Modeling and Control of Voltage Source Converters Connected to the Grid
Abstract: This thesis deals with the modeling and control of pulse width modulated (PWM) voltage source converters connected to the grid. When voltage source converters are connected to the grid, the power quality and the dynamic performance are affected by the line filter connected between the converter and the grid, and by nonlinearities caused by the switching converter. In the thesis, the dynamic performance and power quality of converters connected to the grid by first-order L-filters and third-order LCL-filters are focused on. For each line filter, predictive vector control principles that allow the independent control of the active and the reactive powers are developed and verified by measurements. It is shown that a similar dynamic performance can be obtained with both line filters. To obtain a high power quality, it is advantageous to use LCL-filters. The thesis also deals with different nonlinearities and their influence on dynamic performance. Measured small-signal frequency responses are compared with Bode diagrams obtained from linear analytical models. The analytical models are created by using a technique based on state space equations that is developed in the thesis. In the thesis, switching frequencies and sampling frequencies from 5 to 7 kHz are used. At such switching and sampling frequencies, nonlinearities and cross coupling caused by the uniform PWM method and the coordinate transformation in the control system do not have any significant effect on the small-signal frequency responses for frequencies below 1 kHz. It is, however, shown that a more ideal performance of the vector control system can be obtained by compensating for errors due to the non-ideal commutation caused by the blanking time and on-state voltage drops across the non-ideal IGBT valves of the converter. Moreover, measurements have shown that non-modeled losses in the line filter inductors have an impact on the small-signal frequency responses. When voltage source converters are used as active filters, the performance of the active filters are affected by phase shifts in the current control system, and also by the cross coupling between the control of the active and the reactive currents. Different principles for compensating for the phase shifts have been evaluated. Measurements show that it is possible to compensate for the phase shifts and thereby obtain efficient active filters also at moderate switching frequencies such as 5 to 7 kHz.
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