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745
Transformer Management Relay
5–62
I
P
Impedance grounded
wye winding
FIGURE 5–22: Resistance Grounded Wye Wiring
An internal ground fault on an impedance grounded wye winding (see second figure
above) produces a fault current (I
impedance and the position of the fault on the winding with respect to the neutral
point. The resultant primary current (I
30% of the winding since the fault voltage 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 percent differential element and thus the
fault could go undetected. The graph below shows the relationship between the
primary (I
) and fault (I
) currents as a function of the distance of the fault point
P
F
from the neutral and FIGURE 5–24: RGF and Percent Differential Zones of Protection
outlines the zones of effective protection along the winding for an impedance
grounded wye.
Ip(x)
Ifault(x)
FIGURE 5–23: Fault Currents vs. Points from Neutral
Rg
FIGURE 5–24: RGF and Percent Differential Zones of Protection
http://www.GEindustrial.com/multilin
Delta Winding
Wye Winding
FAULT
I
F
) dependent on the value of the ground
F
) will be negligible for faults on the lower
P
100
90
80
70
60
Ifault
50
40
30
Ip
20
10
0
0
10
20
30
40
50
60
x = distance of fault from neutral
WINDING
35%
RGF
DIFFERENTIAL
ZONE
ZONE
S4 Elements
70
80
90
100
GE Multilin
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