Power cycling test is one of the important tasks to investigate the reliability performance of power device modules in respect to temperature stress. From this, it is able to predict the lifetime of a component in power converters. In this paper, representative power cycling test circuits, measurement circuits of wear-out failure indicators as well as measurement strategies for different power cycling test circuits are discussed in order to provide the current state of knowledge of this topic by organizing and evaluating current literature. In the first section of this paper, the structure of a conventional power device module and its related wear-out failure mechanisms with degradation indicators are discussed. Then, representative power cycling test circuits are introduced. Furthermore, on-state collector–emitter voltage $(V_{{\rm{CE\_ON}}})$ and forward voltage $(V_{F})$ measurement circuits for wear-out condition monitoring of power device modules during power cycling test are presented. Finally, different junction temperature measurement strategies for monitoring of solder joint degradation are explained.

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Power Cycling Test Methods for Reliability Assessment of Power Device Modules in Respect to Temperature Stress

Remaining useful lifetime prediction and extension of Si power devices have been studied extensively. Silicon carbide (SiC) power devices have been developed and commercialized. Specifically, SiC mosfet s have been utilized for the next generation high-voltage, high-power converters with smaller size and higher efficiency, covering various mainstream applications, including photovoltaic systems, electric vehicles, solid-state transformers, and more electric ships and airplanes. However, the SiC-based devices have different failure modes and mechanisms compared with Si counterparts. Therefore, a comprehensive review is critical to develop accurate lifetime prediction and extension strategies for SiC power converter systems. The SiC power device component-level failure modes and mechanisms are first investigated. Different accelerated lifetime tests and component-level lifetime models are then compared. Power converter system-level offline lifetime modeling techniques and software tools are further summarized. Besides, the SiC power converter condition monitoring strategies and health indicators are surveyed. The online measurement challenges are also studied. Furthermore, the system-level lifetime extension strategies are reviewed. By integrating device physics, statistical modeling, reliability engineering, and mechanical engineering with power electronics, this article is intended to provide a comprehensive overview, address existing challenges, and unfold new research opportunities regarding the SiC power converter real-time lifetime prediction and extension.

Overview of Real-Time Lifetime Prediction and Extension for SiC Power Converters

Grid-tied inverters are widely used for interfacing renewable energy sources or storage devices to low-voltage electrical power distribution systems. Lately, a number of different control techniques have been proposed to address the emerging requirements of the smart power system scenario, in terms of both functionalities and performance. This article reviews the techniques proposed for the implementation of current-controlled or voltage-controlled inverters in microgrids. By referring to a voltage source inverter with an ${LCL}$ output filter, different control architectures are classified as single, double, and triple loop. Then, the functionalities that are needed or recommended in the grid-connected, islanded, and autonomous operating modes of the grid-tied inverter are identified and their implementation in the different control structures is discussed. To validate the analysis and to better illustrate the merits and limitations of the most effective solutions, six control strategies are finally implemented and experimentally compared on a single-phase, grid-connected inverter setup.

Review and Comparison of Grid-Tied Inverter Controllers in Microgrids

Medium-voltage (MV) multilevel converters are considered a promising solution for large scale photovoltaic (PV) systems to meet the rapid energy demand. This article focuses on reviewing the different structures and the technical challenges of modular multilevel topologies and their submodule circuit design for PV applications. The unique structure of the converter’s submodule provides modularity, independent control of maximum power point tracking (MPPT), galvanic isolation, etc. Different submodule circuits and MPPT methods to efficiently extract the PV power are reviewed. The integration of the multilevel converters to PV systems suffers unbalanced power generation during partial PV shading conditions. Several balancing strategies to solve this problem are presented and compared to give a better understanding of the balancing ranges and capabilities of each strategy. In addition, the paper discusses recent research advancements, and possible future directions of MV converters-based large-scale PV systems for grid integration.

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Medium Voltage Large-Scale Grid-Connected Photovoltaic Systems Using Cascaded H-Bridge and Modular Multilevel Converters: A Review

This article focuses on failure modes and lifetime testing of IGBT modules being one of the most vulnerable components in power electronic converters. IGBT modules have already located themselves in the heart of many critical applications, such as automotive, aerospace, transportation, and energy. They are required to work under harsh operational and environmental conditions for extended target lifetime that may reach 30 to 40 years in some applications. Therefore, addressing the reliability of IGBT modules is of paramount importance. The paper provides a comprehensive review on IGBT modules dominant failure modes, and long-term reliability. A detailed discussion on accelerated testing, and lifetime and degradation characterization considering thermo-mechanical stress is also presented in details.

A Review on IGBT Module Failure Modes and Lifetime Testing

This paper presents a series of experiment results on the ageing effects of cyclic junction temperature variations ( $\Delta T_{j}$ ) of low amplitudes in power modules, to help the capturing of module reliability characteristics and the derivation of lifetime models in the future. Power cycling tests, for nonaged and aged modules, are designed to illustrate the failure mechanisms. Insulated gate bipolar transistor (IGBT) modules in actual converters are usually operated in a $\Delta T_{j}$ range up to 40 °C; therefore, tests are carried out to observe the effects of such narrow $\Delta T_{j}$ stress cycles on the module lifetime. It is found that the relatively minor stress cycles may not be able to directly initiate a crack but can contribute to the development of damage in the die attach solder layer due to stress concentration. Finite element analysis modeling is utilized to verify the stress concentration effect. The experiment results show that the effects of the narrow $\Delta T_{j}$ stress cycles are affected by the ageing status of the module and the stress level itself.

