Bounded droop controller for parallel operation of inverters

In this paper, the stability of parallel-operated inverters in the sense of boundedness is investigated. At first, the non-linear model of parallelled inverters with a generic linear or non-linear load is obtained by using the generalised dissipative Hamiltonian structure and then the robust droop controller, recently proposed in the literature for parallel operation of inverters, is implemented in a way to produce a bounded control output. The proposed controller is called the bounded droop controller (BDC). It introduces a zero-gain property and can guarantee the boundedness of the closed-loop system solution. Therefore, for the first time, the closed-loop stability in the sense of boundedness is guaranteed for parallelled inverters feeding generic non-linear/linear loads. The controller structure is further improved to increase its robustness with respect to initial conditions, numerical errors or external disturbances while maintaining the stability property. Moreover, the controller is tuned to avoid any possible limit cycles in the voltage dynamics. Real-time simulation results for two single-phase inverters operated in parallel loaded with a non-linear load are presented to verify the effectiveness of the proposed BDC

Modelagem e análise da dinâmica de microrredes de distribuição de energia elétrica

Tese (doutorado) - Universidade Federal de Santa Catarina, Centro Tecnológico, Programa de Pós-Graduação em Engenharia de Automação e Sistemas, Florianópolis, 2017.Uma das tendências dentro da eletrônica de potência atualmente é a aplicação de conversores de energia para a operação em microrredes, o que na essência é eletrônica de potência aplicada a sistemas de distribuição de energia elétrica. Tipicamente, microrredes são redes elétricas compostas por diversas fontes e cargas, o que implica em diversas interações fontes-fontes e fontes-cargas. O propósito deste trabalho é modelar, propor técnicas de controle e analisar a estabilidade paramétrica de pequenas microrredes CC e CA. Um dos grandes problemas que existem na operação de microrredes é quando diversas fontes operam em paralelo e compartilham um conjunto de cargas. O controle por droop é a técnica mais popular para lidar com este tipo de problema, mas possui o contraponto de introduzir não linearidades no sistema. Por conta disso, este trabalho propõe o uso de ferramentas matemáticas como a teoria de bifurcações para compreender o comportamento dinâmico de microrredes. Este procedimento permite um entendimento mais amplo quando comparado com técnicas tradicionais baseadas em modelos linearizados, além de fornecer mais informações que permitem otimizar o funcionamento do sistema garantindo a sua estabilidade. Uma parte significativa deste trabalho possui uma natureza teórica, com exceção de um caso de estudo envolvendo o paralelismo de conversores CC-CA, em que resultados experimentais são apresentados.Abstract : Nowadays, one of the trends within power electronics is the application of power converters in microgrids, which in essence means power electronics applied to electric power distribution systems. As microgrids can be dc, ac, or even hybrid networks, they are complex systems with many source-source and source-load-type interactions. The focus of this work is to model, propose control techniques, and to verify the stability of a small ac and dc microgrids through parametric diagrams. Load sharing is one of the major issues within microgrids, and it is defined as how several voltage sources operate in parallel feeding a given load set. Droop control is the standard approach towards the load sharing problem in microgrids, but this control can make the system unstable due to some nonlinearities introduced into the microgrid?s operation. For that reason, nonlinear dynamic analysis techniques are used here to understand the microgrid?s dynamic behavior. In particular, bifurcation theory offers a broader insight into the system dynamics when compared to usual techniques based on linear models. Moreover, bifurcation theory can optimize the microgrid operation and, thus, increase the system stability range due to a better understanding on how a group of parameters can influence the microgrid operation.A significant part of this thesis consists of a theoretical study, with the exception of one case study involving the parallelism of two dc-ac power converters, where experimental results are shown to validate the theoretical premises

Distributed static compensator (DSTATCOM) for voltage support in single wire earth return (SWER) networks

This investigation is concerned with the effectiveness of Distributed STATic COMpensators (DSTATCOMs) at providing voltage support in Single Wire Earth Return (SWER) networks. The reason for the focus on SWER lines is the high cost of upgrading them in the traditional way to solve voltage regulation problems that result from load growth in some of the feeders. A number of aspects of DSTATCOM installation and operation have been explored. These include their location, reactive power circulation, reactive power prioritising, four quadrant operation and the timing of installation and operation. It has been possible to derive analytical expressions only for the case of a single Thevenin source equivalent and a single load in parallel with a DSTATCOM. From one of those expressions it was deduced that, on a voltage increment per kVAr basis, DSTATCOMs are most effective as voltage regulators when they are installed at the customer terminals rather than further upstream into the network. This result has been found to apply generally to all practical SWER lines. Another derived expression predicted a peak value of customer terminal voltage when active power (P) and reactive power (Q) are injected by the DSTATCOM at constant kVA (S). This maximum voltage represents a stability limit for the case where DSTATCOMs are controlled to operate at constant kVA. Load flow studies revealed that in general this stability limit exists for all practical SWER lines. To avoid VAr circulation it is proposed that droop control with hysteretic band is used for DSTATCOM operation. The standard Newton-Raphson load flow formulation has been extended to accommodate DSTATCOMs operating under droop control and with operating point on a defined trajectory on the P-Q plane. Four defined trajectories have been investigated under given hourly load demand profile. These are the Q-only scheme, the constant kVA scheme with Q-priority, the load power factor follow scheme and the power factor correction scheme. For each one of those defined trajectories a modified Jacobian had to be derived. Load flow studies were based on each of the modified formulations. The load flow programs were designed to automatically provide a solution for each hour of a 24-hour demand profile representing the worst case peak demand for a particular year in the life of any practical SWER line. The customer DSTATCOM is either left on line, brought on line, left off line or taken off line, depending on the calculated customer voltage. Those special features of the load flow programs allowed them to be used to determine when and at what customer location DSTATCOMs should be installed and what their ratings should be. While the focus of the thesis has been on undervoltage problems; the proposed solutions and algorithms are applicable to overvoltage problems caused by the Ferranti Effect