Este trabalho apresenta o estudo de um inversor NPC de 6 kW e 380 V, trifÃsico, para interconexÃo de sistemas de geraÃÃo distribuÃda à rede elÃtrica. O inversor à estudado nas frequÃncias de chaveamento de 3, 4 e 6 kHz, sendo interligado à rede elÃtrica atravÃs dos filtros L e LCL. Para identificar qual filtro passivo, L ou LCL, se adequa melhor ao inversor em estudo foram realizadas simulaÃÃes numÃricas para diferentes situaÃÃes, validando o projeto do inversor e a modelagem vetorial desenvolvida. A resposta do controle à satisfatÃria, pois o inversor à capaz de controlar o fluxo de potÃncia ativa e reativa entregues à rede elÃtrica. Diante dos resultados obtidos percebeu-se que o filtro LCL apresentou um menor conteÃdo harmÃnico para as trÃs frequÃncias estudadas em relaÃÃo ao filtro L. Ambos os filtros atenderam aos requisitos da norma Std IEEE 1543, no entanto o filtro L à o mais indicado para o inversor em estudo, pois alÃm de atender a norma, o mesmo apresenta um menor custo quando comparado ao filtro LCL. / This paper presents the study of a 6 kW, 380 V, three phase NPC inverter for interconnecting distributed generation unit to the grid. The inverter is studied with switching frequency of 3, 4 and 6 kHz, and it is interconnected to the grid through L and LCL filters. To identify which passive filter, L or LCL, is best suited to the inverter under study, a mathematical model has been proposed and several numerical simulations have been carried out to validate the design of the inverter and vector model developed. The control response is satisfactory, since the inverter is able to control the flow of active and reactive power delivered to the grid. Based on these results it was noticed that the LCL filter showed a lower harmonic content for the three frequencies studied in relation to the filter L. Both filters met the requirements of the standard IEEE Std 1543, however the filter L is the most suitable for the inverter under study, because the filter L not only meet the standard but also it presents a lower cost when compared to the LCL filter.

Design of filters used in grid-connected inverter applications involves multiple constraints. The filter requirements are driven by tight filtering tolerances of standards such as IEEE 519-1992–IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems and IEEE 1547.2-2008–IEEE Application Guide for IEEE Std 1547, IEEE Standard for Interconnecting Distributed Resources with Electric Power Systems. Higher order LCL filters are essential to achieve these regulatory standard requirements at compact size and weight. This objective of this thesis report is to evaluate design procedures for such higher order LCL filters. The initial configuration of the third order LCL filter is decided by the frequency response of the filter. The design equations are developed in per-unit basis so results can be generalized for different applications and power levels. The frequency response is decided by IEEE specifications for high frequency current ripple at the point of common coupling. The appropriate values of L and C are then designed and constructed. Power loss in individual filter components is modeled by analytical equations and an iterative process is used to arrive at the most efficient design. Different combinations of magnetic materials (ferrite, amorphous, powder) and winding types (round wire, foil) are designed and tested to determine the most efficient design. The harmonic spectrum, power loss and temperature rise in individual filter components is predicted analytically and verified by actual tests using a 3 phase 10 kW grid connected converter setup. Experimental results of filtering characteristics show a good match with analysis in the frequency range of interconnected inverter applications. The design process is stream-lined for the above specified core and winding types. The output harmonic current spectrum is sampled and it is established that the harmonics are within the IEEE recommended limits. The analytical equations predicting the power loss and temperature rise are verified by experimental results. Based on the findings, new LCL filter combinations are formulated by varying the net Lpu to achieve the highest efficiency while still meeting the recommended IEEE specifications. Thus a design procedure which can enable an engineer to design the most efficient and compact filter that can also meet the recommended guidelines of harmonic filtering for grid-connected converter applications is established.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / This work provides as a main contribution a procedure to design robust state feedback current controllers applied to inverters connected to the grid by means of LCL filters, providing results which comply with performance requirements from the IEEE Standard 1547. First, the plant is modeled in the state space and discretized including a delay of one sample from the control signal implementation, and including in the augmented system a set of resonant controllers of arbitrary dimension. The uncertainty on the grid inductance is modeled, leading to a polytopic description of the augmented system. A state feedback controller is then used. The gains of this controller are determined by means of conditions based on linear matrix inequalities, which ensure the location of the eigenvalues in circular region inside the unit circle for the closed-loop system. The simulation results with this controller in the time domain and also in the frequency domain indicate good transient and steady state performances, tracking of sinusoidal reference and rejection of harmonics from the grid. The simulation results are corroborated experimentally, using a digital signal processor for the implementation of the control law. An example of design of a non robust controller, designed for a nominal value of the grid inductance, is presented, showing that this controller leads to instability for values of grid inductance different from the nominal, which emphasizes the importance of the use of robust controllers for this application. Also is presented a performance analysis based on the