The Lightning Performance of an overhead transmission line is frequently little changed by upgrading the line voltage. However, upgrading of transmission line should be accompanied by an analysis of the lightning performance of transmission line with respect to shielding failures for lines and or back flashovers. For transmission lines with existing overhead earth wires, it must be ensured that increase in cross arm length or displacement of conductor away from the tower body is followed by the subsequent alignment of earth wire so that the shielding angle remains within the stipulated values.
Features of Lightning performance in High Voltage Transmission Lines
Similarly, voltage upgrade of a transmission line with one overhead earth wire may require the installation of two earth wires to protect against the increased horizontal displacement of conductors on the upgraded line.
The installation of two overhead earth wires to reduce shielding failures will contribute additional wind structural loads to suspension, tension and termination structure and additional tension leads to tension and or termination structures.
In this regard, structures may require substantial modification and structural capacity assessments to accommodate the additional overhead earth wires.
For the back flashover analysis for half of the power frequency voltage sine wave cycle, the phase voltage adds to the lightning impulse voltage. For the other half of the power frequency voltage sine wave cycle, the phase voltage subtracts from the lightning impulse voltage.
How to reduce Back Flashovers in Transmission Line
If there are minimal modifications to the line, the net effect is for the impact of the increased line voltage to essentially cancel out Over a period of time, leaving a similar lightning back flashover trip out rate.
However, in order to limit the acceptable outage rate of the upgraded line to a value lower than the existing transmission line, it may be required to reduce the incidence of back flashovers through one of the following strategies;
- Increasing the insulator arcing distance and or increasing the conductor to structure clearance thereby reducing the probability of back flashover.
- Reducing the structure earth resistance, thus reducing the structure voltage to earth and the voltage across the insulators and reducing the probability of back flashover; the reduction in structure earth resistance can be brought about by increasing the number of parallel paths through additional shield wires or use of counterpoise earthing. Additional arrangements in the form of transmission line arresters may be provided to further reduce the probability of back flashovers in the upgraded lines.
- Reduce the Conductor Earth wire separation which improves the coupling and increases the conductor voltage relative to the earth thereby reducing the voltage across the insulators and the probability of back flashover.
Increasing the line voltage may increase fault current levels, thereby making It necessary to increase the diameter of the existing shield wire which, in addition to lightning performance and protection, also provides a path for the flow of fault currents.