Technical and Economic Aspects of Tripole HVDC

This paper shows how, with simple modifications using either conventional or bidirectional valves, bipole and monopole systems can be connected in parallel and operated as a three pole configuration in which no earth return current flows in either normal or pole-out conditions. While slightly less efficient in terms of equipment utilization, the system reduces line losses for a given MW transfer and has better overload and internal redundancy; both of which enhance (n-1)-constrained loading of the parallel ac system. Because it makes full thermal use of three conductors, the tripole system improves both the technical and economic cases for converting existing three-phase ac lines to dc. Doing so increases power transfer much more than is possible with compensation or phase shifting transformers. The paper reviews the basic principles inherent in the tripole HVDC system and compares characteristics of both bipole and tripole alternatives in the context of an ac network.