Field-effect limits and design parameters for hybrid HVDC/HVAC transmission line corridors.

Abstract

New generation centres and servitude constraints in Southern Africa have promoted interest in the possibility of dc and ac lines in close proximity to each other, and even sharing structures. Certain interactions of HVDC and HVAC transmission circuits have been analysed by other authors, but this has mainly focused on conversion of existing double-circuit transmission lines to hybrid lines, and has mostly been in European contexts. The dissertation reviews this prior work, and extends it to a Southern African context. First a framework is developed to describe engineering considerations for ac and dc lines in terms of power transfer capability, air insulation, corona-related phenomena, electric and magnetic fields, and behaviour under fault conditions. A study system amenable to analysis is developed, consisting of two hybrid dc/ac transmission corridors, each with a 1000MWac line and a 3000MWdc bipole. The ac current flowing in the pole conductors due to coupling is estimated, and found to be an issue requiring mitigation. Line transposition is adequate mitigation during normal operation, but 50 Hz blocking filters or special converter controls are needed under fault conditions. A parametric study of conductor surface gradients for the study system was carried out; some calculated dc values seemed misaligned with service experience, possibly due to ionic current flow and space charge. Using the corona saturation method, ion density, ion current and electric field at ground level are estimated. Corona-related phenomena are investigated using empirical equations from the literature and the calculated conductor surface gradients. Behaviour under hybrid dc/ac energisation is not fundamentally different to corona behaviour under pure ac or dc energisation. For ac, sound pressure levels were calculated for each phase, but for dc only the positive pole was considered, because the contribution from the negative pole is negligible. Radio interference for both ac and dc is calculated using empirical expressions. Optimal voltage and conductor sizes are investigated using generalised cost models developed by Cigré. It is found that the study system could be optimised better. Feasible improvements to the system, limits on some field-effect related parameters appropriate to Southern African conditions are suggested. These consist of different conductor bundles, and a reduction in the power transfer to 800 MW for the ac circuits and 2000 MW for the dc circuits. Based on the analysis, it is concluded that there is scope for hybrid dc/ac corridors in the region, and the theoretical basis for doing this is exemplified.