How do we select the CT ratio and CT connection for a differential Protection relay for installation at the primary/ secondary considering the phase angle of a delta/ zigzag power transformer having 30 MVA, 132/ 11.5 KV, Vector Group DZn10?
BTW if someone seriously talks about planning 30 MVA HV/MV transformer protection he should know the answer. This type of transformer is typically protected using dedicated digital tranformer protection relays and there's not only the transformer to protect. Chances are that there will also be an OLTC to control. Not even talking about the whole cooling and thermal monitoring, fire protection,...
Today's digital, mocroprocessor-controlled relays are pretty flexible in their capabilities. If you work with something like the Schweitzer Engineering SEL-487 relay, all the CTs are connected in wye configuration. You select ratios based on full-load current of the particular transformer winding on which they are installed, with relay accuracy class. The transformer voltage ratio and vector group then gets programmed into the relay, and it will know what to do with the relative phase angles of the currents. All you have to do then is know how to program the various protection parameters - Not something for amateurs! Hire a good protection engineer to do that for you!
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The same applies to other digital protection relays like for example ABB REG670 or Siemens SIPROTEC series relays. The art is programming the relays, it's really something for top experts because it's not that rare to find misconfigured protection relays and also testing is all but simple (have a look at Omicron products).
Differential Protection
Re: deferential Protection
