What is Voltage Unbalance?

What is voltage unbalance
3 mins read

What is voltage unbalance?

At most basic, voltage unbalance is the condition when the voltages in all three-phases are not equal. The voltages altogether three-phases ought to have a similar amplitude and phase-shifted by 120° one another. Voltage unbalance occurs if any of these two conditions not meet.

However, voltages are seldom perfectly balanced between the phases but they should not be excessive.


What causes voltage unbalance?

Voltage unbalance can be from three different sources,

  • The utility.
  • The factory where motors are running.
  • The motor itself.

Reasons for voltage unbalance due to the utility are:

  • Lack of symmetry in a transmission line.
  • Large single-phase loads (for example arc furnaces, welding machines, etc.).
  • Faulty power factor correction capacitor bank.
  • Open-delta transformer.
  • Open-delta voltage regulator.
  • Unbalance single-phase load distribution among the three phases.

Reasons of voltage unbalance due to factory or facility or plant:

  • Mismatched transformer taps.
  • Unbalance single-phase load distribution among the three phases within the factory.

Causes of voltage unbalance due to the motor itself are:

  • Irregular impedance due to resistive and inductive imbalance can cause voltage and current unbalance.
  • Improper or damaged power circuit connections and motor leads contacts.
  • Rotor and stator winding.

Standards for Voltage Unbalance

Commonly three different standards have gain popularity in defining the limits of voltage unbalance. Unfortunately, all of them have variations in limiting values for voltage unbalance.

The IEC 61000-3-x series give limits for unbalance voltage by <2% for LV and MV system. For the HV system, the given limit for unbalance voltage is <1%.

American National Standard ANSI C84.1 recommends that the power system should be designed such that the maximum voltage unbalance should not exceed more than 3%.

Above all mentioned standards and their recommended limits for voltage unbalance, NEMA MG-1-2016 is very stringent. Its recommended limit for the maximum allowed voltage unbalance is 1%. This is very hard to achieve for power utilities and facilities.

Further NEMA MG-1-2016 states that 1% of voltage unbalance can create 6-10% of current unbalance.

This discrepancy brings confusion among power utilities, customers, and motor manufacturers. However, if utilities and motor manufacturers mention the standards they follow, confusion can be avoided. Customers can also make careful consideration to avoid confusion.

What are the effects of voltage unbalance?

NEMA tells that voltage unbalance creates extremely high current unbalance.

  • Voltage-unbalance leads to an increase in the temperature rise of the induction motor. This results in premature aging of the motor.
  • Synchronous generators can also suffer from excessive heat.
  • Transformers, cables, and transmission lines operate with reduced capacity due to voltage unbalance.
  • Transformers can also suffer from overheating. Overheating damages the winding and insulation of the transformer.

How to check for voltage unbalance?

It is easy to check for the voltage balance. There are two different approaches to check voltage unbalance.

  1. First is the manual one. What you have to do is, just measure all three-phase voltages at the motor terminal. Note down it. Take out the average of all measured line-line voltages. The following formula can help you further.

% Voltage Unbalance = (Maximum deviation from average voltage / Average Value) X 100

2. You can use Fluke’s MDA 500 series motor drive analyzers. This will give you real-time        data of voltage unbalance.

Consider the standard that your facility follows and check whether the unbalance crossed its allowed limit mentioned in that particular standard.

Also see, How to calculate Voltage Unbalance?

How to eliminate the voltage unbalance?

If you have detected voltage unbalance next is to eliminate it.

  • Check the motor terminals for any loose connections or damage to cable lugs.
  • Check the power circuit connections of the motor inside the panel.
  • If everything is fine with motors then rearrange or redistribute single-phase loads evenly on a three-phase system especially the lighting loads. I have faced a voltage unbalance problem at the construction site due to metal halide lamps. We haven’t distributed lighting loads evenly. After the equal distribution of lights on each phase the problem of voltage unbalance got resolved.
  • Check for faulty power factor capacitor bank.
  • If still the problem persists then switch off and isolate the load from PCC. This you can only do during shutdown maintenance. Check for the unbalance voltage at the PCC incomer. Doing this will let you know whether the problem is with the transformer or voltage regulator (if you are using it). If it is there, rectify it.

Summing Up

The cost of voltage unbalance is very high if you calculate the energy loss. It is like a leaky faucet; small drips can waste liters of water. Similarly, the problem of voltage unbalance drains your money. It must get rectified.



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