3.14
SEL-387-0, -5, -6 Relay Instruction Manual Date Code 20170601
Differential, Restricted Earth Fault, Thermal, and Overcurrent Elements
Differential Element
Independent Harmonic Blocking (IHBL)
Range: Y, N
The setting IHBL is not used when HRSTR = Y. When HRSTR = N, this
setting specifies how the relay will apply harmonic blocking to the relay
differential elements. Upon energization of a three-phase transformer, at least
two phase currents will contain inrush harmonics. In traditional single-phase
relays each relay compares the harmonic current flowing through the phase
for that relay. The SEL-387 performs harmonic blocking in two ways:
1. Independent Harmonic Blocking (IHBL = Y) blocks the
percentage differential element for a particular phase if the
harmonic (second or fifth) in that phase exceeds the block
threshold. No blocking occurs on other differential elements.
2. Common Harmonic Blocking (IHBL = N) blocks all of the
percentage differential elements if the harmonic magnitude of
any one phase is greater than the blocking threshold.
Common Harmonic Blocking is more secure but may slightly delay
percentage differential element operation because harmonics in all three
phases must drop below the thresholds for the three phases.
Setting Calculation
Connection Compensation Settings
The relay offers connection compensation settings, WnCTC (n = 1, 2, 3, or 4),
to compensate for the phase shift across the transformer. These settings offer a
range, 0–12, that represents 3x3 matrices, CTC(0)–CTC(12), permitting
compensation from 0 degrees to 360 degrees, in increments of 30 degrees,
respectively. The general expression for current compensation is as follows:
where IAWn, IBWn, and ICWn are the three-phase currents entering terminal
“n” of the relay; I1WnC, I2WnC, and I3WnC are the corresponding phase
currents after compensation the relay uses to calculate the operate and
restraint quantities; and CTC(m) is the three-by-three compensation matrix
corresponding to the WnCTC setting. The complete list of compensation
matrices (m = 0–12) and the corresponding correction compensation they
result in are shown in Table 3.1.
I1WnC
I2WnC
I3WnC
CTC m
IAWn
IBWn
ICWn
=
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