Abstract:  ABSTRACT：   When AC-side failures of voltage source converter multi-terminal DC （VSC-MTDC） system leads to power grid voltage sag, this paper designs the coordinated control of the active and reactive power of each converter during the fault to maintain the voltage level of related side point of common coupling （PCC）. In the duration of the fault, the converter near the fault is switched from preference of active current control to preference of reactive current control and generates reactive power depend on the grid voltage sag. When added reactive power does not cause over-current on the AC side, the extra current capacity is used to maintain the active level before the fault. When added reactive power causes over-current on the AC side, this paper proposes a control strategy to avoid the over-current by changing the active power of the converter related to fault side. Meanwhile, this paper comes up with the control strategy based on the droop control to convert the power into control signals that are instructed by input voltage. With the changing of the active power of the converter near the fault, the active power is out of balance and the DC capacitors rises or falls due to the charge or discharge. At the meantime, other converters adjust their own active power instructions without the communication between stations automatically according to the changing of the DC voltage, which can attain the goal of self-adjustment and power automatic-distribution and realize autonomous decentralized control. In the end, this paper constructs the simulation model of the VSC-MTDC system in the PSCAD/EMTDC to verify the effectiveness of the proposed control strategy. 

Key words: VSC-MTDC, voltage-sag, voltage level at PCC, coordination control, current limitation