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Abstract
Various kind of filters have been used to interconnect PWM inverters to the utility grid. Among those filters, LCL filter is widely used for many applications because it can reduce more harmonics of an injected grid current, as compared to other filters. However, it has a poor impact on the stability of the system. Furthermore, it increases the level of difficulty in designing the controller of the system, if only grid current is used for the feedback control. To improve the system controller design, the Weighted Average Current Control (WACC) method has been proposed, where the weighted average of the inverter-side current and the grid-side current is used as a control parameter. This method reduces the order of the system to a first order. However, this method stabilizes the weighted average current instead of an injected grid current. The injected grid current is shown to be only marginally stable. In order to make the system more robust and stable, various damping methods have been introduced such as passive and active damping. In passive damping method, a resistor is connected in series with the LCL capacitor, which helps in damping the LCL resonance. Whereas, in active damping method, an extra-feedback of capacitor voltage or current state are utilized to increase the damping behavior of the system. Due to power losses issue in passive damping method, efficiency of the system decreases. Hence, the WACC with passive damping method is not desirable. Likewise, the WACC with active damping method also has a drawback of a more complex controller design because of the use of an extra-feedback loop in the controller. In order to overcome these limitations in both damping methods, a new current control method has been proposed and implemented, which has been investigated in this dissertation. By applying this new method, the gain of an inverter-side current is shown to be higher than that of the grid-side current. In this method, the current gains help in providing damping to the system, which improves system stability. Therefore, the system with this new method is actively damped with no additional feedback loop. Hence, this new method is named as the Weighted Average Current Controller with Embedded Active Damping (WACC_EAD) method. The robustness of the system under various conditions is also analyzed for WACC_EAD method. The dynamic stiffness of a system for various current control methods have been derived to analyze the stiffness of injected grid current. Simulations using MATLAB Simulink and Typhoon and hardware experimental results using PICCOLO-F28035 controller have been demonstrated to validate the theoretical analysis.