The ferroresonance protection device is a device designed to protect measuring inductive voltage transformers from the harmful effects of ferroresonance in distribution networks of medium voltage from 6 to 35 kV, alternating current with a frequency of 50 Hz with isolated or grounded neutral. Ferroresonance occurs between the inductance of the transformer and the capacitance of the network or its elements. The source of ferroresonance can be switching overvoltages in the network, single-phase earth faults or other transient phenomena. When ferroresonance occurs, overcurrents flow through the high voltage winding (HV) of the voltage transformer, which can lead to its damage and complete failure.
PRINCIPLE OF OPERATION
The devices are included in the open triangle circuit formed by additional secondary windings of three-phase measuring voltage transformers. One pole of the load must be earthed. The devices can be used simultaneously with a protective relay. Parallel connection does not affect the protective functions of the relay.
The main feature of the device is that it recognizes whether the fault is an unbalanced situation or ferroresonance occurs. Using the electronic control circuit, the device monitors the open delta voltage of the voltage transformers in real time, and if it is below the set threshold voltage, the device remains offline.
The threshold voltage is set to 20-24V, which means that below this point the device indicates to the voltage transformers that there is no load. So the device does not show any action against the natural asymmetry of the system caused by phase imbalance. When the input voltage exceeds the threshold voltage, the device is activated and the damping action is realized. It features very low resistance for better damping (<7Ω) of ferroresonance.
If the fault is not cleared for some time (depending on the fault current), it recognizes the fault as a ground fault in the event of a fault and in order to protect the voltage transformer from overload and overheating, the device increases the resistance to high resistance values (>> 200Ω) .
The device is equipped with an LED indicator. Once it recognizes ferroresonance damping in the shortest time, the red LED mainly indicates ground faults than threshold voltage.