Characteristics Analysis Of Asymmetrical Arms And Research On Circulating Current Suppression Strategy Under MMC Dynamic Redundant Operation

With the increase of social electricity consumption and the enhancement of public environmental awareness,urban power grid is expanding and renewable energy is growing rapidly.Therefore,new challenges on traditional power transmission is put forward.Voltage sourced converter based high voltage DC transmission(VSC-HVDC)can be an effective solution because of its control flexibility.And the modular multilevel converter(MMC)is widely used in VSC-HVDC field because of its highly modular structure,easy scalability of power level,low cost redundant configuration,high reliability and excellent output waveform quality.In addition,MMC has good prospective application in power quality management,active filtering,medium voltage motor drives and so on.Since there are a large number of sub-modules in MMC arms,normal operation of the system will be affected after a sub-module failure.It is necessary to set a certain number of redundant modules in each arm.Cold spares and hot spares are two common ways to conduct the redundancy.However,the transient time to involve the redundant sub-modules into the work is too long.To make full use of the redundant sub-modules,the redundancy protection scheme applied in this paper is dynamic redundancy control and asymmetric operation after fault sub-modules are cut off.Firstly,the topology of three-phase MMC is introduced and the basic working principles of dynamic redundant operation are analyzed,which lays the foundation of the next analysis.The equivalent circuit model of three-phase MMC is brought out.And the equivalent circuit of single-phase MMC on AC side and DC side are given,respectively.Then carrier phase shift PWM method and nearest level modulation method for dynamic redundancy control and fault tolerance operation are introduced.Secondly,the average switching function model of MMC dynamic redundancy and fault tolerant operation is established.The influence of the fault sub-module on the system is analyzed and the change in circulating current is derived.In the MMC dynamic redundant operation,let the maximum number of inserted sub-module in the arms remain the rated value after fault sub-modules are removed.But the system still appears as asymmetric operating conditions which is similar to that the parameters of upper and lower arm are not equal.The result shows that the equivalent switching frequency of the sub-modules changes after the removal of the fault sub-modules.Thus,the equivalent impedance of the upper and lower arm gets unequal.In addition,the AC side current is asymmetrically distributed into the upper and lower arm on the fault phase.And the result is the appearance of fundamental frequency and odd harmonic circulating current.What's more,the original double and even harmonic circulating current becomes asymmetrical as well.These distorted components flow to the DC side,leading to a fundamental frequency and a second frequency fluctuation on DC current.Aiming at the change of the circulating current after the removal of the fault submodules,two kinds of circulating current suppression schemes are proposed.One is to introduce the arm energy balance control base on the state equation of circulating current.And the other is to carry on the resonant control based on the steady state equivalent model with asymmetrical arms.Finally,a redundancy configuration method is introduced.Simulation with MATLAB\ Simulink and experiment on a single-phase MMC prototype with 16 sub-modules per arm are conducted.The analysis above is proved and the effectiveness of circulating current suppression is verified.