What Happens When a Fuse Blows on the High-Tension Side of a Transformer?

If a synchronous motor is running on the low-tension side then it may stall, and a stall current may take out its protection fuses. If a number of motors are running, the effect may be more serious and they may collectively take out a number of fuses further upstream, back towards this transformer.

It is difficult to say exactly what will happen other than some time will need to be spent by a competent person identifying the location and cause of the fault, and applying sufficient remedies to get the system operating again.

When one phase is lost on a three phase Delte-Wye connection the high side becomes an open-delta (an acceptacle configuration for transformers) the result will be a dimming on all three secondary phases and a further shift in the phase angle. (A three phase transformer bank already causes a 30 degree phase shift from the high to the low side.)

I guess it is not true, because in open-delta the three terminals are still connected and one of the windings is missing, but in my question, one of the terminals is missing, and the three windings are still connected.

Look at the figure below:

See & think over

The answer to this case an anyother, in which the three phase balanced electrical system looses one or even two of the feeding lines, is resolved with "symetrical components", in general this case belongs to "unsymmetrical faults" , in a power system. An excellent textbook for this topic is Elements of Power System Analysis, by William D. Stevenson, I don't know what's the latest edition, I had this as a texbook in my electrical engineering studies, some 30 plus years ago.

Let me tell you, it is not an easy problem to resolve. But, with today computer tools available, shouldn't be much of a problem. The most important thing is, first of all, understand the escence of the problem.

Genaro Chacon

As an addition to Mr.Chacon's answer, i think that symetrical components should be used to solve this unbalanced system because when one phase failed, the system probably continue to run, but it will be in an unbalanced condition. Hence, the delta equivalent of the transformer should be replaced with a wye equivalent by dividing the voltage value with sqrt(3) and shifting the phase angles with 30 degee. After doing these, by using the symetrical components method, the new conditions are able to be seen.

There is also a few programs to simulate this condition and Hadi Saadat's Power Transmission Systems textbook is an also convenient source.

Tuna Yekeler, İstanbul, Turkey

yay

can u show how does it shift 30 degree?? u mean the current is same but voltage leads 30 degree?

Top figure is normal conditions. Bottom figure is a loss of Phase A. It can be seen that the secondary coil 1 and 2 are now in series with the primary voltage across both coils. Each coil will see only 1/2 of the normal primary voltage while coils 1-3 and 2-3 see normal primary voltage.

is the voltage on the secondary side 1/3 of the normal voltage?