The three-phase diagram of connections is given in Figure 8.11; the
connections of the windings of the power transformer can be obtained from the
vector group. If the differential relays are connected on the Y side, then compensation
transformer number 2 should be connected in Yd5 in order to compensate for
the phase difference between the primary and secondary currents. There is no need
for phase compensation between the primary and tertiary windings, and a Yy0
connection for compensation transformer 1 is therefore the most appropriate
arrangement. It should be noted that the common point of the operating coils and
the neutral points of the compensation transformers and CTs must be connected to
only one earthing point in order to avoid maloperation during external faults. If
several earthing points are used, circulating currents could be induced during
external faults causing the relays to pick up inadvertently.