Parallel Operation of TG Sets

If each generator is connected to 33kV through a 2 winding transfo [not auto-transfo], then transfo isolates each generator & you can use high resistance stator star-point earthing to minimise generator damage if there is a stator earth fault.

But do you mean earthing star points of 33 kV HV windings of generator power transformers??

Dear Sir

Thanks for response.

The Gen trafos are solidly earthed on the 33 kV side. And All the TG's are earthed thru NGR/NGT. Also,Noted that earth fault currents will not get transferred to 33 kV side due to the presence of Delta/Star Step up Trafo.

My first query is

1) Whether we can allow multiple grounding ( of TG's) when there are multiple TG's operating in parallel ?

2) And if there is multiple grounding , will there be any third harmonic circulating currents? Is there any limit for allowable % of harmonic currents... ?

Regds

A1) It sounds as though the reference standards for this are either not present or not likely to be applied.

A2a) Yes.

A2b) The theoretical limit would be in the reference standards indicated in A1↑. However, the limit in practice will be the trip settings on the circuit protective devices.

Thank you sir.

Yr question 1 - Yes, resistance grounding of generators isolated by transfos is normal practice.

Q 2/ - There are harmonic currents. If you ground generator to same point as its transfo, there is little opportunity for circulation. If you have identical type gensets at equal loads there will be little imbalance between them to cause circulation

In any case, the usual practice for generators isolated by transfos is high resistance grounding, such that one gen phase earthed, putting V/√3 nominal across earthing transfo/resistor only causes about 5 amp earth current.

This 5 amp is negligible, if it circulates, compared to unavoidable earth currents of HV & LV power. The voltage across earthing resistor does have harmonics & the generator earth fault relay [usually voltage operated] has a harmonic filter so it responds to fundamental only. Harmonic voltages or currents have negligible effect on rms currents handled by earthing system, which must be rated for one line short to earth for seconds.

Thank U sir.

Dear Ravipra01,

I have realised, having seen rwilliams post #10, that in your post #2 you may have meant your two questions to be about when generators are directly in parallel without isolating transfos.

But you did not write "no isolating transfos" and yr question 1) did have "Whether we can allow" confining it to what you have at your plant.

Post #10 is a good description of earthing/protection & reasons for it where generators are directly in parallel in a 3 wire 3 phase system. The operation with a single earth limiting current to ~200A is because it minimises earth fault current to limit damage.

Two earths - each of adequate current to operate automatic earth fault protection - would increase damage if earth fault occurs. This is more important than circulation of currents between grounds, which is minor compared to actual ground currents.

Finally, there is good reason why high resistance grounding is not used in these systems (additional to there being insufficient earth current for reliable protection relay operation) - capacitance to ground of insulated cables reaches a level where L-C resonances cause damaging overvoltages following switching & faults. Use of a medium resistance damps resonances to a low Q while high resistance does not.

You may find the earthing/protection practices in distribution below 33KV in your plant of interest.

67model

Before the question can be answered correctly you need to supply:

1. the vector group,

2. whether there is a tertiary,

3. whether there is high or low resistance grounding.

Good interview question.

Sir,

1) The vector group of the GenTrafo is YNd1.

2) There is no tertiary winding in the GT.

3) For GT the 33 kV side is Y and the neutral is solidly grounded.

Regds

I was referring to the grounding resistance of each generator's neutral, and its purpose; i.e., protective relaying and/or limiting ground fault current.

Sir,

Both protective relay as well as limiting EF current.

Regds

for multiple medium resistance grounds, i.e. 4160V machines, the standard at one plant was to leave only one resistor in service, remaining resistors disconnected from ground.

The resistor connected to the step-up transformer was left connected, typically. Generators could then be scheduled off and on without worrying about grounding configuration. We did have to remember we had no ground fault protection when running separated from the rest of the plant, unless one of the generator resistors was connected. The higher damage that occurs when a ground fault eventually goes phase to phase was the risk.

The 3rd harmonic current that circulated in the multiple grounded resistors was enough to vaporize rain water from the one resistor outside, however was not enough to harm the resistor, nor enough to cause the protective relaying to pick up.

That resistor was a large one, 1000A for 10 second rating, though the ground protective relaying eventually was capable of sensing a 400A fault, and could have been as low as 200A for typical medium resistance grounded systems.

These medium resistance grounded power systems have motors applied at the medium voltage. The reason for medium resistance grounding was to reduce expensive iron damage in the motor stators during fault conditions, while allowing enough current for selective tripping (coordination) of the possible faulty components back to the source. Zero sequence (donut) sensors are typically applied as the first zone of coordination, with time discriminating for higher level components.

Thanks for an elaborate response, Sir.

However, I suspect that at 11kv, your grounding method is considered high resistance, due to high charging currents that would normally flow in the ground circuit, at that voltage. Medium resistance grounding up to 5kV class is common, not sure if the continuous current that a resistor sees with a 6900v system makes for a practical selection.

I have to assume that the 3rd harmonic currents are still circulating in the high resistance grounded system, but at a lower magnitude, and perhaps insignificant, as 67 Model suggests. The only guidance I have seen is to ensure the continuous rating of the resistor is higher than the calculated/estimated charging current. Most of my references cited empirically based values, modern computer based models can likely give you more accurate values.

i think it would be interesting for your audience to learn just what your neutral grounding transformer primary current is, when operating one two or 4 paralleled generators on your 11kv system. A simple clamp on ammeter on the ground side of the transformer should do the trick.

We assume your generators are feeding a delta winding of your step up transformer, with perhaps no artificial neutral / zig-zag transformer....?

Thanks for an elaborate response, Sir.