zero sequence

[kobavr]

hi,
How can we calculate the zero sequence impedance of a
transformer? Can we do by using nameplate values?

 

[davidbeach]

The best method is a zero sequence impedance test. Absent that, there are references, St. Pierre's Short Circuit book the most complete to my knowledge, that have tables with the general relationship between positive and zero sequence impedances of many different transformer configurations. See faq238-1287.

 

[kobavr]

Thank you. Do you have a link to download St. Pierre's Short Circuit book.?

 

[davidbeach]

You buy it in hard cover from the author.

 

[magoo2]

You can also test it. You need to have a variable voltage source. Set up a short ciruit test.

In other words, jumper the three high-side phases together. Also, jumper the low-side phases together. Hook up a variable voltage source to the primary and connect it between one of the primary terminals to the one of the low voltage terminals. Want to have a CT around one of the phase leads before it gets to the jumper.

Gradually increase the voltage from 0 until you get rated secondary current. Suppose if you have a 33 kV primary with a 5% impedance suspected. It should take about 953 V to give you rated secondary current. The actual %Z will be the voltage needed to give rated current divided by the line-to-neutral current time 100.

To get a %R and %X, you need to have a wattmeter in the circuit or a means of measuring power factor.

 

[davidbeach]

magoo2, that could work for a wye-wye transformer, need to tie the X0-H0 terminals together also. For any transformer with at least one wye, pick the highest voltage wye, jumper all those leads together, short out other windings, and drive voltage between phase and neutral on the wye until rated current is achieved, then measure voltage between phase and neutral of the transformer, per cent of nominal voltage is per cent impedance. No need to test a transformer with only delta windings, Z0 can be approximated as [∞] in that case.

 

[magoo2]

David,

You are quite right. You can't energize a jumpered delta winding and expect to get any current to flow on the other side. Nonetheless, if it's a delta wye transformer connection, you can energize the wye side in this manner and short out the delta side.

For a zero sequence model, knowing that you have an infinite impedance on the delta side is important, but you also need to know what you get on the wye side. My suggestion will satisfy that part.

 

[davidbeach]

Just want to make sure that nobody else tried to do it from the delta side.