Improvement of a three-phase converter for combined power supply systems for local objects with a photovoltaic solar battery

Розглянуто принципи формування струму багатофункціонального трифазного перетворювача для комбінованих систем електроживлення локальних об’єктів з фотоелектричною сонячною батареєю та підтриманням близького до одиниці коефіцієнту потужності у точці підключення до мережі за цілодобового використання. Запропоновано використання комбінованої модуляції з релейним регулятором струму, що дозволяє забезпечити відповідні стандартам показники струму мережі із найменшими втратами енергії в ключах. Розроблено структуру керування, що забезпечує реалізацію. Моделювання в системі "мережа-перетворювач-нелінійне навантаження" з оцінкою втрат потужності в ключах за різних принципів формування струму підтверджує ефективність запропонованих рішень.The purpose of the work is to improve the principles of forming the output current of a grid inverter to ensure compliance with the standards of parameters of a grid current without increasing the energy losses in the inverter's keys. The application is considered in a multifunctional three-phase converter with round-the-clock use for combined power supply systems of local objects, which provides a power factor close to unity at the point of connection to the AC grid. The methods of the electric circuit theory were used in combination with computer simulation. The analysis of the known principles of forming of an instantaneous value of output current of inverter by the deviation from the referenced value at a constant and variable key switching frequency is performed. For a grid three-level inverter, the use of combined unipolar and bipolar modulation with a relay current controller is suggested. The structure of the current controller is designed to provide the distribution of the unipolar and bipolar modulation intervals in accordance with the referenced value of inverter current and the mains voltage. A mathematical model is developed for studying the processes in the system, which include "a grid - the converter - non-linear unbalanced load", with an estimate of the power losses in the keys of a grid inverter in accordance with their currents and data sheets used, for a different principles of current formation for a two-level and three-level bridge a grid inverter. The results of the simulation confirm the possibility of ensuring the harmonic composition of the grid current corresponding to the standards with the lowest energy losses in the keys in all modes of the converter operation when using a three-level NPC inverter with clamped diodes. Further improvement in performance is possible when using the structure of three-level TNPC inverters

Удосконалення перетворювального агрегату системи електроживлення з фотоелектричною сонячною батареєю

Удосконалення перетворювального агрегату системи електроживлення з фотоелекричною сонячною батареєю шляхом зменшення втрат енергії та виключення перехідних складових струму за зміною режиму роботи. Синтез структури системи керування перетворювача на базі аналізу процесів у електричних колах з урахуванням зміни напруги, втрат енергії в напівпровідникових ключах та з використанням комп’ютерного моделювання. Обґрунтовано можливість зменшення втрат енергії в ключах перетворювального агрегату і покращення гармонійного складу вихідного струму мережевого інвертора; розроблено структуру системи керування перетворювача з регулюванням напруги на вході мережевого інвертора та перемиканням з роботи паралельно з мережею в автономний режим і навпаки; розроблено комп’ютерну модель системи: «перетворювальний агрегат з сонячною батареєю, мережа і навантаження» з блоком визначення втрат енергії в ключах схеми. Удосконалено: структуру системи керування перетворювального агрегату з регулюванням напруги у ланці постійного струму; структуру генератора з суміщенням функції фазового автопідстроювання частоти. Отримано залежність значення напруги на вході мережевого інвертора відповідно напрузі мережі за умови підтримання якості струму в точці підключення до мережі. Отримані рішення є основою для проектування перетворювачів комбінованих систем електроживлення з поновлювальними джерелами електроенергії.Совершенствование преобразовательного агрегата системы электроснабжения с фотоэлектрической солнечной батареей путем уменьшения потерь энергии и исключения переходных составляющих тока при изменении режима работы. Синтез структуры системы управления преобразователя на основе анализа процессов в электрических цепях с учетом изменения напряжения, потерь энергии в полупроводниковых ключах и с использованием компьютерного моделирования. Обоснована возможность уменьшения потерь энергии в ключах преобразовательного агрегата и улучшения гармонического состава выходного тока сетевого инвертора; разработана структура системы управления преобразователя с регулированием напряжения на входе сетевого инвертора и переключением с работы параллельно с сетью в автономный режим и наоборот; разработана компьютерная модель системы «преобразовательный агрегат с солнечной батареей, сеть и нагрузка» с блоком определения потерь энергии в ключах схемы. Усовершенствовано: структуру системы управления преобразовательного агрегата с регулированием напряжения в звене постоянного тока; структуру генератора с совмещением функции фазовой автоподстройки частоты. Получено зависимость значения напряжения на входе сетевого инвертора согласно напряжению сети при условии поддержания качества тока в точке подключения к сети. Полученные решения являются основой для проектирования преобразователей комбинированных систем электропитания с возобновляемыми источниками электроэнергии.Improving of the converter unit for the power supply system with a photovoltaic solar battery by reducing power losses and eliminating transient components of the current when the operating mode changes. Synthesis of structure of the converter control system based on the analysis of processes in electrical circuits taking into account voltage changes, energy losses in semiconductor switches and using computer simulation. The possibility of reducing energy losses in the keys of the converter unit and improving the harmonic composition of the output current of the grid inverter has been substantiated; the structure of the converter control system has been developed with voltage regulation at the input of the grid inverter and switching from work in parallel with the grid to the autonomous mode and vice versa; a computer model of the system “converter unit with solar battery, grid and load” with a unit for determining energy losses in keys of circuit was developed. Improved: the structure of the control system of the converter unit with voltage regulation in the DC link; the structure of the generator with the combination of the function of phase-locked loop. The dependence of the voltage value at the input of the grid inverter is obtained according to the grid voltage, provided that the current quality is maintained at the grid connection point. Solutions that are obtained are the basis for the development for converters of combined power supply systems with renewable sources of electricity

