Study on improved droop control strategy of MMC-MTDC system based on novel sliding mode control

Abstract

In view of the strong coupling, nonlinearity and uncertainty of HVDC system, and the poor consistency when the MMC control policy adopts the traditional PI double closed-loop control, an improved droop control policy with the novel sliding pattern controlling is given herein. The controller brings to the dual closed loop control structure, the internal loop takes current error in the dq axis as the sliding mode surface. A sliding mode controller with the novel exponential approach law is designed, which improves the convergence speed of the traditional exponential approach law and eliminates the chattering phenomena. Then the consistency of the system is identified by Lyapunov method. The outer-loop adopts a modified sag control policy. After comparing with the voltage reference value, the droop factor is proposed. Finally, an emulation model including MMC-MTDC with four terminal is constructed on the PSCAD/EMTDC, which is emulated and validated under stable and transient conditions. The consequences reveal that the given control policy not only has a good convergence effect, but also improves the kinematic respond speed. It can also reduce the direct current voltage deviation, and enhances the stability and reliability of the system.