Gating of the thyristors in an arcless tap changing regulator

Abstract

A voltage regulator using electronic control of two static switch circuits to permit tap changing without arcing at the switched electrical contacts, without interrupting the load current, and without inducing sizable circulating currents. As a mechanical drive connects a new tap to the auxiliary winding, current through the new winding is blocked by an auxiliary static switch. The drive next opens the main switch delivering current to the load. The opening of the main switch immediately allows the main static switch circuit to 'turn on' to prevent arcing and load interruption until the next current zero. The main static switch control senses no current passing through the main switch and ceases gating, and therefore conduction, of the main static switch. Since neither the main static switch nor the auxiliary static switch is conducting current from the auxiliary winding, the load voltage begins dropping and the voltage across the auxiliary static switch begins rising. When a threshold across the auxiliary static switch is sensed by the overvoltage circuit, the auxiliary static switch control gates the auxiliary static switch 'on' connecting the new tap voltage to the load without interruption. As the auxiliary static switch carries the load current, the main switch may be switched to the new tap connection arclessly. After the new tap is connected, the main switch closes. Any arcing of bouncing main contacts is prevented by a main static switch half cycle conduction shunting.