Predicting stochastic harmonics of multiple converters in a power system (microgrid)

The microgrid concept integrates Renewable Energy Systems (RES) to the Electrical Power System (EPS) as a means to produce clean energy, meet consumer energy demands and preserve the depleting fossil fuels reserves. These RES are usually interfaced to the grid using power electronic converters (such as Voltage Source Converters) to achieve the required control and conversion of power. Nevertheless, Voltage Source Converters (VSCs) produce both current and voltage harmonics which negatively impact on the Power Quality (PQ) of a microgrid and may cause damage or malfunctions of equipment. This thesis focuses on the impact of VSC harmonics on the power quality of a microgrid. It also investigates various factors that affect the harmonics generated by VSCs with the aim of predicting their impact on the PQ of the microgrid. The PQ of the microgrid is represented as a measure of the level of harmonic distortion of the voltage and current at the Point of Common Coupling (PCC) to the grid. The harmonic mean was used as a measure to determine if the VSCs harmonic level meets the IEEE Standard 519 harmonic limits. The level of harmonic distortion of many VSCs can be significantly affected and difficult to predict in the presence of uncertainties, which may arise due to design parameter choice or system parameter changes. This necessitates the use of statistical techniques to quantify VSC harmonic distortion level in the presence of uncertainties. A common statistical approach is to employ Monte Carlo Simulation (MCS), although accurate it is time consuming and burdensome for systems containing a large number of variables. This thesis utilizes the Univariate Dimension Reduction (UDR) technique formulated from an enhanced Unscented Transform (UT) equation in predicting the harmonic distortion level of large numbers of VSCs in a microgrid, when some system or design parameters are only known within certain constraints. The UDR technique drastically reduce the computation time and burden associated with the MCS approach and avoids assumptions that leads to system simplification required to implement other analytical methods. Various microgrid configuration and statistical distributions similar to practical system variations of RES are considered in order to achieve a good evaluation of the UDR performance in predicting the VSC harmonics. The UDR performance was also evaluated experimentally using a practical microgrid lab containing 3 VSCs. The MCS approach was used as a benchmark for the predicted UDR results. In all cases the UDR predicted results were obtained with significant time saved as compared to the MCS approach and the UDR results showed a good match with the MCS approach

Predicting stochastic harmonics of multiple converters in a power system (microgrid)

The microgrid concept integrates Renewable Energy Systems (RES) to the Electrical Power System (EPS) as a means to produce clean energy, meet consumer energy demands and preserve the depleting fossil fuels reserves. These RES are usually interfaced to the grid using power electronic converters (such as Voltage Source Converters) to achieve the required control and conversion of power. Nevertheless, Voltage Source Converters (VSCs) produce both current and voltage harmonics which negatively impact on the Power Quality (PQ) of a microgrid and may cause damage or malfunctions of equipment. This thesis focuses on the impact of VSC harmonics on the power quality of a microgrid. It also investigates various factors that affect the harmonics generated by VSCs with the aim of predicting their impact on the PQ of the microgrid. The PQ of the microgrid is represented as a measure of the level of harmonic distortion of the voltage and current at the Point of Common Coupling (PCC) to the grid. The harmonic mean was used as a measure to determine if the VSCs harmonic level meets the IEEE Standard 519 harmonic limits. The level of harmonic distortion of many VSCs can be significantly affected and difficult to predict in the presence of uncertainties, which may arise due to design parameter choice or system parameter changes. This necessitates the use of statistical techniques to quantify VSC harmonic distortion level in the presence of uncertainties. A common statistical approach is to employ Monte Carlo Simulation (MCS), although accurate it is time consuming and burdensome for systems containing a large number of variables. This thesis utilizes the Univariate Dimension Reduction (UDR) technique formulated from an enhanced Unscented Transform (UT) equation in predicting the harmonic distortion level of large numbers of VSCs in a microgrid, when some system or design parameters are only known within certain constraints. The UDR technique drastically reduce the computation time and burden associated with the MCS approach and avoids assumptions that leads to system simplification required to implement other analytical methods. Various microgrid configuration and statistical distributions similar to practical system variations of RES are considered in order to achieve a good evaluation of the UDR performance in predicting the VSC harmonics. The UDR performance was also evaluated experimentally using a practical microgrid lab containing 3 VSCs. The MCS approach was used as a benchmark for the predicted UDR results. In all cases the UDR predicted results were obtained with significant time saved as compared to the MCS approach and the UDR results showed a good match with the MCS approach

