Thermal up–rating of transmission lines and substation equipment

Abstract

The new regulated electricity supply industry of South Africa is undergoing a major transformation that requires a redefined approach to increase the utilisation of existing transmission line assets. South Africa’s existing 275 kV transmission line network was designed conservatively. It is suspected that the lines are being operated at temperatures well below than which they were designed for. Therefore, in certain cases they could be uprated by operating them above their present power rating such that more power is transmitted without the requirement for new lines. The country is currently experiencing challenging times as additional capacity is needed by the growing economy, increasing the power demands of Eskom’s customers. However, economic and environmental pressures contribute to the difficulty in obtaining new servitudes and the regulatory approval for the construction of new transmission lines. Uprating the 275 kV power network may partly alleviate these predicaments. Thermal uprating a line results in an increase in ampacity, which is the maximum current carrying capacity of a particular transmission line. This means that the power flow will be increased by allowing more current through the conductor which in turn increases the thermal rating (operating temperature) of the conductor, but the resulting increase in power transfer influences the sag which reduce the line clearance. It is possible by means of a non-intrusive method to increase the power transfer capability of transmission assets and at the same time maintaining the safety of the transmission line for the public. Any increase in power transfer will occur without any risk in power equipment or system.