Avr bypass relay welding detection

A method of identifying a welded relay condition in a line-interactive UPS comprising an input to receive input AC power, an output to provide AC output power to a load, a neutral connection, an automatic voltage regulation (AVR) transformer having a primary winding and a secondary winding, a plurality of relays coupled to the input, the output, and the primary winding, the plurality of relays configured to selectively couple the primary winding to the input and the output, a bypass relay configured to selectively couple the primary winding of the AVR transformer to the neutral connection, and a DC/AC inverter circuit coupled to the secondary winding and configured to be coupled to a backup power source and to receive backup DC power from the backup power source, wherein the method comprises acts of:

• operating, in a backup power mode of operation, the plurality of relays, the bypass relay, and the DC/AC inverter circuit to provide the AC output power to the load derived from the backup DC power;
• monitoring the AC output power;
• and
• determining, based on the act of monitoring, that the line-interactive UPS is in the welded relay condition.

An Uninterruptible Power Supply (UPS) system comprising:

• an input to receive input AC power;
• an output to provide AC output power to a load;
• a neutral connection;
• an automatic voltage regulation (AVR) transformer having a primary winding and a secondary winding;
• a plurality of relays coupled to the input, the output, and the primary winding, the plurality of relays configured to selectively couple the primary winding to the input and the output;
• a bypass relay configured to selectively couple the primary winding of the AVR transformer to the neutral connection;
• a DC/AC inverter circuit coupled to the secondary winding and configured to be coupled to a backup power source and to receive backup DC power from the backup power source;
• and
• means for operating the UPS system to provide the output AC power to the load derived from at least one of the input AC power and the backup DC power and for identifying that the bypass relay is in a position de-coupling the primary winding from the neutral connection when the UPS system is operated to provide the output AC power derived from the backup DC power.