Fault current increase due to the progressive undergrounding of a HV subtransmission network

The undergrounding of HV subtransmission networks in urban/suburban areas is bound to increase fault currents, especially for ground faults if system neutral is solidly grounded. A parametric fault analysis based on symmetrical components is applied to a sample HV system with three lines and two EHV points of supply, based on a simple meshed Italian subtransmission network. Results are then validated by detailed three-phase ATP-EMTP simulations. Values yielded by the parametric study confirm the expected increase of all fault levels, reaching up to 36% and 69% for three-phase and 1-phase-to-ground faults, and even 104% for the ground current of phase-to-phase-to-ground faults. All calculated values are however tolerable, with fault currents well within the operating envelope of devices and components; cross-bonded sheaths also significantly relieve substation grounding mats. Some consequences of undergrounding such as reduction of earth-fault factor may be actually beneficial.