Differential protection for converter transformers

This paper will present differential protection schemes used for special Converter Transformers. These transformers are typically used to feed MV drives, power electronic or FACTS devises. Converter transformers are industrial power transformers which may have phase angle shift different from standard 30o or a multiple of 30o. Typical example is 24-pulse converter transformer with additional phase angle shift Θ of ±7,5°. Such special industrial transformers typically have three windings, but sometimes even up to five windings. The converter transformer additional phase angle shift Θ is typically obtained by special connections of its HV winding (e.g. extended-delta or zig-zag). However, if numerical differential protection for standard three-phase power transformers is directly applied for differential protection of such converter transformer, it will not be possible to compensate for additional, non-standard phase angle shift Θ caused by special winding connections. As result a permanent false differential current would appear. For the worst case when Θ=15° the false differential current of up to 26% of the through-load current will appear. Thus, the minimum pickup of the differential protection must be increased to at least double of this value and as a consequence differential relay will not be sensitive for the low level internal faults within the protected converter transformer. Therefore a special differential protection solution is often required. The paper will provide information how to design such differential schemes and present current waveforms from DRs captured in actual installations.