4.11
Date Code 20170814
Instruction Manual
SEL-700G Relay
Protection and Logic Functions
Group Settings (SET Command)
Figure 4.7
Differential Element Output Logic
Harmonic Restraint.
Consider the harmonic restraint feature (HRSTR = Y)
if your practices require independent harmonic restraint. Blocking features are
discussed in more detail later in this section.
For harmonic blocking, the harmonic content of the differential current must
exceed the individual (PCT2 or PCT4) threshold values, i.e., the thresholds are
treated as independent measurements of each harmonic value. For harmonic
restraint, the values of the second- and fourth-harmonic currents are summed,
and that value is used in the relay characteristic. Consider, for example, the
simple case of Slope 1, i.e., a straight line through the origin. The general
equation for a line is:
y = m • x + c
More specifically, in the SEL-700G:
IOP = SLP1 • IRT + c (for sensitive differential logic)
IOP = SLP12 • IRT + c (for high security differential logic)
Because the line starts at the origin, the value of c is normally zero. The sum
of the second- and fourth-harmonic currents, I1H24 in Figure 4.5, now forms the
constant c in the equation, raising the relay characteristic proportionally to the
harmonic values. Based on the status of the HSM Relay Word bit, 87HRn is
driven by either 87HSNn (HSM = 0) or 87HSCn (HSM = 1), indicating
operation of the harmonic restrained differential element, n, and tripping can
take place.
Harmonic Blocking.
Set HRBLK = Y to enable the harmonic blocking
feature. While the restrained differential elements are making decisions, a
parallel blocking decision process occurs regarding the magnitudes of
specifics harmonics in the IOP quantities.
Figure 4.6 shows how blocking elements (87BL1, 87BL2, and 87BL3)
supervise the restrained differential elements if the second-, fourth-, or fifth-
harmonic operating current is above its set threshold. The blocking prevents
improper tripping during transformer inrush or allowable overexcitation
conditions. The SEL-700G1 uses common (cross-phase) blocking. Common
blocking prevents all restrained elements (87Rn) from tripping if any blocking
element is picked up.
However, an independent blocking is used for the fifth-harmonic current. In
this logic, an individual element will only disable tripping of that element.
Tripping can take place if 87Rn is asserted and the corresponding blocking
elements are deasserted.
An additional alarm function for fifth harmonic to warn of overexcitation (not
shown in Figure 4.6) employs a separate threshold (TH5P) and an adjustable
timer (TH5D).
87U1
87U2
87U3
87U
87HR1
87HR2
87HR3
87HB
87R
HRSTR = Y
(setting)
87HR
Relay
Word
Bits
Relay
Word
Bits
Relay
Word
Bits
Relay
Word
Bits
Summary of Contents for SEL-700G Series
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