Robust multivariable control design for HVDC systems considering AC grid impedance uncertainties

Abstract

High-voltage direct current (HVDC) transmission system based on modular multilevel converter (MMC) technology is considered as an attractive solution for the interconnection of two AC grids to enhance the security of supply. Several studies have concluded that the AC grid impedance is a key parameter for the control system design of the MMC. Such important design parameter, however, is subject to wide variations due to the time-varying nature of AC grid and the future modifications and expansions. Therefore, the grid impedance has to be considered as an uncertain parameter during control design stage. To this end, this paper suggests a robust multivariable controller which is able to maintain system stability and performance for an extended range of grid impedance uncertainties. To ensure that the controller operates robustly in a wide range of grid impedances, structured singular value of the system (known as 𝜇) is minimized via an iterative procedure. In such procedure, the uncertainties are separated from the nominal linear model of the system and the weighing functions are tuned to fulfill the robustness criteria defined by the small-gain theorem. The effectiveness of the suggested controller and the design procedure is validated by means of simulation results.