Voltage Imbalance: How to Deal With This Power System Fault

In industrial plants, electrical power issues are pretty common. Current harmonics, voltage unbalance, and current unbalance some of them. 

These issues lead to abnormal functioning of the electric power systems.

Of all of them, the unbalanced voltage condition is the most hazardous.

Let’s learn about voltage imbalance issues and how to prevent them.

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What Is Voltage Imbalance?

There is no particular definition of voltage imbalance, it is simply the voltage difference between different phases. 

Generally, in three-phase motors or polyphase systems, the voltage between different phases should be equal or nearly equal.

However, due to some issues, three-phase voltages become unequal, resulting in negative or zero sequence currents.

Major Effects of Voltage Imbalance

Sometimes, there are extensive voltage imbalances.

As a result, it impacts polyphase motors and other electrical loads. 

Unbalanced voltage mainly causes motor failure due to extreme heat.

Because the voltage unbalances produce high unbalance currents, these currents produce heat and increase the winding temperature.

As a result, it can damage motor insulation. 

Also, a severe imbalance in voltage can lead to the overheating of components in the motor, and there can be severe or permanent damage to the motor.

Motor failures, in turn, lead to user facility downtime.

Voltage imbalance also creates negative sequence voltage, and this negative voltage produces opposite torque.

As a result, there is vibration and noise in the motor. 

Sometimes, imbalances in power systems also lead to transformer failure, and relay malfunction is also one of its adverse effects.

motor winding

Image: motor winding

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Causes and Sources

Several factors affect voltage imbalance in a distribution line, which is either general or motor-related.


  • Unequal distribution of single-phase loads
  • Overloading in feeders due to electrical faults
  • Faulty equipment
  • Unbalanced power source voltage


  • Wrong tapping in the transformer
  • Unbalanced load in three phases
  • An Unequal impedance of the three-phase distribution system
  • Unbalanced loading of capacitors

Image: overload

Voltage unbalance standards

Specific standards decide the limit of voltage unbalance, and ANSI recommends a 3 percent unbalance in voltage for electrical systems.

You must take this percentage under no-load conditions.

However, according to Pacific Gas and Electric, this voltage imbalance percentage should not exceed 2.5.

According to NEMA MG-1-1998, there is just a 1 percent unbalance limit, and this rule is the strictest.

NEMA is an association that represents motor manufacturers.

Per the NEMA rule of 1% voltage imbalance, a current unbalance 6-10%.

On the other hand, some makers fix the current imbalance value to less than 5%.

It is essential to get a valid warranty, which means the makers’ requirements are stricter than NEMA MG-1.

At times, disputes arise between customers and makers due to this difference.

Thus, you need to check the service guidelines of the utility at a specific location.

Testing for Voltage Unbalance

To test the voltage unbalance, you must measure phase-to-phase voltage.

The 3-phase system has a connection across phases.

Thus, do not measure phase-to-neutral voltages.

Take the phase-to-phase voltage readings with a voltmeter. 

According to IEEE, it is a ratio of the positive and negative sequence components.

Now, use this formula to calculate the percentage of voltage unbalance.

Voltage unbalance percent = 100* (maximum voltage deviation/average voltage)

The average voltage is the average of voltages across all three phases. 

This formula identifies the unbalanced voltage magnitude present in the system.

If there is any, you must determine the problem source.

The unbalanced situation can be due to the motor or the power.

Follow these steps to know the source of unbalance:

  • Firstly, measure and note down the current through each load
  • Secondly, rotate all power lines (three) by one position. However, please do not change the order; it will change the motor’s rotation.
  • Now, again measure the current across all leads in this new position.
  • Now, again rotate all power lines by one more position.
  • Again, record the current across all lines in the new position
  • For every three rotations, calculate the average value of the current. Observe the power line/motor lead combination that shows the maximum deviation from the average current.
  • Finally, compare all three power lines with the most current deviations. If the combination always has the same motor lead, the problem is with the motor. On the other hand, the same power line in combination indicates a problem with the power supply.
electrician testing industrial machine

Image: electrician testing industrial machine

Voltage Unbalance Mitigation

The issues of power quality are obvious in distribution networks.

You cannot make voltage imbalance as Zero in a distribution system because of three reasons:

  • Firstly, the connection and termination of single-phase loads are random
  • Secondly, due to the uneven distribution of loads in the three-phase system
  • Finally, due to the asymmetry of the power system

However, you can mitigate it after a thorough voltage imbalance study.

To reduce the effects of voltage unbalance, you can use the following:

Utility level methods.

  • Redistribute single-phase loads across all phases.
  • Reduce unequal impedance due to transformers and lines
  • Decrease single-phase regulators to correct the imbalance. However, you must use them carefully.
  • Use active and passive electronic systems to correct voltage imbalances.

Plant-level methods:

  • Do load balance?
  • Avoid connecting sensitive equipment to systems with single-phase loads
  • Make sure that you size the AC side and DC link reactors properly.
  • It reduces the effect of voltage imbalance in speed drives.
  • Have passive networks.


The impacts of Voltage imbalance are harmful to motors.

Thus, you must adequately find and correct the problem.

When you balance voltage, the life cycle of equipment becomes better.

As a result, you save time, energy, and maintenance costs.

Thus, it would be best if you properly tested the electrical equipment.

We can help you in managing your electrical systems.

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