Tomomi Narita

Parameters of lightning strokes: a review

The paper presents the statistical data of the significant parameters of lightning flash, collected by many researchers over many years around the world. The significant parameters of a lightning flash are: peak current, waveshape and velocity of the return stroke, the total flash charge and /spl int/I/sup 2/dt. Negative first strokes have traditionally been considered to produce the worst stress on the system insulation. The subsequent negative strokes have significantly lower peak current but shorter wavefronts. This may stress the system insulation more. The positive strokes have about the same median current value as the negative first strokes and longer fronts, thus producing less stress. However, their duration is longer than that of the negative strokes. Therefore, the system insulation may be damaged because of the lower volt-time characteristic for long-duration waves. The positive strokes may also cause more thermal damage because of their significantly higher charge and /spl int/I/sup 2/dt. The relationship between the return-stroke velocity and the current peak is a significant parameter in estimating lightning-induced voltages and also in estimating the peak current from the radiated electromagnetic fields of the lightning channel. For better accuracy, the current and the velocity should be measured simultaneously. Better methods to measure the stroke current need to be developed. Correlation coefficient between various lightning parameters is another important parameter which will affect the analysis significantly. Lightning characteristics should be classified according to geographical regions and seasons instead of assuming these characteristics to be globally uniform.

Observation of current waveshapes of lightning strokes on transmission towers

Considerable cost reductions have been achieved through rationalizing insulation design for substation equipment based on the application of high-performance surge arresters and improvement in analysis techniques. For further rationalization in insulation design and cost reduction, it is important to evaluate the lightning stroke current waveshape in detail. Some studies to observe current waveshapes have been conducted before, but they were ones conducted on independent towers and do not provide many data in a large-current region. So, the authors have been observing lightning stroke currents larger than 9 kA on 60 transmission towers (mainly 500 kV lines) since 1994. In this paper, they present some lightning parameters for insulation design of substations, drawn from current waveshape data, which have been collected so far. In addition, they report on the relationships among the peak value, the front steepness and the front duration, which heavily affect the insulation design of substations.

Observation and analysis of lightning surges at substations connected with UHV designed transmission lines

On UHV-class transmission lines with sections of 500 kV class insulation, lightning observations have been made to study the generation and the propagation of overvoltage into substations. Voltages and currents were recorded at substations and at junction-tower footings during three flashovers on the UHV system. These observations were then matched to EMTP calculations by adjusting the average soil resistivity and the average conductor height in the propagation model. It is also found that the attenuation due to corona of voltages was small because it is expected to be minimal for 8-conductor bundles.