Critical Assessment of the Feasibility of Shielding Overhead Medium Voltage Lines in South African Conditions

Lightning is one of the most common causes of power outages on overhead medium voltage (MV) power lines. Lines located in areas with high ground flash density or in open country are particularly susceptible to being struck by lightning. The current design of overhead MV lines in South Africa has a more than adequate lightning performance, but one may be able to optimize this performance. One possible way of doing so is to install shield wires. This paper studies some of the key aspects that should be considered for practical shielding of overhead MV lines, such as ground flash density, line structure footing impedance, number of shield wires, performance when compared to unshielded lines and capital cost. The effect of soil ionization is also considered. It was found that for shielding to be effective on overhead MV lines in South Africa, the footing impedance must be very low, and the basic insulation level (BIL) of the line must be sufficiently high. If the footing impedance cannot be lowered any further, then the line insulation^* must be over-designed to give a BIL of a line of much higher voltage, assuming that the structure is fully bonded and earthed and that the poles are unseasoned. The costs of shielding MV lines are not likely to be justified in South African conditions.