4.14
SEL-700G Relay
Instruction Manual
Date Code 20170814
Protection and Logic Functions
Group Settings (SET Command)
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-700G1 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. The
differential element automatically goes into high security mode when the
transformer is de-energized and IRNn asserts. The relay will stay in this mode
for 10 cycles after energization is detected. See Figure 4.5 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 voltage and generator frequency may vary
somewhat during startup, overexciting the transformers. Transformer
overexcitation produces odd-order harmonics that can appear as differential
current to a transformer differential relay. The SEL-700G1 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.
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-700G1 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
harmonic restraint logic and the HBLK := Y setting to enable the harmonic
blocking logic.
Common harmonic blocking provides superior security against tripping on
magnetizing inrush during transformer energization, yet allows faster
differential element tripping for an energized transformer fault. Differential
tripping through the harmonic restraint logic is slightly slower, but provides a
dependable tripping function when energizing a faulted transformer that might
otherwise have the differential tripping element blocked by common harmonic
blocking logic.
High Security Mode Settings HSM, O87P2, and HSMDOT.
Uneven CT
saturation during external events, such as faults or transformer energization,
can lead to differential operation in some installations. While the preferred
method to avoid these unwanted operations is to properly size and match the
CTs, the high security mode SEL
OGIC
control equation, HSM, can be
Summary of Contents for SEL-700G Series
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