Decoupled Power Control With Indepth Analysis of Single-Phase Electric Springs

Electric spring (ES) as a new effective way to solve the power quality issues caused by the uncertainty of wind and photovoltaic (PV) power, has the advantages of small volume, flexible configuration and low cost. Aiming at improving the dynamic responses of the existing power control for ES-2, a new control with in-depth analysis on the decoupling of the active and reactive powers is proposed in this paper. By introducing second order generalized integrator phase locked loop (SOGI-PLL) and fictitious-axis emulator (FAE) into the control algorithm, the virtual orthogonal voltage and current signals were constructed and the mathematic model of ES-2 in the dq axis synchronous rotating reference frame was established. Then, the control system consisting of three closed loops, namely active power loop, current loop and ES voltage loop, is arranged. Among the three loops, a damped proportional resonance (PR) controller is adopted in the ES voltage loop to ensure the accurate control of the output voltage of ES-2. Instead, traditional PI controllers are used for the current and power loops. Finally, the effectiveness of the proposed decoupled power control is validated by both simulation and experimental results

Stability analysis for single phase converters in microgrids applications

Este trabajo de investigación está dirigido a la comprensión y análisis de estabilidad de los esquemas de control para convertidores monofásicos y el Phase Locked loop (PLL), con el objetivo de identificar el impacto de dichos enfoques en la dinámica del sistema de conversión. Se diseña un sistema que consiste en un convertidor de fuente de voltaje monofásico (SPVSC) interconectado a la red, en el software SIMULINK / MATLAB utilizando el modelo promediado del convertidor. Se explica una técnica de control clásico basada en el concepto de control de vectorial, además se implementa y compara con otras técnicas de control. El objetivo principal de esta tesis de maestría está orientada hacia el análisis de estabilidad de los convertidores monofásicos dentro de dos estrategias de control clásicas, control vectorial y control proporcional resonante. Finalmente, se propone una técnica de control y se aplica al SP-VSC, éste se basa en la teoría de Lyapunov, que cumple las propiedades de estabilidad de un sistema

Advanced Control Strategies for Voltage Source Converters in Microgrids and Traction Networks

Increasing concerns regarding global warming caused by greenhouse gases, which are mainly generated by conventional energy resources, e.g., fossil fuels, have created significant interest for the research and development in the field of renewable energies. Such interests are also intensified by the finitude availability of conventional energy resources. To take full benefit of renewable energy resources, e.g., wind and solar energy, interfacing power electronics devices are essential, which together with the energy resources form Distributed Generation (DG) units. If properly controlled and coordinated, the optimal and efficient operation of DG units, which are the main building block of rapidly emerging microgrid technologies, can be ensured. In fact, the optimal and efficient operation of any energy conversion systems, e.g., microgrids, traction networks, etc., necessitates some sorts of control strategies. Being structured into two main parts and exploiting two-level Voltage Source Converters (VSCs), this thesis introduces several control strategies in the context of microgrids and electrified traction networks. Although the proposed approaches of this thesis are mainly tailored for two-level VSCs, the methods are equally applicable to other converter technologies. In the first part, adopting an optimization-based loop shaping approach, a vector current control strategy for three-phase grid-tied VSCs is proposed. The proposed control strategy is able to independently regulate the direct and quadrature (dq)-components of the converter currents in a fully decoupled manner and shows very fast dynamic response similar to the existing methods. In order to extend the applicability of the proposed vector control method to single-phase systems, a countermeasure is also proposed. In single-phase systems, to form the orthogonal component of the current needed to create the dq-axes, the converter current is phase-shifted a quarter of a fundamental period. This phase-shift is the reason of strongly coupled dq-axes and oscillatory dynamic response in such systems. To obviate the need for the problematic phase-shifting, adopting a Fictive Axis Emulator (FAE), the orthogonal fictive current is created concurrent to the real one. In such a case, utilizing the proposed decoupled vector control strategy and the FAE, the dq-currents of single-phase converters are also regulated in a fully decoupled manner. Moreover, in this part, using a generalized version of the optimization-based loop shaping approach, three voltage control schemes are proposed for the voltage regulation of islanded microgrids. Since the dedicated loads of islanded microgrids are not fixed, the loop shaping is simultaneously carried out for various operating points of interests, i.e., for various combinations of the load parameters. Two single-stage control strategies and a cascade one are proposed: (i) a single-stage PI-based Multi-Input Multi-Output (MIMO) controller, (ii) a single-stage PI-based MIMO controller in conjunction with resonant terms, which is able to compensate for the adverse impacts of nonlinear loads, and (iii) a cascade PI-based MIMO controller. The cascade control scheme utilizes the proposed decoupled vector control strategy as its inner loop for regulating the converter current. In the second part, this thesis focuses on electrified traction networks and addresses a power quality problem in such networks, i.e., catenary voltage fluctuations. The Active Line-side Converter (ALC) of modern locomotives is utilized as STATic COMpensator (STATCOM) in order to inject reactive power to compensate for the adverse effects of catenary line voltage fluctuations. To determine the proper amount of reactive power, several control strategies belonging to the PI-controllers family are proposed: (i) a P-controller, (ii) a PI-controller, and (iii) a gain-scheduled PI-controller. Among the proposed approaches, the gain-scheduled strategy provides the best performance. The gain-scheduling is performed through identifying the catenary inductance at the connection point of the locomotive to that. The inductance identification is carried out by the injection of harmonic current through the ALC and monitoring its effect on the locomotive voltage. Despite its acceptable performance, the gain-scheduled approach shows several shortcomings. Therefore, utilizing the optimization-based loop shaping technique, a high-order voltage support scheme is also proposed. The proposed high-order scheme does not need any online tuning and/or modification while provides excellent performance for various operating points

