4.30
SEL-787 Relay Instruction Manual Date Code 20150130
Protection and Logic Functions
Basic Protection
Step 5. For each CT, multiply the burdens calculated in Step 1 by the
magnitude, in secondary amperes, of the expected maximum
symmetrical fault current for an external fault. Select a nominal
accuracy class voltage for each CT that is greater than twice the
calculated voltage.
If necessary, select a higher CT ratio to meet this requirement,
then repeat Step 2 through Step 5. This selection criterion helps
reduce the likelihood of CT saturation for a fully offset fault
current signal.
Please note that the effective C voltage rating of a CT is lower
than the nameplate rating if a tap other than the maximum is
used. Derate the CT C voltage rating by a factor of ratio
used/ratio max.
Restricted Earth Fault
Element
Restricted earth-fault (REF) protection comes from a zero-sequence
directional element that provides sensitive detection of ground faults near the
neutral of a grounded wye-connected transformer winding. To provide REF
protection, the element compares the direction of a polarizing current, derived
from the line-end CTs, with the operating current, obtained from the neutral
CT. Figure 4.11 shows the characteristic of the REF element, with the shaded
area indicating the tripping area.
Figure 4.11 REF Directional Element
Because the REF element employs a neutral CT at one end of the winding and
a set of three CTs at the line end of the winding, REF protection can detect
only ground faults within that particular wye-connected winding. The element
is restricted in the sense that protection is limited to ground faults within a
zone defined by neutral and line CT placement.
The REF element uses comparison of zero-sequence currents, so the line-end
CTs must be connected in wye for the element to function. Delta-connected
CTs cancel out all zero-sequence components of the currents, eliminating one
of the quantities the REF element needs for comparison.
The SEL-787 has one REF element, REF1. The settings are identified in
Table 4.6. The operating quantity associated with the REF1 element is tied to
the relay input IN1 (E09–E10) as identified in Table 2.5.
Figure 4.12 through Figure 4.15 show the REF1 element logic diagrams.
Figure 4.12 shows the REF1 element enable output, REF1E. The operating
quantity is the output current from the neutral CT connected to the relay input
IN1 (E09–E10). The polarizing quantity is the zero-sequence current from the
selected terminal CTs that are part of the scheme. Both the operating and
polarizing currents are normalized for use within the logic. The normalization,
as shown in Figure 4.13, is explained later.
Polarizing Current
(Line CTs)
Operating Current
(Neutral CT)
Internal
Fault
Area
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