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Section 8
Impedance protection
8.13.4
8.13.4.1
392
since it increases the magnitude of fault current in healthy circuit after the opening
of first circuit breaker. The so called current reversal phenomenon may cause
unwanted operation of protection on healthy circuit and this way endangers even
more the complete system stability.
To avoid the unwanted tripping, some manufacturers provide a feature in their
distance protection which detects that the fault current has changed in direction and
temporarily blocks distance protection. Another method employed is to temporarily
block the signals received at the healthy line as soon as the parallel faulty line
protection initiates tripping. The second mentioned method has an advantage in
that not the whole protection is blocked for the short period. The disadvantage is
that a local communication is needed between two protection devices in the
neighboring bays of the same substation.
Distance protection used on series compensated lines must have a high overreach
to cover the whole transmission line also when the capacitors are bypassed or out
of service. When the capacitors are in service, the overreach will increase
tremendously and the whole system will be very sensitive for false teleprotection
signals. Current reversal difficulties will be accentuated because the ratio of mutual
impedance against self-impedance will be much higher than for a non-compensated
line.
If non-unit protection is to be used in a directional comparison mode, schemes
based on negative sequence quantities offer the advantage that they are insensitive
to mutual coupling. However, they can only be used for phase-to-earth and phase-
to-phase faults. For three-phase faults an additional protection must be provided.
Setting guidelines
General
The settings for Distance measuring zones, quadrilateral characteristic
(ZMFCPDIS) 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 ZMFCPDIS.
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
/Z
ratios of the various sources.
0
1
Line distance protection REL670 2.2 IEC
1MRK 506 369-UEN B
IP14962-1 v1
GUID-B9958CEF-90ED-4644-B169-C6B4A018193B v1
Application manual
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