This paper addresses the harmonic stability caused by the interactions among the wideband control of power converters and passive components in an ac power-electronics-based power system. The impedance-based analytical approach is employed and expanded to a meshed and balanced three-phase network which is dominated by multiple current- and voltage-controlled inverters with LCL- and LC-filters. A method of deriving the impedance ratios for the different inverters is proposed by means of the nodal admittance matrix. Thus, the contribution of each inverter to the harmonic stability of the power system can be readily predicted through Nyquist diagrams. Time-domain simulations and experimental tests on a three-inverter-based power system are presented. The results validate the effectiveness of the theoretical approach.

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Modeling and Analysis of Harmonic Stability in an AC Power-Electronics-Based Power System

Impedance networks cover the entire of electric power conversion from dc (converter, rectifier), ac (inverter), to phase and frequency conversion (ac-ac) in a wide range of applications. Various converter topologies have been reported in the literature to overcome the limitations and problems of the traditional voltage source, current source as well as various classical buck-boost, unidirectional, and bidirectional converter topologies. Proper implementation of the impedance-source network with appropriate switching configurations and topologies reduces the number of power conversion stages in the system power chain, which may improve the reliability and performance of the power system. The first part of this paper provides a comprehensive review of the various impedance-source-networks-based power converters and discusses the main topologies from an application point of view. This review paper is the first of its kind with the aim of providing a “one-stop” information source and a selection guide on impedance-source networks for power conversion for researchers, designers, and application engineers. A comprehensive review of various modeling, control, and modulation techniques for the impedance-source converters/inverters will be presented in Part II.

Impedance-Source Networks for Electric Power Conversion Part I: A Topological Review

Active power decoupling methods are developed to deal with the inherent ripple power at twice the grid frequency in single-phase systems generally by adding active switches and energy storage units. They have obtained a wide range of applications, such as photovoltaic (PV) systems, light-emitting diodes (LEDs) drivers, fuel cell (FC) power systems, and electric vehicle (EV) battery chargers, etc. This paper provides a comprehensive review of active power decoupling circuit topologies. They are categorized into two groups in terms of the structure characteristics: independent and dependent decoupling circuit topologies. The former operates independently with the original converter, and the latter, however, shares the power semiconductor devices with the original converter partially and even completely. The development laws for the active power decoupling topologies are revealed from the view of “duality principle,” “switches sharing,” and “differential connection.” In addition, the exceptions and special cases are also briefly introduced. This paper is targeted to help researchers, engineers, and designers to construct some new decoupling circuit topologies and properly select existing ones according to the specific application.

Review of Active Power Decoupling Topologies in Single-Phase Systems

The current-controlled grid-connected inverter with LCL filter is widely used in the distributed generation system (DGS), due to its fast dynamic response and better power quality features. However, with the increase of power injected into the grid, control performances of the inverter will be significantly influenced by the nonideal grid conditions. Specifically, the possible wide variation of the grid impedance challenges the system stability. Meanwhile, background harmonics of the grid can greatly distort the injected current. Therefore, the control of the inverter should be designed with strong stability-robustness and high harmonic-rejection-ability, both of which correlate closely with the inverter output impedance. However, it is difficult to shape the output impedance into the one with a desirable characteristic simply by adjusting the current loop gain. In this paper, an impedance shaping method is proposed with virtual impedances, and the current control loop can be designed independently. The implementation and parameter design of the virtual impedances are studied under the practical considerations. With this proposed method, the grid-connected inverter can work stably over a wide range of the typical inductive-resistive grid impedance and exhibit strong rejection ability of grid-voltage harmonics. Experimental results from a 6-kW single-phase grid-connected inverter confirm the effectiveness of the proposed method.

Impedance Shaping of the Grid-Connected Inverter with LCL Filter to Improve Its Adaptability to the Weak Grid Condition

Impedance-source networks cover the entire spectrum of electric power conversion applications (dc-dc, dc-ac, ac-dc, ac-ac) controlled and modulated by different modulation strategies to generate the desired dc or ac voltage and current at the output. A comprehensive review of various impedance-source-network-based power converters has been covered in a previous paper and main topologies were discussed from an application point of view. Now Part II provides a comprehensive review of the most popular control and modulation strategies for impedance-source network-based power converters/inverters. These methods are compared in terms of theoretical complexity and performance, when applied to the respective switching topologies. Further, this paper provides as a guide and quick reference for researchers and practicing engineers in deciding which control and modulation method to consider for an application in a given topology at a certain power level, switching frequency and demanded dynamic response.

