For this example with a fault between T and B, the measured impedance from the T
point to the fault will be increased by a factor defined as the sum of the currents
from T point to the fault divided by the IED current. For the IED at C, the
impedance on the high voltage side U1 has to be transferred to the measuring
voltage level by the transformer ratio.
Another complication that might occur depending on the topology is that the
current from one end can have a reverse direction for fault on the protected line.
For example, for faults at T the current from B might go in reverse direction from
B to C depending on the system parameters (see the dotted line in figure
194),
given that the distance protection in B to T will measure wrong direction.
In three-end application, depending on the source impedance behind the IEDs, the
impedances of the protected object and the fault location, it might be necessary to
accept zone 2 trip in one end or sequential trip in one end.
Generally for this type of application it is difficult to select settings of zone 1 that
both gives overlapping of the zones with enough sensitivity without interference
with other zone 1 settings, that is, without selectivity conflicts. Careful fault
calculations are necessary to determine suitable settings and selection of proper
scheme communication.
Fault resistance
GUID-D42B3721-E63B-411E-A7DD-5F606ECA4576 v1
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 arc resistance can be calculated according to Warrington's formula:
EQUATION1456 V1 EN-US (Equation 357)
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 zone 2 to get a reasonable
margin against the influence of wind.
I
is the actual fault current in A.
In practice, the setting of fault resistance for both phase-to-earth 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.
8.13.3 Series compensation in power systems
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The main purpose of series compensation in power systems is virtual reduction of
line reactance in order to enhance the power system stability and increase
1MRK 506 369-UEN B Section 8
Impedance protection
Line distance protection REL670 2.2 IEC 373
Application manual
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