ABB RELION 670 SERIES Applications Manual page 405

1MRK 506 369-UUS -
Line distance protection REL670 2.2 ANSI
Application manual
Where:
Z and Z is the line impedance from the A respective C station to the T point.
AT CT
I and I is fault current from A respective C station for fault between T and B.
A C
V2/V1 Transformation ratio for transformation of impedance at V1 side of the transformer to the
measuring side V2 (it is assumed that current and voltage distance function is taken
from V2 side of the transformer).
Z is the line impedance from the T point to the fault (F).
TF
Z is transformer impedance.
Trf
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 V1 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 120), 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
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:
Section 8
Impedance protection
399