Contribuições ao estudo de conexão de sistemas fotovoltaicos à rede elétrica sem filtros passivos: projeto de controladores digitais para redução do conteúdo harmônico

Abstract

The current thesis contributes to the analysis, modelling and design of discrete time controllers which aim is to control a 30 kWp photovoltaic dispersed generation system connected to the electric grid without passive filters. In fact, the interface voltage– sourced converter (VSC) is connected to the grid using only the leakage inductance of a single–phase transformer bank as harmonic filters. Initially, discrete time models are developed to the converter’s DC–side as well as to the AC–side. The AC–side modelling is performed on αβ0 and dq0 coordinate systems. On the other hand, the DC–side dynamics are modeled on the dq frame according to the power balance between the converter’s terminals. Based on the models obtained, strategies to control the converter in the current mode control on the αβ and dq are developed and a methodology to design the controllers are addressed in details. Experimental results shown that only the use of linear controllers, proportional–integral (PI) and proportional–resonant (PR), tuned on the fundamental component are not sufficient to guarantee the quality of the generated currents according to international standards. This is due to the operation of the connection transformer in a nonlinear region. In order to overcome this drawback, two solutions are taken into account: (i) inclusion of several parallel resonant controller in αβ frame; and (ii) inclusion of a repetitive controller in parallel with the PI controller in the dq frame. Experimental results shown that both strategies are suitable to compensate the harmonic components on the output current. Finally, a strategy is proposed to control the system under asymmetrical faults (fault–ride through) and to mitigate the voltage oscillation on the DC–side during unbalance conditions. The proposed controller is composed of a PI part and two resonant parts, which controls the average and the oscillating voltage components, through the injection of positive and negative sequence currents into the grid, respectively. Simulation results shown that the proposed controller is suitable to mitigate the DC–side voltage oscillations without jeopardizing the system stability.