Grid Connected Distributed Generation System with High Voltage Gain Cascaded DC-DC Converter Fed Asymmetric Multilevel Inverter Topology

The paper presents distributed generation (DG) system in grid connected mode of operation with asymmetric multi-level inverter (AMLI) topology. Cascaded type DC-DC converter is employed to feed proposed AMLI topology. The DG output voltage (generally low voltage) is stepped up to the required level of voltage using high-gain DC-DC converter. Proposed AMLI topology consists of capacitors at the primary side. The output of high-gain DC-DC converter is fed to split voltage balance single-input multi-output (SIMO) circuit to maintain voltage balance across capacitors of AMLI topology. Cascaded DC-DC converters (both high-gain converter and SIMO circuit) are operated in closed-loop mode. The proposed AMLI feeds active power to grid converting DC type of power generated from DG to AC type to feed the grid. PWM pattern to trigger power switches of AMLI is also presented. The inverting circuit of MLI topology is controlled using simplified Id-Iq control strategy in this paper. With the proposed control theory, the active power fed to grid from DG is controlled and power factor is maintained at unity. The proposed system of DG integration to grid through cascaded DC-DC converters and AMLI structure is validated from fixed active power to grid from DG condition. The proposed system is developed and results are obtained using MATLAB/SIMULINK software

A Two Stage Hierarchical Control Approach for the Optimal Energy Management in Commercial Building Microgrids Based on Local Wind Power and PEVs

The inclusion of plug-in electrical vehicles (PEVs) in microgrids not only could bring benefits by reducing the on-peak demand, but could also improve the economic efficiency and increase the environmental sustainability. Therefore, in this paper a two stage energy management strategy for the contribution of PEVs in demand response (DR) programs of commercial building microgrids is addressed. The main contribution of this work is the incorporation of the uncertainty of electricity prices in a model predictive control (MPC) based plan for energy management optimization. First, the optimization problem considers the operation of PEVs and wind power in order to optimize the energy management in the commercial building. Second, the total charged power reference which is computed for PEVs in this stage is sent to the PEVs control section so that it could be allocated to each PEV. Therefore, the power balance can be achieved between the power supply and the load in the proposed microgrid building while the operational cost is minimized. The predicted values for load demand, wind power, and electricity price are forecasted by a seasonal autoregressive integrated moving average (SARIMA) model. In addition, the conditional value at risk (CVaR) is used for the uncertainty in the electricity prices. In the end, the results confirm that the PEVs can effectively contribute in the DR programs for the proposed microgrid model

