Analysis of power redispatch schemes for HVDC grid secondary voltage control

In a droop-controlled High Voltage Direct Current (HVDC) grid, the line voltage drops throughout the system influence the voltage control. This paper introduces and compares five distinct methods for establishing droop setpoints in order to restore the voltage profile after a contingency and evaluates the ability of these methods to track the setpoints. Furthermore, the influence of the network and of the droop characteristics on the offsets is derived analytically. To ensure accurate setpoint tracking, the problem can be circumvented by redefining the redispatch as an optimization aiming at minimizing the changes in converter powers whilst achieving a predefined average system voltage. In order to avoid the need for solving the non-linear power flow problem, the voltage deviations from the post-contingency operation can also be scaled to obtain an approximate solution. Simulations on a 4-terminal meshed HVDC grid demonstrate the degree to which the different methods allow to track the setpoints.