Research On Three-Phase Unbalanced Controller For MMC-STATCOM

Nowdays,the demand for electric energy is increasing in all fields,and the power electronic devices are becoming more and more mature.,which causes the existing power quality problem even worse,such as reactive power pollution,voltage fluctuation,flicker and three-phase unbalanced.As the latest technology of reactive power compensation,STATCOM can solve these power quality problems well,and has become the focus object.At present,multilevel technology is widely adopted in STATCOM devices,which is commonly used in cascade star structure,cascade triangle structure and modularized multilevel structure.Comparing with traditional two-level technology,it has the advantages of modularization design,easy expansion,low output harmonics and so on,which provides technical support for high-voltage and high-power situation.For compensating unbalanced loads,the topology of MMC is superior to the other two cascaded structures.Therefore,the unbalanced control strategy for MMC based on STATCOM(UCMS)is studied in this paper,combining with the theoretical investigation,and simulation analysis,the reliability of the unbalanced control strategy is demonstrated.The main research contents are as follows:Firstly,according to the different effects of three-phase imbalance on traditional two-level STATCOM and multilevel STATCOM,it is shown that internal unbalanced control should be considered in the multilevel topology.The performance of three multilevel STATCOM unbalanced compensation is analyzed and compared.Therefore,determination has made: the selection of the MMC chain structure.Secondly,according to the existing UCMS with the AC side and internal circulation are controlled separately and the MMC-STATCOM mathematical model,the STATCOM unbalanced controller based on MMC bridge current is proposed without a dedicated inner circulating current controller,which makes the overall control structure simple and easy to realize.This paper expounds the principle of MMC-STATCOM comprehensive compensation in ideal and non-ideal cases with vector diagrams,and the DC capacitor charging,DC capacitor voltage equalization control and strategy of the MMC bridge arm current are introduced in detail.According to the Bode diagrams,the effect of parameters setting on the performance of notch filter and two kinds of resonant frequency quasi-PR controller are analyzed,and also gives the basis of parameters tuning.Finally,a 9-level three-phase STATCOM is built in the EMTDC/PSCAD environment,and carries out the simulation of the unbalanced control theory.The simulation result verifies the feasibility and effectiveness of the topology structure and unbalanced control strategy used in this paper.