Condition Monitoring For Metallized Polypropylene Film Capacitor In MMC

The advantages of modular multilevel converter(MMC)make it popular in key applications such as medium and high voltage converter.The metallized polypropylene film(MPPF)capacitors with superior performance are widely used in current MMC projects,the aging and failure of those capacitors are still inevitable.Therefore,in order to ensure the safe and stable operation of MMC,it is necessary to monitor the condition of each capacitor in MMC.In this paper,a series of researches on the online condition monitoring of MPPF capacitor in MMC are carried out.Firstly,the working principle and control strategy of MMC are summarized as the research foundation.After analyzing the common control strategies of MMC,it is found that all control strategies need to collect the bridge arm current and voltage of each submodule,so as to generate the switch sequence of each submodule.These three quantities are critical for on-line capacitor monitoring.Secondly,because MPPF capacitor degradation is usually related to the capacitor's equivalent series capacitance(ESC)reduction.An online monitoring method of MPPF capacitor's ESC based on model reference adaptive control(MRAC)is then proposed.The capacitor voltage and current of the submodule are obtained by the control system in advance.Actual capacitors are used as the reference model,and a mathematical model that tries to follow the reference as the adjustable model.The two models accept the same capacitor voltage as input and are expected to output the same current.The error between the output signals is used as feedback to modify the parameters of the adjustable model until it is zero.The capacitance value is then used to signify the capacitor condition.Then,considering that the equivalent series resistance(ESR)of the capacitor of MPPF increases due to the self-healing process and end-spray detachment.Two parameters(ESC and ESR)of MPPF is chosen to be monitored since they are tied firmly with its deterioration and failure.A new SM impedance based measurement technique is proposed to monitoring the condition of MPPF capacitors in MMC.Measurement technique extracts arm current,switching sequence and voltage signal of SMs which are already existed in control system,SMs capacitor voltage and current is then calculated by above signals.After processing its various frequency components by fast Fourier transform(FFT)algorithm,ESC and ESR of MPPF capacitor can be figured out with desired frequency.Finally,a 2.1k VA MMC system was established in MATLAB/Simulink,and a model assisted testing platform of MMC was established.Simulation and experimental results verify the effectiveness and accuracy of the two monitoring methods.The first method can measure a large number of data in a short time with high accuracy,but the differential operation will increase the computational burden of the controller.The second method avoids the differential calculation,and can get more information of the capacitor,which is beneficial to the capacitor health diagnosis.But the measurement accuracy is a little poor.The two methods have their own advantages and can be extended to the condition monitoring of MPPF capacitor in other applications.