Offset Mho Characteristic; Fault Configuration - Siemens 7SG164 Ohmega 400 Series Manual

1.1.2 Offset Mho Characteristic

1.1.3 Fault Configuration

A distance relay must measure the impedance correctly for all types of power system faults (single-
phase, two-phase and three phase). For each fault type the effective impedance at the relaying point
will be different because the path that the current takes will be different in each case, as illustrated in
figure 3 below ;
©2010 Siemens Protection Devices Limited
7SG164 Ohmega 400 Series Impedance Functions
The offset Mho characteristic is shown in figure
2. The characteristic is set with a forward reach,
ZF and a reverse reach, ZR. This type
characteristic may be selected for Zone 3, and it
provides time delayed back-up for faults behind
the relaying point.
In addition to this, the origin is not a boundary
condition as it is for zones 1 and 2, so the offset
zone can be used in schemes to provide positive
operation for marginal conditions (see Switch-
onto-fault logic).
of
Chapter 2 Page 5 of 21