Abstract:
A family of three phase switched mode AC voltage controller topologies,
namely- Buck, Boost, Buck-Boost, ĈUK, and SEPIC, has been proposed in this
research. The topologies are functionally equivalent to switched mode DC-DC
converters. Two three phase AC bidirectional switches have been used to get the
SPDT switching configuration in three phase AC-AC circuits similar to those of DC
chopper circuits. The three phase true AC bidirectional switch consists of a
unidirectional controlled switch, such as, an IGBT, enclosed by two three phase
diode-bridges on either side. This is equivalent to three single phase AC bidirectional
switches and it provides separate three phase input/output terminals for the switch
such that three phase currents can be carried by the same switching device in both
directions. The new bidirectional switch gives design advantages like reduction of
number of controlled switching devices and associated snubbers, isolated gate driver
supplies and control circuits for PWM signals. It also provides the benefits of having
simple control circuit, less switching loss, reliable operation and reduction in cost
and complexity. Performance of the new AC voltage controllers have been
investigated by varying the duty cycle of the gate pulses of high frequency and
adjusting the circuit parameters. Simulation results show the ability of the proposed
converters to do step-up and/or step-down AC-AC conversion with low input current
THD and high input power factor at enhanced efficiency. A study on the closed loop
feedback control system on the proposed converter topologies by simulation shows
the ability of simple PI-controller to regulate the output voltage of the converters
with sufficient accuracy. In response to sudden disturbance of load or input voltage
the controller adjusts the duty cycle of the gate pulses to recover the desired output
voltage quickly. With the feedback control the performance of the converters has
been found satisfactory at desired operating conditions. To prove the workability of
the proposed converter concept, the three phase Boost AC voltage controller has
been implemented practically in a low voltage, low power prototype setup.
Experiments have been conducted with resistive and inductive loads in open and
closed loop controls. Some adaptations have been made to the three phase AC-AC
voltage converters to further reduce the semiconductor devices and to include
possible neutral connection with source and/or with load. The modified converter
circuits have been made by two modified three phase bidirectional switches with
neutral. The research outcome is expected to result in a family of simple, low cost,
high performance three phase AC-AC voltage controller topologies suitable for ACAC
applications.
