Unfolding Inverter Research Articles

A current-source DC-AC converter and control strategy for grid-connected PV applications

This paper presents a two-stage current-source DC-AC converter for grid-connected PV applications which is composed of an input step-up stage, followed by a step-down stage and an unfolding inverter. A decentralized control strategy of the DC-DC stage allows maximizing the renewable energy harvest using an Incremental Conductance MPPT algorithm and synthesizing an output current to be injected into the grid with low harmonic distortion. Double-loop PI controllers are used for the boost stage. The DC bus voltage of the buck stage is regulated using a PI controller, and an inner Proportional-Resonant (PR) controller tracks a sinusoidal reference. The PR controller proposed in this paper, includes a reduced number of resonant stages meeting the energy quality required by standards, which results in good stability margins. Finally, a SOGI-FLL algorithm synchronizes the inverter operation with the grid. Experimental results show an excellent dynamic response of the system, and the injected current complies with the IEEE Std. 1547–2018 specifications regarding harmonic content using a control law with a low computational burden.

Control Scheme for Leading Power Factor Operation of Single-Phase Grid-Connected Inverter Using an Unfolding Circuit

A single-phase grid-connected inverter with an unfolding circuit consists of a first-stage dc/dc converter, which generates fully rectified sinusoidal waveforms, and a second-stage unfolding inverter, which switches every 180° of line frequency waveform. This converter exhibits low switching loss and high efficiency. Digitally controlled unfolding inverters can be used in numerous applications, such as power supplies to resistive loads and single-phase grid-connected inverters. The operating principle of an unfolding inverter is typically synchronous voltage and current. However, a reactive power supply function with a leading power factor (pf) (lagging pf from the grid perspective) is typically required for grid-connected inverters to suppress the terminal overvoltage. Limited studies have been focused on the operation of inverters with pf less than unity. Such an operation results in large overshoot and oscillation in the output voltage because of the difficulty in coping with large current fluctuations after voltage zero crossing. To this end, we propose a novel control scheme that enables leading pf operation without additional circuitry and overcomes the aforementioned limitations. When the unfolding inverter is operated with leading pf, a special mode called “all-conduction mode” occurs after voltage zero crossing. The proposed scheme implements appropriate dc current control to terminate this mode, resulting in a smooth transition to the normal mode steady state with pf less than unity. The scheme verifiably showcases exceptional efficiency in the conducted simulations and experiments.

Interface Converters for Residential Battery Energy Storage Systems: Practices, Difficulties and Prospects

Recent trends in building energy systems such as local renewable energy generation have created a distinct demand for energy storage systems to reduce the influence and dependency on the electric power grid. Under the current market conditions, a range of commercially available residential energy storage systems with batteries has been produced. This paper addresses the area of energy storage systems from multiple directions to provide a broader view on the state-of-the-art developments and trends in the field. Present standards and associated limitations of storage implementation are briefly described, followed by the analysis of parameters and features of commercial battery systems for residential applications. Further, the power electronic converters are reviewed in detail, with the focus on existing and perspective non-isolated solutions. The analysis covers well-known standard topologies, including buck-boost and bridge, as well as emerging solutions based on the unfolding inverter and fractional/partial power converters. Finally, trends and future prospects of the residential battery storage technologies are evaluated.

Reactive Power Control for Single-Phase Grid-Tie Inverters Using Quasi-Sinusoidal Waveform

This paper presents a reactive power control technique for single-phase photovoltaic (PV) inverters, especially unfolding inverters. The proposed system retains the benefit of the unfolding inverters having low material cost and semiconductor losses, and tackles the drawback of the standard unfolding inverter not having capability of reactive power injection. It is important to note that reactive power delivery is mandatory for PV inverters according to the recent announced regulations. The concept is based on changing the shape of the grid current waveform but keeping the same zero crossing points as in the unity power factor condition. The current waveform is governed by real power and reactive power, at the price of an acceptable deformation. The operating principles of the proposed technique and mathematical derivations of the grid current function are provided in this paper. Experimental results in a grid-tie inverter prototype have shown a good agreement with the derived theory, and they confirm the feasibility of using the proposed technique in grid-tie inverters.

Current-Fed Interleaved Phase-Modulated Single-Phase Unfolding Inverter: Analysis, Design, and Experimental Results

This paper presents a novel direct current-fed interleaved phase-modulated single-phase unfolding inverter for fuel-cell applications. Two active-clamped zero-voltage switching current-fed half-bridge isolated converters are interleaved in parallel input and series output configuration. Phase modulation is proposed to control these two converter cells, which generate a rectified sinusoidal pattern at the dc link. A simple H-bridge unfolding dc-ac inversion is used to produce single-phase sine voltage at line frequency switching using square-wave control. This not only simplifies the inversion stage in terms of modulation but also reduces the switching losses. Direct front-end current-fed based unfolding inverter is not reported in literature yet. Detailed analysis and design of the proposed direct current-fed single-phase unfolding inverter has been presented. Simulation waveforms using PSIM 9.0.4 are given to verify the presented analysis and design. Experimental results on a laboratory prototype rated at 200 W are demonstrated to support the investigation, to validate the accuracy of the proposed analysis and design, and to test the performance.