Restricted Ground (Differential Ground) - GE TRANSFORMER MANAGEMENT RELAY 745 Instruction Manual

5.6 S4 ELEMENTS
PATH: SETPOINTS S4 ELEMENTS
W1 RESTD GND FAULT
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Restricted Ground Fault protection is often applied to transformers having impedance grounded wye windings. It is
intended to provide sensitive ground fault detection for low magnitude fault currents which would not be detected by the
percent differential element.
5
An internal ground fault on an impedance grounded wye winding (see second figure above) produces a fault current (I
dependent on the value of the ground impedance and the position of the fault on the winding with respect to the neutral
point. The resultant primary current (I
will not be the system voltage but the result of the transformation ratio between the primary windings and the percentage of
shorted turns on the secondary. Therefore, the resultant differential currents could be below the slope threshold of the per-
cent differential element and thus the fault could go undetected. The graph below shows the relationship between the pri-
5-62
Courtesy of NationalSwitchgear.com
RESTRICTED GROUND
ð
W1 RESTD GND FAULT
ENTER
FUNCTION: Disabled
ESCAPE
W1 RESTD GND FAULT
ESCAPE
TARGET: Latched
MESSAGE
W1 RESTD GND FAULT
ESCAPE
PICKUP: 0.08 x CT
MESSAGE
W1 RESTD GND FAULT
ESCAPE
SLOPE: 10%
MESSAGE
W1 RESTD GND FAULT
ESCAPE
DELAY: 0.10 s
MESSAGE
W1 NEUTRAL TIME OC
ESCAPE
BLOCK: Disabled
MESSAGE
Figure 5–21: RESTRICTED EARTH GROUND FAULT PROTECTION
Delta Winding
I
P
Impedance grounded
wye winding
Figure 5–22: RESISTANCE GROUNDED WYE WIRING
) will be negligible for faults on the lower 30% of the winding since the fault voltage
P
745 Transformer Management Relay

5.6.8 RESTRICTED GROUND (DIFFERENTIAL GROUND)

W1(3) RESTD GND FAULT
Range: Enabled, Disabled
Range: Self-Reset, Latched, None
Range: 0.02 to 20.00 x CT in steps of 0.01
Range: 0 to 100% in steps of 1
Range: 0.00 to 600.00 s in steps of 0.01
Range: Logc Inpt 1 to 16, Virt Inpt 1 to 16, Output Rly 2 to
8, SelfTest Rly, Vir Outpt 1 to 5, Disabled
87T
Rg
87TG
Wye Winding
FAULT
I
F
5 SETPOINTS
)
F
GE Multilin