Identification of the optimum protection co-ordination in medium voltage co-ordination in medium voltage distribution system of Sri Lanka

Abstract

Majority of the faults in the transmission and distribution network are transient and these faults can be cleared with proper installation of protective devices with appropriate protection settings. It is important to clear the faults as soon as possible by keeping the healthy network undisturbed while avoiding damages to lives and instruments. It was observed that applying protection settings to Medium Voltage network at Grid Substations and Medium Voltage distribution feeders are done by two separate parties without coordination between them. Monthly tripping summary of 33 kV feeders of Grid Substations of Ceylon Electricity Board revealed that some feeders getting disturbed abnormally. Further, it is observed that Auto Reclosers are installed in downstream of these 33 kV feeders to respond to the transient faults but they are not yielding expected results. Hence, applying most appropriate protection setting to these Auto Reclosers and relays are very much required for the higher reliability of the Medium Voltage network. Two 33 kV feeders which were mostly disturbed were analyzed deeply and found that most of the feeder trippings are owing to Earth Fault. Further, with installation of temporary Digital Disturbance Recorder, it was observed that most of the faults have lasted less than 100 ms. Plotted Over Current and Earth Fault co-ordination curves for Medium Voltage distribution network disclose that requirement of revising the settings while identifying the most suitable way of applying Auto Reclosers to the 33 kV feeders. Four scenarios were studied to identify the optimum way of installing Auto Reclosers and protection setting for this Medium Voltage network. Detailed analysis proved that 33 kV feeder with two downstream Auto Reclosers is the optimum solution. Then, the most suitable protection settings for the Medium Voltage network were derived for a typical Grid Substations. Furthermore, an algorithm was defined to find the optimum protection settings for any Grid Substations. Application of these setting to a selected 33 kV feeder viz Feeder 5 of Badulla Grid Substation, proved that the new settings are extremely effective.