Sel SEL-787 - Figure 4.4 Differential Element Decision Logic

4.9
Date Code 20150130 Instruction Manual SEL-787 Relay
Protection and Logic Functions
Basic Protection
Figure 4.4 Differential Element Decision Logic
Restrained elements (87R1, 87R2, and 87R3) determine whether the IOP
quantity is greater than the restraint quantity using the differential
characteristic shown in Figure 4.1. This characteristic is modified by
increasing the restraint current threshold as a function of the second- and
fourth-harmonic content in the input currents for the harmonic restraint
elements (87HR1, 87HR2, and 87HR3). Set HRSTR = Y to activate the
harmonic restraint element 87HR.
In element 87Rn, for example, the IOPn and IRTn quantities determine
whether the relay trips. The logic enclosed within the dotted line of Figure 4.4
implements the Figure 4.1 characteristic. The differential element calculates a
threshold as a function of IRTn. IOPn must exceed this threshold to assert
87Rn. The function uses the SLP1, SLP2, and IRS1 setting values, along with
IRTn, to calculate the IOP value. The relay uses SLOPE2 in place of SLOPE1
when the Relay Word bit INRn is asserted. This features provides a high
security mode of operation for 10 cycles when the transformer is energized.
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, the bit 87On
asserts. The AND gate condition is then 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
blocking decision logic, which is described later.
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,
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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