Trip Characteristic Settings - Siemens 7UT51 Instruction Manual

5
Motor or Generator Differential Protection (87M/G)
The differential protection function uses the
instantaneous values of I
location of the "operating point" on the trip
characteristic shown in Figure 5.3. The shape of the
trip characteristic is configured using settings.
The 87M/G function will pickup if either (1) the
operating point of any phase moves above 85% of the
setting 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
possible for the saturation-detection algorithms to
become active prior to actual trip conditions, as well as
for the relay to record waveform data
(see Section 14.7 on page 147) 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 event annunciations report
which phase(s) have picked up or tripped (see
Section 5.8 on page 75).
After pickup or trip occurs, the 87M/G function will
dropout (or reset) once the operating point stays
below 70% of the trip setting for two full cycles.
5.4.1

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 setting in
diff
Address 1703 before an 87M/G trip can occur. This
value can be set lower than Address 1603 setting (for
72
and I to plot the
diff rest
> 4 I ). This makes it
rest n
transformer protection) since differential currents, due
to tap-changers and matching errors, are not
expected; however, settings below 0.2 are reasonable
only if the CT sets on both sides of the machine are
identical and equally burdened.
1703 87 PICKUP
Minimum trip level of the 87M/G differential protection
function specified as a fraction (or multiple) of the
rated nominal current of the motor/generator, I
–
Range: 0.05 2.00 I
n
Default: 0.10
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 1706:
1706 87SLOPE 1
Slope of lower sloping segment of 87M/G
trip characteristic (Slope 1). This segment
intercepts the horizontal axis at the origin.
–
Range: 0.10 0.50
Default: 0.12
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 current transformer saturation.
(CT saturation due to a high through current caused
by an external fault is dealt with separately, see
Section 5.5 on page 74) The baseline intercept and
slope are set in addresses 1707 and 1708.
1707 87BASE PT 2
Baseline intercept of the upper sloping
segment of 87M/G trip characteristic (Slope 2).
–
Range: 0.0 10.0 I
Default: 2.5
1708 87SLOPE 2
Slope of upper sloping segment of
87M/G trip characteristic (Slope 2).
Range: 0.25
Default: 0.5
7UT51 v3
.
n
n
–
0.95
PRIM-2330C