The power system is predominantly in steady-state operation or in a state that could with sufficient accuracy be regarded as a steady-state. In a power system, there are always small load changes, switching actions, and other transients occurring so that in a strict mathematical sense most of the variables are varying with the time. However, these variations are most of the time so small that an algebraic, i.e. not a time-varying model of the power system is justified. A short circuit in a power system is clearly not a steady-state condition, such an event can start a variety of different dynamic phenomena in the system, and study these dynamic models are needed. However, when it comes to calculating the fault currents in the system, steady-state (static) models with appropriate parameter values can be used. A fault current consists of two components, a transient part, and a steady-state part, but since the transient part can be estimated from the steady-state one, fault current analysis is commonly restricted to the calculation of the steady-state fault currents.