I am trying to what should be setting for overcurrent relay installed in a transformer with multitap.

the rated current at lowest tap is X amps
and at the highest tap is 2X amps.

 

Paru... I presume: a) your question is a theoretical one, based on X-Amp and 2X-Amp taps located in the xfmr's primary (that is, primary voltage could be 100% or 50% of some nominal value); and b) your question is not related to taps in feeder conductors emanating from the xfmr's secondary.

If my presumptions are correct, and the xfmr's:

1. primary voltage is over 600V.

2. secondary voltage is over 600V

3. rated impedance is less than 6%.

4. primary protective device is a circuit-breaker.

5. secondary protective device is a circuit-breaker.

Then, the primary protective device setting can be as high as 600% of the primary winding's current-rating (ignoring primary supply cable.)

Regards, Phil ([email protected])

 

Hi Phil,

the transformer is for a furnace having 33kv on primary. Based on various taps the nominal currents on primary is between for example between 1500 to 2000A.

if the I> and I>> setting in over current relay is 1.1In and 2In, as long as i assume protection of the HT breaker is concerned, it is OK.
But what about the protection of transformer at lower tap?

 

Paru… you really want information for setting the OV/OC protection relay for a transformer serving an EAF (Electrical Arc Furnace.)

It has multiple primary voltage taps. And, for those taps you listed the associated currents for just the 10 or so lower voltage taps, that is, 1.1In to 2.0In.

Is your goal to provide OV/OC protection because you have realized the difficulty in providing differential protection?

Please provide: the xfmr's nameplate data; the relay chosen; and the size and ratio of the CTs?

Regards, Phil

 

Paru... further to my earlier e-mail, how large are the secondary bus-bars connected to the xfmr's secondary bushings, before connection to the electrodes?

Regards, Phil