430 MVA GSU Tripping on 51G1N (Neutral Ground Overcurrent) 1

[Shark96]

We have a 430 MVA GSU (24/220kv Ynd)that tripped as soon as it was taken on load at around 30 MW. The only relay that picked up was "51G1N" HV side Neutral ground overcurrent relay. No other protections operated. The following activities were done:

1. Performed complete testing of the transformer including winding resistance, insulation resistance and turns ratio test. All came out okay.
2. DGA of transformer oil with no abnormal gases.
3. Tested the relay itself and its function at normal setpoint which is 0.1 Amps (120 Amps on primary neutral)

We energized the transformer again and it tripped on the same protection within 30 seconds. We have gone through every circuit that could be faulty except for the actual Neutral CT testing. The CT is inside the transformer body, we only have access of the secondary neutral conductor coming out of the transformer. The CT testing will also be performed but assuming the CT is fine what could be the source of this fault? The transformer's 220kV side is star connected with neutral that is solid/fixed grounded.

Any other information required can be provided.

Thanks.

 

[Skogsgurra]

With the information given "at 30 MW load..." I would suspect an actual fault in one of the loads. It doesn't trip below 30 MW, does it?

Is it possible to start with other circuits to see if it trips at 30 MW at all loads? Or did I misunderstand you?

I don't think that there are much harmonics at that level. But if there are, they don't balance well and can cause current from N to GND.



Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[mgtrp]

Do the protection relays provide any oscillography? That'd be the first place that I'd be looking for clues.

Neutral CTs don't usually produce false currents, even if misconnected, so I'd be wondering about the possibility of there being an open circuit on the HV side of the transformer (perhaps one of the circuit breakers or disconnectors not closing properly).

 

[RRaghunath]

30MW corresponds to ~120A at 220kV. This suggests that one of the phase CTs is wired to the 220kV 51G1N instead of 220kV neutral CT of the transformer.
Was phase and neutral CT wiring right up to relay (with appropriate current readings in the relay) verified as part of commissioning!!

Rompicherla Raghunath

 

[ScottyUK]

With that voltage ratio you should be able to circulate useful (i.e. measurable) current in the HV CT's using an LV generator connected to the delta primary and using the HV earth switches to steer current through the winding(s) that you want to prove. It will require a biggish LV generator - probably a couple of MVA. If you directly control the field current using a benchtop DC power supply then the generator forms a very powerful and very controllable 3-phase primary injection set. In principle you could use the main generator itself, but the steam turbine (or GT/ST if it's a single shaft machine) won't like extended running at no load so using a separate LV generator is a lot easier to arrange.

 

[prc]

I had met with similar situations. The neutral bushing CT polarity was wrong. Actual connection inside was interchanged compared to terminal board markings. Nowadays we provide a one turn primary test turn on the BCT to detect such issues easily.

 

[ScottyUK]

Hi prc - I had that exact problem on a GSU transformer. The field wiring was correct, but the wiring from the internal bushing CT to the terminal box was transposed. The restricted earth fault relay on the HV star winding used to trip in response to faults on the local transmission system. Once the transformer is in position, connected with bus duct either side, and filled with oil the testing gets harder to arrange!

 

[dpc]

If the element that is tripping is truly the 51N, that would be non-directional, so the polarity of the neutral bushing CT shouldn't really matter.

 

[marks1080]

If there is no legit fault I'd consider harmonics. I believe for zero sequence you're looking for 5th, 11th, 17th, etc.... This protection might be deployed to protect against over heating of the rotor.

 

[JG2828]

Can you attach the oscillographic records? Comtrade file or similar? Was this transformer energized and fine prior to it tripping, or is this a new installation?

 

[HamburgerHelper]

Zero sequence harmonics are odds tripling harmonics. 3,9, 15, ect. At least in SEL relays, I believe they operate only on the fundamental component for 51 elements.

------------------------------------------------------------------------------------------
If you can't explain it to a six year old, you don't understand it yourself.

 

[marks1080]

HH Thanks! Yes, I was getting my Zero and Negative sequences mixed up.... again... must be a day ending in 'Y' lol.

Sorry for that Shark96.

 

[FPelec]

My first guess would be a wiring error: as also suggested by RRaghunath, probably one of the phase CTs secondary currents is used to feed the 51G protection.
Fault recordindings should, however, clarify this issue!

Si duri puer ingeni videtur,
preconem facias vel architectum.

 

[slavag]

Please attach connection scheme.

 

[RRaghunath]

Shark96, You asked a question and then seem to have forgotten!
Won't you like to share your own findings with us!!

Rompicherla Raghunath

 

[Shark96]

Sorry guys I was stuck without an internet connection. We finally found the problem and it was a high resistance path in one of the GSU 220kV CT secondary phases. One of the "pole flash-over relay" coils was in the secondary circuit and it was open.
I have attached the three line drawing and marked the 51G1N relay with a RED circle. This is the relay that was operating. I have marked the coil of 'B' phase of the pole flash-over relay which was found open that caused unbalance in the secondary of the CT. The CT unbalance neutral current flows through the "51G1N" relay (operates it) and through grounds. I have shown this path with yellow arrows. Hope it is clear to everyone.
Apologies again for replying late. Thanks for your help.

 

[RRaghunath]

The open circuiting of phase CT secondary wiring would have caused operation of HiZ REF protection (87N-HV).
This would not have affected the 51G1N as per my understanding of the drawing.

Rompicherla Raghunath

 

[slavag]

Sorry, not understand.
51GN is direct connection,

 

[Shark96]

RRaghunath you are right. Actutally REF should have operated but in actual the S1 of Neutral CT is grounded in the field (wrongly so we are assuming), which led the current path as I have shown. Hope it is clear now. If we remove the ground on S1, the same fault would have caused REF operation. I have attached the drawing again with the ground added on S1.