Fault resistance
The performance of distance protection for single phase-to-ground faults is very
important, because normally more than 70% of the faults on transmission lines are
single phase-to-ground faults. At these faults, the fault resistance is composed of three
parts: arc resistance, resistance of a tower construction, and tower-footing resistance.
The arc resistance can be calculated according to Warrington's formula:
EQUATION1456 V1 EN (Equation 217)
where:
L represents the length of the arc (in meters). This equation applies for the distance protection zone
1. Consider approximately three-times arc foot spacing for the zone 2 and wind speed of
approximately 50 km/h
I is the actual fault current in A.
In practice, the setting of fault resistance for both phase-to-ground (RFPE) and phase-to-
phase (RFPP) should be as high as possible without interfering with the load
impedance in order to obtain reliable fault detection.
3.6.5.2 Setting guidelines
General
The settings for the Full-scheme distance protection, quadrilateral for earth faults
(ZMMPDIS, 21) function are done in primary values. The instrument transformer ratio
that has been set for the analogue input card is used to automatically convert the
measured secondary input signals to primary values used in ZMMPDIS (21) function.
The following basics should be considered, depending on application, when doing the
setting calculations:
• Errors introduced by current and voltage instrument transformers, particularly
under transient conditions.
• Inaccuracies in the line zero-sequence impedance data, and their effect on the
calculated value of the ground-return compensation factor.
• The effect of infeed between the IED and the fault location, including the
influence of different Z0/Z1 ratios of the various sources.
Section 3 1MRK504116-UUS C
IED application
298
Application manual
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