Transformer differential protection is a primary relaying scheme designed to detect internal faults, such as short circuits between windings, ground faults, or insulation failures, with high sensitivity and speed. Its operation is based on Kirchhoff’s current law, which states that the sum of currents entering a node must equal the sum exiting it under normal conditions.

The mechanism involves installing current transformers (CTs) on both the primary and secondary sides of the transformer. These CTs measure the current flowing into and out of the transformer, converting high currents into proportional low currents suitable for relay measurement. The secondary currents from the CTs are fed into a differential relay, which compares their magnitudes and phases.

Under normal operating conditions or external faults (e.g., faults beyond the transformer’s terminals), the currents on the primary and secondary sides are balanced, meaning the difference between them is near zero. The differential relay remains inactive. However, during an internal fault, the fault current creates an imbalance: the current entering the transformer exceeds the current exiting it (or vice versa), generating a differential current.

The differential relay detects this imbalance and initiates a trip signal, isolating the transformer from the grid within milliseconds. To account for transformer characteristics such as turns ratio, phase shifts (in three-phase transformers), and magnetizing inrush currents (which occur during startup and can mimic fault currents), modern relays include compensation features. These may involve adjusting CT ratios, phase-shifting circuits, or harmonic restraint—ignoring currents with harmonic components typical of inrush while responding to the purely sinusoidal currents of genuine faults.

This rapid and selective operation minimizes damage to the transformer and prevents fault propagation, making differential protection indispensable for ensuring the reliability of power systems.

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