Traditional Voltage Source Converters Research Articles

Dual Control Structure of Modular Multilevel Converters for Power System Support

This paper presents an analysis of two philosophies to control the modular multilevel converter (MMC). Most MMC control strategies are based on the philosophy used in the traditional voltage-source converter, in which the dc-bus voltage is controlled through the ac-side power; this approach is referred to here as the classical control approach. Because the MMC has an additional degree of freedom, an alternative control philosophy has recently been proposed, in which the dc-bus voltage is controlled through the dc-side power. In this paper, a comparison of these control approaches is presented and their main differences are described. Considering the increasing requirements imposed by system operators to converter-interfaced systems, guidelines are also given to provide supplementary services such as virtual inertia provision, short-term frequency support, virtual capacitor emulation, and oscillation damping. The provided insights are useful for selecting the appropriate MMC control strategy and for developing new supplementary controls for facilities based on the MMC topology.

The Case for Soft Switching in Four-Quadrant Power Converters

This article discusses the use of soft-switching techniques in isolated and non-isolated converters, showing that the new range of grid-connected applications require higher performance power conversion than traditional voltage source converters can provide. The article begins with a review of soft-switching techniques, discusses the challenges of achieving soft switching in four-quadrant converters, and shows the impact that soft switching can have on the next generation of multi-port converter applications, such as energy storage, photovoltaic (PV)/battery/grid systems, and microgrids.

A predictive control method for reducing power backflow in DC/AC matrix converter

In recent years, DC/AC matrix converter with higher power density have become an attractive alternative to traditional voltage source converters. The traditional PI control has been employed to achieve accurate control, but its dynamic response and stability need to be improved. In this paper, a model predictive control (MPC) with reducing power backflow is introduced in an isolated DC/AC matrix converter with a loosely coupled high-frequency transformer (LCHFT). Prediction model and optimal function model are established to improve the dynamic response of the system and reduce the power backflow.

ANFIS Controller for Voltage Compensation in Renewable Energy Applications

this article presents the adaptive neuro fuzzy inference system (ANFIS) based controller for voltage compensation in micro grid. The micro grid has more number of voltage source converters for distribute the electric energy to the small consumers. If any kind of load change in a system has a very small impact in the output voltage of the system, which is extremely sensible. This is due to unbalanced load present in connection point, further increases this problem. In order to minimize the unbalanced load effect an ANFIS based controller has been connected with the traditional voltage source converter control circuit.here, the ANFIS controller fix the reference current gain equal to the unbalanced voltage. This reference current received from ANFIS controller is added with the voltage control loop output to fix the changed current reference for a current control loop. The proposed control logic is verified both simulation and experimentally.

Reliability evaluation of modular multilevel converter based on Markov model

The modular multilevel converter (MMC) is now the most attractive topology for medium and high voltage power conversion applications with several advantages over the traditional voltage source converter (VSC). However, due to a large number of sub-modules (SMs) in the MMC, system reliability is a big challenge in its practical application, where each SM may be considered as a potential point of failure. In this paper, a reliability evaluation based on the Markov model is proposed for the MMC. The failure rates of the power electronic devices and SMs are firstly analyzed. Then, the Markov model and the state transition equation of the system are built in detail. A general reliability evaluation function is established, in which the mean time to failure and reliability evaluation of the MMC with redundant SMs are also discussed. Finally, a practical direct current (DC) distribution example for reliability evaluation is analyzed, and the results verify that the reliability evaluation based on the Markov model could provide a useful reference for project design.

Modeling, control, and reduced-order representation of modular multilevel converters

This paper presents a reduced-order model of the modular multilevel converter (MMC) for electromechanical transient simulations and small-signal analysis. The MMC model is firstly developed in detail; then, simplifications are introduced to reduce it to eleventh- and fourth-order models. The dynamic behaviors of the traditional voltage-source converter and the MMC are also compared. A thorough description of the MMC control system is presented including the inner current control loops, the outer voltage control loops, and the strategy to balance the floating capacitor voltages. Control systems in continuous- and discrete-time domains are given to enable their use in power system simulations and in practical implementations, respectively. Several tests are performed to compare the steady-state and transient response of the detailed and the reduced models. The results show that the fourth-order reduced model can properly capture the input-output dynamics of a complete MMC and significantly reduce the computational cost of large-scale power system simulations with multiple ac/dc converter stations.

Feedback Linearization Controller for MMC-HVDC System Connecting to Passive Networks

Compared with traditional voltage-source converter (VSC) in low level topology, modular multilevel converter (MMC) is suitable for supplying passive networks. A stable and well controller is important to ensure the high quality power supply to passive network connecting to MMC. In this paper, a double closed-loop controller containing an inner current loop and an outer voltage loop is designed for the rectifier station. The feedback linearization controller is adopted in the inner current loop, which is much simpler and better than the traditional PI controller. And a constant AC voltage controller is designed for the inverter station. The performance of the proposed controllers under different operation conditions are simulated in PSCAD/EMTDC. The simulation results show that the proposed controllers can ensure MMC-HVDC system provide highquality power supply to passive networks.

Two-switch three-phase ac-link dynamic voltage restorer

This study presents a topology for a pulse width modulated (PWM) three-phase voltage regulator that uses only two driving switches, providing high reliability and making the switching stage simpler and cheaper compared with other available configurations. The proposed topology is suitable for voltage regulation on the distribution power system. It is able to regulate the steady-state voltage and to reject voltage variations such as flicker or sags caused by large motors start. In addition, the compensator features a topology free of energy storage elements. The PWM control is based on dc signals, asynchronous from the grid, simpler than the control used for traditional voltage source converters – it does not require a phase-locked loop, and does not involve trigonometric calculations. As a result of this, the controller can be achieved with an analogue controller or a low-cost microcontroller. The proof of concept is performed with a 220 V three-phase voltage regulator, which is simulated and prototyped in the laboratory.

Automated Optimal Design of Input Filters for Direct AC/AC Matrix Converters

This paper presents a novel method to design the input filter for a direct ac/ac matrix converter using genetic algorithms (GA) optimization. The input filter for a matrix converter is a very important and critical part of the conversion structure and careful design is necessary to ensure high input power quality, compactness, and stability. The GA will optimize structure and parameters of the input filter as a function of different factors such as energy storage, weight, and volume. The effectiveness of this design method is demonstrated through a wide range of simulations using Saber and experimental results on a laboratory prototype. The same methodology could also be adapted and applied to any converter configuration such as, for example, traditional voltage source converters.

Z-source inverter

This paper presents an impedance-source (or impedance-fed) power converter (abbreviated as Z-source converter) and its control method for implementing DC-to-AC, AC-to-DC, AC-to-AC, and DC-to-DC power conversion. The Z-source converter employs a unique impedance network (or circuit) to couple the converter main circuit to the power source, thus providing unique features that cannot be obtained in the traditional voltage-source (or voltage-fed) and current-source (or current-fed) converters where a capacitor and inductor are used, respectively. The Z-source converter overcomes the conceptual and theoretical barriers and limitations of the traditional voltage-source converter (abbreviated as V-source converter) and current-source converter (abbreviated as I-source converter) and provides a novel power conversion concept. The Z-source concept can be applied to all DC-to-AC, AC-to-DC, AC-to-AC, and DC-to-DC power conversion. To describe the operating principle and control, this paper focuses on an example: a Z-source inverter for DC-AC power conversion needed in fuel cell applications. Simulation and experimental results are presented to demonstrate the new features.