Circulating Current Control For Modular Multilevel Converters

As a power conversion device.modular multilevel converter(MMC)has been applied in high voltage direct current transmission system based on its characteristics,such as modularity,lower harmonic and redundancy.Circulating current is introduced by MMC's structure and operating mode.It affects the performance of arm current and submodule capacitor voltage,which decide the parameter design,power semiconductor device selection and cost of an MMC.Submodule failure and unbalanced ac grid voltage are two common conditions due to MMC's structure and applications.Circulating current is changed under different operating conditions.Among existing circulating current control methods,harmonic components suppression is a conventional way for ensuring better arm currents.However,additional benefits can be achieved when specific harmonic components injected into the circulating current.This dissertation focuses on the analysis and control methods of circulating current under balanced ac grid condition,submodule failure condition and unbalanced ac grid condition.Based on the coupling relationship of submodule's voltage and current,the expressions of circulating current and submodule capacitor voltage are derived under balanced ac grid condition.The simulation and experiment results of harmonic components suppression indicate that the peak arm current can be reduced,but the submodule capacitor voltage fluctuations may not be improved.Utilizing the amplitude of second order harmonic circulating current as a variable,the power semiconductor devices' average current and the root-mean-square current,and the submodule capacitor voltage expressions are derived.For reducing the submodule capacitor voltage fluctuations,an optimal solution of second order harmonic circulating current amplitude is calculated.The simulation and experiment results of circulating current amplitude control in dq synchronous rotating coordinates show its validity in capacitor voltage fluctuations reduction.For improving the submodule capacitor voltage fluctuations more effectively,a harmonic component injected into the circulating current is proposed.The harmonic component reference is calculated based on an equivalence principle of arm energy fluctuations,and it is calculated by the instantaneous ac side voltage and current.The validity and feasibility of this method is verified by experiments.The submodule fault-tolerant control methods and circulating current control method under an MMC without redundancy are investigated.For keeping the system operating normally during submodule fault conditions,two fault-tolerant control methods are introduced.One is generalized discontinuous pulsewidth modulation,the other is amplitude-limited modulation.The circulating current characteristic is analyzed under the two fault-tolerant control methods.A control strategy of ac circulating current components suppression is proposed based on a second-order generalized integrator.The validity and feasibility of two fault-tolerant control methods and ac circulating current components suppression strategy are verified by simulation and experiments.The circulating current control methods under grid imbalances are investigated.Firstly,the ac-side power ripples and circulating current are analyzed.For inhibiting them simultaneously,a control method of circulating current harmonic components suppression is proposed.For reducing the submodule capacitor voltage fluctuations during grid imbalances,a strategy of harmonic components injected into the circulating current is proposed.It relies on the calculation of positive-sequence and negative-sequence voltages vectors.The validity and feasibility of two circulating current control methods are verified by simulation and experiments.For comparing the performance of harmonic components suppression and harmonic components injection in circulating current under grid imbalances,the arm energy fluctuations and peak arm currents under the two methods are analyzed.In the analysis,converter control target k and grid imbalance degree ? are taken into consideration.Under a single line to ground fault,the active power and reactive power boundaries of three converter control targets are elaborated based on power ripples,arm energy fluctuations and peak arm currents.The RTDS simulation results of an 800MVA model indicate that the correctness and validity of power boundaries calculation under grid imbalances.Power boundaries can be expanded by circulating current harmonic injection method.