Research On Capacitor Monitoring Strategy For Modular Multilevel Converters

In recent years,modular multilevel converter(MMC)technology has attracted more and more attention in the power industry.As a typical topology of voltage source converter(VSC),MMC has the advantages of low output harmonics,low switching loss and stress,good output power quality,and easy capacity expansion with modular design.It plays an important role in asynchronous grid interconnection,isolated island power supply and distributed energy system.The submodule(SM)of MMC contains a large number of power devices,and the energy storage capacitor is one of the key components for the conversion of electric energy forms.As the running time increases,the parameters of the capacitor will change.The change of capacitor parameters will bring hidden dangers to the normal operation of MMC.The offline capacitor monitoring method requires the LCR measuring instrument and the capacitors need to be removed from the main circuit for offline measurement.Offline measurement has the characteristics of high monitoring accuracy,but MMCs will shut down during capacitor monitoring,which greatly affects the power supply reliability of the MMC system.Therefore,this method has not been widely used.The research on the online monitoring strategy of MMC submodule capacitors has always been one of the hot issues in the industry and academia.Based on previous studies,this paper puts forward two simplified,noninvasive MMC capacitor online monitoring schemes with high precision and wide range of applications.They provide a new method to ensure the reliable operation of MMC capacitors.The research contents and innovative achievements of this paper are as follows:(1)This paper introduces the types and structures of capacitors commonly used in MMCs,analyzes their principle and specific performance of capacitor aging,and establishes a simplified equivalent circuit of capacitors in MMCs,paving the way for the following capacitor monitoring schemes.(2)In this paper,the precision of conventional capacitor parameter monitoring of MMC is analyzed in detail.The mathematical model of three-phase MMCs is established.Based on the model,the control methods of MMC are given,including double frequency circulation suppression,system-level control and capacitor voltage-balancing control.A three-phase MMC physical prototype with a capacity of 1k VA was built in the laboratory.The working principle of MMC‘s voltage sensor unit is analyzed and the mathematical model of the voltage sensor unit is established,based on which the disadvantage of the conventinal method of capacitor parameter monitoring is pointed out.Finally,the influencing factors on the monitoring accuracy of capacitor parameters are explored,and the influence degree of different influencing factors on the capacitor parameter monitoring results is obtained.(3)The inner relation between SM capacitor voltage fluctuation and capacitance monitoring precision of MMC is revealed,then a method of MMC capacitor online monitoring accuracy based on large capacitor voltage ripple is proposed,that is,to reduce the influence of voltage sensor error on monitoring accuracy and improve capacitor monitoring accuracy by increasing the voltage fluctuation of capacitors.In this paper,a submodule capacitor voltage-balancing algorithm under the condition of capacitor monitoring,namely capacitor dc voltage offset algorithm,is proposed.The algorithm can effectively produce large capacitor voltage fluctuation while the arm voltage still showing a sinusoidal function(4)An online MMC capacitor monitoring strategy based on RC discharge circuit is proposed to further improve the precision of MMC capacitor monitoring.By establishing the mathematical model of RC discharge circuit,capacitor monitoring strategy of MMC based on RC discharge circuit is given in detail.The monitored SM needs to be bypassed,and the discharge resistance contained in the SM is applied here together with the monitored capacitor to construct a RC discharge circuit.Then,the duration and precision of capacitor monitoring under different discharge degrees are studied.(5)The advantages of the two different capacitor monitoring schemes proposed in(3)and(4)are combined.The advantages of sppediness and relatively high precision of the capacitor monitoring scheme in(3)are took to rapidly monitor all SM capacitors,and some SM capacitors with obvious capacitance drop are screened out.Then,the capacitor monitoring scheme proposed in(4)is applied to monitor these SM capacitors screened out with high precision,forming a set of low-time-consumption and high-precision MMC capacitor monitoring system.(6)Professional computer simulation software PSCAD/EMTDC and three phase MMC physical prototype in the laboratory were applied to verify the effectiveness of the two MMC capacitor monitoring schemes proposed above.