Some protection, underfrequency load shedding and testing aspects of the ESCOM network

The power supply system of the Republic, as shown in Fig 1.1 consists of the major load and generation centres in the Transvaal with tight interconnection and relatively short lines. Other important load and generation centres are connected to this area by relatively long transmission lines. There are also a number of smaller load areas with little local generation, importing a relatively large proportion of their load from the Transvaal. The long transmission distances and resulting high capital cost of transmission favour single line supplies, at least in the initial development stages. The long transmission distances also make series compensation economically attractive. The large local loads compared with available generation in the outlying areas necessitate underfrequency load shedding. The paper outlines the approach adopted to these problems in the Republic. South Africa has a high lightning incidence (isoceraunic level over 60) and cane and grass fires are prevalent in the dry season. The resulting faults are predominantly single-phase to ground flashovers at the higher voltages (85 per cent of 400 kV and about 60 per cent of 132 kV faults involve only one phase). The high fault incidence coupled with long transmission distances over single lines makes single-phase auto-reclosing an obvious design feature. It is employed on interconnectors and on radial load systems.