Control Strategies of Power Electronic Converters to Improve the Reliability and Stability of Renewable Energy Systems

This dissertation discusses control strategies for power electronic converters that improve the reliability and stability of renewable energy systems. Three approaches are proposed to improve the control performance of a dc-dc converter and a distributed generation (DG) inverter under different operation modes and fault conditions. First, a seamless control for a dc-dc converter with both discontinuous conduction mode (DCM) and continuous conduction mode (CCM) is proposed. The plant models in DCM and CCM are different in the frequency domain. Therefore, it is difficult to design a controller with stable operation and fast response in both modes. The proposed controller can make mode transitions between DCM and CCM seamlessly with a mode tracker, and then the boost converter can autonomously operate by selecting the appropriate control loop in both modes. Second, a seamless control for the DG inverter with both a grid connected (GC) mode and a standalone (SA) mode is presented. With increasing renewable DGs, fast and stable mode transition technologies are necessary not only for sending the power to the grid in the GC mode, but also for protecting DGs from grid fault conditions in the SA mode. The proposed controller consists of a current controller and a feedforward voltage controller to minimize the grid overvoltage and improve the voltage response.Third, a control strategy to suppress a dc power oscillation of the DG inverter under grid voltage unbalance is discussed. Due to voltage unbalance, the dc power oscillation is generated, which impacts the lifespan of the renewable energy sources. A modified synchronous reference frame based current control with improved current reference is proposed. With the proposed current loop, the dc power oscillation is reduced effectively. The proposed control strategies reduce the impact of the renewable energy and the load under faults or disturbance conditions. And the stable operation of the power electronic converters will also enhance the stability and reliability of the renewable energy, the grid, and the load.