Excitation Problem with Parallel Operation of Turbo Generator

https://drive.google.com/drive/folders/1nD9rpfOtHuJyjO2kbaxGYp_N7WaXXEAq?usp=sharing

Please relate the content of the telephone discussions that have taken place with the original equipment manufacturer on this topic?

Dear PWSlack,

We have taken up matter with OEM M/s GE and their comments are awaited.

BR,

Kashif

Having re-read your message, I realise your current problem is overvolt alarm, although you have had overvolt trip before. So voltage increase is small %. Could be droop circuit problem or switching which modifies volt ref. according to plant state e.g. circuit sheet 5 - 52G?? contact or trouble with wiper contact on many pots on sheet 4. It does not look like a protection relay fault because you see over & under excitation occurring.

Kashif,

You wrote that your initial interpretation of problem was under-excitation fault on a machine.

This would lower system voltage not raise it.

My thought is that TG701, the machine which previously gave an overvoltage fault, gives increased voltage/excitation causing voltage increase and alarm. Because both machine relays see same voltage it is chance which has most sensitive/fast response. Note that an increase in gain or increased firing by the AVR electronics would be corrected by the feedback, suggesting problem is with the reference value or more likely, the feedback components.

You need to investigate the correct machine, the one which over-excited (increased lagging VArs). My last message gave suggestions to investigate the droop and generator voltage setting switching/adjustment variable resistors.

67model

That statement is of doubtful veracity, for were (a) telephone conversation(s) to have taken place, those comments would have been available immediately, and not remain outstanding since <...July 02...>.

That's rich!

Someone decided that anonymous was the best choice for a post begins with, "Who are you...".

If excitation failed on one generator, other would have to supply excitation current for faulty one (as an induction motor) + lagging load current - that looks like a voltage reduction.

Have you observed the field current meters on the machines during these events?

Loss of voltage control (low voltage from VT) on one machine would cause it to overvolt (over excite), but this could cause other machine to reduce excitation to compensate.

Do you have any VAr sharing coupling between machines or an external active/reactive load sharing/frequency control box?

Loss of excitation relay may not operate if one machine goes overvolt - neither machine has lost excitation.

The date on the circuits is 2015 - have you contacted GE?

Unfortunately, I cannot read any graphs from the G60 .cfg files, but I do not think it will give you a cause. The fact that you had a previous overvolt fault suggests fault in voltage control - loss of feedback, jump-up of reference, excitation amp fault causing high excitation current. These things are amenable to investigation on running unit, off-load. Fault in "lagging current droop" possible, but it is a simple circuit and I do not think it would raise volts to overvolt trip level.

Check all excitation panel wires & connections for bad connection by ohmmeter/pulling&pushing while running off-load. Any connections added/altered by recent excitation related modification are particularly suspect.

Did this unit work OK for long period before any recent modifications?

Dear 67Model

Thank you for your valuable comments . . The required details are as :

1) Have you observed the field current meters on the machines during these events? Nopes there is no logging on field ammeters and the event last only for 3-4 seconds therefore we were not able to observe field ammeter.

2) Do you have any VAr sharing coupling between machines or an external active/reactive load sharing/frequency control box? The sharing is reactive current compensation.

3) Did this unit work OK for long period before any recent modifications? We did no modification in excitation circuit since long time. There is no 59VHZ alarm from last 16 hours :)

https://drive.google.com/drive/folders/1nD9rpfOtHuJyjO2kbaxGYp_N7WaXXEAq?usp=sharing

Please find attached revised link including G60 relays fault data osc (OSC_254_39_20150527_085720 for TG-701) OSC_254_440_20160324_113935 for TG-702 Generator

"...Reactive power of one turbo generator TG-702 goes into negative value at / just before the time of alarm and other generator supplies excessive reactive power..."

The situation that you describe is typical when there are two machines tied together, share common protective relaying, and the rate of reactive output of one machine does not match the other. As 67 Model points out, in paralleled machines, if one machine cannot meet the demand put on it by the load, then the other tries to make up for the difference. The question is, what is causing the drop in excitation on one machine? Unfortunately your drawings do not not include any details on the AVR/Excitation system, its inputs/outputs, and/or settings, so all that can be offered are generic suggestions.

Some places to look:

Polarity of all PTs and CTs,

Settings of both AVRs must be identical; gain, damping, sensitivity, droop, etc.

Both generators must be in the same mode; parallel, isolated, only one Master,

If one generator consistently reaches a point where its reactive power starts falling, then there could be thermally or load related issues such as: shorted turns in the rotor winding, overheating connections, failing diodes/fuses, poor contact makeup, etc., etc.

GE knows their equipment very well, so make sure that everything that on your side of the boundaries are in perfect working order, otherwise you will be responsible for any repairs outside of the scope of the rebuild.