Preface

Everything we consume or use requires energy to produce and package, to distribute to shops or front doors, to operate, and then to be got rid of<sup>i</sup> . The global demand for energy is expected to increase but conventional energy sources are limited and have carbon emissions to the environment. The utilization of renewable energy sources such as wind energy, or solar energy, among others, is currently of greater interest. Nevertheless, since their availability is arbitrary and unstable this can lead to frequency variation, to grid instability and to a total or partial loss of load power supply. On the other hand, the presence of a static converter as output interface of the generating plants introduces voltage and current harmonics into the electrical system that negatively affect system power quality, which is dealt with in another book. By integrating distributed power generation systems closed to the loads in the electric grid, we can eliminate the need to transfer energy over long distances through the electric grid. Moreover, they are an alternative source of energy to meet rapidly increasing energy consumption, but they are not appropriate to be directly connected to the main utility grid.

At the same time, they have led to electrical problems. The non-linear components that are used in electronic equipment and other charges can considerably affect the efficiency of an electrical system. An analysis and understanding of the electrical problems and the damage that can be generated to the systems and infrastructures are at the focus of attention due to the progress and introduction of digital systems. Ensuring optimal power quality after a good design and good devices, means productivity, efficiency, competitiveness and profitability.

In the following chapters the reader will be introduced to different power generation and distribution systems with an analysis of some types of existing disturbances and a study of different industrial applications such as battery charges; additionally, different software tools developed for power quality understanding, evaluation or monitoring are analyzed. The book is divided into four sections: power generation and distribution systems, disturbances and voltage sag, software tools, and industrial applications; a brief discussion of each chapter is as follows.

i Worldwatch Institute.

Chapter 1 evaluates the possible drawbacks to power-supply stability and quality regarding the increasing number of renewable energy sources, both in grid-connected and stand-alone configurations, in order to prevent possible problems by using a proper design and management of these generation units. A first solution to a renewable source equipped with an energy storage system in stand-alone is presented; secondly, a grid connected application of diffused hybrid generation units comprising a wind turbine, a photovoltaic generator and a battery bank is analysed. From the study of both solutions some guidelines for the design of renewable generation units are obtained, focusing on the aspect that influences the power quality of the electrical system. Chapter 2 studies a combined flexible wind source-based model, considered as not having the capability to control voltages, and shunt Flexible AC Transmission Systems (FACTS) devices. This model is proposed to dynamically adjust the active power delivered from the wind source and the reactive power exchanged between the shunt FACTS and the network to enhance power quality. To do this, dynamic shunt compensators (STATCOM) modelled as a Solar Photovoltaic (PV) node are used to control the voltage by a flexible adjustment of the reactive power exchanged with the network. Chapter 3 presents the modelling and simulation of PV-based inverter systems. PV based-operating conditions are concerned with the real situation, such as the input effects of radiation and temperature on voltage and PV current and load and grid variation. In order to build models for nonlinear devices without prior information, system identification methods are set out, using only measured input and output waveforms to determine the model parameters by a Hammerstein-Wiener nonlinear model system identification process. After obtaining appropriate models, an analysis and prediction of power quality is carried out, taking account of steady state and transient condition from different scenarios. Chapter 4 presents a mathematical model of external interventions upon a system henceforth called Renewal Processes. It has been done after basic research in the field of operational research and maintenance management, with contributions and applications in optimization strategies of Reliability Based Maintenance (RCM) for Electrical Distribution Systems (EDS). These are performed in order to reduce the life-cycle cost of the electrical installations and prevent maintenance strategies belonging to RCM and solve the technical and economic objectives of the exploitation of distribution systems and systems that use electrical energy. Chapter 5 introduces the Distributed Power Generation System (DPGS) as an alternative source of energy to meet rapidly increasing energy consumption, but not suitable to be connected directly to the main utility grid. In the chapter a virtual grid flux oriented vector control (outer loop controller) is proposed together with three different types of current controllers (inner current loop) hysteresis current controller, current regulated delta modulator and modified ramp type current controller - focusing on DC link voltage control, harmonic distortion, constant switching frequency and the unity power factor operation of the inverter. While the inner control loop controls the active and reactive grid current components, the outer control loop determines the active current reference by controlling the direct voltage.

