Analysis and Control of DC Voltage Dynamics Based on a Practical Reduced-Order Model of Droop-Controlled VSC-MTDC System in DC Voltage Control Timescale

The DC voltage dynamics of droop-controlled voltage source converter based multi-terminal HVDC system (VSC-MTDC) are more complicated due to their transient overshoot and steady-state deviation. Firstly, to solve the DC voltage dynamic problems of the droop-controlled VSC-MTDC system with strong adjacent AC system, a practical reduced-order model of a VSC-MTDC system is established in DC voltage control timescale. Besides, the droop-based outer controller is considered as a droop admittance with lag-lead characteristic, which provides a physical explanation of DC voltage overshoot caused by droop controller coefficients. Moreover, the maximum DC voltage deviation that may occur under any power disturbance can be easily estimated by applying the frequency response analysis to the reduced-order model. Secondly, the damping of droop-controlled VSC can be easily enhanced by adding a simple compensator at the input of the DC voltage feedback loop. In this way, the droop admittance can present like a pure conductance for limiting DC voltage deviation under power disturbances. Finally, electromagnetic transient simulations are conducted to validate the accuracy of the reduced-order model of VSC-MTDC systems and the effectiveness of the proposed damping compensation method for DC voltage overshoot.