Power Quality Improvement and Low Voltage Ride through Capability in Hybrid Wind-PV Farms Grid-Connected Using Dynamic Voltage Restorer

© 2018 IEEE. Translations and content mining are permitted for academic research only. Personal use is also permitted, but republication/redistribution requires IEEE permission.This paper proposes the application of a dynamic voltage restorer (DVR) to enhance the power quality and improve the low voltage ride through (LVRT) capability of a three-phase medium-voltage network connected to a hybrid distribution generation system. In this system, the photovoltaic (PV) plant and the wind turbine generator (WTG) are connected to the same point of common coupling (PCC) with a sensitive load. The WTG consists of a DFIG generator connected to the network via a step-up transformer. The PV system is connected to the PCC via a two-stage energy conversion (dc-dc converter and dc-ac inverter). This topology allows, first, the extraction of maximum power based on the incremental inductance technique. Second, it allows the connection of the PV system to the public grid through a step-up transformer. In addition, the DVR based on fuzzy logic controller is connected to the same PCC. Different fault condition scenarios are tested for improving the efficiency and the quality of the power supply and compliance with the requirements of the LVRT grid code. The results of the LVRT capability, voltage stability, active power, reactive power, injected current, and dc link voltage, speed of turbine, and power factor at the PCC are presented with and without the contribution of the DVR system.Peer reviewe

Offshore Wind Farm-Grid Integration: A Review on Infrastructure, Challenges, and Grid Solutions

Recently, the penetration of renewable energy sources (RESs) into electrical power systems is witnessing a large attention due to their inexhaustibility, environmental benefits, storage capabilities, lower maintenance and stronger economy, etc. Among these RESs, offshore wind power plants (OWPP) are ones of the most widespread power plants that have emerged with regard to being competitive with other energy technologies. However, the application of power electronic converters (PECs), offshore transmission lines and large substation transformers result in considerable power quality (PQ) issues in grid connected OWPP. Moreover, due to the installation of filters for each OWPP, some other challenges such as voltage and frequency stability arise. In this regard, various customs power devices along with integration control methodologies have been implemented to deal with stated issues. Furthermore, for a smooth and reliable operation of the system, each country established various grid codes. Although various mitigation schemes and related standards for OWPP are documented separately, a comprehensive review covering these aspects has not yet addressed in the literature. The objective of this study is to compare and relate prior as well as latest developments on PQ and stability challenges and their solutions. Low voltage ride through (LVRT) schemes and associated grid codes prevalent for the interconnection of OWPP based power grid have been deliberated. In addition, various PQ issues and mitigation options such as FACTS based filters, DFIG based adaptive and conventional control algorithms, ESS based methods and LVRT requirements have been summarized and compared. Finally, recommendations and future trends for PQ improvement are highlighted at the end

Análise de ressonância em sistemas de potência com parques eólicos baseados em GIDA

