Determination of Directional Protection Curve and Equation for Radial Distribution Systems

With the increase in energy demand, the size of distribution systems that are traditionally fed by radial connections in which energy flows from source to the load grows significantly. In order to meet the energy demand and increase the security of the system, the use of cables in distribution systems is in rise. Due to extensive usage of cables and underground cables nuisance tripping of protective devices are imminent and classical non-directional over-current relays can not provide overall selectivity for the system. As a result, directional protection practice is becoming widespread in distribution systems. The directional protection practice is more expensive than the non-directional protection practice because it requires additional measuring transformers. It should be known in which situations the distribution system needs directional protection practice. In this study, a sample radial distribution system is analyzed by using the real parameters. Areas where directional protection is required for single-phase earth fault are identified. The need for directional protection has become a requirement due to two basic parameters. These are neutral earthing resistance and cable length. The effect of cable cross-section, number of