52There is a potential problem arising from CT secondaries in Wye when those CTs are measuring currents on the Wye-connected side of a power transformer, and that is the problem of zero sequence currents. A “zero sequence” set of currents is equivalent to in-phase currents flowing through all three lines of a three-phase power system, lacking the normal 120 degree phase shift from each other. The mathematical foundations of this concept are beyond the immediate scope of this section (for more information, refer to section 5.8.4 on “Symmetrical Components” beginning on page 821), but suffice to say zero-sequence currents are found in certain fault conditions as well as circuits containing “triplen” harmonics (i.e. harmonic frequencies that are some multiple of 3× the fundamental, e.g. 180 Hz, 240 Hz, 540 Hz for a 60 Hz power system). Zero-sequence currents flow through the neutral conductor in a 4-wire Wye-connected system, but circle through the phase elements of a Delta-connected system. This means a Wye-Delta connected transformer where a fourth conductor attaches to the center of the Wye winding set may experience line currents on the Wye side that are not seen in the line conductors of the Delta side, and may therefore cause a differential current relay to operate. This is another reason why connecting CTs differently than the power transformer windings they sense (i.e. Delta-connected CTs on a power transformer’s Wye side) is a good idea: any zero-sequence currents within the power transformer’s Wye-connected winding will circulate harmlessly through the Delta-connected CT secondaries and never enter the 87 relay. For digitally compensated 87 relay installations where all CTs are Wye-connected, the relay must also be configured to mathematically cancel out any zero-sequence currents on the Wye-connected side of the power transformer.