Analysis and design of vector control for VSC-HVDC connected to weak grids

Voltage source converter based high voltage direct current transmission (VSC-HVDC) is considered one of the most suitable technologies to integrate renewable energies. However, connecting VSC to a weak grid is challenging since traditional vector control tends to become unstable under high power demand conditions. In this paper, an improved vector control method is proposed wherein a feed forward branch based on steady state and small signal analysis of the VSC system is added under weak grid situations. The feed forward branch promotes faster reactive power response, thus enhancing the stability of the VSC system. Since the improved vector control uses the same inner loop as traditional vector control, the proposed method allows for the ability to retain fault current suppression capabilities. Furthermore, the control parameters of the outer loop of the improved vector control need not vary according to the variation of the operating points, which makes it easy to implement. The feed forward branch is implemented by solving a nonlinear equation or through use of a look-up table. The influence of the estimation errors of short circuit ratio (SCR) on the control performance is also studied. The effectiveness of the improved vector control is demonstrated through small signal model analysis and time domain simulations.