Power Converter of Electric Machines, Renewable Energy Systems, and Transportation

Power converters and electric machines represent essential components in all fields of electrical engineering. In fact, we are heading towards a future where energy will be more and more electrical: electrical vehicles, electrical motors, renewables, storage systems are now widespread. The ongoing energy transition poses new challenges for interfacing and integrating different power systems. The constraints of space, weight, reliability, performance, and autonomy for the electric system have increased the attention of scientific research in order to find more and more appropriate technological solutions. In this context, power converters and electric machines assume a key role in enabling higher performance of electrical power conversion. Consequently, the design and control of power converters and electric machines shall be developed accordingly to the requirements of the specific application, thus leading to more specialized solutions, with the aim of enhancing the reliability, fault tolerance, and flexibility of the next generation power systems

Advance control of multilevel converters for integration of distributed generation resources into ac grid

Premi extraordinari doctorat curs 2011-2012, àmbit d’Enginyeria IndustrialDistributed generation (DG) with a converter interface to the power grid is found in many of the green power resources applications. This dissertation describes a multi-objective control technique of voltage source converter (VSC) based on multilevel converter topologies, for integration of DG resources based on renewable energy (and non-renewable energy)to the power grid. The aims have been set to maintain a stable operation of the power grid, in case of di erent types of grid-connected loads. The proposed method provides compensation for active, reactive, and harmonic load current components. A proportional-integral (PI) control law is derived through linearization of the inherently non-linear DG system model, so that the tasks of current control dynamics and dc capacitor voltage dynamics become decoupled. This decoupling allows us to control the DG output currents and the dc bus voltage independently of each other, thereby providing either one of these decoupled subsystems a dynamic response that signi cantly slower than that of the other. To overcome the drawbacks of the conventional method, a computational control delay compensation method, which delaylessly and accurately generates the DG reference currents, is proposed. The rst step is to extract the DG reference currents from the sensed load currents by applying the stationary reference frame and then transferred into synchronous reference frame method, and then, the reference currents are modi ed, so that the delay will be compensated. The transformed variables are used in control of the multilevel voltage source converter as the heart of the interfacing system between DG resources and power grid. By setting appropriate compensation current references from the sensed load currents in control circuit loop of DG link, the active, reactive, and harmonic load current components will be compensated with fast dynamic response, thereby achieving sinusoidal grid currents in phase with load voltages while required power of loads is more than the maximum injected power of the DG resources. The converter, which is controlled by the described control strategy, guarantees maximum injection of active power to the grid continuously, unity displacement power factor of power grid, and reduced harmonic load currents in the common coupling point. In addition, high current overshoot does not exist during connection of DG link to the power grid, and the proposed integration strategy is insensitive to grid overload.La Generació Distribuïda (DG) injectada a la xarxa amb un convertidor estàtic és una solució molt freqüent en l'ús de molts dels recursos renovables. Aquesta tesis descriu una técnica de control multi-objectiu del convertidor en font de tensió (VSC), basat en les topologies de convertidor multinivell, per a la integració de les fonts distribuïdes basades en energies renovables i també de no renovables.Els objectius fixats van encaminats a mantenir un funcionament estable de la xarxa elèctrica en el cas de la connexió de diferents tipus de càrregues. El mètode de control proposat ofereix la possibilitat de compensació de les components actives i reactives de la potencia, i les components harmòniques del corrent consumit per les càrregues.La llei de control proporcional-Integral (PI) s’obté de la linearització del model inherentment no lineal del sistema, de forma que el problema de control del corrent injectat i de la tensió d’entrada del convertidor queden desacoblats. Aquest desacoblament permet el control dels corrents de sortida i la tensió del bus de forma independent, però amb un d’ells amb una dinàmica inferior.Per superar els inconvenients del mètode convencional, s’usa un retard computacional, que genera les senyals de referència de forma acurada i sense retard. El primer pas es calcular els corrents de referència a partir de les mesures de corrent. Aquest càlcul es fa primer transformant les mesures a la referència estacionaria per després transformar aquests valors a la referència síncrona. En aquest punt es on es poden compensar els retards.Les variables transformades son usades en els llaços de control del convertidor multinivell. Mitjançant aquests llaços de control i les referències adequades, el convertidor és capaç de compensar la potencia activa, reactiva i els corrents harmònics de la càrrega amb una elevada resposta dinàmica, obtenint uns corrents de la xarxa de forma completament sinusoïdal, i en fase amb les tensions.El convertidor, controlat amb el mètode descrit, garanteix la màxima injecció de la potencia activa, la injecció de la potencia reactiva per compensar el factor de potencia de la càrrega, i la reducció de les components harmòniques dels corrents consumits per la càrrega. A més, garanteix una connexió suau entre la font d’energia i la xarxa. El sistema proposat es insensible en front de la sobrecarrega de la xarxaAward-winningPostprint (published version

