Investigation into open phase faults on transmission circuits.
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Open phase faults are not commonly encountered on the South African power transmission systems. However, when these faults do occur it results in major power system interruptions and spurious tripping of healthy (circuits without faults) circuits to trip without giving any indication of the type or location of the fault. This results in lengthy restoration times to find the open phase fault without any fault detection devices. The intention of this study was to investigate open phase faults and assess the impact thereof. An investigation was conducted on an open phase fault on the bussection of the transmission high voltage yard at Koeberg Power Station. The investigation utilised the Koeberg network configuration, power system data, and protection settings in order to simulate the fault and validate the results with the simulation model. The simulation model was tested by simulating short circuit faults with the current feeder circuit protection scheme characteristics and settings. The results of the investigation confirmed that the current feeder protection schemes do not take open phase fault detection into account. The back-up earth-fault protection which is normally utilised to detect and trip for high resistance faults, did indeed detect and trip for open phase faults where the unbalance currents summation was above the minimum setting threshold of 300 A although the fault clearing times was extremely long. However, this was not the case for all instances. The feeder tripped due to zero sequence currents instead of negative sequence currents. In addition the impact of open phase faults was investigated on the Koeberg generator circuit to confirm that the generator would be protected against negative sequence currents and trip based on the generator protection philosophy, the coordinated and configure generator protection settings. The literature research comprised of the present feeder protection philosophies, a review of currently used feeder protection schemes, available new feeder protection schemes, technologies available or technologies that have the potential to detect an open phase fault. An evaluation of the currently used protection schemes and new protection schemes available was conducted. Considerations with respect to the protection scheme flexibility, adaptability with regards to coordination and configuration of the protection scheme in conjunction with the feeder protection philosophies, modifications and additions to the current feeder protection schemes were considered.