B-H characteristic of large power transformers

[Bronzeado]

For electrical people and transformer experts in this forum,

In general, the complete B-H curve (based on the V-I curve) of large power transformers (above 100MVA) can not measured in manufacture laboratories. This is because of these labs has not anought short-circuit power to saturate deeply large transformers.

Normally, the V-I (B-H) curve is measured in a winding assembled on the ferromagnetic core by supplying voltage up to 120% of the transformer nominal voltage. After that, it is measured the V-I (B-H) curve in the same winding without the ferromagnetic core (air core reactance) and then these two curves are joined graphically.

My questions are:

1) At which point these two curves should be joined?

2) Could that point be considered as the "knee point" of the B-H curve?

These questions heve been posted to magnetic people as well.

Best regards,

Herivelto Bronzeado

 

[edison123]

I have not heard of such B-H curve tests being done in trafos. May be trafo experts like prc will chip in.

Muthu

http://www.edison.co.in

 

[prc]

Usually nobody asks to measure the B_H of transformer as edison pointed out.In Reactors it used to be .Then it is measured by applying DC voltage.The point at which transformer core gets completely saturated is at 2.0 Tesla.So you may join the two graphs at this point.

 

[pie314]

You need to perform the test up to the voltage that will excite the core at 2 Tesla's, and then join the two curves at that point. This will produce a B-H curve. The knee of the curve will be at 1 Tesla.

 

[Bronzeado]

Thank you folks, for your replies,

prc, we use to ask for the B-H curve in our transformer type test as we need this curve to study transformer energizing to asses the impact of the inrush current on the power system.

As the B-H curve given by the manufacturer is obtained in a way I described in my first post, I wonder if anybody knows another method or if that method may be considered correct.

hktrial, the manufacturer lab has no sufficient power to saturate a big transformer with ac voltage. At 2 tesla the current is very high, almost a short circit current.

How can I join the two curves at 2 tesla if I do not have this point? Should I extrapolate the points in the curve measured up to 120% of the transformer nominal voltage?

Best regard,

Herivelto Bronzeado

 

[pie314]

Bronzeado

I agree that it is a difficult measurement, but it is not a short circuit. The best way is to use the calculated BH curve which should give values up to 2 Tesla's
Use your measured values up to 120% volts to check the calculated values.