4.32
SEL-700G Relay
Instruction Manual
Date Code 20170814
Protection and Logic Functions
Group Settings (SET Command)
Ground Differential Element
Basic generator protection in SEL-700G relays includes the ground
differential element (87N) that operates based on the difference between the
measured neutral current and the sum of the three-phase current inputs. The
87N element provides sensitive ground fault detection on resistance-grounded
and solidly grounded generators, particularly where multiple generators are
connected directly to a load bus. This element should not be applied to protect
high-impedance grounded generators.
The relay provides two definite-time delayed ground current differential
elements designed to detect ground faults on resistance grounded and solidly
grounded generators. Because these elements are current-based, they cannot
provide ground fault coverage for 100 percent of the stator windings. They do,
however, offer selective ground fault protection because they do not respond
to ground faults beyond the generator phase current transformers. This quality
makes the element ideal for protecting generators connected to multiple-unit
buses or generators connected to a load bus, such as might be found in an
industrial installation.
The relay uses the neutral CT connected to the relay
IN
input to measure the
generator neutral current. It then calculates the residual current, which is the
sum of the three-phase current inputs (from CTs located at generator
terminals). The relay adjusts the residual current by the ratio of the CTR and
CTRN settings to scale the residual current in terms of the secondary neutral
current. It then calculates the difference. Normally, under balanced load or
external ground fault conditions, the difference current should be zero. In the
event of an internal ground fault, the difference current is nonzero. If the
difference current magnitude is greater than the element pickup setting, the
element picks up and begins to operate the definite time-delay. If the
difference current remains above the pickup setting for the duration of the
definite time-delay, the time-delayed element Relay Word bit asserts.
The relay configures the 87N element to use the appropriate residual current
(IG) automatically based on the X_CUR_IN setting, as shown in Figure 4.15
and Figure 4.16.
Figure 4.15
Effect of X_CUR_IN Setting on Residual Current (IG)
NOTE:
You must locate phase CTs
(either X or Y-side) on the terminal
side of the generator for the 87N
elements to be effective (see
Figure 2.18 and Figure 2.23 for
examples of CT location). Also, do not
attempt to use this element with
high-impedance grounded
generators, because the primary
residual current they supply during a
ground fault is too low for secure,
dependable protection.
X_CUR_IN = NEUT
IG = -1 * IGY * CTRY/CTRN
Generator
IN
Y-CTs
Neutral CT
IGY
X_CUR_IN = TERM
IG = IGX * CTRX/CTRN
Generator
X-CTs
Neutral CT
IGX
X-CTs
IN
Summary of Contents for SEL-700G Series
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