On modeling utility substations equivalent source

Modeling the equivalent source impedance of utility substations, which supply customer facilities, is crucial in order to calculate the fault current levels at the service entrance, and, accordingly, properly rate all of the equipment, which may be involved in a fault, both three- phase and single-phase-to-ground. Electrical designers should receive, therefore, reliable data from the electric utility in this respect. Erroneous data, or the misinterpretation of such data, may lead to oversized or undersized electrical apparatus, which would respectively unduly increase the installation costs, or reduce electrical safety. In addition, in the case of a single-phase, line-to-ground fault on the high-side of the customer´s substation, there will be circulation of current through the grid and appearance of touch and step voltages on the user´s premises. If such a fault occurs immediately outside of the zone of protection of the user´s most upstream protective device, at the service entrance of the facility, the fault may not be cleared, except by the utility´s device. In this case, magnitude and duration of the ground-fault do not depend on the customer, but on the utility distribution and protection systems, which may or may not be known to the user. The customer must request these pieces of information as he needs a criterion to design, and assess, the efficiency of the grounding grid in the facility being designed. This paper seeks to clarify the problem, and propose the adoption of a standard form, containing the basic input data required to be provided by utilities at the beginning of the design phase.