Sakae Taniguchi

Observation results of lightning shielding for large-scale transmission lines

Lightning strokes to transmission lines have been estimated using the electro-geometric model proposed by Armstrong and Whitehead. The observed results of the lightning to large-scale transmission lines are reported here to validate the calculated results, in which ultra high voltage (UHV) designed transmission lines and 500 kV transmission lines were selected as the subjects of the observation. Lightning observations were carried out for direct strokes to phase conductor caused by shielding failures, as well as strokes to ground wires that have rarely been previously reported. The observed results showed that total number of direct lightning strokes to phase conductor were nearly identical to the calculated results based on the conventional method, while observed strokes to upper phase were larger and strokes to lower phase were smaller than those derived from calculations. It was also revealed that lightning strokes to ground wires of UHV designed transmission lines and 500 kV transmission lines were 5.1 times and 2.7 times larger, respectively, than calculated results based on the conventional method.

Discharge characteristics of 5 m long air gap under foggy conditions with lightning shielding of transmission line

The electric geometry model suggested by Armstrong and Whitehead was used to calculate the performance of lightning-shielded transmission lines. However, since the ultra high voltage (UHV) designed transmission lines were brought into 500 kV operation, it has become evident that their actual performance is different from the calculated predictions in terms of their actual faults, lightning observation results, etc. It is thought that the UHV designed transmission lines are subjected to foggy or rainy conditions when lightning strikes, because they often pass through mountainous areas at higher altitudes. However, according to the electric geometry model, the striking distance is determined by the lightning-stroke current value only, and the model does not consider the environment of discharge path for the relevant lightning strokes. These considerations suggest that foggy conditions may affect transmission line lightning shielding performance. In this study, discharge tests were conducted using a scaled-down transmission line with a 5 m air gap. The discharge point to the simulated conductor was analyzed in both dry and foggy conditions. Tests were also conducted with a DC bias applied to the simulated power lines, to take the operating voltage of the lines into consideration. These experiments have shown that the effect of fog on the discharge ratio to the conductors is negligible.

A contribution to the investigation of the shielding effect of transmission line conductors to lightning strikes

The electrogeometric model has been used to calculate the transmission line lightning shielding characteristics. However, actual UHV design transmission line accidents and the lightning observation results indicate that there are differences between what actually happens and theoretical results. In this paper, long-gap discharge tests were conducted to better understand lightning shielding characteristics, mainly of UHV design transmission lines. As far as the test method is concerned, the arrangements of transmission line conductors were reduced, and simulated as ground electrodes, and a switching impulse voltage was applied to rod electrodes. As a result, discharges occurred most frequently to the conductors with the smallest gaps with the rod electrodes. On the other hand, it was seen that the electric field strength of the transmission lines and their position with respect to ground might have some influence. These results might also be valuable for long air gap dielectrics.

Improved method of calculating lightning stroke rate to large-sized transmission lines based on electric geometry model

The characteristics of the lightning shielding of large-sized transmission lines have been calculated based on an electric geometry model (EGM) proposed by Armstrong and Whitehead. However, the characteristics of lightning strokes to large-sized transmission lines observed in recent years differed from calculation results, hence the present study was conducted to improve the conventional calculation method. First, the relationship between the striking distance and the lightning current was reviewed. Specifically, the air gap discharge characteristics based on test data with complementing the large gap region were applied. In addition, the return stroke velocity distribution obtained by actual measurement was newly applied. Second, the distribution of lightning waveforms was reviewed; the distribution of peak values of lightning current with the distribution of front duration was used in this study. With these improvements, the calculated lightning stroke rate to power lines and ground wires differs less from actual observations than that calculated by the conventional method.

Air-Gap Discharge Characteristics in Foggy Conditions Relevant to Lightning Shielding of Transmission Lines

The features of lightning shielding of transmission lines have been calculated using the electrogeometric model (EGM) proposed by Armstrong and Whitehead. Discrepancies, however, have been found in the case of ultra-high voltage (UHV)-designed transmission lines. UHV-designed transmission lines often pass through high-altitude mountain areas, where foggy or rainy conditions are common when there is lightning. While the electrogeometric model determines the striking distance based on the lightning stroke current, environmental conditions, and how they might affect lightning discharge paths, they are not considered. It is possible, therefore, that foggy conditions may cause differences in the performance of transmission-line lightning shielding. This study used scale models of transmission lines for discharge tests with a 1-m air gap and analyzed differences in discharge paths and locations in dry and foggy conditions. The results showed that the polarity of the discharge had a significant influence, but differences in dry and foggy tests were negligible.

Flashover Characteristics of Long Air Gaps with Negative Switching Impulses

The electrogeometric model (EGM) proposed by Armstrong and Whitehead has been used to calculate the lightning shielding features of transmission lines. While the striking distance equation in the EGM uses the 50% flashover voltage characteristics of negative switching impulses for rod-rod gaps, those characteristics for long air gaps have not been sufficiently examined. The present study has therefore focused on clarifying the 50% flashover characteristics of negative switching impulses for long air gaps. Consequently, saturating characteristics have been obtained for rod-plane gaps compared with almost linear characteristics for rod-rod and rod-conductor gaps. Furthermore, new knowledge was obtained indicating that, for air gaps of around 4 to 5 m, the 50% flashover voltage may possibly be lower for rod-plane gaps than for rod- rod and rod-conductor gaps.

Method of calculating the lightning outage rate of large-sized transmission lines

To evaluate the lightning outage risk to large-sized transmission lines, the lightning outage rate is predicted by calculating the probable lightning stroke rate. Conventionally, the increased potential caused by lightning strokes is calculated based on distributed constant circuit theory, and the calculated results are compared with the air gap withstand voltage between the arc horns of phase conductors, usually having six phases, to evaluate whether flashover will occur, and thus calculate its frequency of occurrence. However, the lightning outage rate of large-sized transmission lines calculated by this conventional method provides a poor match with reality. Consequently, the present study was conducted to improve the calculated prediction of the lightning outage rate. The lightning outage rate is a composite figure that includes back flashover caused by lightning strokes to ground wires and flashover caused by lightning strokes to phase conductors. The calculation of each of these cases has been improved. The main improvements include corrections to the withstand voltage to match the differences at larger and smaller facilities, as well as an improved method of calculating the development from a one-phase ground fault (1LG) of a phase conductor to a two-phase ground fault (2LG). Thanks to these improvements, the calculated lightning outage rate was closer to the actuality than that calculated by the conventional method. A trial calculation of the lightning outage rate with the voltage increased from 500 kV to UHV was also performed using the improved calculation method. Even though both 1LG and 2LG due to flashover caused by direct lightning strokes and back flashover increased to a certain extent, the lightning outage rate was about 30% lower than that of the present 500 kV transmission lines.