Schweitzer Engineering SEL-387-0, Instruction Manual

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3.25
Date Code 20170601 Instruction Manual SEL-387-0, -5, -6 Relay
Differential, Restricted Earth Fault, Thermal, and Overcurrent Elements
Restricted Earth Fault Element
Restricted Earth Fault Element
Application
Description
Use the Restricted Earth Fault (REF) element to provide sensitive protection
against ground faults in your wye-connected transformer winding. The
element is “restricted” in the sense that protection is restricted to ground faults
within a zone defined by neutral and line CT placement.
Operating
Characteristic
Restricted Earth Fault (REF) protection is a technique for sensitive detection
of ground faults in a grounded wye-connected transformer winding. Because
it employs a neutral CT at one end of the winding and the normal set of three
CTs at the line end of the winding, REF protection can detect only ground
faults within that particular wye-connected winding. For REF to function, the
line-end CTs must also be connected in wye, because the technique uses
comparison of zero-sequence currents. Delta-connected CTs cancel out all
zero-sequence components of the currents, eliminating one of the quantities
the REF element needs for comparison.
The REF implementation in the SEL-387 uses a directional element (32I) that
compares the direction of an operating current, derived from the line-end CTs,
with the polarizing current, obtained from the neutral CT. A zero-sequence
current threshold and positive-sequence restraint supervise tripping. You can
apply REF to a single wye winding in a transformer or to an entire
autotransformer winding with as many as three sets of line-end CT inputs. The
neutral CT connects to one of the three current inputs for Winding 4 (
IAW4 ,
IBW4
, or
ICW4
), leaving only three three-phase winding inputs for normal
differential or overcurrent protection purposes.
Figure 3.11 shows the REF simplified enable/block logic. The upper logic
group determines whether to enable the REF directional element by assertion
of the 32IE Relay Word bit. The two enabling quantities are assertion of the
E32I equation and a magnitude of the neutral CT secondary current (
IRW4
)
greater than the pickup setting, 50GP. The topmost part of this logic is a
blocking function. This function asserts if any of the winding residual currents
used in the REF function are less than a positive-sequence current restraint
factor, a0, times the positive sequence current for their respective winding.
Such a winding residual current value might occur with “false I0” or if zero
sequence current for that winding exceeds 50GP. False I0 can occur in cases of
CT saturation during heavy three-phase faults. If the blocking logic asserts,
the CTS Relay Word bit asserts. To prevent 32IE assertion when CTS asserts,
set the E32I setting = !CTS. Note that the SEL-387-0 Relay prior to firmware
version R602 does not support the CTS Relay Word bit.
The lower logic group adjusts the winding residual currents to a common
sensitivity level with the neutral CT, calculates a phasor sum of the
appropriate currents, and compares this sum to the 50GP pickup value. If the
sum is greater than the pickup level, Relay Word bit 50GC asserts. This bit
indicates that the winding currents are present in sufficient magnitude.