With the rapid development of new-type power systems, the proliferation of AC–DC transmission corridors and inter-provincial tie-lines has intensified interconnection among provincial grids, posing challenges for tertiary voltage control. Conventional methods often treat external AC–DC systems as PQ equivalents, which underutilizes VSC-HVDC reactive power capability and neglects inter-provincial coordination, leading to degraded boundary voltages and higher losses. This paper proposes a tertiary voltage control optimization for large-scale AC–DC interconnected systems that explicitly models LCC-HVDC and VSC-HVDC converters. By incorporating converter characteristic equations, the model exploits VSC-HVDC reactive power flexibility, while inter-provincial tie-line power constraints ensure consistency with scheduled transactions. Coordinated tie-line control variables are then provided to regional dispatchers, enabling enhanced coordination with provincial centers. Case studies on the IEEE 39-bus and IEEE 118-bus systems verify that the proposed approach improves voltage stability and reduces network losses, demonstrating its effectiveness for coordinated grid operation.

Reactive power optimization, tertiary voltage control,VSC-HVDC, tie-line active power constraints