Control of a Single-Phase Grid-Connected Voltage Source Inverter with LCL Filter
Eren, Suzan Zeynep
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This thesis presents new control approaches for improving the performance, stability, and efficiency of a single-phase grid-connected voltage source inverter (VSI) with an LCL filter that is used in renewable energy power conditioning applications. There are two main controllers that need to be designed: an external DC-bus voltage controller to balance the power flow coming into the VSI, and an internal current controller to control the current injected by the VSI into the utility grid. This thesis aims to find well-tailored control approaches for the aforementioned control loops. First, the stability and behavioral characteristics of the open-loop VSI with an LCL filter are explored using a Poincaré map, and the open-loop system is found to have marginal stability. A current control method is proposed, called composite nonlinear feedback (CNF), which offers significantly improved overall performance compared to the state-of-the-art proportional resonant (PR) controller with state feedback. To reduce the overall number of sensors in the system, two different observers are implemented to estimate the VSI state variables: (1) the Luenberger observer (LO), and (2) the sliding mode observer (SMO). To balance the system power flow, a new DC-bus voltage droop control method is proposed, that provides fast performance during transients. This control approach includes a novel discrete DC-bus voltage sensing technique, which effectively removes the double frequency ripple from the DC-bus voltage signal and prevents it from propagating into the current control loop. A variant on the DC-bus voltage droop control method, called adaptive droop control is proposed, which adaptively changes the droop gains in order to regulate the DC-bus voltage to a constant value. Finally, another variant on the DC-bus voltage droop control method is proposed, called optimized adaptive droop control, which adaptively changes the gains of the controller in order to minimize the overall system power losses. A stability analysis is conducted using the singular perturbation control theory, which allows a nonlinear dynamical system to be broken down into subsystems with different time scales. The results of the stability analysis confirm that the proposed closed-loop grid-connected VSI with an LCL filter is locally stable.