Orientador: Walmir de Freitas FilhoDissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Elétrica e de ComputaçãoResumo: O incremento da penetracao de parques eolicos (PE) baseados em geradores de inducao duplamente alimentados (GIDA) no sistema de potencia eletrica pode trazer problemas de qualidade de energia relacionados com ressonancia de alta frequencia no intervalo de 180 Hz e 1500 Hz. Dois problemas potenciais sao analisados: Ressonancia instavel pela carateristica de amortecimento negativo dos conversores tipo fonte de tensao (VSC); e ressonancias mal amortecidas pela excitacao de componentes de distorcao de tensao na rede proximos a 5...., 7...., 11.... e 13.... harmonicas. Estas ressonancias sao estudadas com numerosas simulacoes de transitorios eletromagneticos (TEM) ja que levam em conta as carateristicas dinamicas do circuito mas com alto nivel de detalhe e custo computacional. Este documento apresenta o desenvolvimento de metodos simplificados para estudar as ressonancias. Inicialmente, esta dissertacao confirma que em frequencias entre 180 Hz e 1500 Hz, o GIDA pode-se modelar como uma impedancia linear com o modelo average dos VSCs. Posteriormente, desenvolve-se um grafico para identificar rapidamente se um parque eolico vira instavel. Este grafico estabelece uma regiao de risco: se as caracteristicas do parque eolico e da rede (relacao de curto circuito e relacao de potencia compensacao reativa) estao localizadas dentro desta regiao, o parque pode virar instavel. Este problema pode-se mitigar com uma escolha adequada de parametros de controle, e por tanto, nao deveria representar um problema geral para o operador de rede. Mesmo assim, a ressonancia mal amortecida pode ocorrer e deve-se considerar ao operar e projetar PEs baseados em GIDA. Dois graficos adicionais sao propostos para analisar o risco de ressonancias mal amortecidas no sistema. Os graficos estabelecem a regiao de risco correlacionando o nivel de curto-circuito da rede no ponto de acoplamento comum, com a capacidade nominal do parque eolico e o nivel de compensacao de potencia reativa. Dois parametros delimitam a regiao: A frequencia de ressonancia, e a amplificacao de tensao com respeito a distorcoes harmonicas na rede. Os tres graficos propostos nesta dissertacao podem-se obter analiticamente com os modelos de impedancia dos PEs sem necessidade de simulacao. Um metodo baseado em medicao tambem foi desenvolvido para obter os graficos sem conhecimento previo das caracteristicas do parque. Estes metodos para obter os graficos foram validados com simulacoes detalhadas de transitorios eletromagneticos. Os graficos propostos podem serfacilmente obtidos e consultados por engenheiros, e apresentam grande potencial para facilitar a analise de ressonancia em sistemas com PEsAbstract: The increase of the penetration of wind parks (WP) based on doubly-fed induction generators (DFIG) in electric power systems may bring problems associated with high-frequency resonances in the range between 180 Hz and 1500 Hz. There are two main phenomena to be analyzed: unstable resonances, when a resonance occurs in frequencies where the DFIG voltage source converters (VSC) have negative damping characteristic; and weakly damped resonances close to frequencies such as 5th, 7th, 11th and 13th harmonics, which are typically present as background grid distortions. Traditionally, these resonances are studied with electromagnetic transient (EMT) simulations as they are able to account for all dynamic characteristics of the circuit. However, numerous EMT simulations are generally required in the studies, which leads to high computational costs. This dissertation develops simplified approaches to address these resonances. Prior to developing these methods, it is first confirmed that, at frequencies between 180 Hz and 1500 Hz, the DFIG can be modeled as a linear impedance using the average model of the VSCs. Then, a chart is developed to quickly identify if a WP can become unstable. This chart establishes a risk region: if WP and grid characteristics (short-circuit level to wind park size ratio and reactive compensation to wind park size ratio) lay inside this region, the WP can become unstable. It is found that this risk region can be significantly reduced by properly designing the control parameters of the converters. Therefore, high-frequency instabilities are unlikely to become a general concern for utilities. Nevertheless, weakly damped harmonic resonances can still occur and must be considered when operating and designing DFIG-based WPs. Two additional charts are proposed to analyze the risk of weakly damped harmonic resonances in the system. The charts establish a risk region by correlating the short-circuit level of the grid at the point of common coupling, with the WP rated power capacity and reactive power compensation level. Two parameters delimit this risk region: Resonance frequency and voltage amplification with respect to background harmonic distortions in the grid. All three charts proposed in this dissertation can be obtained analytically, based on wind park impedance models, without the need for running any simulation. A measurement-based method is also developed to obtain these charts without prior knowledge of any wind park characteristic. These methods for obtaining the charts were properly validated with detailed EMT simulations. The proposed charts can be easily obtained and consulted by engineers, and have the potential to greatly facilitate resonance assessment in systems with WPsMestradoEnergia EletricaMestre em Engenharia Elétric

Harmonic compensation in a grid using doubly fed induction generators

Ideally, electric utilities are expected to deliver a sinusoidal voltage with a constant rated frequency, while customers are expected to draw a sinusoidal current with unity power factor. The recent widespread use of harmonic producing equipment in industrial applications, especially non-linear loads, has increased the distortion of electric currents and voltages in transmission and distribution systems. This thesis proposes a method of using multiple reference frame theory for measuring and mitigating harmonic currents of nonlinear loads using a doubly fed induction generator. The most significant low-order harmonics to be compensated are calculated using a multiple reference frame harmonic observer. This observer is simulated using Matlab® Simulink® and then implemented using the Texas Instruments TMS320F28335 digital signal processor. Experimental and simulation results are provided to verify the analysis of the observer by comparing the results with calculations from the Fourier spectrum. Along with active and reactive power generation, an algorithm is proposed to inject currents in the rotor for the mitigation of harmonics in the system. Simulation results are presented to demonstrate the performance of this proposed method. These results validate the effectiveness of the method in compensating the targeted harmonics in the system. This method of measuring and compensating harmonics discussed in this thesis is accurate, straightforward, easily implemented and effective in the mitigation of any harmonic in the system. The currents obtained in the fundamental reference frame can be further employed for the control of active and reactive power flow --Abstract, page iii

