Enhancement of voltage quality in a passive network supplied by a VSC-HVDC transmission under disturbances

When a VSC-HVDC transmission is connected to a passive network, the receiving AC system is weak and is subject to power quality problems such as voltage sag and swell. Dynamic performance of voltage control would affect the voltage quality in the receiving AC system directly. The direct voltage control (DVC) method is simple but fails to quickly eliminate voltage fluctuation caused by load current change. One solution is to add a feed-forward controller to the DVC. The inverter nonlinearities, however, degrade the performance of the feed-forward controller. This paper presents a modified direct voltage control to enhance control dynamics in the receiving AC system by overcoming the effect of inverter nonlinearities. In the proposed control scheme, the influence of inverter nonlinearities on the performance of the feed-forward controller is first discussed. A compensation method for nonlinearities of the inverter is then designed in the d-q rotating axis. Moreover, to overcome parameter sensitivity of the feed-forward controller, a feed-back loop with a PI controller is implemented. Simulation results from PSCAD/EMTDC showed that the VSC-HVDC system using the proposed control scheme, as compared to that with the conventional control methods, has a better capability to mitigate voltage fluctuation during system disturbances. (C) 2013 Elsevier Ltd. All rights reserved.