The nature of power systems with meshed transmission networks. The strong coupling between the voltage phase angles and active power flows, as well as the strong coupling between the voltage magnitudes and reactive power flows. Proof of the mutual independency between these two couplings. Automatic primary and secondary control of active power and frequency of isolated and interconnected power systems. Automatic control of voltage and reactive powers. The basic and classical power flow model and calculation. Modeling and calculation of faulted power systems affected by one of following five basic short-circuits: line-to-ground, line-to-line, line-to-line-to-ground, line-to-line-to-line, and line-to-line-to-line-to-ground.