Protected zone
Comm. Channel
IEC05000040_2_en.vsd
IED
IED
IED
Comm. Channel
Comm. Channel
IEC05000040 V2 EN-US
Figure 7. Example of application on a three-terminal line with 1½ breaker arrangements
The current differential algorithm provides high
sensitivity for internal faults and it has excellent
stability for external faults. Current samples from all CTs
are exchanged between the IEDs in the line ends
(master-master mode) or sent to one IED (master-slave
mode) for evaluation.
A restrained dual biased slope evaluation is made where
the bias current is the highest phase current in any line
end, giving a secure through-fault stability even with
heavily saturated CTs. In addition to the restrained
evaluation, an unrestrained (instantaneous) high
differential current setting can be used for fast tripping
of internal faults with very high currents.
A special feature with this function is that applications
with small power transformers (rated current less than
50% of the differential current setting
IdMin
) connected
as line taps (that is, as shunt power transformers),
without measurements of currents in the tap, can be
handled. The normal load current is considered to be
negligible, and special measures must be taken in the
event of a short circuit on the LV side of the
transformer. In this application, the tripping of the
differential protection can be time-delayed for low
differential currents to achieve coordination with
downstream overcurrent IEDs. The local protection of
the small tap power transformer is given the time
needed to disconnect the faulty transformer.
A line charging current compensation provides
increased sensitivity of line differential protection.
Line differential protection 3 or 6 CT sets, with in-
zone transformers LT3CPDIF , LT6CPDIF
M14932-3 v9
Two two-winding power transformers or one three-
winding power transformer can be included in the line
differential protection zone. In such application, the
differential protection is based on the ampere turns
balance between the transformer windings. Both two-
and three-winding transformers are correctly
represented with vector group compensations made in
the algorithm. The function includes 2
nd
and 5
th
harmonic restraint and zero-sequence current
elimination. The phase-segregated differential
protection with single-pole tripping is usually not
possible in such applications.
IED
IED IED
Protected zone
Comm. Channel
Comm. Channel
Comm. Channel
IEC05000042_2_en.vsd
IEC05000042 V2 EN-US
Figure 8. Example of application on a three-terminal line with an in-line power transformer in the protection zone
Line differential protection RED670
1MRK 505 379-BEN N
30 Hitachi Power Grids
© Copyright 2017 Hitachi Power Grids. All rights reserved
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