Protection coordination study for an 11 kV overhead feeder

Feeder F7 leaves Hillcrest substation, which has two 10 MVA 33/11 kV transformers, and runs 14 km of overhead line to a cluster of villages and a dairy co-operative. There is a pole-mounted recloser at 6 km and eleven fused spurs feeding 46 distribution transformers of 100 to 500 kVA. Peak feeder load is about 240 A.

Customer minutes lost on F7 are the worst in the district. The outage log for the last year tells the story: - 31 of 44 breaker trips started on a spur, where the spur fuse should have cleared the fault without taking out the whole feeder; - the recloser and the substation relay both tripped on 9 faults beyond the recloser; - one broken conductor lay on dry ground for 40 minutes without any protection operating.

The protection engineer wants a proper study before the settings are changed. You have the network data: source fault level at 33 kV of 750 MVA at maximum and 400 MVA at minimum, transformer impedance 10% on 10 MVA, 11 kV neutral earthed through a resistor limiting earth-fault current to 750 A, and line impedance per kilometre for the main line and spurs, given in task 1.

You will build the network model, calculate maximum and minimum fault currents at every protection point, select CTs, set the substation IDMT overcurrent and earth-fault elements, set the recloser fast and delayed curves, choose spur fuse sizes, plot all the curves on log-log axes, and check grading margins. You will do it in Python from first principles, so no commercial software is needed.

Done looks like a study report and a settings sheet: fault table, CT and relay settings, recloser and fuse choices, time-current plots with margins marked, a sensitive earth-fault proposal for the broken-conductor case, and a list of what to verify on site. If one device cannot both protect its zone and grade with its neighbour, say so and propose a fix.