AFR 31 is an active load to protect metering VT against ferro-resonance in MV power distribution systems
The AFR 31 Smart Load is an instrument designed to protect metering voltage transformers against adverse effects of ferro-resonance in medium voltage power distribution systems with ungrounded or indirectly grounded neutral wire. Ferro-resonance comes up between transformer inductance and conductor's or switching element's capacitance. Connecting, disconnecting, ground faults or other transitional effects may be the trigger events. Ferro resonance oscillations cause significant over-voltage and current surges as a result of transformer magnetic circuit saturation. This most often leads to metering transformer destruction.