Ground Fault Analysis for the New 36 Kv-50 Hz “Res” Collection Systems

The upsurge connection requests of renewable energy sources (RES) to the Italian National Transmission System (RTN) largely involves plants in the (10 ÷ 100) MW installed capacity range. However, the presently standardized connection solutions might result in the underutilization of the existing high-voltage (HV) network infrastructure. Aiming at an optimal allocation of both economic and technical available resources Terna, the Italian Transmission System Operator (TSO), has introduced the unified Un=36 kV voltage level. This should lead to an increased hosting capacity of the network and reduction in land occupation, while allowing a simplified management of connection requests. Given the expected prevalence of cable lines, a compensated neutral operation (i.e. adjustable Petersen coil in parallel to a neutral grounding resistor) is required to limit ground fault currents in the future 36 kV network. The paper addresses the fault analysis on a 36 kV prototype network serving a hypothetical onshore wind farm of nearly 100MW installed capacity. A parametric analysis of single phase-to-ground (1 LG) faults for different neutral grounding modes (ungrounded, compensated neutral) was carried out by means of a detailed ATP-EMTP model. Results evidence the suitability of the zero-sequence directional overcurrent relay schemes presently used in the Italian public distribution systems for the new 36 kV networks.