Abstract:
The goal of this thesis is to obtain a simple and easily implement able scheme
for VAR compensation and harmonic reduction in a power system. The basic
idea of compensation in a power system having specified voltage is analysed. The
requirement of a current source compensator is explained. A basic current source
compensator is designed to meet the requirement. A current source capable of
supplying reactive power is composed of an inverter supplied by a dc current
source. For proper operation the inverter switches are provided short periods,
termed the overlapping guard band, during which all the switches are closed in
order to prevent the dc current from being interrupted. The desired compensation
current is produced by pulse width modulation of a high frequency carrier
by the extracted template. A lowpass filter is designed to stop the high frequency
carrier but pass the compensation current of the inverter's output. A damping
resistor is incorporated in the output filter. The dc current source is obtained
by rectifying the supply voltage and connecting a dc inductor in series. A transformer
is necessary for isolation and for stepping down the voltage to a suitable
level. The transformer volume is reduced by designing it for operation at a high
frequency. An additional line frequency rectifier and high frequency inverter are
needed to supply the transformer. An analytical model of the compensator based
on differential equations is provided. Results of compensation of various linear
and nonlinear loads and combinations are given using the analytical model and
two different circuit simulation softwares. Agreement of these results is evidence
that the compensator is correctly designed both theoretically and practically.
Comparison with a Voltage Source Compensator working under the same conditions
shows that the Current Source Compensator performs at least as well. A
practical design is achievable if the converter losses can be reduced. Reduction
of converter losses by many methods is standard practice but only elimination of
the intermediate high frequency stage has been done as an option. All previous
schemes are complicated and not generalized and none of them will be simpler or
more readily irnplementable than the scheme reported in this work.
