This book presents the reader, whether an electrical engineering student in power electronics or a design engineer, a selection of power converter control problems and their basic digital solutions, based on the most widespread digital control techniques. The presentation is primarily focused on different applications of the same power converter topology, the half-bridge voltage source inverter, considered both in its single- and three-phase implementation. This is chosen as the test case because, besides being simple and well known, it allows the discussion of a significant spectrum of the most frequently encountered digital control applications in power electronics, from digital pulse width modulation (DPWM) and space vector modulation (SVM), to inverter output current and voltage control, ending with the relatively more complex VSI applications related to the so called smart-grid scenario. This book aims to serve two purposes: (1) to give a basic, introductory knowledge of the digital control techniques applied to power converters; and (2) to raise the interest for discrete time control theory, stimulating new developments in its application to switching power converters.

Digital control in power electronics

DC microgrids and distribution systems: An overview

Modular multilevel converter (MMC) is one of the most promising topologies for medium to high-voltage high-power applications. The main features of MMC are modularity, voltage and power scalability, fault tolerant and transformer-less operation, and high-quality output waveforms. Over the past few years, several research studies are conducted to address the technical challenges associated with the operation and control of the MMC. This paper presents the development of MMC circuit topologies and their mathematical models over the years. Also, the evolution and technical challenges of the classical and model predictive control methods are discussed. Finally, the MMC applications and their future trends are presented.

Evolution of Topologies, Modeling, Control Schemes, and Applications of Modular Multilevel Converters

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

Grid-tied voltage source inverters using LCL filter have been widely adopted in distributed power generation systems (DPGSs). As high-order LCL filters contain multiple resonant frequencies, switching harmonics generated by the inverter and current harmonics generated by the active/passive loads would cause the system resonance, and thus the output current distortion and oscillation. Such phenomenon is particularly critical when the power grid is weak with the unknown grid impedance. In order to stabilize the operation of the DPGS and improve the waveform of the injected currents, many innovative damping methods have been proposed. A comprehensive overview on those contributions and their classification on the inverter- and grid-side damping measures are presented. Based on the concept of the impedance-based stability analysis, all damping methods can ensure the system stability by modifying the effective output impedance of the inverter or the effective grid impedance. Classical damping methods for industrial applications will be analyzed and compared. Finally, the future trends of the impedance-based stability analysis, as well as some promising damping methods, will be discussed.

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Damping Methods for Resonances Caused by LCL-Filter-Based Current-Controlled Grid-Tied Power Inverters: An Overview

A novel repetitive controller directly combined with an open loop SPWM inverter is presented in this paper. To cope with the high-resonant peak of the open loop inverter that may cause instability, a zero-phase-shift notch filter other than the inverse transfer function of the inverter or a conventional second-order filter is incorporated in the controller. The proposed method has good harmonic rejection and large tolerance to parameter variations. To further reduce the steady-state error, a low-pass-filter Q(z) algorithm is applied. The DC bias problem is also taken into consideration and solved with the repetitive controller itself. The method is implemented with a digital signal processor and achieves low THD% (1.4%-1.7%) with nonlinear loads and fast error convergence (3-5 fundamental periods). It proves to be a cost-effective solution for common UPS products where high-quality output voltage is more stressed than fast dynamic response.

Direct repetitive control of SPWM inverter for UPS purpose

Single-phase pulsewidth modulation rectifiers suffer from ripple power pulsating at twice the line frequency. The ripple power is usually filtered by a bulky capacitor bank or an LC branch, resulting in lower power density. The alternative way is active power decoupling, which uses an active circuit to direct the pulsating power into another energy-storage component. The main dc-link filter capacitor can, therefore, be reduced substantially. This paper proposed a new scheme of active power decoupling. The circuit consists of a third leg, an energy-storage capacitor and a smoothing inductor. The topology combined the advantages of high energy-storage efficiency and low requirement on control bandwidth. Both the pulsating power from the ac source and the reactive power of the smoothing inductors are taken into consideration when deriving the power decoupling scheme. The active power filter's (APF) capacitor voltage control system consists of inner loop pole-placement control and outer loop proportional-resonant control. To enhance the steady-state performance, the capacitor voltage reference is modified in a closed-loop manner. Simulation and experimental results show that the proposed APF scheme has good power decoupling performance and is more suited for high-power applications where switching frequency is limited.

Active Power Decoupling for High-Power Single-Phase PWM Rectifiers

In a modular multilevel converter (MMC), the interaction between switching actions and fluctuating capacitor voltages of the submodules results in second- and other even-order harmonics in the circulating currents. These harmonic currents will introduce extra power loss, increase current stress of power devices, and even cause instability during transients. Traditional methods for circulating current harmonic suppression have problems such as limited harmonic rejection capability, limited application area, and complex implementation. This paper presents a plug-in repetitive control scheme to solve the problem. It combines the high dynamics of PI controller and good steady-state harmonic suppression of the repetitive controller, and minimizes the interference between the two controllers. It is suitable for multiple harmonic suppression, easy to implement, and applicable for both single-phase and three-phase MMCs. Simulation and experimental results on a single-phase MMC inverter proved the validity of the proposed control method.

A Repetitive Control Scheme for Harmonic Suppression of Circulating Current in Modular Multilevel Converters

Modular multilevel converter (MMC) has become one of the most promising converter topologies for future high-power applications. A challenging issue of the MMC is the voltage balancing among arm capacitors. A good overall control system is also vital for the MMC, which should be based on sound mathematical model, readily adaptable for different applications, and capable of high performance. This paper presents a general control structure for MMC inverters, which is suitable for both voltage-based and energy-based control methods, and includes voltage balancing between the upper and lower arms. A new method for voltage balancing among arm capacitors, which is based on an improved pulse-width modulation, is also presented. The proposed method avoids some major disadvantages found in present voltage balancing methods, such as dependence on computation-intensive voltage sorting algorithms, extra switching actions, interference with output voltage, etc. Furthermore, all switching actions are evenly distributed among power devices. The proposed control system as a whole can serve as a promising solution for practical applications, especially when the number of submodules is fairly high. Simulation and experimental results verify the effectiveness of the proposed methods.

An Improved Control System for Modular Multilevel Converters with New Modulation Strategy and Voltage Balancing Control

Control of the LCL -type three-phase grid-connected converter is difficult due to high resonance peak of the LCL filter. Active damping is the state-of-the-art solution to this problem, but the damping performance will be affected by the inherent time delay of digital control, especially for high-power low switching frequency applications. Based on a discrete-time stability analysis of an LCL -type converter with capacitor-current-feedback active damping, a simple and effective time delay compensation method, which is based on area equalization concept, is proposed. The method can reduce the negative impact of the computation delay significantly. It has the potential to serve as a general solution to time delay compensation of a digitally controlled PWM converter. The validity of the proposed method is proved by experimental results.

Jian Xiong

Papers

Direct repetitive control of SPWM inverter for UPS purpose

Active Power Decoupling for High-Power Single-Phase PWM Rectifiers

A Repetitive Control Scheme for Harmonic Suppression of Circulating Current in Modular Multilevel Converters

An Improved Control System for Modular Multilevel Converters with New Modulation Strategy and Voltage Balancing Control

A Time Delay Compensation Method Based on Area Equivalence For Active Damping of an LCL -Type Converter