Many distributed energy resource (DER) systems are remotely located and are often interfaced at low or medium voltage levels through power electronics converters such as voltage-sourced converters (VSC). Therefore, a weak-grid situation is encountered where the voltage and frequency at the point of DER coupling can experience fluctuations. A power converter designed to operate in normal and strong grid conditions may not perform satisfactorily during such weak and distorted grid conditions. Hence, considering the full dynamics of the system during weak-grid conditions in the design of converter control is indispensable to ensure the stability of the DER and the grid. For instance, the phase-locked loop (PLL) has been identified as a critical component of the VSC controller that can compromise the DER performance during weak-grid conditions. This dissertation investigates and enhances the performances of inverters connected to weak and polluted grids. It primarily presents a novel approach of enhancing the inverter current controller by including the PLL state variables among the entire system state and use them to optimally generate the control input for the VSC. This mitigates the loop interactions between the PLL and other control loops resulting in a mitigation of the oscillations that could cause system instabilities. The procedure is accomplished using the recently developed linear model of the enhanced PLL (EPLL) for single-phase applications and using a model of the three-phase PLL developed in this dissertation. Extensive simulation and experimental results are presented to evaluate and validate the proposed control approaches. Full practical models of all system components are considered for simulation studies. The experimental tests are done on a practical inverter connected to the utility grid. Significant improvement of the inverter performance in weak-grid conditions is confirmed. This dissertation offers a systematic way of integrating and designing the PLL and controller in a VSC to achieve a robust performance in weak-grid conditions.

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.

No Brasil, a necessidade crescente por sistemas de geração solar fotovoltaica conectados à rede de distribuição levou ao desenvolvimento de normas brasileiras para a conformidade de inversores. Assim, de forma a atender a população em geral, foi publicada a Portaria 357, de 01 de agosto de 2014, do Instituto Nacional de Metrologia, Qualidade e Tecnologia (INMETRO), que inclui inversores para conexão à rede de potência nominal até 10 kW no Programa Brasileiro de Etiquetagem (PBE). Por possuir os equipamentos necessários para a realização dos ensaios de conformidade com a portaria 357, o Laboratório de Sistemas Fotovoltaicos (LSF) do Instituto de Energia e Ambiente da Universidade de São Paulo passa então a ser o primeiro laboratório acreditado pelo INMETRO para a realização desses ensaios. Este trabalho tem como objetivo resumir as experiências de aplicação das normas brasileiras no ensaio de inversores para o PBE no LSF, sendo levantados os resultados obtidos no ensaio de 15 inversores ao longo de três anos de operação da bancada de ensaio para sistemas conectados à rede. A partir deste levantamento conclui-se que, embora as normas brasileiras exijam rigor em seus requisitos, existem inversores capazes de atender aos requisitos normativos brasileiros, assim como existem inversores que necessitam de implementações nos respectivos firmware de controle para conseguirem atender todos os requisitos. Uma vez que as normas brasileiras são recentes, este trabalho também destaca pontos das normas que se beneficiariam de adequações no corpo do texto, a fim de melhorar as rotinas de ensaios em inversores e critérios de conformidade. / In Brazil, the increasing demand for grid-tie photovoltaic systems lead to the development of the Brazilian standards for inverter quality and evaluation. In order to meet the general population demand, the National Institute for Metrology, Quality and Technology (INMETRO) published the normative nº 357 01/08/2014 to include inverters up to 10 kW in the Brazilian Labeling Program (PBE). For having all the equipment needed for inverter conformity evaluation in accordance with the normative nº 357, the Laboratory of Photovoltaic Systems (LSF) of the Institute of Energy and Environment of the University of Sao Paulo becomes the first INMETRO accredited laboratory for conducting the inverter evaluation in Brazil. This works main goal is to summarize the experience acquired for the appliance of the Brazilian standards for inverter testing in the LSF. The results obtained for the test of 15 different inverters during the first three years operating the inverter test bench are showed. Using the data shown this work concludes that, although the Brazilian standards are rigorous on its criteria, there are inverters capable of complying with the standards requirements and there are inverters that need further software and hardware development in order to comply with all the requirements. Since the Brazilian standards are in its first version, this work also aims to point out aspects of the standard that would benefit from text revising in order to improve the test conditions and conformity criteria.

