Power electronics plays an important role in a wide range of applications in order to achieve high efficiency and performance. Increasing efforts are being made to improve the reliability of power electronics systems to ensure compliance with more stringent constraints on cost, safety, and availability in different applications. This paper presents an overview of the major failure mechanisms of IGBT modules and their handling methods in power converter systems improving reliability. The major failure mechanisms of IGBT modules are presented first, and methods for predicting lifetime and estimating the junction temperature of IGBT modules are then discussed. Subsequently, different methods for detecting open- and short-circuit faults are presented. Finally, fault-tolerant strategies for improving the reliability of power electronic systems under field operation are explained and compared in terms of performance and cost.

Study and Handling Methods of Power IGBT Module Failures in Power Electronic Converter Systems

In this paper, finite-set model-predictive control (FS-MPC) methodologies for a grid-connected three-level neutral-point-clamped converter are investigated. The proposed control strategies produce fixed switching frequency, maintaining all the advantages of predictive control such as fast dynamic response, inclusion of nonlinearities and restrictions, and multivariable control using a single control loop. The first of the proposed FS-MPC strategies is based on a multiobjective cost function, designed to regulate both the inverter currents and the balancing of the dc-link capacitor voltages. The second FS-MPC strategy is derived from the first one, and it is based on a cost function that regulates only the grid current, with the balancing of the capacitor voltages being realized by controlling the duty cycles of the redundant vectors. The proposed control systems are experimentally validated using a 5-kW prototype. The experimental results show a good performance for both strategies, in steady-state and transient response, with a total harmonic distortion lower than $\text{2}\%$ for the currents supplied to the grid.

Finite-Set Model-Predictive Control Strategies for a 3L-NPC Inverter Operating With Fixed Switching Frequency

This paper proposes an adaptive proportional-integral-derivative (PID) speed control scheme for permanent magnet synchronous motor (PMSM) drives. The proposed controller consists of three control terms: a decoupling term, a PID term, and a supervisory term. The first control term is employed to compensate for the nonlinear factors, the second term is made to automatically adjust the control gains, and the third one is designed to guarantee the system stability. Different from the offline-tuning PID controllers, the proposed adaptive controller includes adaptive tuning laws to online adjust the control gains based on the gradient descent method. Thus, it can adaptively deal with any system parameter uncertainties in reality. The proposed scheme is not only simple and easy to implement, but also it guarantees an accurate and fast speed tracking. It is proven that the control system is asymptotically stable. To confirm the effectiveness of the proposed algorithm, the comparative experiments between the proposed adaptive PID controller and the conventional PID controller are performed on the PMSM drive. Finally, it is validated that the proposed design scheme accomplishes the superior control performance (faster transient response and smaller steady-state error) compared to the conventional PID method in the presence of parameter uncertainties.

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Adaptive PID Speed Control Design for Permanent Magnet Synchronous Motor Drives

This paper presents an open-circuit fault detection method for a grid-connected neutral-point clamped (NPC) inverter system. Further, a fault-tolerant control method under an open-circuit fault in clamping diodes is proposed. Under the grid-connected condition, it is impossible to identify the location of a faulty switch by the conventional methods which usually use the distortion of outputs because the distortion of the outputs is the same in some fault cases. The proposed fault detection method identifies the location of the faulty switch and the faulty clamping diode of the NPC inverter without any additional hardware or complex calculations. In the case of the clamping diode faults, the NPC inverter can transfer full rated power with sinusoidal currents by the proposed fault-tolerant control. The feasibility of the proposed fault detection and the fault-tolerant control methods for the grid-connected NPC inverter are verified by simulation and experimental results.

Open-Circuit Fault Diagnosis and Fault-Tolerant Control for a Grid-Connected NPC Inverter

In this paper, benchmark of Si IGBT, SiC MOSFET, and Gallium nitride (GaN) HEMT power switches at 600-V class is conducted in single-phase T-type inverter. Gate driver requirements, switching performance, inverter efficiency performance, heat sink volume, output filter volume, and dead-time effect for each technology is evaluated. Gate driver study shows that GaN has the lowest gate driver losses above 100 kHz and below 100 kHz, SiC has lowest gate losses. GaN has the best switching performance among three technologies that allows high efficiency at high-frequency applications. GaN-based inverter operated at 160-kHz switching frequency with 97.3% efficiency at 2.5-kW output power. Performance of three device technologies at different temperature, switching frequency, and load conditions shows that heat sink volume of the converter can be reduced by 2.5 times by switching from Si to GaN solution at 60  $^{\circ }$ C case temperature, and for SiC and GaN, heat sink volume can be reduced by 2.36 and 4.92 times, respectively, by increasing heat sink temperature to 100  $^{\circ }$ C. Output filter volume can be reduced by 43% with 24, 26, and 61 W increase in device power loss for GaN-, SiC-, and Si-based converters, respectively. WBG devices allow reduction of harmonic distortion at output current from 3.5% to 1.5% at 100 kHz.

