Backfeeding generator transformer 2

[edison123]

A 11 KV, 25 MW hydro generator was refurbished with new stator & rotor windings along with replacement of old rotating dc exciter with static exciter. As a pre-commissioning requirement, Gen. OCC & SCC tests have to be done for which excitation is required.

After isolating the generator from the 11 KV bus, can we back charge the Generator Transformer (GT is rated 11 KV / 132 KV, 28 MVA) to avail the 11 KV supply for the static exciter transformer ? Any possible problems because of reverse power flow in the GT ? Total excitation requirement is about 100 KW.

We will be bypassing the reverse power protection during these tests.

 

[ScottyUK]

Hi edison,

I don't think you will have any problem with reverse power to the turbine provided the generator line terminals are disconnected from the GSU transformer. That would eliminate any means of motoring the generator. It seems an unusual way of doing this though: where is the regular field flashing supply for normal operation? Normally this would come from the turbine's critical services battery, but you don't mention such a supply.



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[edison123]

Thx Scotty.

They are still working on the field flashing 11 KV supply. But the scc & occ tests are kinda urgent like yesterday.

My only worry is that the GT has been idle for more than 4 months now due to the generator rewind. If 132 KV is switched in, will it bugger off ?

 

[ScottyUK]

Hi edison,

I'm awake early: 0320hrs over here! I had half-written a reply based on 'GT' being gas turbine...

Has the GSU transformer had any intrusive maintenance done on it? Were its DGA results good prior to being switched out? It isn't any different to, say, a bulk supply point transformer on the distribution network and there shouldn't be anything you need to do differently in terms of energising it. Obviously keep people out of the immediate vicinity while switching, but I don't imagine that you will have any problems at all.

Some will argue that it might be prudent to switch it in on the HV side overnight when the site is quiet and allow it to 'soak' just to get some warmth back in. The load you are presenting from the excitation system is so trivial that it is arguable whether this would bring any benefit.



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If we learn from our mistakes,
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[edison123]

Thx Scotty for your "red-eyed" reply. I am as addicted to this site as you are.

There was no maintenance done on the GSU trafo (btw, what does GSU stand for ?). That is worrying me since nobody knows the status of this 40 year old trafo.

Anyway, I plan to do an IR check before thinking of bunging in the 132 KV.

The load, as you say, is peanuts.

 

[ScottyUK]

GSU: Grid Step-Up

Definite good plan with IR test although I doubt you'd see anything other than a pretty gross fault. No maintenance could be good in that no-one will have disturbed anything, but in a four-month outage I'd have at least expected the tapchanger and diverter to get overhauled. I assume DGA trending to see if anything untoward has been developing over the long-term is just a pipedream?

Switch it in remotely and let it soak overnight, just in case.


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If we learn from our mistakes,
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[scottf]

Hmm....I always thought that GSU stood for Generator Step-Up Unit.

 

[ScottyUK]

scottf,

You've prompted me to see what the mighty Google has to say about GSU transformers. The majority verdict from the industry is that it stands for "Generator Step Up". I guess we were both almost right. I've fallen into bad ways listening to colleagues on this site!

As far as I am aware the term "unit transformer" applied to a generator module is usually an abbreviation of "unit auxiliary transformer" which is typically a fairly large transformer stepping down generator terminal voltage to a standard distribution voltage such as 6.6kV or 11kV for the auxiliary plant associated with the turbine and generator such as condensate pumps, FD/ID fans, etc.



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If we learn from our mistakes,
I'm getting a great education!

 

[edison123]

Scotts !!

Thank you both for GSU.

Will come back here after we are finished with this test.

 

[edison123]

It should have been Great Scotts!!

(for hundredth time, I wish this forum has editing facility)

 

[RRaghunath]

edison123,

I see the generator step-up transformer is 40years old. Better do simulation check of all the protections on the 132kV panel before energising the transformer. This will minimise the damage, if the transformer were to go defective while taking the charging inrush currents.

I say this because the transformer being generator step up duty, would not have faced charging inrush till date and 40 years of life might have made the transformer winding insulation (cellulose paper)brittle robbing the transformer of the ability to take the inrush and the consequent dynamic forces on the windings.

