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2–33
Application - 2
Ground Fault Detection using 59G and Broken-
Delta VTs
The 59G may be used to detect system phase
voltage unbalance in conjunction with three VTs.
To do so, the VT secondaries are connected in
“broken” delta; i.e., they are in delta except that one
corner is open, and the 59G device is inserted, as
illustrated in Figure 2-23.
In this case, voltage at 59G in Figure 2-24 will be
zero so long as the three-phase voltages are
balanced, but will rise above zero with any zero-
sequence unbalanced condition, as will be expected
with any real world utility ground fault.
A
B
C
Power
Transformer
a
b
c
59G
Resistor
To Dispersed Storage
and Generation (DSG)
To Utility
Figure 2-23
Ground Fault Detection Using 59G and Broken-Delta VTs
When the relay burden is small, the transformers in this scheme will be subject to ferroresonance and high
voltage oscillations unless a shunt resistor is used. The shunt resistor will damp high transient voltage
oscillations, and will usually hold peak values to less than twice normal crest voltage to ground (
Applied
Protective Relaying, Westinghouse Electric Corporation, 1982).
s
e
u
l
a
V
l
a
c
i
p
y
T
o
i
t
a
R
T
V
R
0
2
1
/
0
0
4
2
0
5
2
Ω
0
2
1
/
0
0
2
4
5
2
1
Ω
0
2
1
/
0
0
2
7
0
9
Ω
0
2
1
/
0
0
4
4
1
0
6
Ω
Table 2-3
Typical Shunt Resistor Values
Содержание M-3520
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