GE MiCOM P40 Agile, Technical Manual, Page: 141 / 962

From the equation above, it can be seen that the measured voltage has a significant impact on the decision
making process.
The ability of distance protection to measure accurately for a given reach point fault, depends on the voltage at
the relaying location being above a minimum value at the time of the fault. If the voltage is above this minimum
value, it is generally used to polarize the distance protection and indicate the direction of the fault. This is called
self-polarization.
If the voltage collapses below the minimum threshold necessary to make a sensible decision, alternative methods
of polarization to determine the direction of the fault are needed. Two methods that are applied are cross-
polarization and memory polarization. If a fault doesn’t affect all phases, the voltage signals on the healthy phases
can be used for the directional decision. This is called cross-polarization. If the fault causes all phase voltages to
collapse, a stored record of the pre-fault voltage can be used to make the directional decision. This is called
memory polarization. Memory polarization, cross-polarization, and self-polarization can sometimes be used in
combination.
2.3 IMPEDANCE CALCULATION
Careful selection of the reach settings and tripping times for the various measurement zones enables correct
coordination between Distance protection devices. Basic Distance protection will comprise instantaneous
directional Zone 1 protection and one or more time delayed zones. A basic distance protection scheme is likely to
feature 3 zones of protection, but numerical distance protection devices may have several more zones, some set
to measure in the forward direction and some set to measure in the reverse direction.
Some numerical distance protection devices measure the fault voltage and current directly then calculate the
impedance, after which they determine whether operation is required according to impedance boundaries defined
on an R/X diagram. Many numerical IEDs emulate their traditional electro-mechanical counterparts. Rather than
calculating the absolute impedance, they compare the measured fault voltage with a replica voltage derived from
the fault current and the zone impedance setting to determine whether the fault is within zone or out-of-zone.
Typically, a comparator will compare either the relative amplitude or relative phase of input quantities to
determine impedance limits. Limits may be either straight line characteristics (quadrilaterals), or circular
characteristics (Mhos).
2.4 IMPLEMENTATION WITH COMPARATORS
The distance protection in this product uses measured values of voltage and current, together with setting values
such as line impedance, to determine whether fault conditions exist. The determination as to whether a fault
condition exists is performed by so-called ‘comparators’. These comparators use voltage and current inputs in
conjunction with impedance settings to decide whether a fault is in a particular zone. Multiple zones can provide
protection for the protected line as well as providing back-up protection for connected lines.
All distance zone calculations in this product are constructed using one or more comparators. Each comparator
uses two vector quantities which are generally referenced as S1 and S2. S1 and S2 comparators are used to
construct either circular (Mho) and/or Quadrilateral characteristics. In the case of Mho characteristics, a single
comparator is used to make a tripping decision. In the case of Quadrilateral characteristics, multiple comparators
are used to make a tripping decision.
2.5 POLARIZATION OF DISTANCE CHARACTERISTICS
The distance zone characteristics are polarized (directionalized) to reflect the characteristic angle of the line. Some
of the zones of the distance protection are forward looking, some are reverse looking, and some are of the offset
type. Polarization is generally achieved by directional self-polarization, but memory-polarization, or cross-
polarization, might be adopted for close-up zero-voltage faults.
P446SV Chapter 7 - Distance Protection
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