H1 norm that illustrates the effect of parametric uncertainties on the closed-loop disturbance rejection capacity. An extension for the control of three-phase inverters are presented. The three-phase robust controller is validated by means of simulations and experimental results similar with the results obtained for the single-phase case. / Este trabalho traz como principal contribuição um procedimento de projeto de controladores de corrente robustos por realimentação de estados aplicados a inversores conectados à rede por meio de filtro LCL, fornecendo resultados que cumprem exigências de desempenho da norma IEEE Standard 1547. Primeiramente, a planta é modelada no espaço de estados e discretizada, incluindo um atraso de uma amostra na implementação do sinal de controle, e um conjunto de controladores ressonantes de dimensão arbitrária. A incerteza na indutância da rede é modelada, levando a uma descrição politópica do sistema. Um controlador por realimentação de estados é então utilizado. Os ganhos deste controlador são calculados por meio de condições baseadas em desigualdades matriciais lineares, que garantem a alocação dos autovalores de malha fechada em regiões circulares dentro do círculo de raio unitário. Os resultados de simulação deste controlador indicam bom desempenho em transitórios e em regime permanente, rastreamento da referência senoidal e rejeição de harmônicas da rede. Os resultados de simulação são comprovados experimentalmente, utilizando um processador digital de sinais para implementação da lei de controle. Um exemplo de projeto de um controlador não robusto, projetado para um valor nominal da indutância da rede, é apresentado, mostrando que este controlador leva à instabilidade para valores de indutância de rede diferentes do nominal, o que reforça a importância do uso de controladores robustos. Também é apresentada uma análise de desempenho utilizando a norma H1 para ilustrar o efeito de incertezas paramétricas na capacidade de rejeição de distúrbios do sistema em malha fechada. Uma extensão para o controle de inversores trifásicos é realizada. O controlador robusto trifásico é validado por meio de simulações e resultados experimentais semelhantes aos obtidos para o caso monofásico, indicando a viabilidade da técnica também para o caso multivariável.

Conselho Nacional de Desenvolvimento Científico e Tecnológico / This dissertation deals with the design of a robust current control loop applied to voltage source inverters connected to the grid thought LCL-filter used in distributed generation systems. The utilization of the LCL-filter bring the need of damping the characteristic resonance of the filter. Two alternatives are generally considered in the literature. The passive damping, that add a passive element, generally a resistor or a additional passive element in the filter circuit and the active damping, that introduce a specific controller in the inverter current control loop. The active damping present more flexibility in the implementation and do not present energy losses like passive damping, so is the preferred damping method in high power applications. The main challenge to the damping method is keep the performance and avoid instability and controllers interactions even when impedance variations occur in the grid. In this dissertation, are proposed two control schemes for achieve the active damping of the filter resonance. In the first one, a robust partial state feedback is derived to allocate the poles of the LCL-filter inside the unity ratio circle for a given grid impedance variation. The feedback gains are obtained by a LMI condition that assure robust pole location in a pre-establish region of the unity ratio circle. In the second method, is proposed a predictive state estimator based on the multirate observers theory. This estimator allow feeding back the estimated stated by the measure of only one state variable. Moreover, the predictive action of the observer, eliminate the time delay of the control loop improving the stability margins of the system. Experimental results of a setup of 10kW DSP based are presented. / Esta dissertação de mestrado trata do projeto de uma malha de controle de corrente robusta para inversores alimentados em tensão conectados à rede através de filtro-LCL, utilizados em sistemas de geração distribuída. A utilização do filtro-LCL traz consigo a necessidade do amortecimento adequado da ressonância característica deste filtro. Dois tipos de amortecimento são apresentados na literatura para solucionar este problema. O amortecimento passivo, que consiste na introdução de um resistor ou filtro passivo adicional no circuito e o amortecimento ativo, que é baseado na introdução de um controlador digital específico na malha de controle de corrente do inversor. O amortecimento ativo, por apresentar maior flexibilidade de implementação e não apresentar perdas de energia, é o método preferencialmente utilizado. O principal desafio na utilização do amortecimento ativo é manter um desempenho satisfatório na presença de incertezas na impedância da rede no ponto de conexão. Neste trabalho, são propostas duas técnicas de amortecimento ativo robusto. Na primeira delas, uma retroação parcial de estados robusta é obtida para alocar os pólos da planta numa região contida no círculo unitário para uma dada faixa de variação de impedância da rede. Os ganhos da retroação de estados são obtidos a partir de um sistema de desigualdades matriciais lineares (LMI), que garantem alocação robusta dos pólos do sistema afetado por incertezas paramétricas e por atraso de transporte. No segundo método é proposto um estimador preditivo de estados baseado em observadores por múltiplas amostras, que permite fazer a retroação de estados estimados a partir da medida de apenas um dos estados da planta. Além disso, elimina o atraso de transporte da implementação digital aumentando a robustez do sistema. Resultados experimentais de um inversor de 11 kW controlado por um DSP de ponto fixo, conectado à rede com um filtro-LCL são apresentados para validar as analises desenvolvidas e demonstrar o bom desempenho dos controladores propostos.

