This application gives rise to a similar problem that was highlighted in section Fault
infeed from remote end, that is increased measured impedance due to fault current
infeed. For example, for faults between the T point and B station the measured
impedance at A and C will be:
DOCUMENT11524-IMG3509 V3 EN (Equation 102)
Z Z Z
I I
I
Z
U
U
C Trf CT
A C
C
TF
= + +
+
⋅
⋅
2
1
2
DOCUMENT11524-IMG3510 V3 EN (Equation 103)
Where:
Z
AT
and Z
CT
is the line impedance from the A respective C station to the T point.
I
A
and I
C
is fault current from A respective C station for fault between T and B.
U2/U1 Transformation ratio for transformation of impedance at U1 side of the transformer to
the measuring side U2 (it is assumed that current and voltage distance function is taken
from U2 side of the transformer).
Z
TF
is the line impedance from the T point to the fault (F).
Z
Trf
is transformer impedance.
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.
1MRK 502 071-UEN - Section 8
Impedance protection
Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062 227
Application manual
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