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Single-Phase T-Type Inverter Performance Benchmark Using Si IGBTs, SiC MOSFETs, and GaN HEMTs

Transformerless topologies of many topologies are widely used in photovoltaic (PV) systems because these topologies have many advantages in terms of the weight, size, and efficiency. A three-level inverter has an outstanding performance and is advantageous in the switching device selection than a two-level inverter. In the transformerless PV systems using the three-level inverter, the PV systems should suffer from the leakage current and generate the neutral-point voltage unbalance. To solve two problems, this paper proposes modulation techniques to reduce the leakage current and balance the dc-link voltages. The cause of the leakage current in the three-level inverter is analyzed. The proposed technique LMZVM using the large, medium, and zero vectors reduces the common mode voltage that causes the leakage current than that of the conventional PWM. Moreover, the proposed technique LMSVM using the large, medium, and small vectors balances the dc-link voltages with reduced CMV as the same in LMZVM. The effectiveness of the proposed techniques is verified by comparing its results with those of the convectional PWM. The results are obtained through simulations and experiments.

New Modulation Techniques for a Leakage Current Reduction and a Neutral-Point Voltage Balance in Transformerless Photovoltaic Systems Using a Three-Level Inverter

In wind turbine generation (WTG) systems, a back-to-back converter with a neutral-point-clamped (NPC) topology is widely used because this topology has more advantages than a conventional two-level topology, particularly when operating at high power. There are 12 switches in the NPC topology. An open-switch fault in the NPC rectifier of the back-to-back converter leads to the distortion of the input current and torque vibration in the system. Additionally, an open-switch fault in the NPC inverter of the back-to-back converter causes the distortion of the output current. Furthermore, the WTG system can break down in the worst case scenario. To improve the reliability of WTG systems, an open-switch fault detection method for back-to-back converters using the NPC topology is required. This study analyzes effects of inner and outer open-switch faults of the NPC rectifier and inverter and describes a novel open-switch fault detection method for all possible open-switch faults in the back-to-back converter.

Open-Switch Fault Detection Method of a Back-to-Back Converter Using NPC Topology for Wind Turbine Systems

Multilevel converter topologies are widely used in applications of wide-power range. The T-type topology, which is one of many three-level topologies, has an advantage in terms of efficiency compared to the neutral-point-clamped (NPC) type. In applications using the T-type topology, interest in reliability has been increased recently. Therefore, the open-switch fault detection method and tolerance control for T-type rectifiers, which are different from those of T-type inverters, are necessary to improve the reliability of applications. When the open-switch fault of switches connected to a neutral point occurs, an NPC-type rectifier cannot restore distorted input currents; however, a T-type rectifier is able to eliminate input current distortion completely. In this paper, an open-switch fault detection method is proposed and can find a position of the open-switch fault. Moreover, two tolerance controls based on space vector modulation are proposed and their characteristics are analyzed. The effectiveness and performance of the proposed open-switch fault detection method and two tolerance controls are verified by simulations and experiments.

An Open-Switch Fault Detection Method and Tolerance Controls Based on SVM in a Grid-Connected T-Type Rectifier With Unity Power Factor

This paper proposes a new modulation strategy that balances the neutral-point voltage for three-level neutral-clamped inverter systems. The proposed modulation replaces the P-type or N-type small switching states with other switching states that do not affect the neutral-point voltage. The zero and medium switching states are employed to help the neutral-point voltage balancing. This method little bit increases the switching events and output total harmonic distortion. However, this method has a strong balancing ability at all regions. Further, it is very simple to implement in both space vector modulation and carrier-based PWM methods. Simulation and experimental results verify the validity and feasibility of the proposed new modulation strategy.

June-Seok Lee

Papers

New Modulation Techniques for a Leakage Current Reduction and a Neutral-Point Voltage Balance in Transformerless Photovoltaic Systems Using a Three-Level Inverter

Open-Circuit Fault Diagnosis and Fault-Tolerant Control for a Grid-Connected NPC Inverter

Open-Switch Fault Detection Method of a Back-to-Back Converter Using NPC Topology for Wind Turbine Systems

An Open-Switch Fault Detection Method and Tolerance Controls Based on SVM in a Grid-Connected T-Type Rectifier With Unity Power Factor

New Modulation Strategy to Balance the Neutral-Point Voltage for Three-Level Neutral-Clamped Inverter Systems