Abstract:
This dissertation presents the means to evaluate lightning performance of pole-mounted
distribution transformers and Medium Voltage (MV) distribution lines. The main intention is to
provide a general bench-mark procedure for analysis of lightning performances of pole-mounted
transformers and MV distribution lines and thereby to ensure proper installation and reduction
of the high lightning failure rate existing in Sri Lanka.//
In installation of HV surge arresters, the separation length between the arrester and the high
voltage bushing is of paramount importance. Thus the calculation of the separation lengths and
failure rates are first discussed and the drawbacks of the present method of installation are
presented. The required separation depends on several factors such as tower configuration,
system voltage, tower surge impedance, the length of the earth wire of the arrester etc. Effect of
ground flash and residual voltage of arresters impart a considerable contribution to the
transformer protection. An alternative approach for this is also presented. The role of the earth
resistance and its contribution to High Voltage and Low Voltage failures due to lightning are
discussed\ indicating typical modes of failure. The determination of a safe value for the earth
resistance is also presented//
Flashover due to lightning induced surge voltages is of paramount importance in evaluating
reliability and power quality of Medium Voltage distribution lines. First, analysis has been done
on flashovers due to direct lightning. Next a comprehensive analysis has been done to develop a
method to evaluate lightning performance against lightning induced flashes. Accuracy of the
algorithms to calculate lightning induced line flashes highly depends on parameters such as
wave front time of return stroke current, magnitude of the return stroke current and its
probability distribution and the speed of the return stroke current. Having selected the most
suitable values for these parameters, a technique has been developed to determine line flas hover
rates due to induced lightning. Next, cumulative effect of both direct and indirect lightning surges
has been discussed. Analysis has been done to study dependence of line flas hover rates on line
insulation levels,isokeraunic level, line height, wave front time, corona damping, shielding due
to near by objects etc. Variation of line flas hover rates with above parameters has been
discussed. Theoretical results have been compared with practical data as a measure to check
adequacy of the developed analysis technique.
The report finally presents simplified preventive measures that can be implemented in Sri Lanka.