Grid-connected inverters have wide application in the field of distributed generation and power quality. As the power level demanded by these applications increase, the design and performance evaluation of these converters become important. In the present work, a 50 kVA three-phase back-to-back connected inverter with output LCL filter is built to study design and performance evaluation aspects of grid-connected inverters. The first part of the work explores the split-capacitor resistive-inductive (SC-RL) passive damping scheme for the output LCL filter of a three-phase grid-connected inverter. The low losses in the SC-RL scheme makes it suitable for high power applications. The SCRL damped LCL filter is modelled using state space approach. Using this model, the power loss and damping are analysed. A method for component selection that minimizes the power loss in the damping resistors while keeping the system well damped is proposed. Analytical results show the losses to be in the range of 0.05-0.1% and the quality factor to be in the range of 2.0-2.5. These results are validated experimentally. In the second part of the work, a test method to evaluate the thermal performance of the semi-conductor devices of a three-phase grid-connected inverter is proposed. The method eliminates the need for high power sources, loads or any additional power converters for circulation of power. Only energy corresponding to the losses is consumed. The capability of the method to evaluate the thermal performance of the DC bus capacitors and the output filter components is also explored. The method can be used with different inverter configurations -three-wire or four-wire and for different PWM techniques. The method has been experimentally validated at a power level of 24kVA. In the third part of the work, the back-to-back connected inverter is programmed as a hardware grid simulator. The hardware grid simulator emulates the real-time grid and helps create grid disturbances often observed at the point of common coupling in an ac low voltage grid. A novel disturbance generation algorithm has been developed, analysed and implemented in digital controller using finite state machine model for control of the grid simulator. A wide range of disturbance conditions can be created using the developed algorithm. Experimental tests have been done on a linear purely resistive load, a non-linear diode-bridge load and a current-controlled inverter load to validate the programmed features of the grid simulator.

The necessity to incorporate renewable energy systems into existing electric power grid and need of efficient utilization of electrical energy are growing every day. A shunt connected Voltage Source Inverter(VSI) capable of bidirectional power flow and fast control has become one of the building block to address such requirements. However with growing number of grid connected VSI, new requirements related to harmonic injection, higher overall efficiency and better performances during short term grid disturbances have emerged as challenges. For this purpose a grid connected three phase four wire VSI with LCL filter can be considered as a general module to study different control approaches and system behavior under ideal and non-ideal grid conditions. This work focuses on achieving enhanced performance by analyzing effect of non-ideal conditions on system level and relating it to individual control blocks. In this work a phase locked loop structure has been proposed which is capable of extracting positive sequence fundamental phase information under non-ideal grid conditions. It can also be used in a single phase system without any structural modification. The current control for the three phase four wire VSI system has been implemented using Proportional Resonant (PR) controller in a per phase basis in stationary reference frame. A simplified controller design procedure based on asymptotic representation of the system transfer function is proposed. Using this method expressions for controller gains can be derived. A common mode model of the inverter system has been derived for low frequencies. Using this model a controller is designed to mitigate DC bus imbalance caused by sensor and ADC channel offsets. A multi-rate approach for digital implementation of PR controller with low resource consumption, that is suitable for an FPGA like digital controller ,is proposed. This multi-rate method can maintain resonance frequency accuracy even at low sampling frequency and can easily be frequency adaptive. Anti-wind up methods for PI controller have been studied to find suitable anti-wind up methods for PR controller. The tracking anti-wind up method is shown to be suitable for use with a PR controller. The effectiveness of this method under sudden disconnection and reconnection of VSI from grid is experimentally verified. A resonant integrator based second order filter is shown to be useful for active damping of LCL filter resonance with a wide range of grid inductance variation. The proposed method utilizes the LCL filter capacitor voltage to estimate resonance frequency current. Suitability of fundamental current PR controller for active damping alone, and with the proposed method show the superiority of the proposed method especially for low switching frequencies. Design oriented analysis of the above topics are included in the thesis. The theoretical understandings developed have been verified through experiments in the laboratory and can be readily implemented in industrial power electronic systems.