No Brasil, a necessidade crescente por sistemas de geração solar fotovoltaica conectados à rede de distribuição levou ao desenvolvimento de normas brasileiras para a conformidade de inversores. Assim, de forma a atender a população em geral, foi publicada a Portaria 357, de 01 de agosto de 2014, do Instituto Nacional de Metrologia, Qualidade e Tecnologia (INMETRO), que inclui inversores para conexão à rede de potência nominal até 10 kW no Programa Brasileiro de Etiquetagem (PBE). Por possuir os equipamentos necessários para a realização dos ensaios de conformidade com a portaria 357, o Laboratório de Sistemas Fotovoltaicos (LSF) do Instituto de Energia e Ambiente da Universidade de São Paulo passa então a ser o primeiro laboratório acreditado pelo INMETRO para a realização desses ensaios. Este trabalho tem como objetivo resumir as experiências de aplicação das normas brasileiras no ensaio de inversores para o PBE no LSF, sendo levantados os resultados obtidos no ensaio de 15 inversores ao longo de três anos de operação da bancada de ensaio para sistemas conectados à rede. A partir deste levantamento conclui-se que, embora as normas brasileiras exijam rigor em seus requisitos, existem inversores capazes de atender aos requisitos normativos brasileiros, assim como existem inversores que necessitam de implementações nos respectivos firmware de controle para conseguirem atender todos os requisitos. Uma vez que as normas brasileiras são recentes, este trabalho também destaca pontos das normas que se beneficiariam de adequações no corpo do texto, a fim de melhorar as rotinas de ensaios em inversores e critérios de conformidade. / In Brazil, the increasing demand for grid-tie photovoltaic systems lead to the development of the Brazilian standards for inverter quality and evaluation. In order to meet the general population demand, the National Institute for Metrology, Quality and Technology (INMETRO) published the normative nº 357 01/08/2014 to include inverters up to 10 kW in the Brazilian Labeling Program (PBE). For having all the equipment needed for inverter conformity evaluation in accordance with the normative nº 357, the Laboratory of Photovoltaic Systems (LSF) of the Institute of Energy and Environment of the University of Sao Paulo becomes the first INMETRO accredited laboratory for conducting the inverter evaluation in Brazil. This works main goal is to summarize the experience acquired for the appliance of the Brazilian standards for inverter testing in the LSF. The results obtained for the test of 15 different inverters during the first three years operating the inverter test bench are showed. Using the data shown this work concludes that, although the Brazilian standards are rigorous on its criteria, there are inverters capable of complying with the standards requirements and there are inverters that need further software and hardware development in order to comply with all the requirements. Since the Brazilian standards are in its first version, this work also aims to point out aspects of the standard that would benefit from text revising in order to improve the test conditions and conformity criteria.

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.

Power distribution facilities all over the world have been committed towards making the grids smarter in order to reduce the risks of grid failures and provide an affordable, reliable, and sustainable supply of electricity to the end consumers. The smart grid concept involves incorporation of monitoring, analysis and control functions into the existing power distribution infrastructure. One of the foremost steps in realizing the smart grid concept is the integration of information and communication technologies with power system engineering. Various communication technologies are available, out of which Powerline Communication (PLC) has been found to be most suitable owing to its least intensiveness on additional infrastructure. Existing methods use PLC as a separate communication physical layer to establish com- munication between components in a micro/sub-micro grid. However, these methods poses the problems of a separate physical layer requirement to establish communication between inverters, attenuation of the information signal by the EMI filters present in various loads and equipments connected to the micro grid, requirement of signal repeaters at regular distance intervals and requirement of a separate server for monitoring and control. In order to simultaneously utilize the incorporation of front end inverters into the grid and achieve inter-inverter communication, a PLC method for the grid connected inverters based on a harmonic injection into the grid current is proposed in this thesis. The harmonic injection is accomplished by considering the grid current as a composite vector with components rotating at different speeds. The lower harmonic spectrum space can be chosen to avoid the attenuation problems associated with the EMI filters. In the proposed method, as the choice of the harmonic space is flexible, it is possible to even adopt a dynamically changing harmonic space to optimize THD. The advantage of the method is that it simultaneously achieves communication along with grid interfacing of DGs without any requirement of extra hardware. Also, since the principle of information exchange amongst inverters is the same as that of the power transfer, there is no added complexity involved in the inverter control system due to the proposed PLC method. The principle of the Composite Space Vector on which the proposed PLC method is based upon has been explained in detail along with the frame transformation equations. The control scheme to achieve the power transfer and the information exchange for the grid connected inverters is explained. The design procedure for various circuit elements and the control loop parameters has been explained. The thesis also discusses the various factors affecting the choice of the modulating signal and the speed of communication achievable in the proposed PLC method. For both the three phase and single phase systems, simulation results have been presented for the proposed PLC method under different grid conditions and different harmonics as the modulating signals. The simulations have been performed using the MATLAB SIMULINK SimPowerSystems toolbox. The simulation results have been experimentally verified through a laboratory prototype. The laboratory prototype consists of individual IGBT based inverters controlled through the Texas Instruments TMS320F2812 DSP based digital controller.