Power transformer diagrams show electrical connections, winding arrangements, vector group, tap changer configuration, core layout, terminal designations, and grounding points. They help engineers understand how the transformer interfaces with switchgear, protection, and grid equipment. Diagrams may include auxiliary circuits for cooling fans, pumps, temperature sensors and OLTC control. They are essential for installation, protection setting, testing and troubleshooting because they reveal both primary and secondary relationships and the physical or functional arrangement of components.