BECKWITH ELECTRIC Syncrocloser M-3410A, Instruction Book

Page: 87 / 225

M-3410A Instruction Book
3–16
27G/59G Undervoltage/Overvoltage, Ground
Circuit or Zero Sequence
The Ground Circuit Under/Overvoltage functions
(27G/59G) provide protection for ground faults on
systems supplied from an ungrounded source.
Applications of 27G/59G, shown in Figure 3-20 and
Figure 3-21 are for detecting ground faults on the
ungrounded utility side of the power transformer.
Protection schemes are applied based on using
one or three voltage transformers (VT).
Ground Fault Detection using a Broken-Delta
VT and the 59G Function
The 59G may be used to detect system phase
voltage unbalance in conjunction with three VTs.
To do so, the VT secondaries are connected
“broken” delta; i.e., they are in delta except that
one corner is open, and the 59G device is inserted
(as illustrated in Figure 3-20).
In this case, voltage at 59G in Figure 3-20 will be
zero, as 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 ground fault.
Ground Fault Detection Using One Phase-to-
Ground VT and 59G/27G
▲
CAUTION : This scheme should be used with
caution, since it can result in high overvoltages due
to ferroresonance and neutral inversion.
An alternate, but not recommended, scheme uses
the 27G and 59G devices with one VT rated for line-
to-line voltage, but connected from any one phase
to ground as shown in Figure 3-21. This scheme
will detect the most common line-to-ground faults
on systems supplied by an ungrounded source, in
the following manner:
• A fault on the phase that includes the VT
will pull that phase voltage low and initiate
operation of the 27G element.
• A fault on either phase without the VT will
result in line-to-line voltage (or S3 x normal
line-to-ground voltage) appearing at the
VT, intiating operation of the 59G element.
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,
Westinghouse Electric Corporation, 1982). Typical
Shunt Resistor values presented in Table 3-1.
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 dampen high transient
voltage oscillations, and will usually hold peak
values to less than twice normal crest voltage to
ground (
Protetive Relaying Principles and
Applications, Second Additon, J. Lewis Blackurn).