BECKWITH ELECTRIC M-3425A, Instruction Book

Page: 224 / 482

4–93
System Setup and Setpoints – 4
64B/F Field Ground Protection
64F Field Ground Detection
Typical connections for Field Ground Protection
applications (including hydro turbine-generator
and brushless generators) is given in Figure
4-70. This function requires the connection of an
external coupler (M-3921). To improve accuracy
and minimize the effects of stray capacitance, the
M-3921 Field Ground Coupler should be mounted
close to the exciter. Connections from the coupler
to the relay should use low capacitance shielded
cable, and be as short as possible. Cable shield
should be terminated at the relay end to the
Relay Ground Stud (See Figure 5-9, External
Connections). If cabling between the coupler and
relay exceeds 100 feet, provisions should be made
for in circuit calibration to nullify the effects of cabling
capacitance. See Section 6.4, Auto Calibration, for
calibration procedure.
The Field Ground function provides detection of
insulation breakdown between the excitation field
winding and the ground. There are two pickup and
time delay settings, and one adjustable injection
frequency setting for the 64F function. The
adjustable frequency is provided to compensate for
the amount of capacitance across the field winding
and the ground so that the function accuracy is
improved. The minimum time delay should be
set greater than (2/IF + 1) seconds. Where IF =
Injection frequency. Ranges and increments are
presented in Figure 4-71.
Table 4-8 gives typical frequency settings based on
the rotor capacitance. The rotor capacitance can be
measured with a capacitance meter by connecting
the meter across the field winding to ground.
64F #1 PICKUP
kOhm
64F #1 DELAY
Cycles
64F #2 PICKUP
kOhm
64F #2 DELAY
Cycles
Factors Affecting 64F Performance
Some excitation systems include shaft voltage
suppressors which include capacitors that are
installed between the +/- field and ground. The
effect of these capacitors is given by the following
equation:
R
J
___1___
(2
π
IF C)
where:
R = Parallel winding-ground resistance
IF = Injection frequency setting
C = Capacitance value
To minimize this effect the following my be
implemented:
• The injection frequency setting can be
reduced, however accuracy decreases
as a result.
• With the concurrence of the exciter
manufacturer, surge capacitors rated at
a lower value may be installed.
This setting should not exceed 80% of the ungrounded
resistance value to prevent nuisance tripping. Typical setting
for the 64F #1 pickup element for alarming is 20 Kohms.
Typical delay setting for tripping is 800 cycles.
Typical setting for 64F #2 pickup element for tripping is 5
Kohms.
Typical delay setting for alarming is 180 cycles.