Flexible Control of Power Flow in Multiterminal DC Grids Using DC-DC Converter

This paper proposes an efficient control framework that utilizes de-de converters to achieve flexible power flow control in multiterminal de (MTDC) grids. The de-de converter employed in this paper is connected in cascade with the de transmission line, and is therefore named cascaded power flow controller (CPFC). In this paper, a two-layer control strategy is developed for the operation and control of voltage source converter stations and CPFC station in MTDC grids. At the primary control layer, a novel differential voltage droop control is developed, while at the secondary control layer, a modified de power flow algorithm-employing the new CPFC frameworkis implemented. The overall control strategy enables the CPFC to regulate the power flow in the de transmission line. The primary control guarantees the transient stability of the CPFC, and the secondary control system ensures the desired steady-state operation. The proposed voltage droop control framework helps the MTDC grid to remain stable in the event of a communication failure between the primary and secondary control layers. Static analysis and dynamic simulations are performed on the CIGRE B4 de grid test system, in order to confirm the effectiveness of the proposed control framework for power flow regulation in MTDC grids.