Sparse Matrix Converters for Interfacing Variable Speed Generators with the Grid

Abstract

During the last decade, variable-speed wind turbines based on Permanent Magnet Synchronous Generators (PMSGs) attracted considerable interest. Many power electronic converters are used to interface the PMSG with the grid such as back-to-back voltage source converters, dc-link boost chopper converters, and AC/AC converters. The AC/AC converters, where no storage elements are used, can be classified into three topologies. The first topology is the ac voltage controllers, known as AC choppers, which can be used to change only the effective value of the output voltage without controlling the output frequency. The second type is the cycloconverter which is used when the output frequency is much lower than the input source frequency. The third topology is the Matrix Converters (MCs) which converts the AC input to AC output with variable amplitude and frequency. The MCs are divided into Direct Matrix Converter (DMC) and Indirect Matrix Converter (IMC). The DMC consists of nine bidirectional switches (i.e. eighteen transistors and eighteen diodes), one switch between each input phase and each output phase. The disadvantages of the DMC are the high number of devices and the complexity of its control and commutation process. The IMC is a two-stage converter topology which is similar to the conventional rectifier-inverter topology but without DC-link energy storage elements. Sparse Matrix Converter (SMC) is introduced to reduce the rectifier stage circuit complexity; the number of transistors is reduced to fifteen transistors instead of eighteen transistors of the IMC topology. The Space Vector Modulation (SVM) technique is used to control the SMC. However, the SVM demands a computational complexity which sophisticates its practical implementation. The carrier based Pulse Width Modulation (PWM) technique is implemented to control the IMC. This technique supersedes the SVM by its simplicity and low computational demand. This thesis presents a simple control system for SMC to interface PMSG based wind turbine unit with the power grid. A carrier based PWM technique is extended and adopted for the SMC. Details of the proposed switching strategy and derivations of the modulation functions for both the rectifier and inverter stages of the SMC are presented. The inverter stage is controlled to regulate the speed of the PMSG for maximum power extraction. In addition, the rectifier stage of the SMC is controlled to deliver the generated active power from the PMSG to the grid at the required power factor to satisfy the reactive power demand. The proposed control system is capable to manage active and reactive power flow in a decoupled manner. The injected reactive power to the grid is restricted by the limit on the power factor angle of the rectifier stage. An active damping principle is applied for SMC to damp out the oscillations caused by the input LC filter. Numerical simulations are conducted to investigate the effectiveness and the fast dynamic performance of the proposed interface system with and without the active damping principle.