Transformerless Grid-Tied Impedance Source Inverters for Microgrids

Renewable energy source (RESs) diffusion into the power system is continuously increasing, where the world cumulative installed capacity of solar and wind energy sources increased from around 63.2 GW in 2005 to around 903.1 GW in 2017 according to International Renewable Energy Agency (IRENA). The energy utilization from these RESs implies the use of what is called power conditioning stage (PCS). Such PCS acts as an interfacing layer between the RES side and the customer side, i. e. the load or the grid. These PCSs can utilize many different configurations depending on the employed RES, where the two-stage architecture is commonly used with solar photovoltaic (PV) systems due to the low or variable output voltage. Such two-stage architecture is usually implemented using a boost converter in order to regulate the PV source output voltage and maximize the output power, and a voltage source inverter (VSI) in order to achieve the inversion operation. On the other hand, impedance source inverters represent a different family of the existing PCSs, which are called single-stage power converters as they embraces the boosting capability within the inversion operation. This family of PCSs is seen as an interesting and competitive alternative to the twostage configuration, which are mandatory for low or variable voltage energy sources, such as PV and fuel cell energy sources. Therefore, these impedance source inverters have been utilized in many different applications, such as distributed generation and electric vehicles. This family of PCSs, i. e. impedance source inverters, has experienced a fast evolution during the last few years in order to replace the conventional two-stage architecture since the first release of the three-phase Zsource inverter (ZSI) in 2003. Consequently, many research activities have been established in order to improve the ZSIs performance from different perspectives, such as overall voltage gain, voltage stresses across the different devices, continuity of the input current, and conversion efficiency. Among these different topological improvements, the conventional ZSI and the quasi-ZSI (qZSI), are the most commonly used structures. Accordingly, the objective of this thesis is to study and reinforce the performance of this family of PCSs. Hence, the work in this thesis starts first by addressing the challenges behind eliminating the low frequency transformer in grid-tied PV systems in order to improve the conversion system efficiency, where a new measurement technique for the dc current component is proposed in order to effectively mitigate this dc current component. Then, the performance of the classical impedance source inverters has been assessed by studying all the possible modulation schemes and proposing a new one, under which the efficiency of these classical impedance source inverters have been improved. Furthermore, the partial-load operation of these impedance source inverters, considering the three-phase qZSI, has been studied and the possible ways of achieving a wide range of operation have been investigated. Due to the seen demerits behind the classical impedance source inverters, an alternative new topology, which is called split-source inverter (SSI), is proposed, under which these demerits have effectively been mitigated or eliminated. Then, the challenges behind grid-tied operation of this single-stage dc-ac power converters has been investigated considering the SSI topology. It is worth to note that all the prior mentioned contributions have been validated experimentally. Finally, this thesis is divided into two chapters, where the first chapter introduces an extended summary of the work done concerning the thesis topic, while the second part includes some selected papers from the publications that have been developed during the doctoral study. These selected papers give all the details of the work done in each section in the extended summary

Applications and Experiences of Quality Control

The rich palette of topics set out in this book provides a sufficiently broad overview of the developments in the field of quality control. By providing detailed information on various aspects of quality control, this book can serve as a basis for starting interdisciplinary cooperation, which has increasingly become an integral part of scientific and applied research