Characterization of a Medium Voltage Test Line in the Free State Province of South Africa

The existing insulation coordination philosophy used on the medium voltage overhead power lines of Eskom, South Africa's largest electricity distributor, consists of bonding pole-top hardware together and connecting this to earth via an insulation coordination gap - to give a lightning impulse withstand voltage of approximately 300 kV as applied to intermediate structures. The results of previous research show that this philosophy performs more than adequately, but that there may be room for optimization by reducing the size of the insulation coordination gap from its present separation of 260 mm to 150 mm. This paper characterizes the site of a line where this shorter separation is being trialled. First observations from the lightning performance of the line indicate that the 150 mm spark gap setting is performing well, but that this may be influenced by strainer structures that have a lower basic insulation level (BIL) due to the use of small stay insulators. An important observation is that guidance given in IEEE Std 1410–2010 may require modification to include the extensive South African experience that the use of bonding, earthing and spark gaps is a valid form of insulation coordination on overhead MV distribution lines.