A linear and decoupled control strategy for VSC based HVDC system

For VSC based HVDC systems, the VSC is a double-input, double-output control object, and if the modulation index and phase angle of PWM are selected as the direct control variable, the VSC will be a nonlinear, coupled control object. In this paper, the voltage across the equivalent converter impedance is decomposed into two orthogonal voltage space vectors, which determine the active and reactive current injected into the AC system respectively. Then the amplitudes of the two vectors are taken as the direct control variables; furthermore, a steady-state model is established with the selected control variable. On this basis, a linear and decoupled control strategy is developed using PI controllers, with feedforward control and proper feedback compensation. Simulation results show the feasibility of the proposed control strategy.