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Holistic diagnosis across special transformers

One side always knows what the other side is doing

Transformers_substation

A phase-to-ground fault on one side of a power transformer can be seen as a phase-to-phase fault on the other side (e.g. Yd connected power transformer), but ABB’s Zoran Gajic set out to see if the same approach can be used with a special transformer, where the phase angle varies.


As well as demonstrating that it can be done, the research paper (which was presented at the Cigre Study Committee B5 Colloquium) shows the formulas necessary to transfer the fault current from one side to the other side of the special transformer based on the actual phase shift angle.

The calculations were done for angles between 60 and -60 degrees, as few special power transformers go beyond such angles, but within those parameters it proves possible to create a set of formulae enabling to predict how a fault current will be transferred to the other side of the transformer.

So, for example, in the case of a 20kA single phase to ground fault, on the load side of a 600MVA, 400kV/400kV, phase shifting transformer with an angle shift of 40 degrees we can calculate the source-side currents will be 85, 45, and 30%, resulting in currents of 17, 9, and 6kA respectively.

The same process can be applied for any type of fault and even for a converter transformer, permitting the calculation of High Voltage side currents from the faults on any low-voltage winding.

It has been shown that individual phase currents on the source side can be bigger (as a percentage) than the load-side currents, in the case of a phase-to-phase or phase-to-phase-to-ground fault on the load side. If this isn’t allowed for in the relay settings it could cause problems with coordinating backup protection, which should be a concern for anyone having commissioning variable-phase special transformers.




Zoran Gajić


Zoran Gajić was born in Serbia, former Yugoslavia in 1965. He received his Diploma Engineer Degree with honors in electrical power engineering from University of Belgrade, Serbia in 1990 and PhD degree in electrical power engineering from Lund University, Sweden in 2008. Read more about Zoran Gajić

Since 1993 he has been working in the area of power system protection and control within ABB Group of companies, where he had various engineering positions. Currently he has position of Global Product Manager for Transformer and Generator Protection with ABB SA Products in Sweden.

Dr. Gajić is a member of various Cigré Study Committee B5 and IEC/MT4 working groups. He has published numerous technical papers in the relay protection area. His main working areas are practical applications of protection relays, computer applications for protection and control of electrical power system, development of advanced protection algorithms for numerical relays and power system simulations. Zoran is holder/co-holder of several patents.



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