% Impedance of Power Transformer

Hi Phil

Can you kindly advise what is the effect of system fault level on transformer design.

I have a 1250kVA, 6.6/0.42kV, 5% transformer for a client who mentioned system fault level at 40kA.

Now the client is asking if the transformer is still OK if the system fault level is 63kA??

You valuable feedback is much appreciated.
 
M

Majid Rasheed

Dear Sir,

I am using 63kVA 20/.4kv, 4% impedance transformer. Will it have fault level current = 45.5 amps on primary side?
Am I right?
Or please tell me the actual way to calculate this. also what are the things which may affect the fault level.
 
M

Miguel Sanchez

I think it is important the definition of impedance as percentage of nominal voltage necessary to apply to the primary(or secondary) winding so to get nominal current flow thru the transformer with the secondary (or primary) winding terminals connected together in short circuit.

It is also very important to understand what makes a transformer have higher or lower impedance.

Impedance on a power transformer is mainly a function of the inductance. The resistive portion is not totally insignificant but it is irrelevant in this analysis.

In a perfect "Ideal" transformer the magnetic field created by one winding is 100% seen and wrapped by the other winding. Every single turn on both windings is affected by 100% of the magnetic field in the core.

The reason why a real transformer shows a value of impedance, more specifically, "inductance" is due to the fact that not all the field is shared by both windings. When each winding keep a portion of its field for itself they inherently build in the transformer two separate series reactances, one on the primary and one on the secondary. These are the reactors that stay inserted in the current path of the short circuit test or in a true short circuit.
 
We specified for a PV plant a 105/115MVA transformer with approximately Z=13% in order to comply with the upper short circuit level of 40kA at 132kV (client substation with 3000A busbars).

Now the client states that 13% is unusually low value and would cause a substantial increase in the zero-sequence fault contribution from the 800MW PV plant (8 x 105/115MVA transformers). For this reason, client asks for transformers with an impedance no less than 20,6%.

May You please give us Your opinion.
Thanks
 
Hi

Can you please let me know how to calculate the percentage impedance of 33000/690, 2600 KVA Transformer.
 
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