Phase angle compensation is discussed in the “Transformer Basics” section. EM transformer relays balance phase angle externally through the connection of the input CTs.
Wye transformer-winding CT inputs are connected in delta and delta-winding CT inputs are connected in wye to balance the 30ฐ phase shift. Frequently, in the case of older transformers that are being upgraded with digital protection, it is easier to retain the delta CT’s connections. The magnitude of the delta CT currents under normal balanced load conditions is 1.73 higher than the individual line current for each of the CT’s phases being subtracted to form the delta—that is,
Ia -Ic =1.73 Ia =1.73 Ic .
Many digital relays do not compensate for the increase in current magnitude so that
overcurrent relaying within the digital relay package being supplied from these same CTs must be compensated by setting the relays at 1.73 higher.
Relay metering also is incorrect by the same value. This has been a source of confusion in some applications.
There are, however, manufacturers that do compensate and provide overcurrent and metering with correct magnitude line currents.