Kerala experiences intense lightning during pre-southwest monsoon and northeast
monsoon. Statistics show that instances of loss of lives, damages to properties and outages
of transmission lines due to lightning are more during the northeast monsoon, beginning
in September and ending in November.
On October 15, 2008, the Mysore-Calicut 220 kV Single Circuit (SC)
transmission line experienced an outage due to decapping of a unit of an insulator string and falling
of conductors. A part of the suspension insulator stack of the 220 kV line got separated
due to breakage and, along with the associated conductor, fell to the ground, causing
severe arcing and short circuit in the nearby houses. Investigation into the causes of failure
of insulator stack of the transmission line during lightning and the remedies to avert
such failures in future are presented in this paper (CUIET, 2008).
The SC Mysore-Calicut 220 kV transmission line has Mild Steel (MS) support with
conductors in horizontal configuration. It has two shield
wires of stranded GI wires.
The lightning failure was on a tower installed on a hill of about 30 m height
(Gopalan, 1979). The tower was surrounded by shrubs and trees of medium height. The soil of the
region is laterite with gravel. There were no trees higher than the transmission line.
The nature of failure shows that the spark originated from the metal cross arm
rather than from the charged conductor. There were severe burn marks on the insulator
shed of the first unit (Figures 1 and 2); smoky streaks were seen on the inside of the
insulator porcelain of the units close to the cross arm. Similar streaks were not seen on the
inside of the units close to the conductor (Figure 3). This reveals the fact that there
might have been a back flashover from the earthed cross arm of the conductor. Back
flashover is typically due to the high tower-top potential that results from lightning strikes on
a tower with high tower footing resistance (Gopalan, 1999). The path of currents due
to the lightning strike to the tower top will be through the tower and the footing
resistance, since the 12 mm diameter GI earth wires (two in number) running on either side of
the tower offer very high impedance to inductive currents (lightning currents are
highly inductive). Therefore, the tower-top voltage is due to the tower footing resistance
and the inductive reactance of the tower. |
