Multi-functional earth-fault protection scheme for compensated MV-networks using novel touch voltage and admittance-based functions

Distribution networks have undergone huge changes during recent years. Due to increased requirements for the security of electricity supply, conventional overhead line networks have been transformed into underground cable networks in large-scale. Due to high phase-to-earth capacitance values of the cables, capacitive earth-fault currents have increased significantly. For this, numerous new compensation (Petersen) coils have been installed, and their unit ratings have been increased. Applied changes affect the electrical behaviour of networks during earth faults, and thus, the dependability and electrical safety provided by the existing traditional protection schemes must be re-assessed. To guarantee high protection performance in mixed networks with high share of cables, the authors suggest an enhanced protection scheme available in modern protection relays. In this scheme novel touch voltage and admittance-based earth-fault protection functions are combined into one multi-functional feeder protection solution to comply with requirements for protection operation speed and sensitivity, not only during normal supply conditions, but also considering abnormal supply conditions and non-constant nature of practical earth-fault current. In this paper, first the main challenges due to network transformation for traditional earth-fault protection performance and for electrical safety due to earth faults are described. Next, the proposed multi-functional protection scheme implemented with novel touch voltage and admittance-based functions is introduced. Finally, the performance of the scheme is demonstrated with recordings from primary earth-fault tests conducted in a practical 110/20kV substation. The results show that a properly configured and set multi-functional protection scheme improves greatly the earth-fault protection performance and ensures dependable and precisely timed protection operation to comply with the applied legislation during all possible network operating conditions.