Preface XI

Chapter 6 presents different types of Power Quality disturbances (Voltage sag, Interruption, transient overvoltage, swell, Harmonic issues and voltage imbalance) and presents statistical voltage sag indices used for the characterization and assessment of power quality, highlighting their importance. Furthermore, a mathematical voltage sag indice applicable to power quality improvement through optimization techniques is developed and verified. Finally, the impact on the voltage profile of heavy induction motor load switching is predicted, and the possibility to mitigate potential power quality violations before they occur is created. Chapter 7 presents a description of the disturbance problem existing in high speed lines and why it is important to detect and apply measures in order to mitigate or eliminate those effects, providing some guidelines in order to lessen the effects that disturbances can cause on conventional DC lines. Described next is the methodology used to measure the effect of the disturbances in conventional facilities and equipment, and some results from real measurement campaigns are shown to enhance the understanding of

Chapter 8 develops a virtual Low Voltage (LV) grid laboratory to evaluate some power quality indicators, including power electronics-based models to guarantee a more realistic representation of the most significant loads connected to the LV grid. The simulated microgenerators are represented as Voltage Source Inverters (VSI) and may be controlled to guarantee near unity power factor (conventional mG), local compensation of reactive power and harmonics (active μG). By comparing the results obtained by using conventional μG, it is shown that active μG have the capability to guarantee an overall Power Quality improvement, allowing a voltage Total Harmonic Distortion (THD) decrease and Power Factor increase. Chapter 9 presents an interactive educational tool using graphic user interface-based Matlab, which has been developed to carry out comprehensive studies on FACTS devices, to understand the basic principle of FACTS devices, and to determine the role that FACTS technology may play in improving power quality. By using this tool, the user understands the effects of the introduction of different FACTS devices like shunt Controllers (SVC, STATCOM), series Controllers (TCSC, SSSC) and hybrid Controllers (UPFC) on a practical network under normal and abnormal situations. The problems of power quality concerned with either voltage or current frequency deviation have increasingly caused a failure or a malfunction of the end user equipment; for this reason, Chapter 10 develops the idea of a power quality monitoring system via the Ethernet Network based on the embedded system with two selected ARM7 microcontrollers, which is analyzed so as to have the power quality monitoring done fluidly and completely. Finally, experimental results are shown displaying the fault signals from AC lines

while being in operation or the process being done afterwards.

Chapter 11 presents the basic issues of switch-mode rectifiers for achieving better performance. After the introduction of the commonly referred harmonic standard, the possible power factor correction approaches are described to comprehend their comparative features, with special interest in the operation principle and some key

the disturbances.

Chapter 6 presents different types of Power Quality disturbances (Voltage sag, Interruption, transient overvoltage, swell, Harmonic issues and voltage imbalance) and presents statistical voltage sag indices used for the characterization and assessment of power quality, highlighting their importance. Furthermore, a mathematical voltage sag indice applicable to power quality improvement through optimization techniques is developed and verified. Finally, the impact on the voltage profile of heavy induction motor load switching is predicted, and the possibility to mitigate potential power quality violations before they occur is created. Chapter 7 presents a description of the disturbance problem existing in high speed lines and why it is important to detect and apply measures in order to mitigate or eliminate those effects, providing some guidelines in order to lessen the effects that disturbances can cause on conventional DC lines. Described next is the methodology used to measure the effect of the disturbances in conventional facilities and equipment, and some results from real measurement campaigns are shown to enhance the understanding of the disturbances.

X Preface

voltage.

Chapter 1 evaluates the possible drawbacks to power-supply stability and quality regarding the increasing number of renewable energy sources, both in grid-connected and stand-alone configurations, in order to prevent possible problems by using a proper design and management of these generation units. A first solution to a renewable source equipped with an energy storage system in stand-alone is presented; secondly, a grid connected application of diffused hybrid generation units comprising a wind turbine, a photovoltaic generator and a battery bank is analysed. From the study of both solutions some guidelines for the design of renewable generation units are obtained, focusing on the aspect that influences the power quality of the electrical system. Chapter 2 studies a combined flexible wind source-based model, considered as not having the capability to control voltages, and shunt Flexible AC Transmission Systems (FACTS) devices. This model is proposed to dynamically adjust the active power delivered from the wind source and the reactive power exchanged between the shunt FACTS and the network to enhance power quality. To do this, dynamic shunt compensators (STATCOM) modelled as a Solar Photovoltaic (PV) node are used to control the voltage by a flexible adjustment of the reactive power exchanged with the network. Chapter 3 presents the modelling and simulation of PV-based inverter systems. PV based-operating conditions are concerned with the real situation, such as the input effects of radiation and temperature on voltage and PV current and load and grid variation. In order to build models for nonlinear devices without prior information, system identification methods are set out, using only measured input and output waveforms to determine the model parameters by a Hammerstein-Wiener nonlinear model system identification process. After obtaining appropriate models, an analysis and prediction of power quality is carried out, taking account of steady state and transient condition from different scenarios. Chapter 4 presents a mathematical model of external interventions upon a system henceforth called Renewal Processes. It has been done after basic research in the field of operational research and maintenance management, with contributions and applications in optimization strategies of Reliability Based Maintenance (RCM) for Electrical Distribution Systems (EDS). These are performed in order to reduce the life-cycle cost of the electrical installations and prevent maintenance strategies belonging to RCM and solve the technical and economic objectives of the exploitation of distribution systems and systems that use electrical energy. Chapter 5 introduces the Distributed Power Generation System (DPGS) as an alternative source of energy to meet rapidly increasing energy consumption, but not suitable to be connected directly to the main utility grid. In the chapter a virtual grid flux oriented vector control (outer loop controller) is proposed together with three different types of current controllers (inner current loop) hysteresis current controller, current regulated delta modulator and modified ramp type current controller - focusing on DC link voltage control, harmonic distortion, constant switching frequency and the unity power factor operation of the inverter. While the inner control loop controls the active and reactive grid current components, the outer control loop determines the active current reference by controlling the direct

