Abstract
This paper deals with a reduced rating delta-polygon
autotransformer based twenty-four-pulse ac-dc converter feeding
variable frequency induction motor drives (VFIMD's) for
improving power quality at the point of common coupling (PCC).
The proposed twenty four-pulse ac-dc converter is realized on the
principle of dc ripple re-injection technique for harmonic
mitigation. The design of the proposed autotransformer is
developed alongwith the necessary modifications required for
making it suitable for retrofit applications, where presently a 6-
pulse diode bridge rectifier is used. The effect of load variation on
VFIMD is also studied to demonstrate the effectiveness of the
proposed ac-dc converter. The extensive tests are conducted on
the developed prototype autotransformer based ac-dc converters.
Different power quality indices of 6-pulse, 12-pulse and proposed
24-pulse ac-dc converters are obtained from simulation and
verified from experimental results.
Index Terms-Autotransformer, multipulse AC-DC converter,
pulse multiplication, power quality improvement, VFIMD.
I. INTRODUCTION
T he revolution in solid state power conversion has
opened an era for enhanced use of variable frequency
induction motor drives (VFIMD's) in various applications
such as air conditioning, blowers, fans, pumps for waste water
treatment plants, cement industry etc. These VFIMD's are now
a days used generally in vector control mode [1] due to their
inherent advantages. These voltage source inverter (VSI)
based induction motor drives are generally fed from the 6-
pulse diode bridge rectifier, which results in injection of
current harmonics in to the ac mains. These current harmonics
while propogating through the source impedance result in
voltage distortion at the point of common coupling (PCC),
thereby affecting the nearby consumers.
There has been an impetus on research in power quality
interface due to stringent power quality regulation and strict
limits on current and voltage distortion imposed by various
standards such as IEC 61000-3-2 [2] and IEEE 519-1992 [3].
This has led to the consistent research in innovating various
configurations of ac-dc converters for mitigating these
harmonics [4]. Different techniques for power quality
improvement such as use of multipulse converters [5-14], use
of multiphase converters [15] and use of dc ripple re-injection
technique [16-17] for pulse multiplication have been reported
in the literature. These techniques for harmonic mitigation are
found to be rugged, reliable and energy efficient over their
counterparts. These methods use two or more converters,
where the harmonics generated by one converter are cancelled
by other converter, by proper phase shift. The autotransformer
based converters result in reduction in rating of the magnetics,
as the transformer transfers only a small portion of the total
kVA of the VFIMD. Different topologies of 12-pulse ac-dc
converters have been reported in the literature for harmonic
mitigation [5-1 1]. But these configurations fail to comply with
above mentioned standards. Increasing the number of pulses
further results in improvement in various power quality
indices, but alongwith the additional cost of different
converters and increased system complexity. Autotransformer
based configurations of 18-pulse ac-dc converters have also
been reported in the literature [12-15]. The Total Harmonic
Distortion (THD) of ac input current in an 18-pulse ac-dc
converter is reported to be 8.6% at full load, which may
further deteriorate at light load [14]. Moreover, the rating of
magnetics required in an 18-pulse converter is of the order of
5500 [15], which is quite high. To achieve an improved
performance in terms of various power quality indices at a
reduced cost, a 24-pulse converter employing dc ripple reinjection
technique for pulse doubling is designed and
developed in this work.
This paper presents a novel reduced rating autotransformer
based twenty-four-pulse ac-dc converter suitable for retrofit
applications (where presently a six-pulse diode bridge rectifier
is used, as shown in Fig.1, referred as Topology 'A'). The
design procedure of the autotransformer is given for making it
suitable for retrofit applications [18]. The dc ripple reinjection
technique used for harmonic mitigation needs two
additional diodes alongwith a suitable tapped interphase
reactor for increasing the number of pulses. The proposed acdc
converter is able to provide almost unity power factor in
wide operating range of the VFIMD. A set of tabulated results
giving the comparison of different power quality indices such
as total harmonic distortion (THD) and crest factor of ac mains
current (CF), power factor (PF), displacement factor (DPF)
and distortion factor (DF), THD of supply voltage at PCC is
presented for a VFIMD fed from an existing 6-pulse ac-dc
converter, 12-pulse ac-dc converter and proposed 24-pulse acdc
converter. Various tests are conducted on the prototype
autotransformers developed in the laboratory alongwith the
interphase reactors and the Zero Sequence Blocking
Transformer (ZSBT). The experimental results validate the
simulation results for both 12-pulse and proposed 24-pulse acdc
converters.

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