It is a general practice to have some
means of adjustment to maintain constant voltage at the output
terminals by compensating for the variations of the input voltage.
This is done by tapping out or adding turns to the primary or input
winding and maintaining the volts per turn, and thus the output
voltage.
This operation is usually performed
when the transformer is de-energized; this is called off-circuit tap
changing. In dry type transformers, the usual method is to bring out
the tap terminals on the outer surface of the coil or on a terminal
board, where the linking to obtain the required turns is done
manually with the unit de-energized.
It is possible, though not usual, to
have tap switches similar to those used in liquid- filled units.
Until recently, dry-type transformers were never supplied with
under-load-tap-changing equipment. This was due to the fact that
under-load tap changing involves breaking of load current at full
voltage, thereby requiring switching equipment with capabilities
comparable to those of circuit breakers.
To do this in air was cumbersome,
bulky, and extremely expensive. But with the increased capacities and
voltages of dry- type transformers, the demand for such equipment has
increased, and recently voltage regulators became commercially
available.
Two different approaches are used to
provide underload voltage regulation. One takes the traditional
approach of the liquid-filled units by providing motor-driven
selector switches combined with a spring activated vacuum diverter
switch.
The other approach uses a separate
regulator winding feeding a buck/boost transformer connected in
series with the primary winding. Voltage regulation is achieved by
means of low-voltage vacuum contactors that modify the tap settings
of the regulating winding of the buck/boost transformer,
circumventing high-voltage switching equipment.
The contactors are usually controlled
by programmable logic controllers (PLC). In cases where high speed
response is required, the second approach has successfully used
thyristors in place of vacuum contactors, thereby achieving a cycle
switching.
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