Trip Characteristic Settings - Siemens 7UT51 Instruction Manual

6
Bus Differential Protection (87B)
The 87T or 87HS function will pickup if either (1) the
operating point of any phase moves above 85% of the
threshold of the trip characteristic for the simultaneous
restraining current, or (2) the restraining current of any
phase exceeds four times the rated nominal current of
the reference winding of the protected transformer
( I > 4 I
). This makes it possible for the
rest n
saturation-detection algorithms to become active prior
to actual trip conditions, as well as for the relay to
record waveform data (see Chapter 14.7) for the
entire fault event, including during external faults
indicated by a high restraining current. The operating
point must be on or above the trip characteristic for a
trip event to occur. Individual pickup and trip events
report which phase(s) have picked up or tripped (see
Section 6.8 on page 86).
The 87T or 87HS function will dropout (or reset) after a
pickup (or trip) when for two full cycles the operating
point stays below 70% of the threshold of the trip
characteristic for the simultaneous restraining current.
6.4.1

Trip Characteristic Settings

Constant error currents can result in a small,
continuous differential current. To prevent this current
from causing a pickup or trip, the differential current,
I , must be at or above the threshold in
diff
Address 1803 before an 87B trip can occur.
1803 87 PICKUP
Minimum differential
87B trip, specified as a multiple of the rated
nominal current of the bus (Address 1301).
–
Range: 0.30 2.50 I
Default: 1.00 I
n
84
-
current that can cause an
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The lower sloped-segment of the trip characteristic
(Slope 1) deals with a differential current resulting from
transformation errors in the CTs or the current inputs
of the relay. The baseline intercept of this segment is
the origin; its slope is Address 1806:
1806 87SLOPE 1
Slope of lower sloping segment of 87B trip
characteristic (Slope 1). This segment
intercepts the horizontal axis at the origin.
–
Range: 0.10 0.50 (unitless ratio)
Default: 0.25
The upper sloped-segment of the trip characteristic
(Slope 2) provides higher pickup levels in the range of
high through currents during normal operation that
may cause CT saturation. (CT saturation due to a high
through current caused by an external fault is dealt
with separately, see Section 6.5 on page 85) The
baseline intercept and slope are set in addresses
1807 and 1808, respectively.
1807 87BASE PT 2
Baseline intercept of the upper sloping
segment of 87B trip characteristic (Slope 2).
–
Range: 0.0 10.0 I
Default: 2.5 I
n
1808 87SLOPE 2
Slope of upper sloping segment of 87B trip
characteristic (Slope 2).
–
Range: 0.25 0.95 (unitless ratio)
Default: 0.50
7UT51 v3
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PRIM-2330C