15 % Transformer Impedance Limitations

[Timmy76]

Hello All

I have 4 115/24 kV 60 MVA transformer banks with 8% impedance. When all the banks are tied the fault levels becomes high. feeder reactors are used to limit fault current on the feeder side. But 24 Kv side has fault levels that require use of 40 KA breakers. But I am planning to increase the impedance of transformer from 8% to 12 or 15% so that I could use 25 ka breakers..

I do understand that there would be 0.12 p.u voltage drop across the transformer for 1p.u of voltage. At the same time tap changers on 24 kv side would correct it. But tap changer can only correct +/- 10% tap.

I would appreciate if you could advise on what other implications would be there to increase transformer impedance to a high value to limit fault current. One of the other method I read about it inserting reactors between transformer and transformer breaker.

Your response is highly appreciated.

 

[mgtrp]

Do some load flow calculations - you'll probably find that the voltage drop is much less than you think if you do the calculations correctly. The transformer impedance is mainly reactive, while the load is (presumably) mainly resistive so the voltage vectors don't add in a straight line.

 

[FPelec]

Timmy76,
12% or 15% are usual impedance values for large HV/MV transformers and do not pose particular manufacturing problems.
As a rule of thumb, considering a power factor 0.9 you should have a voltage drop equal to about half the value of the percentage impedance (6% or 7,5%), thus if the high voltage source is stiff the +/- 10% regulation could be sufficient. Otherwise you have to consider the extra cost of a larger regulating range versus the extra cost of circuit breakers with 40 kA rating.

Si duri puer ingeni videtur,
preconem facias vel architectum.

 

[DTR2011]

Any concern about additional load and no load losses?

 

[Timmy76]

Hi

In this case my X/R ratio is 25 at the feeder after the feeder reactor. Does it impact how i would select the breaker.

 

[waross]

Look at
Percentage Impedance vs Percentage Voltage Regulation
thread238-328234

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

 

[ScottyUK]

I don't deal with ANSI breakers so I'm not very familiar with the C37 set of standards, but that's a reasonably high X/R so you should certainly look at your breaker capability. You might find yourself using the 40kA breaker to deal with the high X/R ratio rather than the 25kA type. Definitely something to explore before you add reactors!

 

[Khalid1970]

Timmy


For the voltage regulation, capacitor banks for the PF correction can help.


This issue is discussed as well in the article I put in my reply to your same related thread of today