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 resistance
is also depending on the presence of ground shield conductor at the top of the tower,
connecting tower-footing resistance in parallel. The arc resistance can be calculated
according to Warrington's formula:
EQUATION1456 V1 EN (Equation 314)
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
30 m/h
I
is the actual fault current in A.
In practice, the setting of fault resistance for both phase-to-ground RFPGZx and phase-to-
phase RFPPZx should be as high as possible without interfering with the load impedance
in order to obtain reliable fault detection.
8.12.3 Setting guidelines
8.12.3.1 General
The settings for Distance measuring zones, quadrilateral characteristic (ZMFPDIS) are
done in primary values. The instrument transformer ratio that has been set for the analog
input card is used to automatically convert the measured secondary input signals to
primary values used in ZMFPDIS .
The following basics must 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 Z
0
/Z
1
ratios of the various sources.
Section 8 1MRK 506 369-UUS -
Impedance protection
372 Line distance protection REL670 2.2 ANSI
Application manual
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