Robust Nonlinear Control of MMC-VSC for Connection of Energy Resources to Weak Grids with Low SCR

This paper discuss the application of robust finite-time nonlinear control scheme to voltage source converters (VSCs), to enhance the robustness and transient response of the VSC connected to a weak ac grid. Recently, modular multilevel converter (MMCs) are extensively used to connect large-scale renewable energy resources to grid, through MMC-VSCHVDC systems. Lyapunov stability criterion is used along with the detailed nonlinear model of the grid-connected VSC to derive an effective control scheme which is independent of the ac system strength. Power flow control and DC link voltage control are investigated. Finite-time convergence of the proposed controller provides very fast and accurate tracking capability to the closed-loop VSC system. The robustness of the system is mathematically proved and substantially analyzed in detail, for external disturbances and parameter variations. Average-value model of the VSC is used for simulation. The proposed scheme is applied to control of a modular multilevel converter VSC-HVDC system in both rectifying and inverter modes. Power control, current regulation, and circulating current suppressing control loops of the MMC-VSC are investigated using the proposed FTC scheme. Moreover, performance of the proposed control scheme is evaluated in connecting the grid-side VSC of a large-scale wind farm to a weak grid with a low SCR value at the PCC. The impact of different grid’s strength and the injected active/reactive power on the variations of the PCC’s voltage of a weak grid are investigated. It is shown that the proposed FTC yields fast transient and resilient response, and improves the robustness of the VSC control system in connection to a weak grid

Keywords:

Voltage source converter , Modular multilevel , converter , HVDC , control Lyapunov function , finite-time control