Demagnetising a transformer core - Practical help 1

[FarrUK]

Hi,

With regards to Off-Load commissioning tests, I have been reading about performing transformer winding resistance measurements and have seen that after performing them, a transformer core demagnetising operation is recommended before the transformer enters service in order to reduce inrush current.

Further reading suggested many methods of doing this, the easiest seems to be having a winding resistance tester with a built in demagnetisation function.

Another method was to pass multiple cycles of reducing DC current through the winding (or windings?) in alternating polarity. I have seen car batteries being mentioned.

My question is this; although I will most likely use a test set with a demagnetising function, how would I do it if I didn't have one, say for example a transformer had been taken out of service for a period of time and I wanted to demagnatise it before it re-entered service. Practical explanations would be very much appreciated identifying the components I'd need and any relevant safety information.

The transformers I will be commissioning are as follow;

• 3Ph 33 kV / 433 V Dyn11
• 1Ph 25 kV / 4 x 3.036 kV I ii ii
• 3Ph 33 kV / 2 x 3.483 kV Yy0d11

This is my first post, any help is appreciated.

 

[prc]

More than kV, KVA rating is critical for residual magnetism. I don't think one should worry too much on residual magnetism to reduce inrush current. Even if you remove residual magnetism, there will be inrush current if you switch at voltage zero on sine wave. But demagnetization may be required at site when you measure low voltage excitation current to check healthiness of windings after a line fault.
Of course as you said, best is to make use of the provision in instruments to demagnetize core. Dedicated equipment is also available for demagnetizing transformer cores.
If nothing is available, use a 24 V battery. Demagnetization will be required only in middle phase for 3 phase 3 limbed cores. Send a DC current between line and neutral of middle phase HV winding. Current required root 2 times the phase value of AC excitation current for rated voltage. Send current for 120 sec- reverse polarity and send a lower current for 100 sec. Again change the polarity reducing current value and duration of current. Caution-high voltage and arc can occur when breaking the inductive current. Provide parallel protective resistors in parallel to switch or use commercial DC breakers.

 

[waross]

Search this forum. This has been discussed several times.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[FarrUK]

Thanks for the replies, I will search the forum more thoroughly to see where this has been discussed before.