ABB RET670 - Setting Guidelines

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
123), 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.
3.6.4.2 Setting guidelines
General
The settings for Full-scheme distance protection, mho characteristic function
(ZMHPDIS) are done in primary values. The instrument transformer ratio that has been
set for the analog input card is used to automatically convert the measured secondary
input signals to primary values used in ZMHPDIS.
The following basics should be considered, depending on application, when doing the
setting calculations:
• Errors introduced by current and voltage instrument transformers, particularly
under transient conditions.
• Inaccuracies in the line zero-sequence impedance data, and their effect on the
calculated value of the ground-return compensation factor.
• The effect of infeed between the IED and the fault location, including the
influence of different Z
0
/Z
1
ratios of the various sources.
• The phase impedance of non transposed lines is not identical for all fault loops.
The difference between the impedances for different phase-to-ground loops can be
as large as 5-10% of the total line impedance.
• The effect of a load transfer between the terminals of the protected line, the fault
resistance is considerable and the effect must be recognized.
• Zero-sequence mutual coupling from parallel lines.
Section 3 1MRK504116-UUS C
IED application
274
Application manual