Research On Control Strategy Of Dc Transmission System Based On Modular Multilevel Converter

With the traditional ac power system replaced by the high-voltage andhigh-power dc transmission system gradually, converter as a core equipment isdeveloping from two level to multilevel. Modular multilevel converter is getingmore and more attention for its characteristics of highly modular, ease of integrationand high quality of output waveform. Each module of modular multilevel converterexists a charge and discharge capacitor, which cause a a series of problems includingthe problem of voltage balancing in operating system, as well as the problem ofcirculating current caused by three energy imbalance.This paper takes the modular multilevel converter as research object. Byestablishment of mathematical modeling, its working principle and characteristicsare detailed analyzed. The causes of three phase circulating current are derived indetail. According to voltage balancing and circulating current suppression control,the parameters of submodule capacitance and bridge-arm inductance are designed.Self-excited and separate-excited precharge ways of submodule capacitor areintroduced.In order to suppress the submodule capacitor voltage fluctuation, this paperproposes a hierarchical voltage balacing control strategy. Meanwhile, to suppressthe three phase circulating current, a quasi proportional-resonant(PR) circulatingcurrent suppression controller based on ?? coordinates is proposed based on thetraditional negative sequence double frequency circulating current suppressionmethod. This method does not need d-q decoupling and feedforward compensationcontrol. However, each order of resonant controller can only correspond to a cut-offfrequency. With the orders increasing, the controller is becoming more complex.Therefore, this paper also puts forward a kind of composite circulating currentsuppression controller of PI and repetitive control in double d-q coordinates. Thismethod can not only realize the positive and negative circulating current suppression,but also has better performance compared with the former two methods ofcirculating current suppression.This paper researches on the overall control strategies of dc transmissionsystem based on modular multilevel converter, and designs the double closed-loopcontrol of system. Firstly, this paper analyzes the dynamic and steady-stateoperating characteristics of system under different working conditions, as well asthe system fault tolerance when the system transmits power to active network.Secondly, according to the characteristics of voltage source converter dctransmission system, stability analysis is made when the system transmits power to passive network. Finally, the results of simulations and experiments prove thevalidity and effectiveness of the proposed method.