3-4 Protection Functions Date Code 20080110
SEL-387E Instruction Manual
IOP1
E87W1
I1W1C1
I1W3C1
I1W2C1
E87W2
E87W3
Σ
⏐ΣI1W
n
C1⏐
E87W1
I1W1C2
I1W3C2
I1W2C2
E87W2
E87W3
E87W1
I1W1C4
I1W3C4
I1W2C4
E87W2
E87W3
n=1, 2, 3
IRT1
IHRT1
I1HB2
I1HB4
Σ
Σ
Σ
Σ
h
⏐ΣI1W
n
C2⏐
⏐ΣI1W
n
C4⏐
⏐I1W1C1⏐
⏐I1W2C1⏐
⏐I1W3C1⏐
100/PCT2
÷2
100/PCT4
Figure 3.3: Differential Element (87-1) Quantities
Figure 3.4 shows how the differential element quantities are used to generate the unrestrained
87Un (87U1, 87U2, 87U3) and restrained 87Rn (87R1, 87R2, 87R3) elements. These elements
are combined to form differential element targets (87-1, 87-2, 87-3).
Unrestrained elements (87U1, 87U2, and 87U3) compare the IOP quantity to a setting value
(U87P), typically about 10 times TAP, and trip if this level is exceeded. Elements 87U1, 87U2,
and 87U3 are combined to form element 87U as shown in the lower right corner of
Figure 3.4.
Harmonic blocking is not performed on the unrestrained elements. Use these elements to protect
your transformer bushings and end windings while maintaining security for inrush and through-
fault conditions. Operating current elements 87On (87O1, 87O2, 87O3) are provided for testing
purposes.
Restrained elements (87R1, 87R2, and 87R3) determine whether the IOP quantity is greater than
the restraint quantity using the differential characteristic shown in
Figure 3.1. Set HRSTR=Y
(harmonic restraint) to modify this characteristic as a function of the second- and fourth-harmonic
content in the input currents.
In element 87Rn, for example, the IOPn and IRTn quantities determine whether the relay trips.
The logic enclosed within the dotted line of
Figure 3.4 implements the Figure 3.1 characteristic.
The differential element calculates a threshold as a function of IRTn. IOPn must exceed this
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