Optimization and analysis of the current control loop of VSCs connected to uncertain grids through LCL filters

Abstract

This thesis focuses on the design and analysis of the control of voltage source converters connected to the grid through LCL filters. Particularly it is centered on grids presenting uncertainty in their intrinsic dynamic parameters and their influence over the inner control loop of a grid converter: the current control. To that end, the thesis follows a three-fold discussion. Firstly, the thesis studies the grid model, its uncertain parameters and presents a proposal to recursively estimate them. The estimation is based on a recursive least-squares optimization procedure applied to the current and voltage measurements, performed in the point of common coupling, expressed in a synchronous reference frame. The synchronization and the reference frame transformation process is specially designed for the proposed system. The optimization process is complemented with an estimation evaluation block that gives a real-time measure of the estimation quality. The influence of those uncertain parameters over the stability of the current control loop of grid converters is the second topic of this thesis. For the case of linear controllers, the analysis is performed by applying the structured singular value mu theory to a parametric uncertainty model that is described in the document. The proposed method extracts safe grid parameters ranges from a previously defined controller and plant model. Special attention is payed to important practical considerations as pure real uncertainty and sampled-data systems analysis. To test the method performance and illustrate its behavior, this dissertation discusses the robustness of three particular examples: a SISO control approach, a MIMO servo-controller approach and a robust H_inf design. For the case of non-linear controllers, the thesis focuses on hysteresis controllers and presents some practical conclusions. After that analysis, the thesis deals with the complementary problem: the design of a robust controller for grid converters connected through LCL filters to grids whose parameters range between known values. As a prior stage, the thesis presents an LQ servo-controller design procedure that may be complemented with the use of state estimators. The control is faced in a synchronous reference frame and directly controls the grid injected current. Once the framework is settled, the thesis proposes a design technique based on a robust Loop-shaping H_inf design procedure complemented with the nu-gap analysis tool. The final part of this dissertation describes the experimental set-up used for testing the presented proposals. After this, a summary of experimental results and waveforms is presented.