A Comparison of Diode-Clamped and Cascaded Multilevel Converters for a STATCOM with Energy Storage

The progression of distributed generation within a bulk power system will lead to the need for greater control of transmission-line power flows. Static synchronous compensators (STATCOMs) provide a power-electronics-based means of embedded control of transmission-line voltage and power flows. The integration of energy storage with a STATCOM can extend traditional STATCOM capabilities to four-quadrant power flow control and transient stability improvement. This paper discusses energy storage systems (ESSs) integrated with conventional and multilevel bidirectional power converters for a hybrid STATCOM/ESS. Conventional, diode-clamped, and cascaded multilevel converter-based STATCOM/ESSs are developed, and their performances for a variety of power system applications are compared using battery energy storage. The advantages and disadvantages of each topology are presented. Both simulation and experimental results are provided to validate the conclusions

Fully decoupled current control and energy balancing of the Modular Multilevel Matrix Converter

The Modular Multilevel Matrix Converter (M3C) is a Modular Multilevel Converter topology which is suitable for high power low speed drive applications. This paper presents a fully decoupled current control which allows an independent input, output and internal balancing current control. To equalize the energy stored in the nine converter arms, an energy and balancing control is presented which includes average, horizontal, vertical and diagonal balancing control loops. Simulation results are used to verify the function of the M3C together with an induction motor drive system. Additionally, the proper function of the recently constructed arm PCB working as single phase multilevel STATCOM is presented. This PCB will be used for each arm in the laboratory prototype of the M3C in the near future

Low capacitance cascaded H-bridge StatCom

The application of Cascaded H-Bridge (CHB) multilevel converter StatCom is well established in the industry. In a conventional CHB StatCom, low frequency ripple on the dc side is limited to 10% by utilizing large capacitance. Having a smaller capaci-tance is advantageous because the system will be smaller, cheaper, and more reliable. However, reducing the capacitors will increase the ripple’s magnitude and causes problems such as, control and filtering issues, and high voltage stress on the semicon-ductors. In this thesis, the next generation of CHB StatCom i.e. Low Capacitance StatCom (LC-StatCom) is developed which is able to operate with ripple magnitudes close to the theoretical limit (smallest capacitors’ size). A feed-forward filtering scheme is developed that is able to effectively filter out large ripples without imposing any delay to the control loop and facilitates design of higher bandwidth capacitors’ voltage controllers. A decoupling theory which outlines requirements for completely decoupling the individual capacitors’ voltage controller from the rest of the control system is introduced. The decoupled control system has a linear cluster capacitors’ voltage controller which is essential for operation of the LC-StatCom The proposed LC-StatCom utilizes this linearity to have a variable capacitors’ voltage reference in order to limit the maximum voltage stress on the semiconductors. The proposed LC-StatCom and innovative solutions are evaluated by simulation and experimental case studies. It is shown that the LC-StatCom can achieve 80% reduction in the capacitors’ size, improve current quality, and reduce the maximum voltage stress on the semiconductors com-pared to a conventional StatCom. The LC-StatCom has a reduced operating area in the inductive region compared to conventional StatComs. In this thesis, to overcome this drawback, a hybrid LC-StatCom that utilizes a series thyristor bypassed inductor is developed. The compensated LC-StatCom developed in this thesis, provides approxi-mately 75% reduction in overall energy storage capacity of passive components

Modeling and control of cascaded bridgeless multilevel rectifier under unbalanced load conditions

“The goal of this project is to model and control a novel unidirectional cascaded multilevel bridgeless rectifier as an active front end in medium and high voltage applications. This topology has many advantages over a conventional cascaded H-bridge rectifier, such as lower implementation cost, higher reliability, and greater flexibility with similar power quality. The complete design process of the proposed converter is developed step by step in order to meet all the desired objectives. The steady-state mathematical model is used to develop a method for the voltage balancing of dc cells. Power factor analysis is discussed to mathematically derive requirements for the number of partially controlled and fully controlled H-bridges in the proposed H-bridge converter. Power loss, efficiency, and cost comparison studies between the traditional cascaded H-Bridge converter and the proposed bridgeless converter demonstrate the advantages. After exploring various well-established control methods, a novel control strategy is proposed to achieve dc voltage balancing, fast and robust grid synchronization, power factor correction, and elimination of zero crossing current distortion under both balanced and unbalanced load conditions. The converter can also be used for reactive power compensation in a grid tied power system if a sufficient number of fully controlled H-bridge modules are included. Processor-In-the-Loop (PIL) simulation has been the utilized to validate the performance of discrete control structure. Simulation and experimental results validate the models and control method”--Abstract, page iii

A novel predictive-fixed switching frequency technique for a cascade H-bridge multilevel STATCOM

There are several control techniques for active power filter applications. This paper presents a new alternative of predictive current control for a cascade H-bridge multilevel converter, where the optimal vector selected by the predictive controller is used for a modulation stage to obtain a fixed switching frequency. Simulation results validate the proposal where almost similar results can be obtained in comparison with the traditional methods.CONACYT – Consejo Nacional de Ciencia y Tecnologí

Finite-fixed switching predictive control technique for a 7-level cascaded H-bridge multilevel active power filter. A comparative study.

