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

Abstract

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-toground faults, and even 104% for the ground current of phaseto-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.

Synergy of the future: high voltage insulated power cables and railway‐highway structures

“…With regard to the AC power transmission, the value of the single phase short-circuit current value depends on the transmission network node [30][31][32] where the fault occurs, and upon the weakness or strength of the supplying network. Its magnitude, in comparison with the three-phase symmetrical short-circuit current, depends on the neutral wire connection (quantifiable as a function of the ratio between the positive sequence sub-transient reactance and the zero sequence one).…”