| dc.description.abstract | Transmission of bulk electric power from the generating stations to the load centres can be carried out only through high voltages transmission lines. One of the main issues in the design and maintenance of extra and ultra-high voltage transmission system is the phenomenon named corona. It is the local electrical breakdown of air in the vicinity of the line conductors and hardware. Even though the design and dimensions of these elements are made considering the corona onset, surface abrasions arising either during installation or operation can lead to intolerable corona. Apart from producing some insignificant chemical reactions and noticeable acoustic noise, they can be a significant source of electromagnetic interference. In the early days, this interference was of concern only to radio and television receptions, however, with extensive use of wide frequency bands for modern applications, it has assumed prime importance.
The EMI due to the transmission line corona has been extensively studied and reliable empirical formulas have been proposed. The basis for all the earlier studies was the experimentally measured corona currents. This approach fails for new line designs especially with higher and higher voltages being employed due to non-availability of experimental data. A second approach assumed corona current to be injected into the line and subsequent analysis was carried out based on transmission line model. However, there were assumptions made on the mode of corona current injection into the conductor and the frequency range involved were also not adequate for the modern-day applications. Applicability of transmission line model for analysis is also questionable.
From a theoretical perspective, the coupling of the field produced by corona to the conductor was hardly investigated and the total field produced by the corona itself was not quantified. In order to address these serious lacunae, the present work was taken up and it can be considered as the first leap towards the correct picturization, as well as, quantification of the problem.
The field produced by the electron avalanche involves noticeable retardation effects. In the literature, only the field produced by arbitrarily moving point charge of fixed strength is given by the Heaviside-Feynman equation. On the contrary, the avalanche involves an arbitrarily moving charge of time varying strength at its head with trailing positive charge, which is almost stationary. Starting from the basics, an analytical expression for the electric field due to an arbitrarily moving point charge of time varying strength is derived which forms a fundamental contribution to Electrodynamics. This is extended to deduce an expression for the total electric field due to an avalanche for the very first time. Suitable validation of the expression is provided through
numerical simulation of electric field integral equation.
Corona discharge is a complex phenomenon having many distinctly different modes which differ in their visual, as well as, electrical characteristics. Innumerable electron avalanches contribute to the measured corona current with their space-charge acting as a moderator. Therefore, in order to model the corona on conductors, an indirect approach based on linear system theory is proposed. An equivalent spatio-temporal dipole distribution was obtained to produce the measured current on the conductor. The general expression derived for the isolated avalanche is extended for this purpose.
Using the above, the means of induction, spatial decay rate of corona current in the close range, its propagation mode and field produced by both avalanche/equivalent dipole and that due to induced current in the conductor, have all been investigated and quantified.
In summary, the contributions made in this work are more of fundamental in nature and would be of significant interest to the high voltage power transmission line, as well as, to the communication engineers. | en_US |
