Field-effect limits and design parameters for hybrid HVDC/HVAC transmission line corridors.
Loading...
Date
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
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.
Description
M. Sc. Eng. University of KwaZulu-Natal, Pietermaritzburg [2015].
Keywords
Hybrid power systems--South Africa., Electric power transmission--South Africa., Field-effect transistors., Theses--Electrical engineering.