Control strategy for hybrid high-voltage DC systems connecting voltage-weak grids

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.