ABB RED650, Applications Manual

Page: 127 / 454

This application gives rise to 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
Z
A
=Z
AT
+ ·Z
TF
I
A
+ I
C
I
A
DOCUMENT11524-IMG3509 V3 EN-US
(Equation 51)
Z Z Z
I I
I
Z
U
U
C Trf CT
A C
C
TF
= + +
+
⋅





⋅






2
1
2
DOCUMENT11524-IMG3510 V3 EN-US
(Equation 52)
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
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
61 ), 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 505 393-UEN B
Section 7
Impedance protection
Line differential protection RED650 2.2 IEC 121
Application manual