ABB RED650, Applications Manual

Page: 128 / 454

Fault resistance
GUID-83E3E475-3243-4308-9A91-B8DD9B47C276 v4
The performance of distance protection for single phase-to-earth faults is very
important, because normally more than 70% of the faults on transmission lines are
single phase-to-earth 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 earth 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:
1.4
28707 L
Rarc
I
×
=
EQUATION1456 V1 EN-US
(Equation 53)
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-earth RFPEZx and
phase-to-phase RFPPZx should be as high as possible without interfering with the
load impedance in order to obtain reliable fault detection.
7.1.3 Setting guidelines
IP14962-1 v1
7.1.3.1 General
GUID-BA20D421-4435-44CA-A6E9-743E461F8C59 v2
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 earth-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.
• The phase impedance of non transposed lines is not identical for all fault
loops. The difference between the impedances for different phase-to-earth
loops can be as large as 5-10% of the total line impedance.
• The effect of a load transfer between the IEDs of the protected fault resistance
is considerable, the effect must be recognized.
• Zero-sequence mutual coupling from parallel lines.
Section 7
1MRK 505 393-UEN B
Impedance protection
122 Line differential protection RED650 2.2 IEC
Application manual