Schweitzer Engineering SEL-387-0, Instruction Manual

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3.27
Date Code 20170601 Instruction Manual SEL-387-0, -5, -6 Relay
Differential, Restricted Earth Fault, Thermal, and Overcurrent Elements
Restricted Earth Fault Element
The directional element compares the polarizing current to the operating
current and indicates forward (internal) fault location or reverse (external)
fault location. The internal/forward indication occurs if the fault is within the
protected winding, between the line-end CTs and the neutral CT. The relay
multiplies each current by the appropriate CT ratio to convert input currents to
actual primary amps. This must be done to properly sum the currents in the
autotransformer winding.
The polarizing current, IPOL, is simply the neutral CT current multiplied by
the neutral CT ratio, CTR4, to produce a primary current value. The operating
current, IOP, is the phasor sum of the winding residual currents, also on a
primary basis. The 32IOP setting determines the appropriate IRW
n , which the
relay multiplies by the associated CTR n . The relay then sums the products.
The 32I element calculates the real part of IOP times IPOL* (IPOL complex
conjugate). This equates to |IOP| times |IPOL| times the cosine of the angle
between them. The result is positive if the angle is within ±90 degrees,
indicating a forward or internal fault. The result is negative if the angle is
greater than +90 or less than –90 degrees, indicating a reverse or external
fault. The relay compares the output of the 32I element to positive and
negative thresholds, to ensure security for very small currents or for an angle
very near +90 or –90 degrees. If the 32I output exceeds the threshold test, it
then must persist for at least 1.5 cycles before the Relay Word bit 32IF
(forward) or 32IR (reverse) asserts. Assertion of 32IF constitutes a decision to
trip by the REF function.
A second path can also assert the 32IF bit. This path comes from the two-
position AND gate at the top-right of
Figure 3.12 . The gate asserts if 32IE is
asserted. This assertion indicates that neutral current is above pickup but
50GC is not asserted, indicating no line-end current flow. This logic covers the
situation of an internal wye-winding fault with the line-end breaker open.
You can perform tripping directly by inclusion of the Relay Word bit 32IF into
one or more of the trip variables, TR
n , as appropriate. If you want additional
security, the relay is programmed to use 32IF to torque control an inverse-time
curve for delayed tripping, as discussed below.
Figure 3.13 shows the output
of the REF protection function. Timing is on an extremely inverse-time
overcurrent curve (curve U4) at the lowest time-dial setting (0.5) and with
50GP as the pickup setting.
Figure 3.13 REF Protection Output (Extremely Inverse-Time O/C)
Relay Word bit 32IF (forward fault) torque controls the timing curve, and
IRW4
operates the timing function. The curve resets in one cycle if current drops
below pickup or if 32IF deasserts. When the curve times out, Relay Word bit
REFP asserts. You can use this bit directly as an input to the appropriate trip
variables, TR
n , to trip the breaker or breakers that feed the fault.
Relay
Word
Bit
Relay
Word
Bits
|IRW4|
50GP
50G4
32IF
REFP
Timing
50GP
(pickup)
U4 Curve
TD 0.5
Reset 1 cycle