ABB RET670 Applications Manual page 303

1MRK504116-UUS C
Application manual
infeed. For example for faults between the T point and B station the measured
impedance at A and C will be
I + I
A C
Z =Z +
A AT
I
A
DOCUMENT11524-IMG3509 V2 EN
æ
I
A
+ +
Z = Z Z
ç
C Trf CT
è
EQUATION1783-ANSI V1 EN
Where:
ZAT and ZCT is the line impedance from the B respective C station to the T point.
IA and IC is fault current from A respective C station for fault between T and B.
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).
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 136), 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.
·Z
TF
ö 2
+
I æ V 2 ö
C
× ×
Z
÷ ç
÷
TB
è V 1 ø
I
ø
C
Section 3
IED application
(Equation 215)
(Equation 216)
297