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Energisation of Power Transformer
4

Energisation of Power Transformer

Energisation of Power Transformer

(OP)
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.

RE: Energisation of Power Transformer

Energizing with a generator as opposed to what?

RE: Energisation of Power Transformer

(OP)
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..

RE: Energisation of Power Transformer

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!
 

RE: Energisation of Power Transformer

2
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  

RE: Energisation of Power Transformer

(OP)
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)?

RE: Energisation of Power Transformer

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

RE: Energisation of Power Transformer

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!
 

RE: Energisation of Power Transformer

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.

RE: Energisation of Power Transformer

Same thought at the same time!
  

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

RE: Energisation of Power Transformer

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

RE: Energisation of Power Transformer

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

RE: Energisation of Power Transformer

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.

RE: Energisation of Power Transformer

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.

RE: Energisation of Power Transformer

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

RE: Energisation of Power Transformer

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.

RE: Energisation of Power Transformer

(OP)
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.

RE: Energisation of Power Transformer

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

RE: Energisation of Power Transformer

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.

RE: Energisation of Power Transformer

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

RE: Energisation of Power Transformer

Manufacturers all brace their machines for a three-phase fault across the terminals. You need to ask the generator manufacturer if you expect the transformer inrush to exceed this value, which in my opinion is unlikely but possible.

When you close the generator CB onto a de-energised transformer the equipment you are stressing is the generator's AVR which will try to control terminal voltage in the presence of an under-damped, highly reactive and distorted inrush current which will likely have a significant DC component. That is a horrible load for any control system to work with, especially when it is optimised for operation under the very different conditions found during normal operation. You can expect some fairly aggressive field current and voltage variation as the AVR tries to control the terminal voltage.

Under a black start situation I would expect that common sense would be to make everything as gentle on the equipment as possible to maximise the chances of the generator successfully re-energising the grid. Stressing any element of the machine when there is an alternative which avoids that condition reduces the chances of the machine reaching, and reminaing in, service.
  

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

RE: Energisation of Power Transformer

(OP)
"Manufacturers all brace their machines for a three-phase fault across the terminals. You need to ask the generator manufacturer if you expect the transformer inrush to exceed this value, which in my opinion is unlikely but possible."

Generator 3-ph short circuit current: 14 In
Expected inrush current on Generator (110MW) during 300MVA transformer energisation : 21.8 X In  ("In"- Generator rated current)

With the above status, Generator may not withstand inrush current. So, alternative method (first close GCB then increse the excitation) described before is the only way to energise the transformer.

RE: Energisation of Power Transformer

21.8x? Wow, that is high. Where did you get that value?
  

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

RE: Energisation of Power Transformer

(OP)
Transformer rating is 300MVA (300/150/150MVA, 220/15/15kV),
Generator rating is 150MVA, 15kV
Transformer inrush current on LV side: approx. 10 times of (300x10^6/(1.732x15000)) which is equvalent of 21.8times of Generator rated current.

RE: Energisation of Power Transformer

(OP)
Now I have a question of 3-winding transformer (300/150/150MVA) inrush current: If we energise the transformer from 300MVA side- inrush current/MVA is approx.10times 300MVA.
If we energise the transformer from 150MVA side- Inrush current / MVA: Is it 10 times of 300MVA or 150MVA?

RE: Energisation of Power Transformer

kssschsekhar, my guess is that the inrush will be 10x 150 MVA when energizing the 150MVA winding. This is because the 150MVA winding probably has a higher leakage inductance/resistance then the 300MVA winding.

But this is all guesswork without the actual numbers and some analysis. To what degree of certainty do you know that 10x inrush figure? Is it one of those rules of thumb or an actual spec from the manufacturer?

RE: Energisation of Power Transformer

Good Q Kssschsekhar , very good.
Im not remeber, I think like to PHovnanian , 10x of 150MVA rated current, but not sure.

For avoid all problems, need build this additional bypass logic and close GCB before energazing GSU, not a nice solution, but solution.

Thank you for start this thread.
Good Luck.
Slava

RE: Energisation of Power Transformer

"For avoid all problems, need build this additional bypass logic and close GCB before energazing GSU, not a nice solution, but solution."

It is a beautiful solution. Unless you are a protection engineer! tongue
  

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

RE: Energisation of Power Transformer

kssschsekhar,
In my previous employment, I have had the same questions regarding GSU-generator energization. What we have was a directly-coupled GSU-generator. I understood the peculiar design was to do away with GSU inrush.
Please read and study davidbeach's post 9 May 09 12:13.

RE: Energisation of Power Transformer

(OP)
Thanks for all your Great partcipation and sharing your experiences.

Today we got reference that exactly similar type of setup is being used in one of the power plants. Logic is "close the circuit breaker first and then ramp up excitation to avoid inrush current."

For finding transformer inrush current, is it require Transient Stability program or EMTAP (Electromagnetic Transient Analysis program)?  

RE: Energisation of Power Transformer

(OP)
Again, I am going to basics. Is it possible to get inrush current on Generator (while charging the transformer) more than three phase short-circuit current?

RE: Energisation of Power Transformer

Quote (kssschsekhar):

Is it possible to get inrush current on Generator (while charging the transformer) more than three phase short-circuit current?
The simple answer (for a given field current) is no. The transformer will provide an impedance that will limit the current to less than the value available for a bolted fault.

The correct answer is 'it depends'. In particular, on the response of the generator's AVR. Since (one would assume) the protection will operate to remove a fault whereas one expects no protection operation during transformer energization, the latter condition could produce greater transient stresses on some system components than the fault.  

RE: Energisation of Power Transformer

Hi, kssschsekhar

About your question:
"If we energise the transformer from 300MVA side- inrush current/MVA is approx. 10times 300MVA. If we energise the transformer from 150MVA side- Inrush current / MVA: Is it 10 times of 300MVA or 150MVA?"

The answer is:
In general, the LV windings have a cross section area smaller than the HV windings. So, if we energise the transformer via the LV windings the inrush current will be bigger (in pu) than if the transformer is energised via the HV windings.

ScottyUK and davidbeach suggest:
"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."

My comments is:
That is really a beautfull solution! If this operation is possible, go for it.

Another solution would be to energise the transformer considering a "point on wave" switching strategy, i.e, making a circuit breaker controlled closing at instants at which no inrush currents are produced.

Finally, if  you want anyway to energise the transformer by colsing the breaker on the unloaded generator, you will  stress both the generator and the transformer with great probability to get overcurrent and/or diferential trips.  

Best regards,

Herivelto Bronzeado
    

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