Earth Fault Protection of Compensated Rural Area Cabled Medium Voltage Networks

Recent storms in Nordic countries have damaged MV distribution networks and caused major outages. Furthermore, new quality requirements of electricity supply, and customers’ demands for more uninterruptable and better quality of supply have led to build weatherproof and reliable networks by replacing overhead lines by underground cables in rural areas. However, the rising level of cabling increases earth fault currents and produces dangerously high touch voltages in surrounding areas. Earth fault current through human body and related consequences depend on its magnitude and duration. In worst case even a low current can be fatal to victim. Because earth fault current consists of increased capacitive component and resistive part due to considered zero sequence series impedance with longer feeders, protection has to be implemented in different ways ensuring safety and selectivity during earth faults. Resistive part can not be compensated with Petersen coils, but it can be limited with decentralized compensation. Moreover, network structure and earthing method impact on the magnitude of earth fault current. Earth fault phenomenon with phase angle and admittance criteria was studied. Typical MV distribution network models using PSCAD simulation software were created. The aim was to find out how earth fault protection should be arranged with defined fault scenarios in different cases and what is the sensitivity that can be reached. The impacts of phase angle errors on protection were also studied in one situation. The results showed that admittance criterion is reliable and sensitive in radial networks, and protection even operates without the parallel resistor in some cases. However, it requires careful setting of certain admittance boundaries. When using phase angle criterion, parallel resistor should be connected or wider tolerance should be set in some cases. Phase angle criterion was not affected by errors, which was accounted for parallel resistor connection. In theory the admittance method was vulnerable to errors, but false operations can be avoided by placing the boundaries with sufficient margins. Consequently, threshold settings and accurate calculations of protection quantities should be done carefully.fi=Opinnäytetyö kokotekstinä PDF-muodossa.|en=Thesis fulltext in PDF format.|sv=Lärdomsprov tillgängligt som fulltext i PDF-format