GE 745 Instruction Manual page 169

CHAPTER 5: SETPOINTS
745 TRANSFORMER PROTECTION SYSTEM – INSTRUCTION MANUAL
I
P
Impedance grounded
FIGURE 5–23: Resistance grounded wye wiring
An internal ground fault on an impedance grounded wye winding (see second figure
above) produces a fault current (I
the position of the fault on the winding with respect to the neutral point. The resultant
primary current (I ) will be negligible for faults on the lower 30% of the winding since the
P
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
distance of the fault point from the neutral and FIGURE 5–25: 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 5–24: Fault currents vs. points from neutral
Delta Winding
Wye Winding
I
F
wye winding
) dependent on the value of the ground impedance and
F
) and fault (I
P
100
90
80
70
60
Ifault
50
40
30
20
10
0
0 10 20 30 40 50
x = distance of fault from neutral
S4 ELEMENTS
FAULT
) currents as a function of the
F
Ip
60 70 80 90 100
5–73