ABB RELION 670 SERIES Applications Manual page 299

1MRK 506 369-UUS -
Line distance protection REL670 2.2 ANSI
Application manual
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 145), 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
zone2 trip in one end or sequential trip in one end.
Generally for this type of application it is difficult to select settings of zone1 that both
gives overlapping of the zones with enough sensitivity without interference with other
zone1 settings that is, without selectivity conflicts. Careful fault calculations are
necessary to determine suitable settings and selection of proper scheme communication.
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:
×
28707 L
=
Rarc
1.4
I
EQUATION1456 V1 EN
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.
Section 8
Impedance protection
(Equation 206)
293