Stability analysis for single phase converters in microgrids applications

Este trabajo de investigación está dirigido a la comprensión y análisis de estabilidad de los esquemas de control para convertidores monofásicos y el Phase Locked loop (PLL), con el objetivo de identificar el impacto de dichos enfoques en la dinámica del sistema de conversión. Se diseña un sistema que consiste en un convertidor de fuente de voltaje monofásico (SPVSC) interconectado a la red, en el software SIMULINK / MATLAB utilizando el modelo promediado del convertidor. Se explica una técnica de control clásico basada en el concepto de control de vectorial, además se implementa y compara con otras técnicas de control. El objetivo principal de esta tesis de maestría está orientada hacia el análisis de estabilidad de los convertidores monofásicos dentro de dos estrategias de control clásicas, control vectorial y control proporcional resonante. Finalmente, se propone una técnica de control y se aplica al SP-VSC, éste se basa en la teoría de Lyapunov, que cumple las propiedades de estabilidad de un sistema

Interconnected Modular Multilevel Converter (IMMC) Using Wide Band Gap Devices for Multiple Applications

This dissertation proposes a high-power density Interconnected Modular Multilevel Converter (IMMC) with sinusoidal output voltage for multiple applications. The proposed converter utilizes wide band gap devices at a high switching frequency to achieve compact size/weight/volume. The proposed converter is modular in construction, employs high frequency L-C components and can be stacked for voltage sharing. The IMMC is proposed for motor drives applications due to the following advantages: sinusoidal output with adjustable voltage and frequency (v/f), no acoustic noise, low EMI and absence of dv/dt related issues due to long motor leads. Two design examples for low voltage drives using Gallium Nitride (GaN) devices and medium voltage drives using Silicon Carbide (SiC) are discussed in this dissertation. The proposed converter is also evaluated for solar micro-inverter applications due to its compact size and the high-quality output. The proposed system connects the inverter to the PV solar panel through a flyback converter for stepping up the voltage to the grid level, isolation and Maximum Power Point Tracking (MPPT). The proposed inverter eliminates the need for a bulky grid-tie inductor or complex LCL filter. The power can be injected to the grid using a small iron-core inductor due to the sinusoidal nature of the output voltage. A grid-tie control using Fictive Axis Emulation (FAE) is implemented on the converter to optimize the power injected to the grid. Moreover, a DC-AC IMMC to integrate two PV power plants through medium voltage DC collection grid (MVDC) system is proposed. The sinusoidal output of the IMMC facilitates the integration of the solar plants. The inductance required to connect the inverter to the grid is less due to the sinusoidal nature of the output of the IMMC

Modular Multilevel Converters with Integrated Split Battery Energy Storage

The electric power grid is undergoing significant changes and updates nowadays, especially on a production and transmission level. Initially, the move towards a distributed generation in contrast to the existing centralized one implies a significant integration of renewable energy sources and electricity storage systems. In addition, environmental awareness and related concerns regarding pollutant emissions have given rise to a high interest in electrical mobility. Advanced power electronics interfacing systems are expected to play a key role in the development of such modern controllable and efficient large-scale grids and associated infrastructures. During the last decade, a global research and development interest has been stimulated in the field of modular multilevel conversion, due to the well-known offered advantages over conventional solutions in the medium- and high-voltage and power range. In the context of battery energy storage systems, the Modular Multilevel Converter (MMC) family exhibits an additional attractive feature, i.e., the capability of embedding such storage elements in a split manner, given the existence of several submodules operating at significantly lower voltages. This thesis deals with several technical challenges associated with Modular Multilevel Converters as well as their enhancement with battery energy storage elements. Initially, the accurate submodule capacitor voltage ripple estimation for a DC/AC MMC is derived analytically, avoiding any strong assumptions. This is beneficial for converter dimensioning purposes as well as for the implementation improvement of several control schemes, which have been proposed in the literature. The impact of unbalanced grid conditions on the operation and design of an MMC is then investigated, drawing important conclusions regarding the choice of line current control and required capacitive storage energy during grid faults. [...

