Control Strategy for Hybrid High-Voltage DC Systems Connecting Voltage-Weak Grids

Abstract

High-voltage dc (HVDC) systems are being adopted for grid connections by grid operators and transmission owners. Among the converter topologies, the line-commutated converter (LCC) is more economical than the voltage-sourced converter (VSC); however, its reactive power cannot be controlled. Hybrid HVDC, a mixed topology containing both LCC and VSC, demonstrates the benefits of both topologies. In this paper, a hybrid HVDC control scheme with an LCC sending end and a VSC receiving end is proposed. It is used to control sending-end reactive power. Using the proposed secondary controller, hybrid HVDC using only shunt devices, filters, or capacitor banks can have the same reactive abilities as VSC HVDCs. This reduces the cost of connecting the voltage-weak grids and ensures that the voltage-weak grids are not afflicted by voltage stresses. The proposed secondary controller was verified to be effective through PSCAD/EMTDC simulations.