Dynamics, Robust Control, and Power Management of Voltage-Source Converters in Hybrid Multiterminal AC/DC Grids

  • Author / Creator
    Davari, Masoud
  • The electric energy sector is moving toward extensive integration of clean and renewable energy sources, energy storage units, and modern loads via highly efficient and flexible multiterminal dc grids integrated within the traditional ac grid infrastructure in both transmission and distribution levels. A voltage-source converter (VSC) is the main technology enabling the interconnection of dc and ac grids. In such demanding applications, effective and robust integration of ac and dc grids, in the presence of coupling nonlinear dynamics, parametric uncertainties, and disturbances, is crucial to maintain the stability and robust performance of the overall ac/dc dynamic system. Motivated by this objective, this thesis addresses the dynamics, robust control, and power management of VSCs in hybrid multiterminal ac/dc grids. Firstly, a robust multi-objective dc-link voltage controller is developed for a bi-directional VSC regulating the dc-link voltage of a multiterminal dc grid; i.e., the VSC operates as a dc-voltage power-port. The proposed controller ensures excellent tracking performance, robust disturbance rejection, and robust stability against operating point and parameter variation with a simple fixed-parameter low-order controller. Secondly, the dynamics and control of VSCs considering the instantaneous power of both ac- and dc-side filters and dc grid uncertainties are addressed in the this thesis. The proposed controller ensures excellent tracking performance, robust disturbance rejection, and robust performance against operating point and parameter variation with a simple fixed-parameter controller. Thirdly, this thesis presents a natural-frame variable-structure-based nonlinear control system for the master VSC applied in multiterminal grids to overcome problems associated with conventional dc-link voltage controllers, which are suffering from stability and performance issues, mainly attributed to the small-signal-based control design approach and the use of cascaded control structure based on the power balance framework that yields unmodeled nonlinear dynamics. Fourthly, this thesis presents a robust vector-controlled VSC that facilitates full converter power injection at weak and very weak ac grid conditions (i.e., when the short-circuit capacity ratio is one). The controller overcomes problems related to the stability and performance of conventional vector-controlled VSCs integrated into very weak ac grids (high impedance grids) because of the increased coupling between the converter and grid dynamics, via the phase-locked loop (PLL). As a result, a detailed ac-bus voltage dynamic model, including the PLL dynamics, is developed and validated in this thesis. Then, the model is used to design a robust optimal ac-bus voltage controller to stabilize the dynamics under operating point variation and grid impedance uncertainty. Fifthly, this thesis addresses the challenges associated with a dc-voltage-controlled VSC interfacing a wind turbine into a dc grid, which is gaining widespread acceptance under weak grid connection or isolated operation. Under weak grid connection or isolated operation, the machine-side VSC regulates the dc-link voltage via changes in the generator speed. However, several control difficulties are yielded; important problems are: 1) the nonlinear plant dynamics with a wide range of operating point variation; 2) the control lever is mainly the generator speed, which complicates the dc-link voltage control dynamics; 3) the presence of uncertain disturbances associated with dynamic loads (e.g., power-converter-based loads) connected to the dc grid and wind speed variation; and 4) the presence of parametric uncertainty associated with the equivalent dc-link capacitance due to connecting/disconnecting converter-based loads. Finally, this thesis presents a robust power sharing and dc-link voltage regulation controller for grid-connected VSCs in dc grids applications to overcome difficulties and problems related to the dynamics and stability of a grid-connected VSC with dc power sharing droop control. Major difficulties are: 1) ignoring the effect of the outer droop loop on the dc-link voltage dynamics when the dc-link voltage controller is designed, which induces destabilizing dynamics, particularly under variable droop gain needed for optimum economic operation, energy management, and successful network operation under converter outages and contingencies; 2) uncertainties in the dc grid parameters (e.g., passive load resistance and equivalent capacitance as viewed by the dc side of the VSC); and 3) disturbances in the dc grid (i.e., power absorbed or injected from/to the dc grid), which change the operating point and the converter dynamics by acting as a state-dependent disturbance. A theoretical analysis and comparative simulation and experimental results are presented in this thesis to show the validity and effectiveness of the developed models and proposed control structures.

  • Subjects / Keywords
  • Graduation date
    Spring 2016
  • Type of Item
  • Degree
    Doctor of Philosophy
  • DOI
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.