Power quality improvements of single-phase grid-connected photovoltaic systems

PhD ThesisThe number of distributed power generation systems (DPGSs), mostly based on photovoltaic (PV) energy sources is increasing exponentially. These systems must conform to grid codes to ensure appropriate power quality and to contribute to grid stability. A robust and reliable synchronization to the grid is an important consideration in such systems. This is due to the fact that, fast and accurate detection of the grid voltage parameters is essential in order to implement stable control strategies under a broad range of grid conditions. The second-order generalized integrator (SOGI) based phase-locked loop (PLL) is widely used for grid synchronization of single-phase power converters. This is because it offers a simple, robust and flexible solution for grid synchronization. However, the SOGI-PLL is affected by the presence of a dc offset in the measured grid voltage. This dc voltage offset is typically introduced by the measurements and data conversion process, and causes fundamental-frequency ripple in the estimated parameters of the grid voltage (i.e. voltage amplitude, phase angle and frequency). In addition to this ripple, the unit amplitude sine and cosine signals of the estimated phase angle (i.e. unit vectors), that are used to generate reference signals in the closed-loop control of grid-connected PV converters will contain dc offset. This is highly undesirable since it can cause dc current injection to the grid, and as a consequence, the quality of the power provided by the DPGSs can be degraded. To overcome this drawback, a modified SOGI-PLL with dc offset rejection capability is proposed. The steady-state, transient and harmonic attenuation performance of the proposed PLL scheme are validated through simulation and experimental tests. The overall performance demonstrates the capability of the proposed PLL to fully reject such dc current injection as well as to provide a superior harmonic attenuation when compared with the SOGIPLL and two other existing offset rejection approaches. It is shown that, the proposed PLL scheme can enhance the overall total harmonic distortion (THD%) of the injected power by 15% when compared to the conventional SOGI-PLL. In addition to the synchronization, grid-connected PV systems require a current control scheme to regulate the output current. Due to the simple implementation, proportional-integral (PI) controllers in the stationary reference frame are commonly used for current controlled inverters. However, these PI-controllers exhibit a major drawback of failure to track a sinusoidal reference Abstract ii without steady-state error, which may result in low-order harmonics. This drawback can be overcome if the PI-controllers are implemented in direct-quadrature (dq) rotating reference frame. In single-phase systems, the common approach is to create a synthesized phase signal orthogonal to the fundamental of the real single-phase system so as to obtain dc quantities by means of a stationary-to-rotating reference frame. The orthogonal synthesized signal in conventional approaches is obtained by phase shifting the real signal by a quarter of the fundamental period. The introduction of such delay in the system deteriorates the dynamic response, which becomes slower and oscillatory. This thesis proposes an alternative way of implementing such PI-controllers in the dq reference frame without the need of creating such orthogonal signals. The proposed approach, effectively improves the poor dynamic of the conventional approaches while not adding excessive complexity to the controller structure. The results show that, in addition to its ability to regulate the current and achieve zero steady-state error, the proposed approach shows superior dynamic response when compared with that of conventional delay-based approach.Libyan Governmen

Design and control of a multicell interleaved converter for a hybrid photovoltaic-wind generation system

The solution for the generating energy derived from non-polluting sources configures a worldwide problem, which is undetermined, complex, and gradual; and certainly, passes through the diversification of the energetic matrix. Diversification means not only having different sources converted into useful energy, like the electricity, but also decentralizing the energy generation in order to fit with higher adequacy the demand, which is decentralized too. Distributed Generation proposes this sort of development but in order to increase its penetration several technical barriers must be overpassed. One of them is related to the conversion systems, which must be more flexible, modular, efficient and compatible with the different energy sources, since they are very specific for a certain area. The present study drives its efforts towards this direction, i.e. having a system with several inputs for combining different renewable energy sources into a single and efficient power converter for the grid connection. It focuses on the design and control of an 11.7 kW hybrid renewable generation system, which contains two parallel circuits of photovoltaic panels and a wind turbine. A multicell converter divided in two stages accomplishes the convertion: Generation Side Converter (GSC) and Mains Side Converter (MSC). Two boost converters responsible for the photovoltaic generation and a rectifier and a third boost, for the wind constitue the GSC. It allows the conversion to the fixed output DC voltage, controlling individually and performing the maximum power point tracking in each input. On the other side, the single-phase 4- cell MSC accomplishes the connection to the grid through an LCL filter. This filter uses an Intercell Transformer (ICT) in the first inductor for reducing the individual ripple generated by the swicthing. The MSC controls the DC-link voltage and, by doing that, it allows the power flow from the generation elements to the network

Design and Control of Power Converters 2019

In this book, 20 papers focused on different fields of power electronics are gathered. Approximately half of the papers are focused on different control issues and techniques, ranging from the computer-aided design of digital compensators to more specific approaches such as fuzzy or sliding control techniques. The rest of the papers are focused on the design of novel topologies. The fields in which these controls and topologies are applied are varied: MMCs, photovoltaic systems, supercapacitors and traction systems, LEDs, wireless power transfer, etc