Implementation of Auto Reclose for Combined Overhead Line with Underground Cable using Distance Relays. A case study at Gajah Tunggal 150kV Substation

More than 70% of fault incidents on overhead lines are temporary faults. Thus it needed to put auto-reclose as a protection scheme to maintain continuity of transmission supply when there is a temporary fault on overhead lines, such as kite, tree, or lightning. While on underground cable, the occurrence of a fault is must be permanent. Therefore auto-reclose is not applied on underground cable. Gajah Tunggal 150kV substation is supplied by using a combination of overhead lines and underground cable. Generally, this transmission should be equipped with an independent cable protection system to detect a fault on the cable thus should not be reclosing the circuit breaker. In this case study, there is no independent cable protection, and there is only one protection for the entire line. The application of the auto-reclose scheme on combined transmission lines needs specific consideration. As short-term mitigation, a distance relay is used with an additional impedance zone setting to discriminate whether a fault occurs in overhead line or underground cable, so it can determine circuit breaker need to reclose or not. To obtain the correct response of distance relay is necessary to apply distance relay setting with measured line impedance data instead of calculated one. Hence, the transmission was shut down to conduct on-site line impedance measurement. The measured data is used as a reference to perform recalculating distance relay setting, then to design and implement an auto-reclose scheme on this transmission line. This implementation is used as an alternative method to provide an auto-reclose scheme on a combined transmission line in the form of overhead line and underground cable, without an independent cable protection system.