4.9
Date Code 20081022
Instruction Manual
SEL-787 Relay
Protection and Logic Functions
Basic Protection
Figure 4.4
Differential Element Decision 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-787:
IOP = SLP1 • IRT + c
Because the line starts at the origin, the value of c is normally zero. The sum
of the second- and fourth-harmonic currents now forms the constant c in the
equation, raising the relay characteristic proportionally to the harmonic
values.
Harmonic Blocking
While the restrained differential elements are making decisions, a parallel
blocking decision process occurs regarding the magnitudes of specific
harmonics in the IOP quantities.
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-787 uses common (cross-phase) blocking by the sum of
_
+
_
+
_
+
_
+
2 CYC
10 CYC
n = 1, 2, or 3
87U1
87U2
87U3
87U
87HR1
87HR2
87HR3
87HB
IOPn
U87P
(setting)
87Un
87Rn
87HRn
f(IRTn)
87R
Σ
c
5HB1
87On
HRSTR = Y
(setting)
87HR
IRTn
O87P
(setting)
I1H24
Relay
Word
Bits
INRn
0.05
Summary of Contents for SEL-787
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