Effects of Varying Fault Impedance on Distance Protection Schemes of 11 KV Distribution Systems

Abstract

Distance protection schemes are used in the protection of transmission and distribution lines and they use distance relay in their operations. The protection scheme is always partitioned into two or more zones and each zone is a certain percentage of the entire length of the line (which may also include the next line). With all things being equal, the tripping of the relays is solely a function of the zones where the fault occurred, that is, the location of the occurrence of the fault. However, it has been shown in this paper through simulations in Power System Computer-Aided Design (PSCAD) that for a LLG fault on the line (in Zone 1), the distance relay/protection system tripped inaccurately in Zone 2 for a fault impedance of 0.1Ω, 0.5Ω and 5Ω and trips accurately in Zone 1 for fault impedance of 1Ω, and 10Ω for the same type of fault and same location. Also, for a fault impedance of 0.1Ω, the system tripped at Zone 1 for LL and 3 phase faults and tripped in Zone 2 for LG and LLG faults for the same fault impedance and at the same location. This indicates that tripping zone in distance protection schemes are not solely dependent on fault locations but also slightly dependent on the fault impedance and type of fault