INDUCTIVE CURRENT BREAKING

[appunni]

Sir,
We have procured a new 66.67 MVA, 11/220KV delta/star transformer which is to be kept as spare. We decided to give 415Volts 3 Phase supply on 220KV side and to short 11KV side in order to get a heating effect during idle conditions. But somebody said me that if one phase became open circuit which will result in breaking of inductive current, it will make voltage rises. When I calculated the short circuit current on the 220KV side side, it was found about 2.5A. So I think breaking of that much current will not make any problem. But actually I do not know what will be the effects of breaking of inductive current in these situations. What will be the effects if the current were of full load quantity? I request for your valuable comments
With regards,
appunni

 

[jbartos]

Suggestion: The situation needs to be analyzed with respect to the hardware selection and system design. If hardware is adequately rated for voltage level and current level, including the current nature, the approach is technically feasible.

 

[heckef]

Sir,

As I see it, this is no problem to worry about.
Opening an inductive circuit can rise the voltages, but if you have connected 415 V (to the 220 kV), this will result in a 20,75 V !!! at the 11 kV-side (= 11 kV / 220 kV x 415 V). Maybe the voltage will rise when opening the circuit, but not by 500 times. And even then, you are within normale working conditions(voltage) of the transfo (500 x 20,7 V < 11 kV)

Regards,
heckef

 

[Kiribanda]

Hi appunni,

This is a very common method used in the field to keep a spare transformer in “ WARM STORAGE “ condition by making use of the rated percentage impedance to avoid any possible insulation breakdown. Since the voltage levels are very low comparing to the rated conditions, the flux density is also very low in the core. As such even one phase failure on the primary will not create a dangerous “ inductive current”.

You have to assure that the jumper wires used in the 11 kV side are strong enough to carry the secondary s/c current and your 415 V circuit breaker/ fuse and the 415 V feeder cable are sized enough to trip if there is an internal fault.

Just an information please? How much is the percentage impedance of the 66.67 MVA/ 220/ 11 kV/ YNd transformer? Is it 12 % at the principle tap?

Regards!
Kiribanda

 

[appunni]

To Kiribanda,
Yes sir. It is 12%.
With regards,
appunni