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VSC-HVDC transmission systems may overcome some unexpected stochastic time variant (STV) parameters uncertainties such as frequency disadjustments and cable parameters uncertainties inherent with AC grids. This paper first presents a mathematical modeling of VSC-HVDC transmission systems. These systems exhibit a nonlinear character, and therefore, two different methodologies for robust nonlinear controllers design based on Asymptotic Output Tracking (AOT) and the Sliding Mode Control (SMC) techniques are investigated to control theVSC-HVDC transmission systems submitted to network nonlinearities introduced by requirements to power flow and line voltage. Robust stabilizing feedback control laws are formulated to govern the DC link voltage and to control the active and reactive powers under STV parameters uncertainties. Both kinds of robust nonlinear controllers are demonstrated and compared through simulation studies on the VSC-HVDC transmission systems using MATLABcopy-Simulink software. Dealing with systems of high power ratings up to 300 MW, based on this comparison, the simulation results significantly clarify the controllers robustness and the control flexibility towards improving the system's dynamic behaviour and enhancing the system's stability under STV parameters uncertainties. Regarding the economical feasibility and the relative structural simplicity, the superiority of the controller based AOT over that of the SMC controller is pointed out.