Distance Protection Functions; Polarised Mho Characteristic; Cross-Polarised Mho - Siemens 7SG164 Ohmega 400 Series Manual

7SG164 Ohmega 400 Series Impedance Functions

1 Distance Protection Functions

1.1 Polarised MHO Characteristic

1.1.1

Cross-Polarised MHO

It is fundamental to the requirements of discrimination that distance protection Zone 1 and 2
measuring characteristics for direct tripping are directional since they are required to detect faults in
the forward direction only.
As with any measuring device, operation on, or very close to, the boundary of operation will be less
decisive than that further inside the characteristic. It can be seen that the characteristic for Zones 1
and 2 pass through the origin, and thus, faults occurring very close to the relaying point will represent
a boundary condition. In order to improve the operating speed, and to ensure correct directional
response for such faults, a method known as cross-polarising is used.
A proportion (30%) of the voltage measured on a phase (or phases) not involved in the fault is added
to the fault voltage used by the comparator (after being shifted 90º to bring it into phase with the fault
voltage). The polarising voltage used will be different for each fault comparator, i.e. red-earth for a
yellow-blue fault, red-blue for a yellow earth fault. For balanced (three-phase) faults the voltage in
each phase will be equal, and so this will have no effect. For unbalanced faults, however, this "cross-
polarising" changes the overall shape of the characteristic into a circle of diameter Z - kZ S − as
F
shown in figure 1, when the current is flowing in the forward direction. It can be seen from this diagram
that the reach along the line angle is unaffected, but off angle, the characteristic expands. This
expansion gives an increasing coverage of the resistive axis, and allows detection of higher resistance
faults than the unpolarised mho characteristics. The healthy phase voltage, and thus the degree of
expansion will depend largely on the source impedance, and thus the shape of the characteristic will
depend upon the System Impedance Ratio (SIR). The higher the SIR, the greater the expansion.
When current flow is in the reverse direction, the shape of the characteristic will change again to give a
small circle of operation in the forward direction (i.e. in the opposite direction from the fault). This
ensures the stability of the relay for close-up reverse faults.
This expansion will only apply for unbalanced conditions. Some models of Ohmega employ a feature
known as Voltage Memory. This provides a polarising vector derived from the pre-fault voltage which
is applied for a limited time, after which the protection is inhibited. This provides a similar expansion for
three-phase faults. Full details about voltage memory are given in later in this section.
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