Coordination of the relay protection settings against phase to phase faults in electric power lines 20 kV

The Ac Time Overcurrent Relay (51) and the Instantaneous Overcurrent Relay (50) protections are the most commonly used relay protections in Bulgaria against phase-to-phase faults in the 20 kV medium voltage electric power lines. These protections are also used and against external short circuit faults in the supply power transformer. In recent years, particular attention has been paid to the continuity and the security of the electric power supply of the costumers, which in turn leads to increased requirements for relay protections regarding their performance, selectivity and sensitivity. The coordination of the settings of the relay protections of the power system elements has an important meaning for normal operation of the electric power system. The paper presents the coordination of the settings of the relay protection against phase-to-phase faults of the power lines with the most used cross sections of the conductors 20 kV and for power transformers 16, 25, 31.5 and 40 MVA.