BECKWITH ELECTRIC M-3425A, Instruction Book

Page: 228 / 482

4–97
System Setup and Setpoints – 4
Where V
20
is the 20 Hz voltage measured across
the neutral resistor R
N
and X
CS
is the capacitive
reactance of the generator stator winding and unit
transformer referred to the grounding transformer
secondary. N is the turn ratio of the grounding
transformer. There are two overcurrent pickup
settings. One operates on the magnitude of total
20 Hz neutral current measured by the relay.
The other pickup setting operates on the real
component of the 20 Hz neutral current where V
20
is the reference. V
20
is the 20 Hz voltage measured
across the neutral resistor R
N
.
The real component
of the 20 Hz current increases in magnitude
during a ground fault on the generator stator
since the insulation resistance decreases.
The
real component of current pickup is disabled
when V
N
is less than 0.1 V @ 20 Hz
. Set the
two pickups utilizing the equations illustrated in
Figure 4-73.
The 20 Hz signal generator has an output of
25 volts and the band pass filter is eight ohms
purely resistive.
Only a small amount of 20 Hz current flows when
the generator is operating normally (that is, no
ground fault) as a result of the stator capacitance to
ground. The magnitude of 20 Hz current increases
when there is a ground fault anywhere along the
stator windings. The 64S function issues a trip
signal after a set time delay when the measured
20 Hz current exceeds a pickup as illustrated in
Figure 4-74.
The 59N Function (90 to 95%) should also be used
in conjunction with 64S protection to provide backup.
▲
CAUTION:
Dangerous high voltages may be
present at the generator terminals if the 20 Hz
injection voltage is not removed when the generator
is taken out of service.
If the 20 Hz injection voltage generator receives
power from the generator terminal voltage, then
the 20 Hz injection voltage generator will be
automatically switched off whenever the generator
terminal voltage is not present.
64S 100% Stator Ground Protection by Low
Frequency Signal Injection

NOTE : The Stator Ground Protection function
(64S) must be selected when the
M-3425A is initially ordered.
The 100% stator ground fault protection is provided
by injecting an external 20 Hz signal into the neutral
of the generator. The protection is provided when
the machine is on-line as well as off-line (provided
that the 20 Hz generator and relay are powered on).
The injected 20 Hz signal will produce a voltage
that appears on the primary side of the grounding
transformer when the machine is online as well as
offline. This scheme requires the following external
components in addition to M-3425A protection
system:
• 20 Hz Signal-generator (BECO Part No.
430-00426) (Siemens 7XT33)
• Band-pass filter. (BECO Part No.
430-00427) (Siemens 7XT34)
• 20 Hz Measuring Current Transformer,
400/5 A CT (BECO Part No. 430-00428)
(ITI CTW3-60-T50--401)

NOTE : Chapter 5, Installation contains low
frequency signal injection equipment
installation information.
The voltage signal generated by the 20 Hz signal-
generator is injected into the secondary of the
generator neutral grounding transformer through
a band-pass filter. The band-pass filter passes the
20 Hz signal
and rejects out-of-band signals. The
output of the 20 Hz band-pass filter is connected
to the V
N
input of the M-3425A relay through a
suitable voltage divider, that limits the M-3425A to
< 200 V ac (the voltage generator may be bypassed
if the expected 50/60 Hz voltage during a phase-
to-ground fault of the generator is >200 V). The
20Hz current is also connected to the I
N
input of the
M-3425A, through the 20Hz current transformer.
The expected 20 Hz current during no fault condition
is given by:
I
NF
= V
20
X
CS
X
CS
= X
C
(Primary)
N 2