This paper presents a finite-fixed predictive control technique applied to the three-wire cascade H-bridge multilevel converters for active power filter applications. The focus of this paper is to examine the impacts of the use of optimum modulation techniques combined with predictive current control in order to increase the control performance in terms of reactive power compensation and total harmonic distortion.CONACYT – Consejo Nacional de Ciencia y Tecnologí

A Multi-Processor Control System Architecture for a Cascaded StatCom with Energy Storage

This paper presents a multi-processor control system for a general purpose five-level cascaded inverter for real time power system applications. Practical design considerations for the digital controller architecture as well as the power converter are discussed and a 3 kVA laboratory prototype is presented. As a case study, a StatCom with battery energy storage was implemented on this multi-processor controlled inverter system. To eliminate the troublesome PI parameter tuning and the limitation of small signal models, which exist in conventional control for StatComs, a new and simple control method based on large signal model was designed to realize four-quadrant power injections into a grid. Experimental results are provided to support the proposed concept

Modular Multilevel Cascaded Flying Capacitor STATCOM for Balanced and Unbalanced Load Compensation

Voltage and current unbalance are major problems in distribution networks, particularly with the integration of distributed generation systems. One way of mitigating these issues is by injecting negative sequence current into the distribution network using a Static Synchronous Compensator (STATCOM) which normally also regulates the voltage and power factor. The benefits of modularity and scalability offered by Modular Multilevel Cascaded Converters (MMCC) make them suitable for STATCOM application. A number of different types of MMCC may be used, classified according to the sub-module circuit topology used. Their performance features and operational ranges for unbalanced load compensation are evaluated and quantified in this research. This thesis investigates the use of both single star and single delta configured five-level Flying Capacitor (FC) converter MMCC based STATCOMs for unbalanced load compensation. A detailed study is carried out to compare this type of sub-module with several other types namely: half bridge, 3-L H-bridge and 3-L FC half bridge, and reveals the one best suited to STATCOM operation. With the choice of 5-L FC H-bridge as the sub-module for STATCOM operation, a detailed investigation is also performed to decide which pulse width modulation technique is the best. This was based on the assessment of total harmonic distortion, power loss, sub-module switch utilization and natural balancing of inner flying capacitors. Two new modulation techniques of swapped-carrier PWM (SC-PWM) along with phase disposed and phase shifted PWM (PS-PWM) are analyzed under these four performance metrics. A novel contribution of this research is the development of a new space vector modulation technique using an overlapping hexagon technique. This space vector strategy offers benefits of eliminating control complexity and improving waveform quality, unlike the case of multilevel space vector technique. The simulation and experimental results show that this method provides superior performance and is applicable for other MMCC sub-modules. Another contribution is the analysis and quantification of operating ranges of both single star and delta MMCCs in rating the cluster dc-link voltage (star) and current (delta) for unbalanced load compensation. A novel method of extending the operating capabilities of both configurations uses a third harmonic injection method. An experimental investigation validates the operating range extension compared to the pure sinusoidal zero sequence voltage and current injection. Also, the superiority of the single delta configured MMCC for unbalanced loading compensation is validated

An Overview of Modelling Techniques and Control Strategies for Modular Multilevel Matrix Converters

The Modular Multilevel Matrix Converter is a relatively new power converter topology appropriate for high-power Alternating Current (AC) to AC purposes. Several publications in the literature have highlighted the converter capabilities such as modularity, control flexibility, the possibility to include redundancy, and power quality. Nevertheless, the topology and control of this converter are relatively complex to design and implement, considering that the converter has a large number of cells and floating capacitors. Therefore multilayer nested control systems are required to maintain the capacitor voltage of each cell regulated within an acceptable range. There are no other review papers where the modelling, control systems and applications of the Modular Multilevel Matrix Converter are discussed. Hence, this paper aims to facilitate further research by presenting the technology related to the Modular Multilevel Matrix Converter, focusing on a comprehensive revision of the modelling and control strategies.Agencia Nacional de Investigacion y Desarrollo (ANID) of Chile Fondecyt 11191163 Fondecyt 1180879 Fondecyt 11190852 Fondef ID19I10370 University of Costa Rica 322-B9242 University of Santiago Dicyt 091813D