Impedance-Source Networks for Electric Power Conversion Part II: Review of Control and Modulation Techniques

An infinite source with series inductance is usually employed as a grid emulator in grid-connected distributed generation systems. Thus, high capacitance of a transmission cable (i.e., underground cable) is too significant to be neglected. As a result, the capacitance and inductance may cause system resonance, which, in turn, challenges system stability. This paper takes offshore wind farm as an example to investigate the resonance issues caused by the submarine transmission cable of the grid-connected generation system. Based on the submarine cable model, a series of considerable resonant peaks is found in the open-loop transfer function of the grid-connected system because of the high-order LC configuration. The resonant peaks are sensitive to the system setup, which is clearly investigated. To overcome the resonances, this paper proposes a cascaded notch-filter-based active damping method to guarantee a good system stability and robustness. Furthermore, the proposed controller employs a proportional-resonant component to reduce the steady-state error of the output current. The simulation and experimental results have validated the findings of resonances and the effectiveness of the proposed controller.

Resonance Issues and Damping Techniques for Grid-Connected Inverters With Long Transmission Cable

This paper proposes a new bidirectional dc-dc converter — buck-boost + Cuk converter — for series connected battery cells. Traditionally, in order to balance n battery cells in a string, (2n−1) switches are required by either the buck-boost converter or the Cuk converter. However, by nicely combine the buck-boost converter and the Cuk converter together, the proposed buck-boost + Cuk converter only requires n switches. It achieves almost half of the switch count reduction, without losing the advantage of modularization or sacrificing the device voltage stress, unlike many other existing one-switch-per-cell topologies. Meanwhile, it still maintains the buck-boost battery charge equalizer's best characteristic — simple pulse width modulation (PWM) — 50% duty cycle. Moreover, it offers a solution to bypass the open-circuit faulty cells, and prolong the life time of the whole battery string. Simulation and experimental results are given to demonstrate the functionality of the proposed buck-boost + Cuk converter.

Modularized buck-boost + Cuk converter for high voltage series connected battery cells

Parallel operation of multiphase bridge legs can reach high current in application to megawatt (MW) power conversion systems. However, direct parallel operation without any current balancing circuit may result in huge circulating current and short circuit because of differences of time delay and parameter tolerance in control circuits and switching speed difference in IGBTs. This paper proposes a multiphase coupling current balancing technique for parallel operation of multiphase legs to achieve multiple MW power converter/inverter modules. Multiphase coupling current balancer (CB) circuit modeling, analysis, and design demonstrate in detail how to implement such CBs. A 4.3-MW/3600-A inverter module is prototyped to illustrate the proposed scheme's implementation and validation. Six phase legs are operated in parallel to achieve the 3600-A rms inverter current rating. Each phase leg employs 1200-A IGBTs to provide 600-A rms output current. To achieve compactness and light weight, a very high permeability magnetic core with no air gap is used for the CB. Simulation and experimental results verify the excellent current balancing ability in parallel operation of multiple MW power modules.

Multiphase-Leg Coupling Current Balancer for Parallel Operation of Multiple MW Power Modules

Minimization of the dc capacitor is an essential step towards developing and manufacturing compact low-cost hybrid electric vehicle (HEV) converter/inverter systems for high temperature operation, long life and high reliability. Previously, most papers have been focused on the AC-DC-AC pulse-width modulation (PWM) converters with relatively complicated close-loop control methods, while few papers have analyzed the DC-DC-AC PWM converters that also commonly exist in HEV systems. The so-called DC-DC-AC PWM converter consists of a bidirectional dc-dc converter and an inverter cascaded together. This paper proposes a carrier modulation method for the previously mentioned dc-dc converter, which connects the battery or any energy storage system to the dc link, in order to match the converter output current with the inverter input current so as to minimize the current ripple going through the dc link capacitor. Compared with the conventional triangle carrier, the proposed carrier is able to shift the converter output current pulses left and right, so they match with the inverter input current, and finally minimize the current ripple flowing through the dc link capacitor at unity power factor by a simple and easy implementation. Simulation and experimental results are provided to validate the effectiveness of the proposed method.

A carrier modulation method for minimizing the dc link capacitor current ripple of the HEV DC-DC converter and inverter systems

This paper presents a phase-shift control method for double-wing multilevel modular switched-capacitor dc-dc converter (DW-MMCCC) to achieve output voltage regulation and zero voltage switching features. By utilizing the circuit stray inductance and the proposed phase-shift control method, the output voltage of DW-MMCCC can be controlled without sacrificing efficiency significantly. The DW-MMCCCs with odd conversion ratio and even conversion ratio are analyzed separately. Simulation results are provided to demonstrate the validity of the proposed method. A 500 W prototype is built; and experimental results is provided to validate the proposed operating method.

Xi Lu

Papers

Resonance Issues and Damping Techniques for Grid-Connected Inverters With Long Transmission Cable

Modularized buck-boost + Cuk converter for high voltage series connected battery cells

Multiphase-Leg Coupling Current Balancer for Parallel Operation of Multiple MW Power Modules

A carrier modulation method for minimizing the dc link capacitor current ripple of the HEV DC-DC converter and inverter systems

Zero voltage switching double-wing multilevel modular switched-capacitor DC-DC converter with voltage regulation