A Novel Optimized Capacitor Voltage Balancing Method For Modular Multilevel Converter

As an important circuit topology for voltage source converters,the highly modular structure of the modular multi-level converter(MMC),easy to run redundant,avoids the direct series of switching devices while improving the output voltage waveform quality,has recently become a hot spot in the field of VSC-HVDC.The voltage of MMC DC side is supported by capacitors,so the capacitor voltages balance control becomes a key problem for MMC operating safely and stably.The conventional voltage balance method based on sorting algorithms can effectively balance the MMC capacitor voltages,but there are also some defects,such as high computation of MMC controller and high switching frequency.In this paper,a novel optimized capacitor voltage balance method for MMC-HVDC system is proposed aiming at the problem existing in traditional capacitor voltage balance method.Firstly,the basic operation principle and modulation method of MMC sub-module and main circuit are introduced.The mathematic model of MMC is established in two-phase dq rotating coordinate system.Based on its strong coupling nonlinearity,the power decoupling control of MMC is designed method.Secondly,the basic principle of MMC capacitor voltage balance is analyzed from the viewpoint of energy balance between AC side and DC side.The control variable method is used to calculate and simulate various factors that may cause capacitance voltage imbalance.And the evaluation index of capacitance voltage balance control is introduced.Thirdly,according to the problem of large computational complexity of traditional capacitor voltage sorting algorithm,the randomized-select algorithm is used to sort,and the mathematical proof and computer experiment of the time complexity of the randomized-select algorithm are carried out.Finally,for the high switching frequency of the traditional method,a renew sort criterion is introduced to make the MMC controller avoiding sorting and maintain trigger pulse unchanged in the control cycle when the capacitance voltages change little,sorting the capacitor voltages employing the randomized-select algorithm whose efficiency is higher.The two-terminal 21-level MMC-HVDC system is built in Matlab/Simulink to verify the proposed method.The designed power decoupling controller is proved to be effective to decouple the output active power and reactive power of MMC by mathematical simplification.In the various factors that may cause the capacitance voltage imbalance,only the difference of the power supply and the switching delay may cause the difference of sub-module capacitance voltages to diverge.Both of the result of mathematic proof and computer experiment verify the time complexity of randomized-select algorithm is O(n),which is one order of magnitude smaller than the traditional algorithm.The simulation results show that the proposed method maintain the sub-modules capacitance voltages balance in both steady and transient state,reduces the computation of the MMC controller effectively and the switching losses of converter station largely.Compared with the existing methods,the proposed method is easy to be applied with only one parameter ?N.