False operation of protective relays in Oman Electricity Power System & its remedies

In this paper, we conduct a study to clarify the difficulties which affected the operations of the protective relays in the Oman Electricity Power System and the steps which were taken to overcome these difficulties. A single line to ground fault on the network may cause the tripping of phase loads and reduce the speed of three phase inductive loads. This in turn causes an over current trip on the 11 and 33 kV feeders after the fault is cleared. The bank capacitors, used in the system, should not trip during the dip of voltage to avoid any further increase. The settings of the earth fault relays should be adjusted taking into consideration the ground resistance and the length of the distribution lines. A load shedding arrangement for 33 kV feeders should be used to avoid the tripping of a second transformer in the feeder transformer arrangement set. Distribution feeders should not be connected with the bus bars of the power stations. Under speed protective devices should not operate on frequencies more than 47.5 Hz. New types of relays should be used after testing it on the existing operating system.