The power calculation is an indispensable element in droop-controlled inverters because the bandwidth of the measured power has a direct impact on the controller performance. This paper proposes a fast and accurate power calculation algorithm based on the combined Second Order Generalized Integrator (SOGI) filters in stationary coordinates for a three-phase system, which takes into consideration the use of nonlinear loads. The power calculation scheme is formed by the two-stage SOGI filters that are employed for obtaining the active and reactive powers required to perform a droop-based inverter operation, respectively. From the two-stage structure, the first SOGI is used as a band-pass filter (BPF) for filtering harmonics and obtaining the fundamental current of the nonlinear load; The second SOGI is used as a low-pass filter (LPF) for extracting the DC-component, which corresponds with the average power. A small-signal model of a two droop-controlled inverters system is built to obtain the dynamical response and stability margin of the system. And compared it with the dynamical behaviour of a standard droop-control method. Next, the proposed power calculation system is designed in order to achieve the same ripple amplitude voltage as that obtained with the standard droop-control method by adjusting the bandwidth gains. Through simulation and hardware in the loop (HIL) validation, the proposed approach presents a faster and more accurate performance when sharing nonlinear loads, and also drives the inverters’ output voltage with lower distortion.
This work proposes an isolated double conversion UPS with power factor correction using a high frequency transformer, and input voltages equal to 110V/220V. The arrangement is suitable to rack structures because it has small size and reduced weight. For both input voltages, the proposed converter has almost the same efficiency processing the same output power. Other relevant features are soft commutation of the controlled switches in the chopper and the boost stage, simple control strategy that can be implemented with well-known integrated circuits, and the use of few batteries in series due to the step-up stage. Qualitative analysis and experimental results obtained from 2kVA laboratory prototype are presented.
A circuit configuration of a single phase non-isolated on-line uninterruptible power supply (UPS) with 110V/220V input output voltage ratings is proposed, allowing the bypass operation without a transformer even if the input voltage is different of the output voltage. The converter consists of an AC-DC/DC-DC three level boost converter combined with a double half bridge inverter. In this type of configuration size, cost and efficiency are improved due to reduced number of switches and batteries, as well as no low frequency isolation transformer is required to realize bypass operation because of the common neutral connection. Both stages of the proposed circuit operate in high frequency, using a passive non-dissipative snubber circuit in the boost converter and IGBTs switches in the double half bridge inverter, with low conduction losses, low tail current and low switching losses. Principle of operation and experimental results for a 2.6kVA prototype are presented to demonstrate UPS performance.
This paper proposes a double conversion UPS system with power factor correction, high frequency transformer isolation and 110V/220V input voltage characteristics. It's suitable for rack type structure because it has small size and reduced weight. For both input voltages, the proposed converter has almost the same efficiency processing the same output power. Other relevant features of this structure are: soft commutation of the controlled switches of the chopper and of the boost stage, simple control strategy that can be implemented with well-known integrated circuits and the use of few batteries in series due to step-up stage. Qualitative analysis and experimental results obtained from 2kVA laboratory prototype are presented.
Resumo-Este trabalho tem por objetivo analisar o modelo do gerador de indução duplamente alimentado no referencial dq síncrono através de estudo teórico. Através da análise dos circuitos que relacionam o rotor e o estator desta máquina nos eixos direto e em quadratura, um modelo matemático em malha fechada será deduzido e analisado em detalhes. Um desenvolvimento matemático baseado em controladores proporcional e proporcionalintegral será proposto. Por fim, resultados experimentais a partir de um protótipo de 8 kW montado em laboratório, onde o comportamento dos controladores diante de variações em degrau na referência de corrente do rotor tanto no eixo direto quanto no eixo em quadratura serão apresentados e discutidos, a fim de validar a teoria proposta.
Resumo -O modelo matemático clássico do gerador de indução duplamente alimentado não considera as perdas no núcleo magnético e suplementares. Porém, ele é válido quando o gerador entrega sua potência nominal, o que não ocorre no processo de sincronização, onde o gerador está a vazio. Em geral, os modelos que consideram essas perdas tornam o equacionamento mais complexo. Neste sentido, este artigo propõe que se considerem estas perdas como agrupadas na resistência do rotor, o que mantém as equações da máquina inalteradas. Ainda, apresenta-se um mé-todo simplificado para a estimação dos parâmetros do circuito do rotor, os quais são utilizados para sintonizar os controladores de corrente durante o processo de sincronização à rede elétrica. Os resultados de simulação e experimentais confirmam a eficácia da abordagem proposta.Palavras-Chave -Estimação de Parâmetros, Gerador de Indução Duplamente Alimentado, Sincronização.
PARAMETER ESTIMATION AND CONTROL OF A DOUBLY-FED INDUCTION GENERATOR DURING THE SYNCHRONIZATION WITH THE GRIDAbstract -The conventional mathematical model of the doubly-fed induction generator does not take into account the magnetic core and the stray load losses. However, this model is valid when the generator delivers its rated power, which does not occur during the synchronization process, as it is a no-load situation. In general, the models that consider these losses make the equations more complex. Thus, this paper proposes to consider these losses as grouped in the rotor resistance, which keeps unchanged the machine equations. In addition, a simplified method is presented for estimation of rotor circuit parameters, which are used to tune the current controllers during the process of synchronization with the grid. The simulation and experimental results confirm the effectiveness of the proposed approach.
This work presents a literature review about control techniques for parallel connected power inverters under microgrid applications. Some control strategies, based on droop control for parallel inverters of distributed generation units in an ac distribution system will be presented in this work. Finally, an important method called Virtual Oscillating Control (VOC) is suggested for connecting voltage source inverters. Inverters are able to work in parallel with a constant-voltage constant frequency system, as well as with other inverters and also in standalone operation. The different power sources can share the load also under unbalanced conditions. Throughout this work several simulation results are presented in order to demonstrate the behaviour the behavior of the different control strategies tested.
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