Energisation of Power Transformer 4

[kssschsekhar]

Dear Folks,
We would like to energise 300/150/150MVA (220/15/15kV) transformer (GSU) with one of the Generators (110MW,15kV) connected to 150MVA winding of the Transformer. Someone please discuss what are the limitations of energising GSU transformer with Generators.
Thanks a Lot.

 

[davidbeach]

Energizing with a generator as opposed to what?

 

[kssschsekhar]

I would like to know " What are the limitations of enerising the transformer with no-load generator". Is it possible to energise a 300MVA transformer with 150MVA Generator(un-loaded).
Your partcipation is highly appreciated. Advanced Thanks..

 

[ScottyUK]

I suspect you normally energise this transformer from the HV side and synchronise the generators independently to the respective LV windings, correct?

Assuming you have circuit breaker on the generator bus it would be much less onerous on the generator if the transformer was energised as the generator excitation builds up, i.e. with the generator CB closed, but you should be able to energise it by closing the generator breaker.


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

 

[slavag]

Hi.
Limitations...
Maybe need check only one point.
Voltages on the not used LV side. I think you have OLTC on the HV side, maybe some influence and possible some overvoltge situation on the not used LV winding.
Just suggestion.
Best Regards.
Slava

 

[kssschsekhar]

During Black start, no power supply is available on HV side. Generator needs to energise GSU transformer. Is GCB the main limitation for energising the GSU transformer by generator (at 100% terminal voltage)?

 

[slavag]

Why not?
Isn't problem, you can energize GSU from HV and LV side, no problem, standard situation.
Only check in synch system option for dead-line for the GCB, it's all.
Best Regards.
Slava

 

[ScottyUK]

If you are in a black start condition then make life easy for the generator and close the GCB before energising the field. The excitation system will gently ramp up the voltage to nominal. There's no point in having a large inrush on the generator when there is no need to do so, even though the generator should be capable of withstanding the inrush without difficulty.


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

 

[davidbeach]

The generator will like it better if the breaker between the transformer and generator is closed before starting the generator, but it should work either way.

 

[ScottyUK]

Same thought at the same time!


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

 

[kssschsekhar]

"Generator capability to withstand inrush current"- How can we confirm this? Is Generator manufacturer confirms Generator withstand capability of inrush current?

 

[dpc]

If you do as suggested and energized generator and transformer at the same time (GCB closed), then there is little or no inrush. It's done all the time. A lot of large generators do not even have a breaker between the generator and GSU.

"The more the universe seems comprehensible, the more it also seems pointless." -- Steven Weinberg

 

[rcw retired EE]

We do it frequently, always trying to do it with the breaker closed as everyone else stated. Some generator suppliers get very nervous about the inrush on an unloaded generator. I never quite understood why.

 

[nawao]

Inrush currents may be seen as earth fault or differential fault by the proteccion devices in some cases depending on the residual magnetism of the transformer. Is wise to energise the transformer as suggested with the GCB closed.

 

[slavag]

Hi.
What is a difference with close of industrial generators on the load in case of black start, it's same, inrush of all unloaded trafos.

Im not like this idea, close GCB on dead-dead situation, it's request additional logic/bypass in synchrosystem and in "mechanical" protections:
for example: close GCB&open field CB=trip.

But all possible: need build additional logic bypass or emergency close GCB with lot of interlocks.

Best Regards.
Slava

 

[slavag]

BTW, not only "mechnical" protection, same for the 27 undervotage protection, in lot of application where used this function ( I don't know, for what 27 in generator protection, except special application)this function blocked by CB position.

 

[kssschsekhar]

I agree with Slavag opinion.
"Some generator suppliers get very nervous about the inrush on an unloaded generator" - So, Generator manufacturer confirmation is required for energising GSU tranformer.

 

[davidbeach]

Connect the transformer and the generator before energizing the generator, or at least before applying the field, and there will be no inrush.

 

[PHovnanian]

kssschsekhar, there has been some good advice regarding energizing the GSU from the generator side. Stress on the generator, protection behavior and black start capabilities have been addressed.

My question is: what next? Is the plan to synchronize the GSU
high side with the outside system? If so, it might be a good idea to review the protection and high side breaker control logic to make sure that it supports this function.

 

[kssschsekhar]

With all the discussion, I understood, huge inrush currents are constrains. How can we find the Generator capability to withstand inrush currents?