Articles publicats per investigadors de l'ETSEIB indexats al Journal Citation Reports: 2013

Informe que recull els 297 treballs publicats per 203 investigadors de l'Escola Tècnica Superior d'Enginyeria Industrial de Barcelona (ETSEIB) en revistes indexades al Journal Citation Reports durant l’any 2013.Postprint (published version

Multiterminal HVDC transmissions systems for offshore wind

Offshore wind is emerging as one of the future energy vectors. Offshore wind power plants locations provide more strong and constant wind speed that allows to extract more power compared to onshore locations. In addition, as wind turbine components transportation is less restricted to terrestrial infrastructure, bigger and more powerful wind turbines can be installed offshore. In Europe, 1,567 MW of offshore wind power was installed in 2013. It represents the 14\% of the total wind power installed in Europe. Offshore wind power plants near the shore can be connected to the main grid by means of conventional AC technology. However, if these wind farms are installed further than 80-100 km, the use of AC equipment is economically infeasible due to reactive power issues. In these applications, HVDC system based on static converters can be used. The projects build and commissioned nowadays are based on point-to-point connections, where, each wind farm or wind farm clusters are connected to the terrestrial grid individually. Consequently, these lines might be understood as an extension of the AC system. If different offshore wind farms are interconnected between them and connected at the same time to different AC systems, for example, different countries, the DC grid is created. This scenario creates one of the most important challenges in the electrical power system since its creation, more than 100 years ago. The most relevant challenges to be addressed are the stability and operation of the DC grid and the integration and interaction with the AC grid. This thesis addresses various aspects related to the future Multiterminal-HVDC systems for transmission of offshore wind power. First, the voltage control and the system operations are discussed and verified by means of emulations using an HVDC scaled experimental platform built for this purpose. Voltage stability might be endangered during contingencies due to the different inertia time constant of the AC and the DC system. DC systems only have the inertia of the capacitors compared to synchronous machines rotating masses of the AC systems. Therefore, in faulty conditions the power transmitted through the DC system must be reduced quickly and efficiently. For this reason, in this thesis a coordinated power reduction algorithm taking advantage of Dynamic Braking Resistors (DBR) connected to onshore converter stations and the ability of the power plants to reduce the generated power is presented. From the AC and DC grids integration point of view, the connection point between the offshore grid and the AC grid might be located remotely leading to a connection with a reduced Short Circuit Ratio (SCR). In the literature, several issues regarding the connection of transistor-based power converters to weak AC grid have been reported. In this thesis am advanced control for Voltage Source Converters connected to weak grids is presented and tested by means of simulations. From the AC and DC grids interactions, the voltage stability is not enough to operate a DC grid. Transport System Operators (TSO) operates the power flow through the cables and the power exchanged between by the power converters. In this thesis, a novel hierarchical power flow control method is presented. The aim of the proposed power flow control is to obtain the desired power flows changing the voltage control set-points while the system stability is ensured. Finally, a control procedure for offshore wind farms based on Squirrel Cage Induction Generators connected to a single power converter is introduced.L'energia eòlica marina emergeix com un dels vectors energètics del futur. Les localitzacions eòliques marines proporcionen vens més forts i constants que les terrestres, cosa que permet extreure més potència. A més a més, els aerogeneradors marins poden ser més grans i més potents ja que es redueixen les limitacions de gàlib existent en les infraestructures terrestres. A tall d'exemple, l'any 2013 a Europa es van instal.lar 1.567 MW de potència eòlica marina, cosa que representa un 14\% de la potència eòlica instal.lada a Europa. Els parcs eòlics marins poden ser connectats a la xarxa elèctrica terrestre utilitzant emparamenta convencional de corrent alterna, però quan la distancia amb la costa excedeix els 80-100 km l'ús d'aquesta tecnologia es torna econòmicament inviable degut a l'energia reactiva generada en els conductors. Per solucionar aquest problema, s'emparen els sistemes en corrent continua basats en convertidors estàtics. Els projectes construïts o projectats a dia d'avui es basen en esquemes de connexió punt-a-punt, on, cada parc eòlic o agrupació de parcs eòlics es troba connectat a la xara terrestre individualment. En conseqüència, l'operació d'aquestes línies es pot considerar com una extensió de la xarxa d'alterna. Però, si s'interconnecten diferents parc eòlics amb diferents xarxes terrestres d'alterna (per exemple, diferents països) en corrent continua, s'obtenen xarxes en corrent continua. Aquest nou escenari crea un dels majors reptes des de la creació dels sistema elèctric de potencia, ara fa més de 100 anys. Entre aquests reptes hi ha l'estabilitat i l'operació dels sistemes en corrent contínua i la seva integració i coexistència amb les xarxes en corrent alterna. En la present tesis s'han estudiat diferents aspectes dels futurs sistemes multi terminal en alta tensió en corrent contínua (HVDC, en anglès) per la transmissió de potencia generada mitjançant parcs eòlics marins. Primerament, es descriu el control de tensió i els modes d'operació dels sistemes multi terminal i es verifiquen en una plataforma experimental construïda per aquest propòsit. L'estabilitat de tensió dels sistemes en corrent continua, es pot veure afectada durant una falta a la xarxa d'alterna degut a la reduïda inèrcia dels sistemes multi terminal, només formada pels condensadors dels convertidors i els cables. Així la potència que no pot injectar a la xarxa ha de ser reduïda de forma ràpida i eficient. Per això, en aquesta tesis es presenta un sistema coordinat de reducció de potència que utilitza la resistència de frenat dels convertidors de connexió a la xarxa i els mètodes de reducció de potència dels parcs eòlics. Des del punt de vista de la integració de les xarxes en continua i en alterna, el punt d'interconnexió pot estar localitzat llunys dels grans centres de generació, la qual cosa implica tenir una potència de curtcircuit molt reduïda. En la bibliografia científica s'han descrit diverses problemàtiques a l'hora de connectar un convertidor de commutació forçada a les xarxes dèbils. Per tal de pal.liar aquests inconvenients, en aquesta tesis es presenta un algorisme avançat de connexió de convertidors a xarxes dèbils basat en control vectorial. Des del punt de vista de les interaccions i interoperabilitat dels sistemes en corrent alterna i continua, no n'hi ha suficient en garantir l'estabilitat, ja que el propòsit finals dels operadors de xarxa és fer fluir una potencia a traves de la xarxa per tal de satisfer la demanda. Per aquest propòsit en aquesta tesis es presenta un control jeràrquic de control del flux de potència que fixa el flux de potència a traves d'una xarxa multi terminal canviant les consignes del control primari, tot assegurant l'estabilitat del sistema. Per tancar la tesis, es presenta un nou controlador per parcs eòlics basats en aerogeneradors de gàbia d'esquirol controlats per un sol convertidor

