Zero Sequence Current

Zero sequence current polarization is preferred for applications with low ground source impedance.

Figure 1-1 illustrates relay connections that obtain zero sequence current (3I0

) from a separate current

transformer in the grounded neutral of a two winding wye-delta power transformer. Figure 1-2 illustrates

relay connections that obtain zero sequence current from a current transformer(s) in the grounded

neutral(s) of a three winding transformer. The magnitude of the zero sequence current used for polarizing

may be different from that of the ground current seen by the measuring element(s) of the relay.

Zero Sequence Voltage

Zero sequence voltage polarization is preferred for higher ground source impedances. Refer to Figures 1

1 and 1-2 for relay connections that obtain zero sequence voltage (3V0

) from a set of grounded wye

broken delta voltage transformers. The voltage polarized relay has a directional polarization adjustment to

match the impedance angle of the protected line to the characteristic angle of the relay. This is the angle

about which the directional angle is centered. A choice of line angles allows the relay to be adjusted to

include the resistance component of the ground fault.