The virtual impedance concept is increasingly used for the control of power electronic systems. Generally, the virtual impedance loop can either be embedded as an additional degree of freedom for active stabilization and disturbance rejection, or be employed as a command reference generator for the converters to provide ancillary services. This paper presents an overview of the virtual-impedance-based control strategies for voltage-source and current-source converters. The control output impedance shaping attained by the virtual impedances is generalized first using the impedance-based models. Different virtual impedances and their implementation issues are then discussed. A number of practical examples are demonstrated to illustrate the feasibility of virtual impedances. Emerging applications and future trends of virtual impedances in power electronic systems conclude this paper.

Virtual-Impedance-Based Control for Voltage-Source and Current-Source Converters

Resistive switching (RS) phenomena are reversible changes in the metastable resistance state induced by external electric fields. After discovery ∼50 years ago, RS phenomena have attracted great attention due to their potential application in next-generation electrical devices. Considerable research has been performed to understand the physical mechanisms of RS and explore the feasibility and limits of such devices. There have also been several reviews on RS that attempt to explain the microscopic origins of how regions that were originally insulators can change into conductors. However, little attention has been paid to the most important factor in determining resistance: how conducting local regions are interconnected. Here, we provide an overview of the underlying physics behind connectivity changes in highly conductive regions under an electric field. We first classify RS phenomena according to their characteristic current–voltage curves: unipolar, bipolar, and threshold switchings. Second, we outline the microscopic origins of RS in oxides, focusing on the roles of oxygen vacancies: the effect of concentration, the mechanisms of channel formation and rupture, and the driving forces of oxygen vacancies. Third, we review RS studies from the perspective of statistical physics to understand connectivity change in RS phenomena. We discuss percolation model approaches and the theory for the scaling behaviors of numerous transport properties observed in RS. Fourth, we review various switching-type conversion phenomena in RS: bipolar-unipolar, memory-threshold, figure-of-eight, and counter-figure-of-eight conversions. Finally, we review several related technological issues, such as improvement in high resistance fluctuations, sneak-path problems, and multilevel switching problems.

Resistive switching phenomena: A review of statistical physics approaches

A switched boost inverter (SBI) can replace a $Z$ -source inverter (ZSI) in low-power applications because it has one less $LC$ pair than the ZSI. This paper presents a class of quasi-SBIs (qSBIs) that offers several advantages when compared with a conventional SBI, including reducing the voltage stress on the capacitor, increasing the boost voltage factor, and improving input current profiles. Operating principles, steady-state analysis, and comparisons with conventional inverters are presented. A prototype based on a TMS320F28335 digital signal processor is built to verify the operating principle of the proposed qSBIs.

A Class of Quasi-Switched Boost Inverters

In this paper, two topologies are presented for the enhanced-boost quasi-Z-source inverters (qZSI), namely continuous input current configuration and discontinuous input current configuration of enhanced-boost qZSI with two-switched impedance networks. Similar to enhanced-boost impedance-source inverters (ZSIs), these proposed inverter topologies possess very high boost voltage inversion at low shoot-through duty ratio and high modulation index to provide an improved quality output voltage. Compared to enhanced-boost ZSIs with two-switched Z-source impedance networks, these proposed inverter topologies share common ground with source and bridge inverter, overcome the starting inrush problem, and draw continuous input current and the lower voltage across the capacitors. Moreover, the input ripple current is negligible. This paper presents the operating principles and analysis of continuous input current configuration enhanced-boost qZSI with two-switched impedance networks and compares with ZSI, switched inductor ZSI, DA/CA-qZSI, and enhanced-boost ZSIs. The theoretical analysis is done and is validated through simulation and experimental results.

