4.12
SEL-787 Relay Instruction Manual Date Code 20081022
Protection and Logic Functions
Basic Protection
differential element automatically goes into a high security mode for 10 cycles
when the transformer is energized, see Figure 4.4 and the associated
description.
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. Transformer overexcitation produces odd-order harmonics
(primarily fifth harmonic), which can appear as differential current to a
transformer differential relay. The SEL-787 measures the amount of fifth-
harmonic current flowing in the transformer. You can set the relay to block the
percentage restrained differential element if the ratio of fifth-harmonic current
to fundamental current (IF5/IF1) is greater than the PCT5 setting. 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 level and delay settings (TH5P and TH5D) use the
presence of fifth-harmonic differential current to assert a Relay Word bit
TH5T. This bit indicates that the rated transformer excitation current is
exceeded. You may consider triggering an alarm and/or event report if fifth-
harmonic current exceeds the fifth-harmonic threshold that you set.
The SEL-787 includes one harmonic blocking and one harmonic restraint
element; you can select either one of them or both. The combination of both
elements provides optimum operating speed. Use HRSTR setting to enable the
harmonic restraint element and HBLK setting for the harmonic blocking
element.
Setting Calculation
General Discussion of Connection Compensation
The general expression for current compensation is as follows:
where IAWn, etc., are the three-phase currents entering terminal “n” of the
relay; IAWnC, etc., are the corresponding phase currents after compensation;
and [CTC(m)] is the three-by-three compensation matrix.
Setting WnCTC = m specifies which [CTC(m)] matrix the relay is to use. The
setting values are 0, 1, 2, …, 11, 12. These are discrete values “m” can assume
in [CTC(m)]; the values physically represent the “m” number of increments of
30 degrees that a balanced set of currents with ABC phase rotation will be
rotated in a counterclockwise direction when multiplied by [CTC(m)]. If a
given set of such currents is multiplied by all 12 of the CTC matrices, the
resulting compensated values would seem to move completely around the
circle in a counterclockwise direction, returning to the original start position.
This is the same as successively multiplying [CTC(1)] times the original
currents, then times each successive compensated result value, a total of 12
times.
If a balanced set of currents with ACB phase rotation undergoes the same
exercise, the rotations by the [CTC(m)] matrices are in the clockwise
direction. This is because the compensation matrices, when performing phasor
addition or subtraction involving B or C phases, will produce “mirror image”
IAWnC
IBWnC
ICWnC
CTC m()[]
IAWn
IBWn
ICWn
•=
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