Differentiële relais

Differential relays protect generators, buses, and transformers.

In a fault-free generator, identical current transformers keep the relay inactive as no current flows through the relay's operating coil.

An internal fault, like a phase-to-ground or phase-to-phase short, disrupts this balance, causing differential current and potentially triggering the relay.

Electromechanical differential relays, like balance beam relays, operate when one side's force exceeds the other. Since no two current transformers are identical, significant differential currents can occur during external faults, even with equal currents.

For bus protection, each phase uses a differential relay. If one activates, all three-phase circuit breakers connected to the bus open.

If no internal fault exists between the current transformers, the differential relay current remains zero. However, a bus fault generates a non-zero differential current, triggering the relay.

Transformers larger than 10 MVA use differential relays for protection.

Specialized adaptations exist for multi-winding transformers and voltage-regulating transformers.