AFR 111 is an active load to protect metering VT against ferro-resonance in MV power distribution systems

AFR 111 smart load is an instrument designed to monitor and protect single measuring voltage transformers (VT) against the potentially harmful effects of ferro-resonance in a high voltage distribution network. Ferro resonance occurs between the inductance of the transformer and the capacitance of the MV lines or switching equipments. Ferro-resonance can be initialized by switching, disconnection, ground connections or by other transient events. During the ferro-resonance a significant over-voltage and concurrent current peaks can occur due to the saturation of the magnetic circuit of the VT. This often leads to a fatal destruction of the VT.

AFR 111 acts as a protection of the VT against the unwanted effects of such events. In comparison with other protection methods AFR is only selectively activated during the occurrence of ferro-resonance and stays inactive during the normal operation.

AFR 111 is intended for alone operated VTs, i. e. for single phase measuring, V connection measuring or for one pole power feeders. AFR 131 is intended for connection into open delta of auxiliary secondary windings of three VTs.