Some power quality (PQ) potential problems have very few outwardly detectable signs. Voltage sags are called blinks in some trouble reports as the lights have a noticeable change in intensity, as does voltage fluctuations that result in light flicker, or swells that can cause incandescent bulbs to go “supernova.” Voltage and current unbalance generally don’t make themselves known to the naked eye. They also may not even draw attention to those watching the power meters at a service entrance, since “that’s the way they always read and nothing has gone wrong.”
I will avoid the ongoing discussion of choosing between the words “imbalance” versus “unbalance,” as well as the mathematics behind the two different formulas used, other than to mention that one method is the maximum deviation from the average of the three phase values, and the other method is the ratio of the negative sequence components to the positive. While the latter is more accurate in highly distorted systems, the key is that the amplitudes and/or the phase relationship between the three phasors aren’t equal. Like other PQ problems, there isn’t a hard-fast limit for what is acceptable. In general, voltage unbalance exceeding 1% is clearly worth investigating, as well as current unbalance exceeding 10% for steady state values. While an instantaneous unbalance may indicate a problem, it is the continuous unbalance conditions that generally result in the unseen condition causing mis-operating or equipment failures.
Normally, the voltage produced by the three phase generators is balanced in both voltage amplitude and phase angle between each phase (120 degrees). It is somewhere along the line from the generator to the loads that the unbalance condition is created. Like other PQ problems, it is typically the current unbalance that results in the voltage problem, though unbalanced impedances in the line and loads also can be the source of the unbalanced voltage. Voltage and current unbalances can also be the result of maintenance issues, such as loose connections and worn contacts, which increase the impedances. One of the most common examples is where a three phase system within a facility has an excessive amount of single phase loads on one or two circuits, rather than distributing them evenly amongst all three circuits. What was once a balanced system when the facility was first occupied often becomes unbalanced as loads change and get re-arranged, unless diligence is paid to measuring and maintaining balance periodically and with each major revision to the electrical system.
The largest share of unbalance related problems occurs in electromechanical devices: motors, generators and transformers. Purely resistive loads rarely have a problem running under such conditions, though it may cause uneven heating or some other unbalanced physical effects. Voltage unbalance at the terminals of a three phase motor causes high current unbalance within the motor, which can be 6–10 times as large as the voltage unbalance. One effect of the unbalance is the increase in negative sequence components, those that try to make the motor turn in the opposite direction from the positive sequence components. Unbalance can lead to torque pulsation, lower than normal torque output, increased vibration and mechanical stress, increased losses, and motor overheating. Continued operation of the motor at elevated temperatures will lead to a breakdown of the winding insulation of the stator and shaft/rotor assembly heating that may cause the rotor to expand, and transfer the excessive heat to the motor bearings and mechanical seals through the shaft. Increased temperature is not just decreased performance but decreased operational life of the equipment. NEMA MG-1 has a table to show how a motor should be derated for different levels of voltage unbalance.
Voltage unbalance can also occur on a distribution system if there are single phase distributed generation resources that aren’t even balanced. The effect of large unequal distribution of single phase loads can cause a significant flow of neutral current in a three phase system. A neutral relay that is set to trip for ground faults may mis-operate, as well as breakers and reclosers in substations. If current overload relay settings are adjusted to compensate of the current unbalance, then the generators may not be adequately protected.
While you can’t see it (though you might hear it or smell it), it is best just to monitor the voltage and current unbalances, and make sure that they are within the operating specifications for all of the equipment powered from such.