Network studies and mitigation of high 132 kV fault currents in eThekwini electricity.
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The growth of the world population has led to an increase in the demand for electricity. This has resulted in the expansion of electric power networks and this evolution brought with it many challenges. One of which is that power networks are experiencing increased fault current levels. This is as a result of growth in demand which has led to interconnected networks and increases in generation capacity. Fault current levels have been increasing steadily and are at a point where mitigation measures have to be evaluated to ensure that equipment operate within designed limits. Alternatively, equipment would have to be replaced with adequately rated equipment. In some cases, replacement would have to take place prematurely, since equipment would not have reached their “end of life”. This study investigates the problem at a 132 kV sub-transmission voltage, and the various factors involved with increasing fault levels and mitigation methods being used. Essentially, mitigation measures increase the impedance in the network, thereby reducing fault currents. Mitigation measures are classified as passive or active, and have varied degrees of effectiveness, usage and network losses. Active measures do not have any effect on the network under normal operating conditions, and only operate during a fault. An example is the superconducting fault current limiter. Passive measures operate under normal and abnormal conditions and affect network parameters. These are usually topological changes which increase the system impedance. Passive measures were chosen for the network studies since active measures are in the developmental stage at the 132 kV voltage level. In this research investigation, the measures tested include: network splitting by creating sub-grids, network reduction, high impedance transformers, introducing a higher voltage network and current limiting reactors. Reducing the 132 kV interconnectivity by creating a northern, southern and central grid reduced the fault levels significantly and does not require any capital investment. However, under abnormal conditions the grids are reconnected to ensure that there is no loss of supply. A solution is to construct a network at a higher voltage level that will support the 132 kV sub grids. A reduction in 275/132 kV transformation lowers the fault levels, while a reduction in generation had little effect on the network. High impedance transformers and current limiting reactors increase the losses in the network, but can be used to limit fault currents to pre-determined values. Electric utilities have to investigate the various measures in order to ascertain the most beneficial to that particular network, given the high cost of infrastructure, the ability to experience outages, space constraints in substations, and the electrical losses that might be incurred. The results obtained from this study carried out on the 132 kV eThekwini Network is presented and discussed.