Current Control Of Parallel Three-phase PWM Converters Under Unbalanced Operating Conditions

In the photovoltaic (PV) power generation systems, the parallel structure ofconverters is an effective way to enhance power level and increases gird-connectioncapacity. With the growing grid-connection transmission, new grid-connection codesput forward higher requirements on the reliability of the system. In applications,differences often exist in the values of the three phase inductors, which is thedominant connection filter of gird converters. As a result, the quality of converteroutput currents would degrade. Meanwhile, the circulating currents between parallelmodules would be affected. In addition, the frequent occurrence of gridunsymmetrical fault will also affect the operation condition of PV system. Therefore,this dissertation mainly focuses on the control strategies of three phase PWMconverters parallel systems.Considering the influence of unbalance factor in filter inductance on the outputcurrent, the mathematical model of three phase PWM converter was build at first.The influence was then analyzed and a current decoupling control strategy wasproposed based on the traditional PI method. The dynamic performance of theproposed strategy was also analyzed compared with the positive-negative doublesynchronous reference frame method (DSRF) and positive PIR method. As the studyindicates, the imbalance in filter inductance would result in unbalanced outputcurrents. Fortunately, with the proposed strategy, the side effects caused byunbalanced filter inductance can be effective solved. Moreover, compared with theDSRF method and PIR method, the dynamic response ability of the proposedstrategy is more satisfactory.The control of parallel three phase PWM converters is a significant content inthis dissertation. When the common-dc link converters are directly connected at ACside, circulating current problem will occur, which would distort converter outputcurrents and reduce the energy conversion efficiency. What’s more, the outputcurrents of different converters would be unequal. In a balanced three phase system,the currents circulating in the parallel modules is mainly zero-sequence circulatingcurrent. For this reason, the influence of unbalanced filter inductance on thezero-sequence circulating current was analyzed on the mathematical model ofparallel structures. Then a feed-forward strategy based on a traditional PI controlmethod was proposed. As the study indicates, when unbalance occurs in filterinductance, the circulating current suppression performance of traditional PI methodwould degrade, while the proposed feed-forward strategy can effectively reject theimpacts caused by unbalanced inductance and acquire satisfactory suppressionperformance. Due to the existence of unbalanced load, the occurrence of grid unsymmetricalfault is frequent. However, both the unbalance factors in grid and inductance wouldlead to unbalanced and even distorted output currents. The normal conditions ofparallel system would be affected. The influence of unbalanced grid and inductanceon positive, negative and zero-sequence components of circulating currents wasanalyzed in this dissertation by symmetrical component method, and a correspondcoordinate scheme was proposed to properly control the parallel system undergeneralized operating conditions. As the study indicates, besides zero-sequencecirculating current, negative-sequence circulating current component would also begenerated under generalized unbalanced operating conditions. The resultedcirculating currents will cause distortion in converter output currents and lead tounequal current flow in different modules. With the proposed coordinated schemethe problem can be effectively solved and the normal condition of parallel systemunder unbalanced condition can be guaranteed.