3-12 Protection Functions Date Code 20080110
SEL-387E Instruction Manual
Fifth-Harmonic Alarm Time Delay Pickup (TH5D)
Range: 0–8000 cycles, in 0.125-cycle steps
With this pickup, you can delay assertion of an alarm for excessive fifth-harmonic differential
current.
DC Ratio Blocking (DCRB)
Set E87Wx=Y1 to make the fourth-harmonic, (PCT4), dc ratio blocking (DCRB), and harmonic
restraint (HRSTR) settings available. Some magnetizing inrush cases contain very little harmonic
content but contain a dc offset. The SEL-387E Relay can detect the dc offset and use it in the
blocking (not restraint) logic. Enable this function by setting DCRB = Y.
Harmonic Restraint (HRSTR)
Set E87Wx=Y1 to make the fourth-harmonic, (PCT4), dc ratio blocking (DCRB), and harmonic
restraint (HRSTR) settings available. Even harmonics (second and fourth) can be used to provide
security against magnetizing inrush currents during transformer energization. Choose between
harmonic blocking and harmonic restraint. Harmonic blocking treats the second and fourth
harmonics independently and blocks the relay when the second- or fourth-harmonic content
(harmonic current as a percentage of the fundamental current) exceeds the PCT2 or PCT4 setting,
respectively. For example, assume the following:
PCT2 = PCT4 = 20 percent, and the harmonics in the differential current are:
second harmonic = 15 percent, fourth harmonic = 7 percent
In this case, the relay does not block because neither harmonic content exceeds its particular
setting. But when the second-harmonic content increases to 21 percent, the relay blocks,
regardless of the value of the fourth-harmonic content present in the differential current.
Increasing the fourth-harmonic content to exceed the PCT4 setting while the second-harmonic
content remains lower than the PCT2 setting yields the same result.
Harmonic restraint is more secure than harmonic blocking since it adds the values of the second-
and fourth-harmonic currents together and raises the relay characteristic by the sum of the two
values. In the example, second-harmonic content + fourth-harmonic content = 15 percent + 7
percent = 22 percent and relay tripping is restrained when it would not have been blocked.
Set HRSTR = Y to select the harmonic restraint function and automatically enable Independent
Harmonic Blocking (IHBL).
Independent Harmonic Blocking (IHBL)
Range: Y, N
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-387E Relay performs harmonic blocking in two ways:
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