Research On The Key Technologies Of Modular Multilevel Matrix Converter In AC/AC System

As a new topology of ac/ac power converter,modular multilevel matrix converter?M³C or MMMC?aims to realize three-phase to three-phase direct conversion,especially suitable for high-power low-speed motor drive and high-power wind energy conversion system,which can directly realize ac/ac power conversion.Therefore it has a broad application prospects in the passive grid and for the grid connection of offshore wind farms.Based on the summary of previous research results,this paper makes an in-depth study on the key problems of M³C application in ac/ac system from the following aspects:?1?The topology and operating principle of 3×3 modular multilevel matrix converter?M³C?and hexagon modular multilevel converter?hexverter?are studied.This paper introduces the double??0 coordinate transformation and the mathematical decoupled model of M³C in this coordinate,then further deduces and analyzes the physical significance of the capacitor voltage and power matrix elements of the branch in the??coordinate system.This paper points out the problem of M³C under the condition of equal frequencies operation between both systems through mathematical analysis,and puts forward to cancel the difference frequency component in its power component by applying additional circulating current.The mathematical expressions of branch voltage,current and power of hexverter are analyzed and established.It is also pointed out that the difference of active power between adjacent branches will be caused by the difference of reactive power between two sides of the system.By comparing the simulation waveforms of M³C under the conditions of non-equal frequency operation and equal frequency operation,it is pointed out that under the condition of equal frequency operation these three branch currents inside one sub-converter are no longer symmetrical.The theoretical analysis shows that the amplitude is not equal due to the cancel each other out between the additional circulating current and the original branch currents.For the design and manufacture of M³C,it is hoped that each branch will maintain the symmetry no matter from the point of the device selection or heat dissipation design.Therefore,from the perspective of symmetry,this paper proposes an additional neutral point voltage to counteract the asymmetry of branch currents caused by applying circulating currents alone.?2?The capacitance parameter design method of the hexverter is studied and the state-space equation of M³C is established.First of all,the frequency components of the branch power are analyzed in detail,including several frequency components:f₁,f₂,f₁±f₂,2f₁,2f₂,0?constant part?.According to the two different conditions of f₁?f₂,Q_g1=Q_g22 and f₁=f₂,Q_g1?Q_g2,the expressions of branch voltage,branch current and the voltage fluctuation of capacitor are derived respectively,and the instantaneous branch power components under different condition are obtained.From the viewpoint of energy pulsation,the paper deduces the energy pulsation of the branch and establishes the energy pulsation function of the branch and the submodule respectively,and then obtains the expression of the capacitance parameter of the submodule through the relationship between the storage energy of the submodule and its rated voltage taking into consideration of the voltage ripple coefficient of the submodule.Secondly,circulating currents paths and the number of independent circulating currents and state variables of M³C are analyzed.Then input variables,four independent circulating currents and state space variables are determined.On this basis,state space equation of M³C is established mathematically.?3?The loss characteristic of the hexverter is studied.The on-state and switching characteristic parameters of IGBT and its freewheeling diode are obtained by fitting the data of IGBT module.According to the output characteristics of the full bridge submodule,the analytical expressions of the conducting range and time of each device are derived,and then the theoretical expressions of on state loss and switching loss are derived.?4?A kind of distributed battery energy storage system based on the hexverter is proposed which mainly focus on SOC balancing control of multiple battery cells,including inter-branch SOC balancing control and single submodule SOC balancing control which one branch.The mathematical model of the battery energy storage submodule is established and its correctness is verified by simulation.Based on the internal model principle,a new SOC balancing control strategy is proposed by controlling the branch currents,which includes the SOC balancing between six branches of the converter and the SOC balancing between 14 cascaded submodules within a single branch.?5?An asymmetric fault control strategy of ac/ac system based on M³C is proposed.Based on the mathematical model of M³C under the double??0 transformation,the decoupled control strategies of primary side and secondary side systems are designed respectively.In view of the asymmetric fault condition of the secondary side ac system,the positive sequence components and negative sequence components of voltages and currents are separated,and the proportional resonant controller?PR?is used to adjust the positive and negative sequence currents at the same time and to realize the decoupled control in the??coordinate system.The capacitor voltage balancing control consists of the balancing control between these three subconverters and the balancing control between three branches within one subconverter,which is realized by adjusting four circulating currents.