Internal faults refer to failures occurring within the tank enclosure, including phase-to-phase short circuits in windings, turn-to-turn short circuits within a single phase winding, short circuits between windings and the core, and broken wires in windings. External faults encompass various phase-to-phase short circuits between the transformer's external leads, as well as single-phase ground faults caused by flashover through the insulation bushings of the leads to the tank enclosure.

What is Zero-Sequence Overcurrent Protection In power systems, zero-sequence protection refers to an electrical protection measure designed to guard against zero-sequence currents caused by ground faults or short-circuit faults. When leakage occurs in a line (below the transformer), the vector sum of currents passing through the transformer is no longer zero, resulting in an output current on the transformer's secondary winding. This principle enables zero-sequence overcurrent protection.

Voltage Transformer A voltage transformer (abbreviated as PT), similar to a transformer, is an instrument used to transform voltage. The primary purpose of a voltage transformer's voltage transformation is to supply power to measuring instruments and relay protection devices.