The electrical power grid operates in equilibrium. The power supply from the generator matches the demand levied by the end-users. The flow of power to enable this demand-supply equilibrium over the transmission lines (T-lines) is governed by the following conditions.
Power System Fundamentals
Power system fundamentals covers short-circuit studies, load flow studies, arc flash studies etc. Articles here cover some of the basics too which are important if you want to understand some of these studies. The basics include per unit systems, symmetrical components, current ratings etc.