This paper presents a novel seven-level inverter topology for medium-voltage high-power applications. It consists of eight active switches and two inner flying-capacitor units forming a similar structure as in a conventional Active Neutral Point Clamped (ANPC) inverter. This unique arrangement reduces the number of active and passive components. A simple modulation technique reduces cost and complexity in the control system design without compromising reactive power capability. In addition, compared to major conventional 7-level inverter topologies such as the Neutral Point Clamped (NPC), Flying Capacitor (FC), Cascaded H-bridge (CHB) and Active NPC (ANPC) topologies, the new topology reduces the dc-link voltage requirement by 50%. This recued dc-link voltage makes the new topology appealing for various industrial applications. Experimental results from a 2.2 kVA prototype are presented to support the theoretical analysis presented in this paper. The prototype demonstrates a conversion efficiency of around 97.2% ± 1% for a wide load range.

As grid codes evolve, inverter-interfaced renewable generation may be required to take greater responsibility for grid support. It may be obliged to source a large current during a short-circuit fault in the grid and provide large dynamic reactive power, beyond the inverter rating, for voltage recovery after the fault clearance. In this sense, the inverter-interfaced generating system would behave more similarly to a conventional synchronous machine. This is not yet mandatory but current technology cannot readily support it if required in the future, because the power semiconductors would overheat unless they are drastically de-rated. This study proposes a method to increase the power semiconductor's transient thermal capacity, and hence its short-term over-current capability, by integrating a phase-change material inside the power semiconductor module. Experiment and thermodynamic simulation across different materials are used to illustrate the feasibility of the concept and the factors that should be considered in such a design. The study shows that 1.5 p.u. (per unit) over-current could be achieved for 30 s, which is typical of synchronous generators; and 3.0 p.u. fault current could be achieved for 3 s. A custom-made 1200 V, 50 A IGBT half-bridge module integrating a liquid metal as the phase change material is compared with a conventional commercially-available module. A validated simulation model is then used to further evaluate the proposal, regarding a 1700 V, 1000 A IGBT module for use in a wind turbine.

Current ripples produced in single-phase onboard charging systems of electric vehicles (EVs) impact the lifetime of their batteries. In this paper, an isolated multi-functional charger topology is proposed. The charging system can be used to charge the auxiliary batteries or serve as an active filter (AF) for the power battery charger. By time-sharing multiplexing it, the high frequency (HF) and low frequency (LF/second harmonic) current ripple can be suppressed in driving and parking charging mode respectively. The proposed topology can also achieve zero voltage switching (ZVS) for all power switches under a full load range in driving charging mode. In addition, the integration and the power density of the charging system can be improved due to the reconstruction of the capacitive energy storage AF in parking charging mode. Finally, a 400W experimental prototype is developed and the experimental results are presented to validate the performance and feasibility of the proposed topology.

In single-phase AC battery chargers, pulsating power at twice of the line frequency is delivered by the AC source resulting in second-harmonic ripple voltage on the DC-link if additional passive or active filter circuits are not used. In this paper, an inductance-based active filter (AF) using the leakage inductance of the machine windings along with a current compensation control method is proposed. No additional passive components are required. The machine used in this paper has a double-layer winding with a center tap in each phase and the winding structure of the machine is special in that the pulsating magnetomotive force of the whole phase winding can be cancelled when currents flow into the central tap. In this paper, the operating principle and control strategy of the AF with the inductive energy storage are analyzed. Finally, a 400W experimental prototype is developed and the experimental results are used to validate the performance and feasibility of the proposed converter and control strategy.

This paper presents a hierarchical balancing algorithm for use in large-scale reconfigurable ac battery packs. Using a decentralized battery management system, the pack is organized in control layers with each layer controller responsible for a small number of slave controllers, thereby significantly reducing communication and centralized processing requirements. The hierarchical balancing algorithm uses all available voltage steps in the pack ensuring a high-quality sinusoidal output voltage while balancing the objects of different layers. Balancing priority can be swapped dynamically between layers according to the maximum state of charge (SoC) variation between the objects of each layer while also ensuring that the output voltage reference is always met even when a cell failure occurs, as long as there are an adequate number of cells available in the pack. An experimental demonstration of the proposed balancing algorithm is presented using an ac battery pack comprising 144 20-Ah lithium-titanate cells. Cell balancing to within 2 mV of the cell open-circuit voltage is achieved between cells with 20% initial SoC variation and 5% capacity difference.

In modular multilevel converters (MMCs), temperature control of semiconductor devices in the submodules (SMs) is a key factor for the safe and reliable operation. Under normal operation, significant temperature differences can exist between SMs due to, for example, aging of semiconductor modules and module parameter mismatch. This paper presents a method for achieving SM thermal balancing by controlling the capacitor voltage of each SM in an arm, while maintaining the sum of the SM capacitor voltages at a constant value in order to regulate the dc-link voltage. The proposed temperature balancing strategy is validated using an experimental MMC setup with three SMs, where an increase in the thermal resistance to ambient of one or more SM semiconductors is created by restricting coolant flow to simulate a partial failure in the cooling system. Increases in the thermal resistance by 21% and 42%, corresponding to temperature increases of 5 and 10 $^{\circ}$C, respectively, are managed by three SMs, using a capacitor voltage margin of 60%.