4.12
SEL-787 Relay Instruction Manual Date Code 20150130
Protection and Logic Functions
Basic Protection
Use the restraint slope percentage settings to discriminate between internal
and external faults. Set SLP1 to accommodate current differences from
steady-state and proportional errors such as power transformer tap-changer,
CT errors, and relay error. Set SLP2 to accommodate transient error caused by
CT saturation.
A two-slope, or variable-percentage differential application, improves
sensitivity in the region where CT error is less and increases security in the
high-current region where CT error is greater. We must define both slopes, as
well as the Slope 1 limit or point IRS1, where SLP1 and SLP2 intersect. If you
want a single slope characteristic, set both SLOPE1 and SLOPE2 to the
desired slope value.
The purpose of the instantaneous unrestrained current element is to react
quickly to very heavy current levels that clearly indicate an internal fault. Set
the pickup level U87P to 8 to 10 times tap. The unrestrained differential
element only responds to the fundamental frequency component of the
differential operating current. It is unaffected by the SLP1, SLP2, IRS1,
PCT2, or PCT5 settings. Thus, you must set the element pickup level high
enough so as not to react to large inrush currents.
Energization of a transformer causes a temporary large flow of magnetizing
inrush current into one terminal of a transformer, without the other terminal
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-787 measures the amount of second-harmonic and
fourth-harmonic currents flowing in the transformer. You can set the relay to
block the percentage restrained differential element if the ratio of the
second-harmonic and/or fourth-harmonic current to fundamental current
(IF2/IF1, IF4/IF1) is greater than the PCT2 or PCT4 setting, respectively.
Also, the 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 common harmonic blocking (cross-phase blocking)
and harmonic restraint logic; you can select either one or both of them. The
combination of both logic functions provides optimum differential element
operating speed and security. Use the HRSTR := Y setting to enable the
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