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.

 

[ScottyUK]

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.


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

 

[kssschsekhar]

"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.

 

[ScottyUK]

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


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

 

[kssschsekhar]

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.

 

[kssschsekhar]

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?

 

[PHovnanian]

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?

 

[slavag]

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

 

[ScottyUK]

"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!


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

 

[burnt2x]

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.

 

[kssschsekhar]

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)?

 

[kssschsekhar]

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?

 

[PHovnanian]

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.

 

[Bronzeado]

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