Integration of wind energy systems into the grid: power quality and technical requirements

The integration of wind energy into the utility network has increased significantly over the past years largely as a result of the increasing environmental concerns arising from the use of fossil fuels, coupled with the anticipated global increase in oil. In South Africa, the wind energy industry is still in its infancy, with the Klipheuwel (about 3.2 MW) and Darling (about 4.2 MW) wind farms being the only grid connected projects in the country. However, grid integration studies carried out in [1] have shown that there are over 7 000 MW potential ideas for wind power in the Western Cape alone and this is a clear indication that there is a growing interest in wind development locally. The Government has also set a 4% target for the development of the renewable energy in the country by 2013. In light of the above, this thesis discusses some of the technical requirements and power quality issues that need to be addressed in order to fully integrate wind power into the network without adversely affecting the operation of the grid. These have been researched through reviewing the various standards and grid codes for wind power that have been implemented in other leading countries, in order to identify some of the requirements that can be adapted to suit our local integration process. Some of the main technical issues that are discussed in this thesis include the strength of the grid (fault levels), permitted penetration levels, choice of wind turbine and the reactive power requirements of the network. All these issues contribute towards the resolution of the impact of wind turbines on the power quality of the network, especially at the point of common coupling or connection (PCC). Various power quality phenomena were discussed in the literature but the ones that were further investigated included the voltage level profile, harmonic distortions as well as reactive power requirements from the wind turbines. These were determined both during the steady operation of the network and during a network disturbance

An improved sliding mode control for reduction of harmonic currents in grid system connected with a wind turbine equipped by a doubly-fed induction generator

Introduction. The implementation of renewable energy resources into the electrical grid has increased significantly in recent years. Wind power is one of the existing resources. Presently, power electronics has become an indispensable tool in wind power plants. Problem. However the associated control usually has an impact on increasing the harmonic distortion, especially on the output voltage. Goal. This paper proposes a new sliding mode control strategy, applied on a rotor-side of a doubly-fed induction generator. The main goal is to meet the electrical power requirements, while responding to the power quality issues. Methodology. The wind energy conversion system must be able to not only track the maximum power point of the wind energy, but also to mitigate the harmonic currents caused by the non-linear loads. To achieve this goal, the power converters are driven by the proposed sliding mode control strategy. The corresponding two gains of the sliding surface are well selected using a particle swarm optimization algorithm. The particle swarm optimization algorithm solves a constrained optimization problem whose fitness function is a prior formulated as the sum of two mean square error criterions. The first criterion presents the tracking dynamic of the reference active power while the second one presents the tracking dynamic of the reference reactive power. The novelty lies in the implementation of the particle swarm optimization algorithm in conventional sliding mode control strategy, in which the proposed-improved sliding mode control strategy is developed. The wind energy conversion system control uses the principal of the vector oriented control to decouple the control of the active power from that of the reactive power. Results. The improved sliding mode control strategy is applied to control separately theses powers in the presence of non-linear loads. The energy assessment of this strategy is analysed using the wind energy conversion system model based on SimPower software. Originality. The obtained simulation results confirm the superiority of the proposed-improved sliding mode control strategy in terms of reference tracking dynamics and suppression of harmonic currents.Вступ. Використання відновлюваних джерел енергії в електричній мережі останніми роками значно зросло. Енергія вітру – один із існуючих ресурсів. Нині силова електроніка стала незамінним інструментом вітряних електростанцій. Проблема. Проте, відповідне управління зазвичай має вплив на збільшення гармонійних спотворень, особливо у вихідній напрузі. Мета. У цій статті пропонується нова стратегія управління ковзним режимом, що застосовується на боці ротора асинхронного генератора з подвійним живленням. Основна мета – задовольнити вимоги до електроенергії, вирішуючи відповідні проблеми з якістю електроенергії. Методологія. Система перетворення енергії вітру повинна мати можливість не тільки відстежувати точку максимальної потужності вітру, але й пом'якшувати гармонійні струми, викликані нелінійними навантаженнями. Для досягнення цієї мети силові перетворювачі керуються запропонованою стратегією управління ковзним режимом. Відповідні два коефіцієнти посилення поверхні ковзання добре вибираються з використанням алгоритму оптимізації рою частинок. Алгоритм оптимізації рою частинок вирішує задачу оптимізації з обмеженнями, функція придатності якої заздалегідь сформульована як сума двох критеріїв середньоквадратичної похибки. Перший критерій репрезентує динаміку відстеження еталонної активної потужності, а другий – динаміку відстеження еталонної реактивної потужності. Новизна полягає в реалізації алгоритму оптимізації рою частинок у традиційній стратегії управління ковзним режимом, в якій розроблена запропонована покращена стратегія управління ковзним режимом. Управління системою перетворення енергії вітру використовує принцип векторно-орієнтованого управління, щоб відокремити управління активною потужністю від управління реактивною потужністю. Результати. Удосконалена стратегія управління ковзним режимом застосовується для роздільного управління цими потужностями за наявності нелінійних навантажень. Енергетична оцінка цієї стратегії аналізується за допомогою моделі системи перетворення енергії вітру на основі програмного забезпечення SimPower. Оригінальність. Отримані результати моделювання підтверджують перевагу запропонованої удосконаленої стратегії управління ковзним режимом з точки зору еталонної динаміки стеження та придушення гармонійних струмів

