I/O expansion module PFTL101B 2.0KN

I/O expansion module PFTL101B 2.0KN

I/O expansion module PFTL101B 2.0KN

In general the zero sequence voltage is higher than the negative sequence voltage at the fault, but decreases more rapidly the further away from the fault it is measured. This makes the -U0 polarization preferable in short line applications, where no mutual coupling problems exist. Negative sequence polarization has the following advantages compared to zero sequence polarization:

• on solidly earthed systems U2 may be larger than U0 . If the bus behind the IED location is a strong zero-sequence source, the negative sequence voltage available at the IED location is higher than the zero-sequence voltage.

• negative sequence polarization is not affected by zero sequence mutual coupling (zero sequence polarized directional elements may misoperate in parallel lines with high zero-sequence mutual coupling and isolated zero sequence sources).

• negative sequence polarization is less affected by the effects of VT neutral shift (possible caused by unearthed or multiple earths on the supplying VT neutral)

• no open-delta winding is needed in VTs as only 2 VTs are required (U2 = (UL12 – a · UL23)/3) The zero sequence current polarized earth directional unit compares zero sequence current I0 of the line with some reference zero-sequence current, for example the current in the neutral of a power transformer. The relay characteristic AngleRCA is fixed and equals 0 degrees.

Care must be taken to ensure that neutral current direction remains unchanged during all network configurations and faults, and therefore all transformer configurations/constructions are not suitable for polarization. In dual polarization, zero sequence voltage polarization and zero sequence current polarization elements function in a “one-out-of-two mode”. Typically when the zero