With wide-spread application of power electronic systems across many different industries, their reliability is being studied extensively. In the dissertation, a mission profile based reliability analysis framework is proposed to quantitatively predict the reliability of power electronic systems. Based on the framework, the reliability of power converters in hybrid electric vehicle (HEV) and voltage source converter (VSC) high voltage direct current (HVDC) power transmission systems is evaluated and the reliability improvement strategies are proposed and verified by simulation and experiment.Firstly, a general framework of reliability analysis of power electronic systems is presented. The framework is applied to quantitative reliability prediction of keypower components, and standard and fault-tolerant power electronic systems in terms of probabilistic or deterministic performance metrics. The reliability evaluation is based on mission profiles for various applications so that the inaccuracy due to the uncertainty of operating conditions of systems can be minimized at the earliest design stage. The strategies to improve the reliability of power electronic system at three possible levels are also systematically analyzed.Based on the framework of reliability analysis, a mission-profile-dependent simulation model based on MATLAB for quantitatively assessing the reliability of HEVs' electric drivetrain is developed. This model takes into consideration the variable driving scenarios, dormant mode, electrical stresses, and thermal stresses. Therefore, more reliable and accurate prediction of system reliability has been achieved. The model is explained in detail and the results of reliability assessment based on a series hybrid electric vehicle (SHEV) are presented.Reliability analysis results of SHEV provides guidance on improving reliability, two control strategies are proposed to increase the mean time to failure (MTTF) of HEV powertrains: 1) variable dc-link voltage control; 2) hybrid discontinuous pulse-width modulation scheme. These novel control schemes reduce the power losses and thermal stresses of power converters, and consequently enhance system reliability. Numerical simulation and experimental results verify the benefits of two proposed control strategies in terms of power losses and reliability.As a safety-critical system, the fault-tolerant operation is desirable for HEVs' power electronic systems. A fault-tolerant power electronic system for series hybrid electric vehicles (SHEVs) is proposed. The introduction of a redundant phase-leg that is shared by three converters in a standard SHEV powertrain allows to maximize the reliability improvement with minimal part-count increase. The new topology features fast response in fault detection and isolation, and post-fault operation at rated power throughput. A scaled-down laboratory prototype has been built and the experimental results further validate the robust fault detection/isolation scheme and uncompromized post-fault performance.Due to aggregation of thousands of power electronic components, failures of power converters in VSC HVDC systems dominate faults of overall systems. Therefore, the accurate prediction of the reliability of HVDC converters is of great importance for design, reliability improvement, and maintenance management of HVDC systems. The failure rates and MTTF of three types of HVDC converters that are used in commissioned VSC HVDC systems have been comparatively evaluated by use of a mission-profile-dependent reliability simulation model. As a mission-critical system, the unplanned stoppage of HVDC systems would result in a significant economic loss, therefore the fault-tolerant operation is necessary for HVDC converters. The fault-tolerant designs of the HVDC converters also have been presented and the reliability of the fault-tolerant designs has been assessed by use of Markov reliability model.
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