Ferroresonance in electricity networks

Ferroresonance is an unstable high voltage that can occur on 3 phase electrical systems under specific conditions, potentially causing the failure of equipment.

Conditions

Ferroresonance can occur when an unloaded 3-phase system consisting mainly of inductive and capacitive components is interrupted by single phase means. In the electrical distribution field this typically occurs on a medium voltage electrical distribution network of transformers (inductive component) and power cables (capacitive component). If such a network has little or no resistive load connected load and one phase of the applied voltage is then interrupted, ferroresonance can occur. If the remaining phases are not quickly interrupted and the phenomenon continues, overvoltage can lead to the breakdown of insulation in the connected components resulting in their failure.

The phenomenon can be avoided by connecting a minimal resistive load on the transformer secondaries or by interrupting the applied voltage by a 3-phase interrupting device such as a ganged (3 pole) circuit breaker.

Ferroresonant regulators

A ferroresonant regulator is a transformer with magnetic shunt and a tuned circuit consisting of a coil and a capacitor arranged so that it provides a constant output when the input voltage or output current varies. The circuit has a primary on one side of a magnet shunt and the tuned circuit coil and secondary are on the other side. The regulation is due to magnetic saturation in the section around the secondary. The saturation causes a lot of heat and the regulation is not as good as solid state devices, so these units are obsolete for most purposes.

By arranging particular magnetic properties of a transformer core, and installing a ferro-resonant tank circuit (a capacitor and an additional winding), a transformer can be arranged to automatically keep the secondary winding voltage relatively constant for varying primary supply without additional circuitry or manual adjustment. CVA transformers run hotter than standard power transformers, because regulating action depends on core saturation, which reduces efficiency somewhat. The output waveform is heavily distorted unless careful measures are taken to prevent this. Saturating transformers provide a simple rugged method to stabilize an AC power supply.