Phase (Voltage) Unbalance Explained

Three-Phase Monitor Relays


How is Phase Unbalance calculated and how do Phase Monitor Relays from Macromatic react to an unbalanced voltage condition?


Per the National Electrical Manufacturers Association (NEMA), phase (voltage) unbalance in a three-phase voltage system is defined as the Maximum Deviation from the Average Voltage divided by the Average Voltage x 100 (for percent):

Unbalance =Maximum Deviation from the Average Voltagex 100
Average Voltage

Assume the three voltages are 451V, 480V & 487V.  The average of the three voltages is (451 + 480 + 487) = 1418 / 3 = 472.7.  The maximum deviation from the average is 472.7 – 451, or 21.7 (of the three voltage values, 451 is the furthest from the average).   The unbalance is calculated as shown:

Unbalance =21.7x 100=4.6%

All Macromatic three-phase voltage monitor relays that have unbalance protection use this method to calculate the monitored unbalance condition.  The relays trip when the calculated unbalance condition is greater than the unbalance setting for a period longer than the trip delay.  Please see the catalog section of each specific Macromatic product for details on what versions offer unbalance protection and whether the settings are adjustable or fixed.