Enhanced-Boost Quasi-Z-Source Inverters With Two-Switched Impedance Networks

By regulating the inverter output current every switching cycle, instantaneous current-sharing control strategies are usually employed in paralleled modular uninterruptible power supplies (UPSs). In this paper, virtual impedance, which is usually utilized in the droop method, is firstly introduced to the instantaneous current-sharing control strategy to achieve good load current-sharing performance. The instantaneous circulating current model of the paralleled system is developed and the circulating impedance is derived as well to clearly identify the intrinsic nature of unequal sharing of load current. By inserting the virtual impedance to the circulating impedance with the help of proper control strategy, good current-sharing capability can be obtained. The parallel inductor, e.g., coupled inductor, which is generally added at the output of each inverter to reduce the circulating current among modules, can be, therefore, eliminated, contributing to reduced weight, volume, and cost. In the meantime, the system output-voltage regulation performance, e.g., amplitude and waveform quality, is not affected by the introduction of the virtual impedance. The designation of the virtual impedance is provided and its implementation is simple but rather effective. Experimental results are also provided to verify the feasibility of the proposed method.

Instantaneous Current-Sharing Control Strategy for Parallel Operation of UPS Modules Using Virtual Impedance

This paper describes a theoretical and experimental study on a control strategy for the parallel operation of single-phase voltage source inverters (VSI), to be applied to uninterruptible power supply. The control system for each inverter consists of two main loops, which both use instantaneous values. The first (parallelism control) employs the feedback of the inductor current from the output filter to modify the input voltage of the same filter and, therefore, to control the power flow of each inverter to the load. Additionally, the second loop (voltage control) is responsible for controlling the output voltage of the LC filter, which coincides with the output voltage of the VSI. Due to the fact that there is no exchange of information among the VSIs regarding their operation points, it is easier to obtain redundant systems. Furthermore, the connection (or disconnection) of inverters in a parallel arrangement is carried out directly, without connection impedance, and can occur at any operation point of the system. The proposed control strategy ensures the proper sharing of the load current and avoids current circulation among the inverters during transient and steady-state operation. Moreover, its design and implementation are very simple. The control technique was verified through experimental results with a maximum load of 10 kVA supplied by three parallel-connected inverters.

A Control Strategy for Parallel Operation of Single-Phase Voltage Source Inverters: Analysis, Design and Experimental Results

This paper presents a simple, effective and robust control strategy applied to parallel connected of single phase voltage source inverters. The parallelism control is only done using internal measures of each inverter. The objective is to obtain a control system where the inverters are independent from each other. The proposed control strategy of each inverter has two control loops by instantaneous average values: one for voltage and another for current. The first one is designed to control the output voltage of the inverter, while the second is designed to lead the parallel operation of the inverters. The parallelism control acts in the voltage before the LC filter and it is based on the feedback of the L inductor current. Its function is to guarantee the correct load current sharing. The control technique was corroborated through experimental results with a maximum load of 10 kVA supplied by three parallel connected inverters.

A control strategy by instantaneous average values for parallel operation of single phase voltage source inverters based in the inductor current feedback

This paper presents a simple, effective and robust control strategy applied to parallel connected single phase voltage source inverters The proposed control strategy has two control loops with instantaneous average values: one for voltage and another for current The first one controls the output voltage of the inverter while the second controller is designed to lead the parallel operation of the inverters The intended parallelism control acts in the voltage before the LC filter and it is based on the feedback of the L inductor current Its objective is to guarantee the correct load current sharing The inverters are independent of each other and there is no communication among them A prototype with three 5 kVA inverters each was built to corroborate the control technique

A control strategy for parallel operation of single phase voltage source inverters

This paper presents a simple, effective and robust control strategy applied to parallel connected single phase voltage source inverters. The proposed control has two instantaneous average values control loops: one for voltage and another for current. The first one controls the output voltage of the inverter while the second is designed to control the parallel operation of the inverters. The proposed parallelism control acts in the voltage before the LC filter and is based in the feedback of the L inductor current. There is no communication among inverters so that one unit is independent from the others. A prototype with 3 inverters of 5 kVA each was built to corroborate the control technique.

Guilherme A. T. Bauer

Papers

A Control Strategy for Parallel Operation of Single-Phase Voltage Source Inverters: Analysis, Design and Experimental Results

A control strategy by instantaneous average values for parallel operation of single phase voltage source inverters based in the inductor current feedback

A control strategy for parallel operation of single phase voltage source inverters

A control strategy for parallel voltage source inverters