RADHD - Restricted earth-fault protection (high impedance)

RADHD is used for providing a high-speed restricted earth-fault protection for transformers, reactors and other power system objects.

The relay is connected to a CT in the neutral point of the transformer and residually connected CT´s in the phases. Applications are not limited by CT saturation for external or internal faults. The saturation voltage of the involved CT´s must be at least twice the selected operating value of the RADHD relay and CT ratio correction can normally not be accepted. A non-linear resistor is used at the CT summation point to protect the CT´s from high peak voltages during internal faults.

The RADHD relay is available in 6 variants, all with 2 reconnectible operate values, and in 2 variants with operate value adjustable at a tapped resistor.

• Primary side fault sensitivity down to 2 - 5 % of main CT rating
• High speed operation, 10 - 20 ms
• Secure operation, not affected by CT saturation
• Trip or flag indication relay included