Voltage unbalance is the difference between the amplitude or phase of the voltage between three  phases. The voltages over every phase in a balanced three-phase system are equal. In Europe the nominal value is 230V. The three angles between the phasors are also equal: 120 degrees.
Voltage unbalance can occur as a result of incorrect load distribution. It can also be the result of defects caused by prolonged inrush currents and short circuits elsewhere in the network, such as a broken neutral conductor.
Voltage unbalance has a major effect on the reliability of the installation, and may even have safety implications, as unexpected large currents can flow through conductors.
Voltage unbalance can lead to
Voltage unbalance in weak grids
Voltage unbalance is mainly caused by unbalance in currents. This can be the result of incorrect distribution of single phase loads. In a weak grid, aboard a ship for example, the need for equal distribution of loads among the phases is especially important. Unbalance in current at the generator will wear the generator out faster.More importantly, it will lead to unbalanced voltage throughout the ship.
Why do motors wear out faster when voltage unbalance occurs?
Motors generate magnetic fields which repulse each other. By constantly moving the point of repulsion, the motor starts to rotate. The rule of thumb is that the current determines the torque of the motor. Unbalance in the voltage leads to unbalance in the current within the motor. This results in uneven torque during every revolution of the motor.
Rules and regulation of voltage unbalance
Most grid operators prescribe regulations related to voltage unbalance at the point of coupling. However, if you own a transformer, or are operating in emergency mode, you are solely responsible for your Power Quality, and therefore also your voltage unbalance.
Solutions to mitigate voltage unbalance
Several solutions exist to reduce voltage unbalance, such as the application of a Scott transformer, Steinmetz circuit or even Static VAr generators or active filters. However the best solution is to prevent unbalance in the current. Generally, unbalance in the current is the source of unbalance in the voltage. A measurement to find the source of the unbalance can provide a solution. Subsequently, the malfunctioning device can be replaced or the distribution of the loads could be improved. Continuous monitoringcan prevent malfunctions, downtime and resulting financial damage.
