Schweitzer Engineering Laboratories SEL-387E - Harmonic Restraint

Date Code 20080110 Protection Functions 3-5
SEL-387E Instruction Manual
threshold to produce tripping. The function uses the SLP1, SLP2, and IRS1 setting values, along
with IRTn, to calculate the IOP value. The differential element decision logic compares the
calculated value, denoted f(IRTn), to the actual IOPn. If IOPn is greater, one input of the AND
gate at the right receives a logic 1. Comparison of IOPn with the O87P setting determines the
second AND input. If IOPn is greater than O87P, Relay Word bit 87On asserts. The
AND gate
condition then is satisfied, and Relay Word bit 87Rn asserts, indicating operation of the restrained
differential element, n. This does not, as yet, produce a trip. The relay still needs the results of
the harmonic and dc blocking decision logic, which is described later.
f(IRTn)
IOPn
U87P
(setting)
IRTn
O87P
(setting)
87Un
87Rn
87On
n = 1, 2, or 3
_
+
_
+
_
+
Relay
Word
Bits
DWG: M3871090
87U1
87U2
87U3
87U
Σ
c
IHRTn
HRSTR=Y
Figure 3.4: Differential Element Decision Logic
Harmonic Restraint
Consider the harmonic restraint feature (HRSTR = Y) if your practices require independent
harmonic restraint. This feature disables common harmonic blocking (IHBL = Y). It also
disables second- and fourth-harmonic blocking since it adds the second- and fourth-harmonic
quantities to the differential characteristic restraint quantity. 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:
cx•
m
y
+=
More specifically, in the SEL-387E Relay:
cIRT•1SLPIOP +=
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.