Experimental Investigation on the Effects of Narrow Junction Temperature Cycles on Die-Attach Solder Layer in an IGBT Module

In the renewable energy generation systems, the grid-connected inverter with LCL filter (GCI- LCL ) is an important device to realize dc–ac power conversion and connect the distributed power generation system and power grid However, the resonance problems of the LCL filter may bring instability to GCI- LCL  Besides, the grid impedance and the time delay in digital controllers also have a harmful influence on the stability of GCI- LCL  To solve these problems, a dual-current active damping control strategy based on the inverter-side current and grid-connected current feedback for GCI- LCL is proposed in this paper, and the systematic parameters design process is investigated Compared with the conventional damping strategies, a resonance suppression for the LCL filter, a larger stability margin, and a better anti-interference capability in a weak grid can be achieved under the proposed strategy In addition, by analyzing the active damping effect of the proposed strategy, a simplified model in a low-frequency band for GCI- LCL is proposed to enhance the anti-interference capability in the weak grid and simplify the parameters design process Finally, a GCI- LCL prototype is constructed and the experiment results verify the correctness and feasibility of the analysis and proposed strategy

The Dual-Current Control Strategy of Grid-Connected Inverter With LCL Filter

The dual-active-bridge (DAB) converter is applied for power transmission and voltage conversion in dc power grid. To improve the quality of input current, an input LC filter is cascaded to DAB converter. However, due to the interaction between LC filter and DAB converter, substantial oscillation and instability occur, leading to the excessive electromagnetic interference, large voltage ripple, and power loss and even operation failure. To solve this issue, an active damping strategy is proposed in this article to reshape the small-signal input impedance by parallel virtual impedance. The input voltage is regarded as a control objective, as well as the output voltage, therefore, a triple-closed-loop control structure is established. Besides, all dynamics of controllers are considered for the input impedance modeling of DAB converter. Moreover, compared with traditional methods, an active damping method for cascaded DAB converter with LC filter is adopted to design the virtual impedance and controller, suppressing resonance of LC filter and achieve a larger bandwidth. Therefore, a more stable and rapid dynamics cascaded system composed of DAB converter and LC filter are achieved. Finally, a prototype is set up and the effectiveness and superiority of proposed strategy are verified by experiments.

An Active Damping Strategy for Input Impedance of Bidirectional Dual Active Bridge DC–DC Converter: Modeling, Shaping, Design, and Experiment

For photovoltaic applications, the flyback microinverter with pseudo-dc-link is popular as a simple topology but brings large transformer turns ratio and thus large leakage inductance, which would deteriorate the converter efficiency. To solve this issue, based on the nonisolated pseudo-dc-link structure, this paper proposed a hybrid boost–flyback/flyback (BF/F) microinverter. This new topology is operated at the BF mode for the most segment of a half grid cycle and the F mode for the rest. During the BF mode, high voltage gain with low voltage stress is easily available in minimized transformer turns ratio. Besides, the leakage energy is recycled and the turn- off voltage spike of the main switch is clamped, as a result of a passive snubber inherently contained in this mode. Given that the BF mode is lack of step-down function, the F mode is developed to regulate the output voltage even for values lower than the input voltage. The operation and characteristic of the hybrid BF/F microinverter in boundary conduction mode are analyzed in detail, and the mathematical expression of reference current is derived theoretically to guarantee high power quality. Finally, a 240 W prototype was implemented to validate the theoretical analysis and the benefits of the proposed topology.

A Hybrid Boost–Flyback/Flyback Microinverter for Photovoltaic Applications

This paper evaluates the use of a back-to-back modular multilevel converter (B2B-MMC) in offshore low frequency (LF) AC transmission. The scheme is compared to two alternative transmission technologies: DC and 50Hz AC. The MMC topology is further compared to a cycloconverter. A high level design of a ±120kVdc, 250MW B2B-MMC system is described, with the total volume and weight of submodule capacitors estimated. The system performance is studied for both normal and fault conditions by computer simulation. Analysis shows that the dynamics of the controller that regulates the DC bus voltage may not be fast enough for the B2B-MMC, and feedforward control may be needed.

Xuemei Wang

Papers

Experimental Investigation on the Effects of Narrow Junction Temperature Cycles on Die-Attach Solder Layer in an IGBT Module

The Dual-Current Control Strategy of Grid-Connected Inverter With LCL Filter

An Active Damping Strategy for Input Impedance of Bidirectional Dual Active Bridge DC–DC Converter: Modeling, Shaping, Design, and Experiment

A Hybrid Boost–Flyback/Flyback Microinverter for Photovoltaic Applications

Offshore low frequency AC transmission with back-to-back modular multilevel converter (MMC)