Interface inductors are a commonly used coupling component for grid-connected DC/AC voltage-sourced power-electronics converters. They provide two main benefits: a voltage-sensing point for converter synchronization and, together with the grid impedance, they filter out the converter's voltage-switching-harmonics from influencing the grid-currents. Despite these benefits, the interface inductors add additional cost, weight, volume, and power losses to the system. Furthermore, the measurement of the synchronization voltage requires additional voltage sensors to be used, further increasing system cost and complexity. In recent years, much has been done to reduce the interface inductor's size and/or weight without compromising filtering performance, such as the use of different magnetic materials, planar windings, or higher converter-switching-frequencies. Also, voltage-sensor-less algorithms have been proposed, such as the Direct Power Control method (which is similar to the Direct Torque Control of induction machines). However, the reduction/elimination of these two items (inductors and sensors) are usually not considered together. Furthermore, many sensor-less control methods usually rely on knowledge of the source impedance, which may be difficult to estimate in grid-connected systems.To eliminate the voltage sensors, the concept of virtual impedance can be employed to fabricate a synchronization point for the converter within software. Since the virtualized synchronization-voltage is based on information from already-available AC current sensors, the external voltage sensors can be removed from the system. In addition to synchronization, additional virtual impedances or transfer functions can be fabricated to enhance the dynamic performance of the system, reduce computational complexity, and/or enhance the stability range. Lastly, if the AC source impedance alone is suitable to provide adequate harmonic filtering of the current (e.g. in a motor/generator connection) and the synchronization point is virtualized, the physical interfacing inductance can be completely removed from the system. The main focus of this research work is to investigate the theory and implementation of using virtual impedances in DC/AC converter control systems. The self-synchronized inductor-less DC/AC converter system utilizing the concept of virtual-impedance is proposed. Also, a method of using only a virtual interfacing-resistance to alter the current control loop, eliminate cross-coupling terms, and reduce computational complexity is proposed. In addition, a method of virtual-impedance-compensating PLL algorithms with better transient response is proposed. Finally, the main application considered for this proposed control method is grid-connected systems, but an alternate virtual-impedance-based method for high-speed sensor-less control of permanent magnet AC machines is also proposed. A 1 kVA three-phase two-level inverter prototype has been designed to experimentally validate some of the proposed control strategies.
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