The Diode-Clamped Modular Multilevel Converter And Its Application In Power Quality Control

Multilevel converters have many advantages such as lower voltage stress,high efficiency,and lower harmonic distortion et,which are attractive in high voltage transmission,power quality control applications.Recently the Modular multilevel converter(MMC)emerges as the most popular multilevel converter.The capacitor voltage imbalance remains to be a key issue in MMC,as found in other multilevel converters.This paper has deeply studied the topology and control strategies of the new self-balancing multilevel converter,as well as the application in power quality control area.The contents of the work are as follows:Firstly,this paper proposed a novel diode-clamped modular multilevel converter.In this converter a branch which consists of a diode and an inductor is used to clamp neighboring sub-modules(SMs),making their voltages be a descending sequence from the top SM to the bottom one in one arm.A voltage balancing control strategy is proposed combining with the phase-shifted carrier PWM,in which the top SM is controlled to be the average voltage value of the SMs.The balancing method is very simple and the control computational burden is slight.A redundancy method is also presented for SM malfunction.In this paper,the voltage balancing factors are analyzed,including the parasitic resistance in clamping diodes and switches,clamping inductance,and the reference voltage of top SM.The calculation method for clamping circuit parameters are carried out.The three-phase DCM2 C prototype is estabished.Secondly,based on the DCM2C a parallel-connected diode-clamped MMC is proposed.Each arm in the new topology has two clusters which contains the clamping branches.All the SM voltages in this arm are clamped into a circle,realizing the capacitor voltage balancing.The current rating is increased owning to the parallel structure.The capacitor voltage fluctuation model is established and analyzed,and the arm current balancing control is studied according to the voltage equation in the arm.The control method of parallel topology as an inverter is also proposed.Thirdly,a novel Var compensation system based on the diode-clamped MMC is proposed.The static synchronous control scheme is applied to inject current in to the grid.The arm power caused by negative-sequence current is analyzed,as well as the equivalent model of converter in uncontrollable period of the pre-charge.The charge method in the controllable period is studied.The control strategy of Var compensation is proposed,in which the positive-/negative-sequence currents are decoupled and controlled using instantaneous power theory.The strategy also combines the voltage balancing method and phase-shifted carrier PWM method.Finally,a novel voltage mitigation system based on the diode-clamped MMC is proposed.The parallel-connected diode-clamped modular multilevel converter is used as the inverter,and super-capacitors are installed in every SMs as the storage devices.Owning to the voltage self-balance capability of the parallel converter,the super-capacitors can be balanced automatically.Three basic voltage mitigation strategies are analyzed,as well as the relation of compensation time and sub-module numbers,capacitance,output voltage amplitude/power.A new integrated voltage mitigation strategy is proposed,which selects the proper compensation method according to the voltage sag depth and load power factor.The new strategy can ensure the good compensation performance,as well as the compensation time.