alarm didn't come for one week but still the machine is in shutdown condition. Did you face this type problem? the alarm came after power recycling it gone? we are waiting to watch after the start up, weather the the same alarm is coming again or not? now floating voltage also showing good. Again thanks for your support.
 
Your problem lies in a switched relay or solenoid that has a leg going to ground. This activation is condition dependent; so first locate that switched circuit.
 
B

Bob Johnston

David is correct. What I suggest you do is keep your meter on to check the ground condition during the next start up and see if you can see when the ground comes on. Try to check Speed relays (14HM, 14HA & 14HS) and Shaft Speed. This may give you some kind of clue where the ground is coming from. Unfortunately this is one of the hardest grounds to find as it can be intermittent. Don't despair, keep trying, you will eventually find it.

Good Luck
 
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