Evaluating the effectiveness of phase overcurrent protection on overhead medium-voltage feeders

Abstract

Traditionally, the effectiveness of a phase overcurrent protection philosophy has been assessed by only considering a fault level versus protection operating time graph (only selectivity). In this research, an improved method was created to evaluate different phase overcurrent protection philosophies for medium-voltage feeders. A focus was placed on reliability, sensitivity, selectivity, speed of operation, performance and minimising risk. The hypothesis stated that it is possible to develop a method that allows for the evaluation of the effectiveness of phase overcurrent protection. To test this hypothesis, an application was created that allows for the analysis of an overcurrent protection philosophy. This application made provision for changes in source impedance, evaluation of protection backup contingencies, different conductor types, user-definable protection equipment, the placement of protection equipment, user-definable protection settings, primary plant equipment damage information, user-definable safety margins and source transformer protection information. The application provides graphs that allow the user to evaluate the protection philosophy in terms of the following criteria:  The protection operating time at specific positions in the network. The PU sensitivity of the feeder-installed protection equipment.  The PU sensitivity of the source transformer protection (backup function).  The let-through energy and associated equipment damage criteria.  The energy-area over the analysed path.  To classify the busbar voltage dip.  To determine the position of the fault on the analysed path for the associated busbar voltage dip.  To quantify the occurrence of a specific voltage dip category on the analysed path. The graphs that were generated by the application allowed for the analysis and optimisation of the applied protection settings. This optimisation includes determining operating time, operating curve selection and the number of auto-reclose attempts. It is possible to determine the preferred protection philosophy using the application. The application does not prescribe how settings are to be calculated, or the placement of the devices; it evaluates if the applied philosophy is protecting the feeder and how well it is protecting it.