Figure 97: Settings Required To Apply A Mho Zone; Earth Fault Characteristic - GE MiCOM P40 Agile Technical Manual

Chapter 7 - Distance Protection
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Figure 97: Settings required to apply a mho zone

7.2.2

EARTH FAULT CHARACTERISTIC

The earth fault characteristic selection is common to all zones, allowing Mho or quadrilateral selection. Generally,
the characteristic chosen matches utility practice. Generally we would recommend a Mho characteristic for line
protection and a quadrilateral characteristic for cable applications.
Quadrilateral earth-fault characteristics are also recommended for all lines shorter than 10miles (16km). This
ensures that the resistive fault arc coverage does not depend on Mho circle dynamic expansion and is a known set
value.
7.3 ZONE REACH SETTING GUIDELINES
The Zone 1 elements of a Distance protection should be set to cover as much of the protected line as possible,
allowing instantaneous tripping for as many faults as possible. In most applications the Zone 1 reach (Z1) should
not be able to respond to faults beyond the protected line. For an underreaching application the Zone 1 reach
must therefore be set to account for any possible overreaching errors. These errors come from measuring errors,
the current and voltage transformers, and inaccurate line impedance data. We therefore recommend that the
reach of the Zone 1 distance elements is restricted to 80% of the protected line impedance (positive phase
sequence line impedance), with Zone 2 elements set to cover the final 20% of the line.
The Zone 2 elements should be set to cover the 20% of the line not covered by Zone 1. Allowing for underreaching
errors, the Zone 2 reach (Z2) should be set in excess of 120% of the protected line impedance for all fault
conditions. Where aided tripping schemes are used, fast operation of the Zone 2 elements is required. It is
therefore beneficial to set Zone 2 to reach as far as possible, such that faults on the protected line are well within
reach. A constraining requirement is that, where possible, Zone 2 does not reach beyond the Zone 1 reach of
adjacent line protection. For this reason the Zone 2 reach should be set to cover up to 50% of the shortest
adjacent line impedance, if possible.
The Zone 3 elements would usually be used to provide overall back-up protection for adjacent circuits. The Zone 3
reach (Z3) is therefore set to approximately 120% of the combined impedance of the protected line plus the
longest adjacent line. A higher apparent impedance of the adjacent line may need to be allowed where fault
current can be fed from multiple sources or flow through parallel paths.
Zone 3 may also be programmed with a slight reverse ("rev") offset, in which case its reach in the reverse direction
is set as a percentage of the protected line impedance too. This would typically provide back-up protection for the
local busbar, where the offset reach is set to 20% for short lines (<30km) or 10% for longer lines.
194
Variable mho
expansion by
polarizing ratio
Zone Reach Z
Time
Delay
t
Load
Line
Blinder
Angle
Angle β
Blinder Radius
P543i/P545i
P54x1i-TM-EN-1