Date Code 20020129
Differential, Restricted Earth Fault,
3-29
Overcurrent, Voltage, and Frequency Elements
SEL-387E Instruction Manual
Figure 3.12 illustrates the logic of the REF directional element, 32I. It is at this stage that the
element decides whether to operate.
Figure 3.12: REF Directional Element
The relay enables the 32I directional element if the output of the AND gate at left-center in
Figure 3.12 asserts. This will occur if the two Relay Word bits 32IE and 50GC assert.
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,
CTR3, 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
IRWn, which the relay multiplies by the associated CTRn. 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 three-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, TRn, 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.
