Research On Control Strategy Of MMC-STATCOM

With the rapid development of power electronics devices in power system,the reactive power of the power grid draws increasing attention.Under the increasing maturity of multilevel converters,high-voltage and high-power STATCOM has become a significant research topic.The multi-level techniques have been proposed and actively used in STATCOM.In this paper,the modular multilevel static synchronous compensators(MMC-STATCOM)is selected as the research object.In this paper,start-up process,modulation technology,control strategy and the output characteristics of MMC-STATCOM are studied.Firstly,control of the start-up process is an important prerequisite for the normal operation of MMC-STATCOM.Based on the analysis of the uncontrolled charging process,the resistance used for current limiting is determined.When the uncontrolled charging process finishes,the module voltage is lower than the capacitor voltage in the steady-state,in which condition,directly starting the converter may induce current over-shoot and distortion.Hence,a controllable start-up process is proposed,in which the modules are categorized based on their status,so some of the modules are bypassed and only the connected modules are charged.Therefore,all the modules can be charged to the required voltage at the steady-state smoothly,and the current over-shoot and over-voltage can be effectively alleviated.In this paper,the modulation technology of MMC-STATCOM is studied thoroughly.Stair-wave modulation can obtain a wide range of output at relatively low switching frequency,and has the advantages of high output quality,low switching frequency,and so on.The application of stair-wave modulation strategy in MMC-STATCOM is studied.In this paper,a comprehensive study is conducted based on the stair-wave modulation of MMC-STATCOM.A switching angle calculation approach based on conjugate gradient method and Newton interactive method is proposed.In this method,the conjugate gradient method is used to calculate the initial value of exact switching angle,and the derived switching angle is used in the Newton interactive method to get the exact switching angle.The proposed algorithm,which combines the merits of wide range of convergence in conjugate gradient method and fast convergence rate in Newton interactive method,and can calculate the switching angle of the stair-wave rapidly.Meanwhile,by using the repetitive squared pulses,the power among different modules can be balanced.A reactive power control strategy is proposed in this paper.Since MMC-STATCOM is a system with multiple variables and highly coupled nonlinear system,conventional linear method is not sufficiently feasible.Based on this fact,a differential geometry method is proposed.In this method,the objective current is firstly decomposed based on the sequences,and then a state feedback linearization approach is proposed based on differential geometry method,and the n onlinear control rate is derived.By linearizing the MMC-STATCOM,the isolated positive and negative sequence control can be achieved.This method is applicable to both balanced and unbalanced grid conditions,and fully decoupled system of MMC-STATCOM can be achieved in large operation range so that accurate active and reactive power control and be realized.Finally,the problem of high-order harmonic distortion in the output current of MMC-STATCOM is studied,and the LCL filter is proposed to use in MMC-STATCOM.However,unlike other topologies,STATCOM is the one featuring a bridge arm inductor,filter inductor and inductance of the bridge arm,which enables the elimination of interphase circulating current and limits the DC bus fault current rise rate.Therefore,the LCL filter design method compared to the other topologies are different.In order to solve this problem,the design method of LCL filter in MMC-STATCOM is proposed,and the effect of LCL filter on the system performance is studied.The high-order harmonic current in the output current is thereby reduced.