GE gas turbine with MARK IV control showing ground fault in MARK IV side. Voltages are 125P is 92V and 125 is 18V. But all CTB module are healthy. Where could be possibly grounded?

 

Umar,

>GE gas turbine with MARK IV control
>showing ground fault <b>in MARK IV side.</b>

And herein lies the BIGGEST problem with troubleshooting grounds on GE Speedtronic turbine control systems: Because the Speedtronic annunciates the ground fault alarm, the ground <b>MUST</b> be in some circuit or device connected to the Speedtronic panel.

This is purely, plainly, simply and patently <b>FALSE!</b> The ground reference circuit in the Speedtronic monitors the entire 125 VDC battery circuit--including anything the battery supplies power to outside of the Speedtronic panel. The Speedtronic monitors the entire 125 VDC power system and annunciates a ground fault alarm whenever a ground develops anywhere on the entire 125 VDC system--not just on circuits and devices connected to the Speedtronic panel.

There is no way to isolate the fire protection system (commonly powered by the same 125 VDC battery powering the Speedtronic panel), or the Emer. L.O. Pump (also commonly powered by the same 125 VDC battery powering the Speedtronic panel); or the Emer. D.C Lighting circuit, or the protective relays and meters in the Generator Control/Protection Panel (also both commonly powered by the same 125 VDC battery powering the Speedtronic turbine control panel). Just because the Speedtronic annunciates a ground fault on the ENTIRE 125 VDC system does NOT mean the ground fault is ONLY on circuits and devices connected to the Speedtronic turbine control panel.

In addition to powering the discrete (contact) input circuits of the Mark IV, the 125 VDC battery also powers any 125 VDC solenoids energized by the Speedtronic turbine control panel. Have a look at the 44 sheets (pages) of the Mark IV Speedtronic elementary to follow the flow of 125 VDC from the input terminals on PTB-1 and PTB-2 to the numerous fuses and from the fuses to the circuits and cards. The <C>, <R>, <S>, and <T> power supplies are not generally the source of grounds, and ONLY devices directly connected to the 125 VDC (through fuses in the <PD> module) can be sources of grounds. That means that 24 VDC-powered transmitters cannot be sources of 125 VDC battery grounds. Thermocouples cannot be sources of 125 VDC battery grounds. LVDTs cannot be sources of 125 VDC battery grounds. ONLY DEVICES POWERED BY 125 VDC CAN BE SOURCES OF 125 VDC BATTERY GROUNDS. Servo-valves cannot be sources of 125 VDC battery grounds.

Use the Mark IV Speedtronic elementary--it's one of the finest documents every produced for GE Speedtronic turbine control systems. It can help you understand MANY things about the Mark IV that are not explained anywhere else. DC power distribution is typically shown on the 44 sheets, but may also be on some other pages in the elementary drawing. Users of Mark Vs and Mark VIs and Mark VIes do not have the luxury of an elementary drawing like the Mark IV Speedtronic elementary.

And don't forget: A ground on any circuit or device powered by the 125 VDC battery can cause the Speedtronic to annunciate a 125 battery ground fault--not just devices and circuits connected to the Speedtronic.

And, 125 VDC is used to power other devices connected to the Mark IV in addition to the discrete (contact) inputs.

Hope this helps!

Write back to let us know how you fare in you troubleshooting.

 

I would agree with all that was said. Don't forget that a dirty battery case can cause this ground also.

Since it appears you are showing a negative ground, if we assume it is in the turbine controls, I would look for a ground on a solenoid or the return wire of an open switch.

Our original Mark IV installations had a single positive from one card going to a pair of redundant switches with the returns each to a different card.This made it next to impossible to isolate to an individual card. When we did our upgrade to the mark VI, we went through extra pains in the wiring to be able to isolate the circuits. One of the additional bonuses of the upgrade is that we have redundant power to the mark VI. We have the normal 125 VDC and a UPS feed to a transformer /rectifier. This allows me to disconnect the 125Vdc battery system and keep everything running. If the alarm goes away when it is isolated, I know the problem is elsewhere and not the turbine controls.

It has also been our experience that most of the time our grounds were in the fire detection system. We used spare wiring and brought these temperature switches into the control panel where they could easily be isolated through cutout type terminals.

Good Luck.

 

I've read good advice given already. Here's my two cents and I hope it helps. Yes go to the 44 pages of the elementary and with a cold turbine prepare to start de energizing sections of the MKIV power distribution module <PDM>. Now, based on the information given, also remember that the MKIV has a floating 125 vdc bus, so when you see the 'split' shift up towards positive, it means that the ground or multiple grounds, are affecting the negative side of the system. CTB field inputs are fed from the positive side of the system (107) common power so are not likely culprits. Here's how I would approach this, and I have more times that I'd like to remember...leave the fuses in that power <RST> computers power supplies. Pull fuses on <RTB> Relay Terminal Board Modules. Remember that the 125v relays that control field solenoids and the like have a constant negative power 108 bus connected to them and are activated by double contact closure on the positive side only to pick them up or drop them out. You start pulling RTB distribution fuses and I'll bet you an isolate to a specific RTB most likely. Once determined reinstall the fuse and go to the associated RTB module and begin to isolate the individual outputs. I'm assuming you've already eliminated the DC MCC by isolating all the starters directly at the buckets themselves. Monitor MKIV before and after any and all changes to the system you make. for example, If you isolate a motor starter and the neg DC ground improves, but doesn't go away, make note, leave it isolated and keep on isolating until it does go away. You may have multiple grounds. Ronald Anderson [email protected] or contact me on linkedIn.