Date Code 20080110 Protection Functions 3-11
SEL-387E Instruction Manual
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 second-harmonic current than do fault currents. This second-
harmonic current can be used to identify the inrush phenomenon and to prevent relay
misoperation. The SEL-387E Relay measures the amount of second-harmonic current flowing in
the transformer. You can set the relay to block the percentage restrained differential element if
the ratio of second-harmonic current to fundamental current (IF2/IF1) is greater than the PCT2
setting.
Fourth-Harmonic Blocking Percentage of Fundamental (PCT4)
Range: OFF, 5–100%, in 1% steps
Set E87Wx=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-387E Relay 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.
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.
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
You may use the presence of fifth-harmonic differential current to assert an alarm output during
startup. This alarm indicates that the rated transformer excitation current is exceeded. You may
also consider triggering an event report if fifth-harmonic current exceeds the fifth-harmonic
threshold that you set.
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