BECKWITH ELECTRIC M-0420, Instruction Book

Page: 35 / 174

2–17
Application – 2
Ground Fault Detection Using 27N and 59N
with One VT
An alternate, but not recommended, scheme uses
the 27N and 59N devices with one VT rated for
line-to-line voltage, but connected from any one
phase to ground as shown in Figure 2-9. This
scheme will detect the most common line-to-ground
faults in the following manner:
1. A fault on the phase that includes the VT
will pull that phase voltage low and initiate
operation of the 27N device.
2. A fault on either phase without the VT will
result in line-to-line voltage (or
S
3 x normal
line-to-neutral voltage) appearing at the VT,
initiating operation of the 59N.
For this scheme to work, the capacitance to ground
of the lines must be fairly closely balanced and
high enough to keep the neutral of the system at
close to ground potential. The shunt resistor helps
to minimize the chance of ferroresonance or neutral
inversion. (
Applied Protective Relaying, Westing-
house Electric Corporation, 1982.)
▲
CAUTION: This scheme should be used with
caution since it can result in high overvoltages
due to ferroresonance and neutral inversion.
V
L-L
C
B
A
Power
Transformer
Fault on phase 'a'
V=0
27N detects
undervoltage
Fault on phase 'b'
V=V
59N detects
overvoltage
To
Utility
L-L
To Dispersed Source of
Generation (DSG)
c
b
a
V
59N 27N
RESISTOR
Figure 2-9 Ground Fault Detection Using 27N and 59N with One VT