Electricians who operate high-voltage cable networks have a lot of trouble with earth faults in individual conductors. They now have a simple device at their disposal that, in the event of an emergency, supplies voltage to the winding of the protection relay, which cuts off the voltage in the damaged cable.
The device is a ring transformer, in the hole of which a three-phase cable with a diameter of up to 200 mm is placed. Under normal conditions, all phases are displaced by the same angles, the magnetic fields from the flowing currents are compensated and the resulting magnetic field around the cable is zero. When one or two cores are closed, the symmetry is violated, so-called zero-sequence currents arise. They induce a voltage in the transformer, which controls the operation of the relay.