3.12
SEL-387-0, -5, -6 Relay
Instruction Manual
Date Code 20170601
Differential, Restricted Earth Fault, Thermal, and Overcurrent Elements
Differential Element
When using harmonic restraint (HRSTR = Y), use the PCT2 setting to scale
the amount of second-harmonic content that will be added to the restraint
slope characteristic. The larger the PCT2 setting, the smaller the increase on
the restraint slope as a result of the measured second-harmonic content. See
for details.
Fourth-Harmonic Blocking Percentage of Fundamental (PCT4)
(All But SEL-387-0)
Range: OFF, 5–100%, in 1% steps
For all relays but the SEL-387-0, set E87W
x
= Y1 to make the fourth-
harmonic (PCT4), dc ratio blocking (DCRB), and harmonic restraint
(HRSTR) settings available. Energization of a transformer causes a temporary
large flow of magnetizing inrush current into one terminal of a transformer,
without other terminals seeing this current. Thus, it appears as a differential
current that could cause improper relay operation. Magnetizing inrush
currents contain greater amounts of even-harmonic current than do fault
currents. This even-harmonic current can be used to identify the inrush
phenomenon and to prevent relay misoperation. The SEL-387 measures the
amount of fourth-harmonic current flowing in the transformer. You can set the
relay to block the percentage restrained differential element if the ratio of
fourth-harmonic current to fundamental current (IF4/IF1) is greater than the
PCT4 setting.
When using harmonic restraint (HRSTR = Y), use the PCT4 setting to scale
the amount of fourth-harmonic content that will be added to the restraint slope
characteristic. The larger the PCT4 setting, the smaller the increase on the
restraint slope as a result of the measured fourth-harmonic content. See
for details.
Fifth-Harmonic Blocking Percentage of Fundamental (PCT5)
Range: OFF, 5–100%, in 1% steps
According to industry standards (ANSI/IEEE C37.91, C37.102),
overexcitation occurs when the ratio of the voltage to frequency (V/Hz)
applied to the transformer terminals exceeds 1.05 per unit at full load or 1.1
per unit at no load. This ratio is a measure of the core flux density.
Transformer overexcitation produces odd-order harmonics, which can appear
as differential current to a transformer differential relay.
Unit-generator step-up transformers at power plants are the primary users of
fifth-harmonic blocking. Transformer voltage and generator frequency may
vary somewhat during startup, overexciting the transformers.
The fifth harmonic blocking is independent of the harmonic restraint setting
(HRSTR), and does not affect the restraint slope characteristic.
Fifth-Harmonic Alarm Threshold (TH5P)
Range: OFF, (0.02–3.2), in 0.01 steps • TAP
Note:
1 A: TAP
MIN
• TH5P
0.05
TAP
MAX
• TH5P
31.0
5 A: TAP
MIN
• TH5P
0.25
TAP
MAX
• TH5P
155.0
Summary of Contents for SEL-387-0
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