Modeling, Stability Analysis, and Control of Distributed Generation in the Context of Microgrids

One of the consequences of competitive electricity markets and international commitments to green energy is the fast development and increase in the amount of distributed generation (DG) in distribution grids. These DGs are resulting in a change in the nature of distribution systems from being "passive", containing only loads, to "active", including loads and DGs. This will affect the dynamic behavior of both transmission and distribution systems. There are many technical aspects and challenges of DGs that have to be properly understood and addressed. One of them is the need for adequate static and dynamic models for DG units, particularly under unbalanced conditions, to perform proper studies of distribution systems with DGs (e.g., microgrids). The primary objective of this thesis is the development and implementation of dynamic and static models of various DG technologies for stability analysis. These models allow studying systems with DGs both in the long- and short-term; thus, differential and algebraic equations of various DGs are formulated and discussed in order to integrate these models into existing power system analysis software tools. The presented and discussed models are generally based on dynamic models of different DGs for stability studies considering the dynamics of the primary governor, generators, and their interfaces and controls. A new comprehensive investigation is also presented of the effects of system unbalance on the stability of distribution grids with DG units based on synchronous generator (SG) and doubly-fed induction generator (DFIG) at different loading levels. Detailed steady-state and dynamic analyses of the system are performed. Based on voltage and angle stability studies, it is demonstrated that load unbalance can significantly affect the distribution system dynamic performance. Novel, simple, and effective control strategies based on an Unbalanced Voltage Stabilizer (UVS) are also proposed to improve the system control and the stability of unbalanced distribution systems with SG- and DFIG-based DGs

