604 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 44, NO. 5, OCTOBER 1997 Harmonic Current Reduction in Three-Phase Bridge-Rectiﬁer Circuits Using Controlled Current Injection Wladyslaw Mielczarski, Senior Member, IEEE, William B. Lawrance, Member, IEEE, Rafal Nowacki, and Donald Grahame Holmes, Member, IEEE Abstract— This paper describes further development of the novel current-injection scheme devised by the authors for at- tenuating line current harmonics in bridge-rectiﬁer circuits. The previous passive ﬁlters are replaced by controlled ﬁlters con- nected between the bridge-rectiﬁer output and the star point of the transformer secondary. These ﬁlters are used to inject controlled third harmonic currents into the transformer wind- ings. Variations in bridge working conditions, due to changes in either load or bridge delay angle, lead to corresponding changes in the ﬁlter operating conditions if the harmonic attenuation is to remain optimal. The control law for the ﬁlters has been derived and the effectiveness of the new method conﬁrmed by both simulation and laboratory tests. Index Terms— Bridge rectiﬁers, distribution networks, har- monic currents, harmonic reduction, quality of electricity supply, third harmonic injection. I. INTRODUCTION T HE application of power electronics in industry has led to increases in both the size and number of nonlinear loads. These loads act as harmonic current sources which pollute the mains supply. Supply authorities, who are responsible for the integrity of the supply, are concerned with this harmonic distortion, and various harmonic standards are now in place [1], [2]. These standards (or recommendations) limit the har- monic voltage distortion at the point of common coupling between the load and the supply system. The cumulative effect of the individual voltage harmonics is termed total harmonic distortion (THD) and is usually limited to 5% at the 22-kV point of common coupling. While the standards specify allowable values for the har- monic voltages, they do not provide information on how to reduce harmonic distortion in the supply network. There are at least two reasons for this omission. Firstly, when several consumers are connected to the point of common coupling, it is difﬁcult to determine the contributions of individuals to the THD. Secondly, the level of harmonic voltage distortion Manuscript received February 6, 1996; revised December 8, 1996. W. Mielczarski and D. G. Holmes are with the Centre for Electrical Power Engineering, Monash University, Clayton, 3168 Australia. W. B. Lawrance is with the Department of Electrical Engineering, Curtin University, Bentley, 6102 Australia. R. Nowacki was with the Centre for Electrical Power Engineering, Monash University, Clayton, 3168 Australia. He is now with Connor, Pincus & Sounders, Melbourne, Australia. Publisher Item Identiﬁer S 0278-0046(97)06535-0. is dependent not only on the harmonic source, but also on the structure of the supply network. Despite this, it is usually incumbent on the consumer to implement a suitable harmonic attenuation scheme. Typically, this involves the addition of banks of passive ﬁlters adjacent to the point of common coupling. The new scheme, shown in Fig. 1, utilizes the familiar concept of current waveshaping by the injection of a third har- monic current into the star point of the transformer-secondary winding [3], [4]. This method utilizes the third harmonic voltage available between the bridge output and the common point of the secondary winding. The initial scheme developed by the authors [5], [6], uses two passive LC ﬁlters tuned to the third harmonic frequency and is intended for application to uncontrolled three-phase bridge rectiﬁers. The method de- scribed in this paper uses back-to-back thyristors in series with the passive ﬁlters, in order to control the phase and magnitude of the injected third harmonic current. These controlled ﬁlters allow the scheme to be used with controlled three-phase bridge rectiﬁers and to minimize the line current harmonic distortion, as either the bridge delay angle varies or the rectiﬁer load ﬂuctuates. While the merit of the circuit using passive ﬁlters was its inherent simplicity, the “modiﬁed” circuit remains relatively simple, requiring, in addition to the tuned ﬁlters, two pairs of back-to-back thyristors and an associated controller. There are also other contributions to reducing harmonic current distortion by the utilization of third harmonic current injection. Optimization of a dc-link-current-modulated inter- face [7] leads to a scheme with three LC ﬁlters connected to the bridge-rectiﬁer input with the injection of a harmonic current into the common point of the LC ﬁlters. Another scheme utilizes a transformer connected in parallel to the bridge with star/closed-delta windings and an injection of third harmonic current into the common point of the star [8]. Although both methods use the same concept of a third harmonic injection [3], [4], their implementation leads to different circuits from the scheme presented in this paper. II. INJECTION OF THIRD HARMONIC CURRENTS Assuming both the amplitude and phase of the injected harmonic current are variable, the current can be described by (1) 0278–0046/97$10.00  1997 IEEE