Open circuit current transformer protection circuit

Abstract

A three-phase power supply has a current transformer in each power line. The transformer secondary winding is connected to a load. Generally, the load is a control means responsive to the current being sensed. A triac in parallel with the secondary winding and load has its gate connected by a voltage sensitive circuit to sense the corresponding secondary winding voltage and to conduct and protect the transformer in the event of load loss. The gate circuit includes a metal oxide varistor and a linear resistor in series connected in parallel with the secondary winding. A second linear resistor is connected to the common node of the first linear resistor and metal oxide varistor. The second linear resistor is in series with a diac and connects the gate to the common node of the varistor and first resistor. A trigger capacitor is connected between the common node of the second resistor and diac and the opposite end of the first linear resistor. On load loss, the secondary voltage rises and the varistor conducts, rapidly charging the trigger capacitor. The diac then conducts and the trigger capacitor supplies a large but controlled gate current to drive the triac 'on.' During the opposite half cycle, this cycle repeats until a load draws enough current to keep the secondary voltage within a critical level.