Anàlisis de les tècniques de control d'injecció d'energia i de les estratègies de filtratge en la qualitat del subministrament elèctric per a la integració d'energies renovables a la xarxa elèctrica

El present Treball Fi de Màster aporta un anàlisi i revisió de la bibliografia científica de les tècniques d’injecció i estratègies de filtratge en la qualitat del subministrament elèctric, des dels canvis més rellevants dels reglaments de la xarxa (Grid Codes) de 2012 fins al 2017. A partir de la selecció i anàlisi de la literatura científica més significativa, s’analitzen i s’observen els àmbits on s’han realitzat millores o aportacions substancials. Al desgranar cadascun dels mètodes, es pot realitzar una taula comparativa que permet classificar les tècniques i/o estratègies segons les seves característiques, per posteriorment analitzar en profunditat tots els aspectes tractats en elles. El treball conclou exposant quins són els mètodes més significatius d’aquest període, juntament amb una justificació de la necessitat de crear tècniques de control específiques per objectius concrets i un seguit d’observacions sobre possibles tendències futures esdevingudes de l’evolució dels propis mètodes

Model Predictive Control of Impedance Source Inverter for Photovoltaic Applications

A model predictive controlled power electronics interface (PEI) based on impedance source inverter for photovoltaic (PV) applications is proposed in this disssertation. The proposed system has the capability of operation in both grid-connected and islanded mode. Firstly, a model predictive based maximum power point tracking (MPPT) method is proposed for PV applications based on single stage grid-connected Z-source inverter (ZSI). This technique predicts the future behavior of the PV side voltage and current using a digital observer that estimates the parameters of the PV module. Therefore, by predicting a priori the behavior of the PV module and its corresponding effects on the system, it improves the control efficacy. The proposed method adaptively updates the perturbation size in the PV voltage using the predicted model of the system to reduce oscillations and increase convergence speed. The experimental results demonstrate fast dynamic response to changes in solar irradiance level, small oscillations around maximum power point at steady-state, and high MPPT effectiveness from low to high solar irradiance level. The second part of this work focuses on the dual-mode operation of the proposed PEI based on ZSI with capability to operate in islanded and grid-connected mode. The transition from islanded to grid-connected mode and vice versa can cause significant deviation in voltage and current due to mismatch in phase, frequency, and amplitude of voltages. The proposed controller using MPC offers seamless transition between the two modes of operations. The main predictive controller objectives are decoupled power control in grid-connected mode and load voltage regulation in islanded mode. The proposed direct decoupled active and reactive power control in grid connected mode enables the dual-mode ZSI to behave as a power conditioning unit for ancillary services such as reactive power compensation. The proposed controller features simplicity, seamless transition between modes of operations, fast dynamic response, and small tracking error in steady state condition of controller objectives. The operation of the proposed system is verified experimentally. The final part of this dissertation focuses on the low voltage ride through (LVRT) capability of the proposed PV systems during grid faults such as voltage sag. In normal grid condition mode, the maximum available power from the PV panels is injected into the grid. In this mode, the system can provide reactive power compensation as a power conditioning unit for ancillary services from DG systems to main ac grid. In case of grid faults, the proposed system changes the behavior of reactive power injection into the grid for LVRT operation according to the grid requirements. Thus, the proposed controller for ZSI is taking into account both the power quality issues and reactive power injection under abnormal grid conditions