Direct current conductor corona modelling and metrology

[摘要] ENGLISH ABSTRACT: Prospects of up-rating existing high voltage direct current (HVDC) transmission schemes, aswell as the conversion of existing alternating current (AC) to direct current (DC) lines and thedevelopment of new HVDC schemes in sub-Saharan Africa, have led to renewed interest in DCresearch. The radio interference (RI), audible noise (AN) and corona loss (CL) performance ofHVDC transmission lines are critical factors when assessing the reliability of the line design.The RI performance is especially important when considering the successful transmission of thecarrier signal of the power line carrier (PLC) system. The PLC system is the main form ofcommunication between teleprotection devices on the Cahora Bassa HVDC scheme.The aim of the dissertation is to devise modelling as well as metrological techniques to characteriseDC conductor corona. A particle-in-cell (PIC) computational code is developed togain a better understanding of the physical processes that occur during corona events. Thenumerical code makes use of the charge simulation method (CSM) andnite element method(FEM) to solve for the Laplace and Poissoneld equations. Higher-order basis functions areimplemented to obtain a more accurate solution to the Poisson equation. The computationaltool yields insight into the mathematical models for the various ionization, attachment andelectron avalanche processes that give rise to corona currents. Together with a designed anddeveloped electrometer-type circuit, the numerical code assists the visualisation of the space charge particle dynamics that form in the electrode gap during corona events. The metrological techniques consider the wideband time domain (TD) as well as the frequencydomain (FD) information of the measured corona pulses in the presence of noise. These arethen compared to the narrowband CISPR standard measurements centred around 500kHz. Theimportance of impedance matching when attempting to derive a wideband excitation functionis investigated. The TD measurements are quite distinct from the well-published FD measurements,and consider the pulse shape, pulse spectrum and pulse repetition rates. The use ofthree possible conductor corona test methods to study direct current conductor RI performanceunder both positive and negative polarities is investigated at high altitude in this dissertation.These include a small corona cage, a short test line and the Eskom Megawatt Park large outdoorcorona cage. Derived wideband and narrowband monopolar DC RI excitation functions at500kHz are consolidated with existing radio noise (RN) measurement protocols and predictionmethods.The use of a corona cage to derive excitation functions for monopolar RI predictions is exploredand it is shown that a small corona cage, due to the build-up of space charge in thesmall distance between the electrodes, cannot be used to predict the RI levels on HVDC transmissionlines accurately. As a consequence of the physics, computational modelling and bothfrequency and time domain measurements, it is now possible to explain why a small cage systemprevents the accurate RI prediction on transmission lines. The large outdoor corona cage andshort test line RI performance predictions agree with existing empirical prediction formulas.

[发布机构] Stellenbosch University