The use of Distributed Generation (DG) or Distributed Resources (DR) is rapidly increasing in modern distribution networks. Many types of these resources generate electrical energy in the form of a dc voltage source. Consequently, they need a dc/ac converter for energy transfer from the dc source to the utility grid. For this purpose, voltage source converters are usually used to connect the dc source to the utility grid, and this task is normally done via a simple L filter. However, higher order filters, like LCL filters, can provide better harmonic attenuation and reduce the filter size at the same time. This thesis is concerned with the modeling and control of grid connected voltage converters. Open-loop and closed-loop state space models of these converters are derived in synchronous reference frame. The dynamic performance of a system with L filter and with conventional PI controllers is shown. Independent active and reactive power control is described. Basic design criteria in designing LCL filters are addressed. Various mathematical expressions and plots are given for design in this respect. Open-loop and closed-loop state space modeling of LCL filter in synchronous reference frame shows that system dynamic response with conventional controllers is not desirable. In order to improve system stability and dynamic response, state feedback along with output feedback is used and performance of such system is demonstrated through modeling and simulation. Application on observer for reducing the number of sensors is demonstrated. Phase locked loop as a nonavoidable part of these systems is explained. A method for improving PLL performance in unbalance environments is proposed. Grid connected converter with L filter is implemented using a DSP. The validity of theoretical analysis and simulations is shown by comparison with experimental results.