PWM Based VSC For Power Quality Assessment of Grid Integrated DFIG-WECS

The socioeconomic indicators are exclusively dependent upon the growth of energy for every country. The rise of population and urbanization on one hand and environmental degradation by fossil fuel sources on the other has realized the world for sustainable sources of energy to meet the energy imbalance. Therefore, high penetration of wind energy is need of the hour to meet the challenge of rising consumption of power. Globally, wind energy trends are witnessing a rapid growth with the passage of time. In this research contribution, the Doubly Fed Induction Generator (DFIG) derived wind power generation has been selected to harness wind energy at variable speed. The uncertain and fluctuating nature of wind flow cause power quality problems. MATLAB simulation model has been designed and response of power quality issues like voltage sag and harmonic distortion is checked both at No load and with load at the point of common coupling (PCC). For smooth integration of wind turbine generator with power grid, pulse width modulated (PWM) voltage source converters (VSC) are used both at the rotor side converter (RSC) and the grid side converter (GSC). Proportional integral (PI) controllers are used along with VSC for high power output

IMPROVEMENT OF POWER QUALITY OF HYBRID GRID BY NON-LINEAR CONTROLLED DEVICE CONSIDERING TIME DELAYS AND CYBER-ATTACKS

Power Quality is defined as the ability of electrical grid to supply a clean and stable power supply. Steady-state disturbances such as harmonics, faults, voltage sags and swells, etc., deteriorate the power quality of the grid. To ensure constant voltage and frequency to consumers, power quality should be improved and maintained at a desired level. Although several methods are available to improve the power quality in traditional power grids, significant challenges exist in modern power grids, such as non-linearity, time delay and cyber-attacks issues, which need to be considered and solved. This dissertation proposes novel control methods to address the mentioned challenges and thus to improve the power quality of modern hybrid grids.In hybrid grids, the first issue is faults occurring at different points in the system. To overcome this issue, this dissertation proposes non-linear controlled methods like the Fuzzy Logic controlled Thyristor Switched Capacitor (TSC), Adaptive Neuro Fuzzy Inference System (ANFIS) controlled TSC, and Static Non-Linear controlled TSC. The next issue is the time delay introduced in the network due to its complexities and various computations required. This dissertation proposes two new methods such as the Fuzzy Logic Controller and Modified Predictor to minimize adverse effects of time delays on the power quality enhancement. The last and major issue is the cyber-security aspect of the hybrid grid. This research analyzes the effects of cyber-attacks on various components such as the Energy Storage System (ESS), the automatic voltage regulator (AVR) of the synchronous generator, the grid side converter (GSC) of the wind generator, and the voltage source converter (VSC) of Photovoltaic (PV) system, located in a hybrid power grid. Also, this dissertation proposes two new techniques such as a Non-Linear (NL) controller and a Proportional-Integral (PI) controller for mitigating the adverse effects of cyber-attacks on the mentioned devices, and a new detection and mitigation technique based on the voltage threshold for the Supercapacitor Energy System (SES). Simulation results obtained through the MATLAB/Simulink software show the effectiveness of the proposed new control methods for power quality improvement. Also, the proposed methods perform better than conventional methods