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Tapchanger on LV side of 275kV Transformer

Tapchanger on LV side of 275kV Transformer

Tapchanger on LV side of 275kV Transformer

(OP)
Hi

We've requested a 50MVA, 275/22kV, delta-star transformer for a mine. Supplier came back and offers a transformer where the tapchanging is done on the 22kV side. This leads to 1 x 3phase tapchanger being used at 11kV instead of 3 x singlephase tapchangers at 275kV. The flux density with a 275kV tapchanger is 1.75 Tesla - constant.

With a 22kV tapchanger the flux density varies from 1.42T at tap 17 to 1.76T at tap 1.

A booster trfr has been included inside the trfr tank to reduce the currents in the 22kV tapchanger. The LV winding effectively consists of a fixed part, the booster and the tapchanger (at the neutral end).

According to manufacturer LV tapchanging is less complex and reduces the voltage stress on the tapchanger. The increased current is offset by the booster though how effective this is I have no idea.

Anybody have any experience with this before? Anything we should look out for or consider?

Thanks.

RE: Tapchanger on LV side of 275kV Transformer

The manufacturer may be right.In fact in US at MV and HV ranges such tap changer arrangement is common.

But I feel probably the price difference with HV tap changer may not be so high when we consider the extra cost of booster + extra core cost due to variable flux regulation.

With Hv tap changer it is also possible to manage with 2 pole tap changer, with one pole accommodating two phases.( Providing tappings at corner of delta with separate regulating winding)

But my question, why you want delta connection on HV side.Why not star with neutral solidly grounded so that you can benefit from graded insulation for Hv winding,simple neutral end tap changer on HV?

RE: Tapchanger on LV side of 275kV Transformer

(OP)
prc,

One of the problems with the HV being a star winding with a solidly earthed neutral is earthfault relaying. The 275/22kV trfr in essence is a zero sequence source for 275kV earthfaults and it can cause upstream distance relays to underreach for line faults close to the transformer if there is arc resistance. Of course there are ways of overcoming it but the situation becomes more tricky if you have several feeders with star HV winding trfrs in relatively close proximity.

I am interested to know what the experience is in US utilities who have employed LV tapchanging with a booster.

Regards.

RE: Tapchanger on LV side of 275kV Transformer

veritas, US utilities are using such arrangement for many years.Definitely it becomes complex.In fact this was the arrangement used when HV tap changers with high current rating were not availableb for EHV auto-transformers,say 30 years back.

In India also industries taking power from utilities used to ask for delta on HV side.But later all of them changed over to star/star with solidly grounded neutral on both sides.

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