GSU unbalanced load considerations or concerns ?

[ACCOTTER]

This is a hypothetical scenario considering that there is no protection is in place to prevent the following:

While backfeeding a combined cycle power plant via the GSU [HS (System Side) connected in Wye and LS (Gen Side) in Delta] the upstream switchyard breaker opens but only:

a) 1 pole opens
b) 2 poles open

What negative effects or concerns, if any, will the GSU (~350MVA) encounter by carrying an unbalanced load for an X amount of time given scenarios a or b?
Appreciate the input, Thanks!

 

[dpc]

I don't the transformer is going to care much, but the entire plant would be single-phased and all of your motors would be at risk. Under scenario b, the motors probably would not continue to run, but they certainly would under scenario a.

Single-phasing is not damaging to a transformer as long as the windings are not overloaded.

 

[ScottyUK]

If it is a GSU then the generator rotor is going to be in trouble long before the transformer.

The effect of a single pole closed depends somewhat on the earthing system employed.


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

 

[ACCOTTER]

Hey ScottyUK,

I didn't mention that the Generator Circuit Breaker is open during these scenario's....

clos

 

[slavag]

Hi Clos.
I assume you say about aux. load is connected on the generator voltage bus between generator CB and GSU trafo. And this load will feedeing by non three phase grid connection.
this load is about 10-20% of GSU trafo rating.
That means nothing do to GSU trafo, As Dave saied:
"all of your motors would be at risk" of thermal overload.
BTW, isn't hypothetical scenario!!!! If you have HV CB build from three single pole CB with three close coils connected in parallel is possible and HAPPEND.
for avoid this situation your CB must include PDF- pole discreapncy function is operated directly to trip coil and installed in the local CB cubicle.
Best Regards.
Slava

 

[AMBMI]

Perhaps I read too quickly but what about the negative sequence element of the generator protective relay?
I think that for this size of machine it should be set about 8%In and it should disconnect the generator quite quickly.
Otherwise we could have a quite dangerous overheating of the generator rotor (wave rotating at 2*fn)

 

[slavag]

Hi AMBMI.
You read quikly , generator CB is open according to Clos.
Best Regards.
Slava

 

[waross]

How is the wye neutral connected?
If the wye neutral is connected to the system neutral, a three transformer bank will continue to supply three phase as an open delta bank with one primary phase missing.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[slavag]

Hi.
Bill. it should be neutral solid grounded wye trafo ( >350MVA).
From experience. We started 10.5MW synch motor via 170/6.9kV 25MVA trafo, two times motor triped only on the unbalance protection. All voltages was O.K. and aux trafo supplied all other small load. After half hours we found reason:170kV CB pole was disconnected .
Regards.
Slava

 

[ACCOTTER]

Yes that is correct slavag...

By the way, you say that you had a similar situation occur and your motors were tripping on unbalance protection. If you were backfeeding motors via the GSU and Unit Aux Tfr, with GCB open, and a single pole on your upstream switchyard CB did not open, why didn’t one of your differential protection zones on either GSU or Unit Aux Tfr pick up before unbalance load protection on your motors picked up? Were the Aux loads low enough to the point were you were unable to pick up the necessary operating vs. restraint currents on your diff protection? Am I missing some information on your scenario?

Thanks and Regards!!

 

[slavag]

Hi Clos.You didn't missing.
It wasn't same situation. it was some pump station: two tafo 25MVA are feedeng three 10.5MW synch motors ( it was test new protection after retrofit).
BUT from protection point of wiew it was same.
Diff protection isn't sensetive to loss of phase situation.
in this case only unbalance protection operated , or NPS current, or NPS voltage.
We had few topics on the issue in the Forum.
Don't forget, for the Y trafo connection: or by protective relay, or by imterposing CT Y windings are conversed to D connection .
BTW, in the newer BBP/BFP protection possible used PDF signals as additional protection functionality.
Best Regards.
Slava