This paper addresses two different robust nonlinear control methodologies based Sliding Mode Control (SMC) for the Voltage Source Converters (VSC) High Voltage Direct Current (HVDC) transmission system's performance enhancement and stability improvement. After developing the system's steady state mathematical model which exhibits a nonlinear character, appropriate feedback control signals based on SMC are formulated to control the active and reactive power towards their specified set point values, hence, desirable unity power factor at both sides of the DC link is attained. Additionally, the DC link voltage is governed to its desired value. The active and reactive powers, as the outputs of the state space equations, are controlled via SMC by using their reference signals directly. Otherwise, either a supplementary proportional-integral (PI) controller or an extra feedback two degrees of freedom proportional-integral-derivative (2DOF-PID) controller is utilized to govern the DC voltage and to generate the direct and quadrature (d-q) currents reference values which are then used for controlling the corresponding active and reactive powers via SMC. Both nonlinear control methodologies based on SMC are compared in terms of behavioral analysis and robustness. Using MATLAB™-Simulink, the results significantly verify that the desired robust controllers based on SMC can achieve favorable tracking performance, and contribute efficiently towards improving the system's dynamic behavior, under either normal operating conditions or parametric uncertainties, for different DC link lengths.