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1MRK 506 369-UEN B
8.4.3
8.4.3.1
Line distance protection REL670 2.2 IEC
Application manual
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 128),
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.
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 earth-return compensation factor.
•
The effect of infeed between the IED and the fault location, including the
influence of different Z
•
The phase impedance of non transposed lines is not identical for all fault
loops. The difference between the impedances for different phase-to-earth
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.
/Z
ratios of the various sources.
0
1
Section 8
Impedance protection
SEMOD154496-1 v2
SEMOD154469-4 v7
249
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