A three-line diagram is a schematic drawing that shows the electrical connections and relationships between the major components of a power system using three lines to represent each phase of the AC system. The three lines represent the three phases of the power system and are typically labeled A, B, and C. The diagram shows the connections between generators, transformers, circuit breakers, and other components, as well as the voltages and currents associated with each component. Three-line diagrams are used in power system design, protection, and control, and are an essential tool for engineers and technicians working with large power systems. They provide a clear and concise representation of the electrical system, enabling operators to quickly identify problems and take corrective action as needed.