ABB REG670 Applications Manual page 208

Section 8
Impedance protection
202
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 90), 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-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:
×
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-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.
Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
1MRK 502 071-UEN -
(Equation 58)
Application manual