For speed and accuracy, modeling on digital computers is a must. The size of the network is important. Even the most powerful computer may not be able to model all the generations, transmissions, and consumer connections of a national grid, and the network of interest is "islanded" with boundary conditions represented by current injection or equivalent circuits. Thus, for performing power system studies on a digital computer, the first step is to construct a suitable mathematical model of the power system network and define the boundary conditions. As an example, for short-circuit calculations in industrial systems, the utility's connection can be modeled by sequence impedances, which remain invariant. This generalization may not, however, be valid in every case. For a large industrial plant, with cogeneration facilities, and the utility's generators located close to the industrial plant, it will be necessary to extend the modeling into the utility's system.