| In recent years,flexible DC transmission and static synchronous compensator technology have been rapidly developed.The Modular Multilevel Converters(MMC)overcome the high switching frequency and high loss of the two-level converter,achieving high-quality voltage output and excellent control characteristics.Consequently,it widely applied by industry.Due to the influence of the installation ambient temperature and power profile,the reliability of the MMC varies in different applications.Moreover,MMC-HVDC system has more submodules(SMs)and a more compact layout,which cause more serious thermal stress.MMC lifetime and reliability modeling are the basis of safe,stable and economic operation of the converter system.Therefore,this paper estimates the SM component lifetime based on physics-of-failure,and evaluate the overall reliability of the MMC system considering and SM lifetime correlation.The following research is carried out in this paper:(1)The current stress of the insulated gate bipolar transistor modules(IGBT and diode),is computed according to the working principle of the MMC.A thermal equivalent network in the form of a Foster model is adopted to link the power losses and junction temperature;the junction temperature of the component is calculated using a thermal equivalent network and verified by the finite element simulation software ANSYS.Next,the rain-flow counting algorithm is applied to extract the low-frequency thermal cycles from the junction temperature;an RC equivalent circuit analytical method for characterizing the fundamental-frequency thermal cycles is proposed.The Bayerer model is employed to predict the IGBTs lifetime.(2)Based on physics-of-failure,this paper proposed a method for estimating the lifetime of DC-link capacitors(DCLC)of SM.According to the MMC mission profile,this method calculates electro-thermal stress of the DCLC,evaluates its lifetime.Considering the fatigue of the DCLC is depend on its parallel states,the sequential Monte Carlo method is used to simulate the state transition process of DCLC bank,and then evaluate its influence on lifetime of DCLC bank.The case study shows that the ambient temperature profile,power transmission profile and failure conduction mechanism affect the lifetime of the SM capacitor bank;the lifetime of SM is limited by the IGBT module or DCLC bank under different mission profiles.(3)The reliability of MMCs is determined both by the component lifetime distribution and the MMC structure.The lifetime Weibull distribution of each component is obtained using the Monte Carlo method.This distribution links the component lifetime estimation and MMC reliability analysis.Finally,considering the SM lifetime correlation,the overall reliability of an MMC system with redundancy is evaluated using a Copula function.The reasons for the impact of SM lifetime correlation on MMC reliability are revealed in a case study. |
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