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Adjusting
General
The Type PBA2 is a 'resonant' or 'in-phase' device having
Class'"A" performance, and hence it must be adjusted or
tuned to match a specific bushing and a specific burden. A
reference voltage of known value and phase angle position
is required to make this initial adjustment. This reference
voltage is usually supplied from the secondary voltage of a
potential transformer of the desired ratio, which is connected
line-to-ground to the same phase of the transmission line
as the bushing to which the potential device is mounted.
However, the reference voltage may be taken from another
potential device which has been previously adjusted, or it
may be taken from the low voltage secondary of the power
transformer provided it is known that voltage is in phase
with the line-to-ground voltage of the transmission line. The
object is to have a reference voltage for comparison, this
reference voltage to be approximately 115 volts (or this value
divided by 1.73) as may be decided upon for the device
output, and this voltage to be in phase with the transmission
line-to-ground voltage.
There are three adjustments provided on the device adjusting
panel; these are power factor correction of the burden, phase
angle adjustment of the transformer to series tune the
bushing reactance, and voltage adjustment to control the
burden voltage. The phase angle adjustment shows its
effect by shifting the phase angle of the secondary or output
voltage. The controls on the adjusting panel are marked
power-factor correction, volt-amperes, phase-angle, and
inductive reactance, and voltage adjustment.
The numbers on the respective adjustments are in terms of
volt-amperes of power factor correction; active turns of
reactance winding and volts-primary of the ratio transformer.
The values are useful in making the device adjustment.
It is desirable to adjust the device with the final burden in
place. If this is not possible then a synthetic burden shall be
used having the identical volt-ampere and power factor
characteristics of the actual burden.
The adjustment procedure requires a voltmeter of fairly high
impedance to check the magnitude of the output voltage,
whether 115 volts or 66.4 volts (note that the ratio of these
two voltages in the output is fixed so that only one of them
need be adjusted and checked). To check the phase angle
of the output voltage requires either a phase angle meter
having two voltage coils, a low voltage voltmeter, or an
oscilloscope. The oscilloscope is probably the most
satisfactory for phase angle adjustment.
6
If the reference voltage being used is the exact magnitude
desired then the entire adjustment can be made by using
two voltmeters (one reading about 150 volts, and the other
about 5 volts) by connecting the ground of the reference
and the ground of the device output together and then
checking voltage between the line terminals, first with the
150 scale voltmeter and finally with the 5 scale voltmeter.
The device will be correctly adjusted when the voltage
between line terminals is a minimum (less than 1 volt).
Procedure
Power Factor Correction
Correct the power factor of the burden to unity, or to a slightly
leading angle. This is the first step and may be done by
switching in an amount of capacitive volt-amperes, to match
the reactive volt-amperes of the burden. In the case of
delta-connected burdens operating from a bank of three
potential devices, it may be simplest to excite the total burden
from a separate voltage source and to adjust the capacitors
to secure unit power-factor in each phase. This can be done
by grounding terminal S3 (Fig. 3) of each device and applying
the three-phase test voltage to the three S1 terminals, or
the three S2 terminals, depending on the voltage. The primary
shall be de-energized for this test and the ground switch
open.
If in this test the protective gap flashes over this may be
stopped by making a radical change in the phase angle
setting, such as by reversing the BUCK-BOOST switch, or
by changing large sections of turns in the tapping arrangement.
As an alternative, the reactive volt-amperes can be calculated
and the corresponding capacitive volt-amperes set on each
device.
Phase Angle and
Voltage Adjustment
There is some interdependence between the phase angle
and voltage adjustment and so these will be considered
together. These are the final adjustments and are best made
with the final burden (power factor corrected) connected in
its final form to the potential device. The device adjustment
is basically a single-phase procedure, even when connected
into a 3 phase group.
ABB
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