Better, I think if the back-charging is avoided.

Best wishes.

Coming to field flashing supplies, I don't think you need 11kV rated supplies. Normally, LVAC or station DC supplies are used as filed flashing supplies and should serve the purpose (for OCC/SCC).

 

[edison123]

Thx raghunath.

My feelings exactly on switching in 132 KV on a trafo that probably got used to all the pampering from the Genny all these years. No one has tried this stunt before.

As for using field flashing battery services for OCC & SCC, they cannot sustain the load for a long time since we will be doing a 12 hour temp rise test during each of these tests.

Other option being looked into is to feed LV supply direct to the thyristor panel of the static exciter. But this way would eliminate the static exciter step down trafo from pre-commissioning tests.

I preferred back feeding since it would sorta test the entire static exciting system (static exciter trafo, thyristor panel, cables etc.) in one shot before the actual commissioning.

I will be back here once we do this on how we did this.

 

[ScottyUK]

rraghunath,

Good point about the inrush on the transformer. I'd expect such a transformer to be very heavily braced as part of its design, given that some of the highest fault levels anywhere on a system at the generator LV terminals (phase-phase, not phase-earth). If the transformer was weak enough for inrush to be a problem I'd hope it would be retired from service as it would probably experience winding movement if a fault occured on the HV side of the transformer. However I like your suggestion of seeing if the protection can be increased in sensitivity for the purposes of this test. Good call.



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If we learn from our mistakes,
I'm getting a great education!

 

[bahrra]

Another issue to be concerned about is grounding. The GSU is generally a delta wye grounded with the ground on the load side. The generator will supply the ground on the generator side between the GSU and the aux. If this is the case and you energize the transformer from the high side, you could experience high voltage on the generator side. Also, there may not be any relay protection between the aux and the GSU for a ground fault.

 

[edison123]

bahrra,

The GT (or GSU) is HV Star and LV Delta (Yd1). No grounds on 132 KV side. No Unit Aux Trafos (UAT's) connected this being a hydro power station with not much auxiliaries.


Year of manufacture 1962. Time to retire this guy.

 

[RRaghunath]

ScottyUK,
Thanks. I would expect the shock that the GSU experiences when energised from 132kV side would be greater than that when the generator is connected and a fault takes place on the 132kV OHL. Two reasons, one that the combined impedance of generator + the GSU and the other that most of the faults in OHLs are phase - to - ground faults (less severe in general).

Bahrra,

I agree with the point you raised - the GSU LV winding, Exciation transformer HV winding and the connecting 11kV cables / bus duct have earth reference only when the generator is in circuit. With generator disconnected, the 11kV system is without earth reference and in the event of an earth fault while the OCC/SCC test is going on no current can circulate.

The earth fault can however be detected and annunciated if there is a residual voltage based protection connected to the open delta of the generator VTs is part of the generator protection scheme. edison123 may like to check.

 

[edison123]

On further discussions, I found out that isolating the generator (along with its grounded neutral), will put an overvoltage on the surge arresters. So, we plan to remove them from the 11 KV circuit. Any opinions on this ?

Bahrra and raghunath,

How to overcome this ground reference problem in the GSU LV side ?

Since new issues have developed on the mechanical front (governors, MIV etc.), the whole excercise might take place only by this month end.

 

[JBinCA]

Edison123,

If your arrestors were selected based upon the phase to ground voltage (which I understand to be the normal means if the gen is solidly grounded), you will have to remove the arrestors from the system to do your test.

I don't know of any good way to supply the ground reference that the generator normally supples in this case. I do know that without that ground reference, the Phase to Ground voltages will float around due to factors that are not in youur control. If you keep the arrestors in the system and voltage floats above their operating point, they wil blow up (by design).

JMB

 

[edison123]

A quick feedback.

After a short circuit dry-out (with the generator feeding the current), we did back-charge the generator trafo . No problem at all.

Thank you guys for your posts.

 

[ScottyUK]

Hi edison,

Good to hear things worked out. What did you do re. earthing the delta winding?

Thanks for the feedback.



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If we learn from our mistakes,
I'm getting a great education!