Chapter 8 develops a virtual Low Voltage (LV) grid laboratory to evaluate some power quality indicators, including power electronics-based models to guarantee a more realistic representation of the most significant loads connected to the LV grid. The simulated microgenerators are represented as Voltage Source Inverters (VSI) and may be controlled to guarantee near unity power factor (conventional mG), local compensation of reactive power and harmonics (active μG). By comparing the results obtained by using conventional μG, it is shown that active μG have the capability to guarantee an overall Power Quality improvement, allowing a voltage Total Harmonic Distortion (THD) decrease and Power Factor increase. Chapter 9 presents an interactive educational tool using graphic user interface-based Matlab, which has been developed to carry out comprehensive studies on FACTS devices, to understand the basic principle of FACTS devices, and to determine the role that FACTS technology may play in improving power quality. By using this tool, the user understands the effects of the introduction of different FACTS devices like shunt Controllers (SVC, STATCOM), series Controllers (TCSC, SSSC) and hybrid Controllers (UPFC) on a practical network under normal and abnormal situations. The problems of power quality concerned with either voltage or current frequency deviation have increasingly caused a failure or a malfunction of the end user equipment; for this reason, Chapter 10 develops the idea of a power quality monitoring system via the Ethernet Network based on the embedded system with two selected ARM7 microcontrollers, which is analyzed so as to have the power quality monitoring done fluidly and completely. Finally, experimental results are shown displaying the fault signals from AC lines while being in operation or the process being done afterwards.

Chapter 11 presents the basic issues of switch-mode rectifiers for achieving better performance. After the introduction of the commonly referred harmonic standard, the possible power factor correction approaches are described to comprehend their comparative features, with special interest in the operation principle and some key issues of Switch-Mode Rectifiers (SMR). Then, the prominences of three single-phase boost SMRs are comparatively evaluated experimentally in serving as front-end AC/DC converters of a Permanent-Magnet Synchronous Motor (PMSM) drive; the results indicate that for all cases, the close winding current tracking performances are obtained, and thus good line-drawn power quality characteristics are achieved. Finally, an SMR-fed Switched-Reluctance Motor (SRM), an SMR-based electric vehicle battery charger and a flyback SMR-based battery plug-in charger are presented to further comprehend the advantages of using SMR. Chapter 12 analyzes a simple and improved battery charger system with power quality improvement function, which solves a configuration where the capacitor is installed in parallel with the battery terminals by a proposed equal charge concept. In an unsolved configuration if anybody connects the battery to the charging terminals, a surge discharge current from the capacitor occurs, damaging the circuitry, connectors and battery due to the high current. By using the principle of operation associated to the equal charge concept, the idea is to bring the potential difference between the charger and the battery to zero before the electrical connection and as a result there will be no surge current flow. The circuit has been simplified by integrating a twoswitch DC/DC converter with a fullbridge converter / inverter and using only one inductor. Finally, to verify the power quality capability of the proposed battery charger, a series of simulations have been carried out.

We hope the different sections of this book will have been to your interest and liking, and that we will have succeeded in bringing across what the scientific community is doing in the field of Power Quality.

Lastly, we would like to thank all the researchers for their excellent works and studies in the different areas of Power Quality.

> **Gregorio Romero Rey Mª Luisa Martinez Muneta** Universidad Politécnica de Madrid Spain
