ABB REL 356 Current Differential Protection
4-6
Measuring Elements and Operational Logic
Figure 4-6. Stub Bus Trip
Weak Feed Trip
As seen from the operation algorithm, both relays will measure the same quantities. It is therefore sufficient with fault
current infeed from one end only for both relays to trip. Trip is supervised by a sensitive change detector, CD. CD will
operate for any current change in any phase or ground larger than 12.5% based on a sample compared to the same
sample one cycle earlier. Note that also a phase shift or a drop in current will thus cause the change detector to operate.
In case there is very little pre-fault load, the current change detector might not operate. It is then recommended to use
the setting CD =
D
I
D
V which enables CD for change in voltage as well. Potential transformers then need to be con-
nected to the relay.
Reclose Initiate Logic
REL 356 provides four reclose initiate (RI) contact outputs to be used with an external recloser. Two reclose block (RB)
contacts are also provided, in case the recloser requires this input instead of RI.
The RI contacts will close for the following trips:
·
Pilot trip from the differential function
·
Stub bus trip
·
Open breaker trip
When Reclose Block Enable (RBEN) is set to Normal Reclose Block (NORB) the RB contacts will close for the following
trips:
·
Reclose into fault
·
Trip from the back-up system
·
Out-of-step trip
·
High set overcurrent trip
·
Transfer trip
When RBEN is set to All Reclose Block (ALRB) the RB contacts will close for all trips.
A pilot trip within 500 msec after closing the breaker is interpreted as having reclosed into the same fault, and further
reclosing is blocked.
High Set Overcurrent Elements
REL 356 includes four channel independent overcurrent elements, IAH, IBH, ICH and IGH, for faster tripping for close in
faults. When potentials are connected to the relay, these units can be made directional, by setting FDOP and FDOG to
IN.
The phase directional unit FDOP is based on the angular relationship of a single-phase current and the corresponding
pre-fault phase-to-phase voltage phasors. The forward direction is identified if the current phasor leads the voltage
phasor. The pair of current and voltage phasors that are compared are I
A
and V
BA
, I
B
and V
CB
, I
C
and V
AC
.
Содержание REL 356
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