Study On Reliability Evaluation Of DC-link Capacitor And Its Enhancement Measures In Wind Power Converter

With the continuous development of wind power technology and the fast growth in the total installation of wind turbines,the requirement of the reliability of wind turbine and its components for the power system stability are also growing.As a storage element,dc-link capacitor is widely used to balance the instantaneous power difference between the rotor converter and the grid converter in wind turbine,and minimize ripple voltage of dc-link capacitor in converter.However,research results show that dc-link capacitor are a weak component of wind power converters,so it is one of important factors that affect the reliability of power converter and wind turbine.Therefore,the reliability evaluation for dc-link capacitor and putting forward some targeted improvements are have important significance.On the basis of the analysis of failure mechanism and electrical thermal model of capacitor,an improved calculation method of dc-link capacitor hot-pot temperature based on fast Fourier transform(FFT)was proposed.Combined with the calculation of capacitor life loss rate,the improved model for reliability evaluation of dc-link capacitor was presented,which can adapted to analyze the reliability of dc-link capacitor in wind turbine.With the established reliability evaluation model,the factors affecting the reliability of dc-link capacitor of wind turbine and converter system are studied,and corresponding improvement measures for increasing the reliability of dc-link capacitor are proposed.The main contents of this paper are as follows:(1)According to the frequency characteristics of equivalent series resistance of capacitor and electrical thermal model of capacitor,a calculation model of capacitor hot-pot temperature is studied based on Fast Fourier Transform(FFT).Compared to the hot-pot temperature calculation method based on root mean square value of capacitor ripple current in time domain,the presented hot-pot temperature calculation method in this paper is available to analyze the electrical stress and thermal stress of dc-link capacitor caused by high frequency ripple current,which provide the basis of reliability evaluation for dc-link capacitor in wind power converter.The accuracy of the proposed model based on the fast Fourier transform is proved by the experiment in this paper.(2)Based on the real operating conditions of dc-link capacitor and combined with the calculation method of capacitor lifetime loss rate,the improved reliability evaluation model is presented in this paper,which can reflect the effect factors of dc-link capacitor reliability during wind turbine operating.Through the analysis of the lifetime loss rate and electrical-thermal stress of dc-link capacitor,the effects of wind turbine parameters on the reliability of dc-link capacitor are studied based on the electrical thermal model of capacitor,such as ambient temperature,wind speed,switching frequency of converter,equivalent series resistance and thermal resistance of capacitor.(3)In order to reduce electrical stress thermal stress of dc-link capacitor for improving the reliability converter and wind turbine,some targeted improvement measures including external factors,internal factors and dc-link design solutions are proposed based on the analysis of effects of wind turbine parameters on reliability of dc-link capacitor in power converter.Finally,the design procedure for dc-link capacitor can be optimized with the objective function of capacitor cost and lifetime,which can offer a technical support for improving the level of reliability-oriented design of dc-link capacitor and the level of operation reliability of wind power converter.