Phase unbalance in a three-phase system occurs when one or more of the line-to-line voltages in a three-phase system do not match. Three-phase power systems and equipment are designed to operate with balanced phases lines. Line-to-line voltages in three-phase circuits typically differ by a few volts, but a difference of more than 1% can damage motors and equipment.
Unbalanced voltages cause unbalanced currents in the motor windings unbalanced currents mean increased current in at least one winding, increasing the temperature of that winding. Elevated temperatures can shorten the life of a motor or equipment, leading to premature failure. Phase unbalance can be caused by unstable utility power, unbalanced transformer banks, unevenly distributed single-phase loads on the same power system, or unidentified single-phase ground faults.
A single phase (lack of phase) caused by utility failure, broken wire, fuse failure, contact damage, or fault overload can also cause destructive unbalanced conditions. Three-phase monitoring relays, also known as open-phase relays, are a cost-effective and easy-to-install solution to prevent costly damage to motors and equipment due to phase imbalances.
Three-phase supervisory relays prevent premature equipment failure by monitoring several common fault conditions including phase imbalance. They notify fault three-phase relay
conditions and provide control contacts to shut down equipment or motors before damage occurs. These relays clearly indicate the presence of faults for quick troubleshooting and reduced downtime.