Voltage Unbalance

What is Voltage Unbalance?

Voltage Unbalance (or imbalance) is a condition where the 3-phase voltages (L1, L2 & L3) differ; either in amplitude, with respect to their usual 120° separation or in some cases, both.

Perfectly balanced 3-phase voltages, separated by exactly 120°, consist only of a positive sequence voltage component.  This voltage observes the standard L1, L2, L3 phase rotation convention and contains no negative sequence (L1, L3, L2) or zero sequence (the zero sequence component for each phase is in exact phase alignment).

This ideal state is rarely realised; either due to local unbalanced loading conditions caused by single-phase loads or even just due to an unbalanced voltage supplied by the local DNO (District Network Operator).

Figure 1 shows the representation of an unbalanced three-phase voltage utilising phase sequence components.  The unsymmetrical three-phase voltages can be synthesised using balanced three-phase symmetrical components consisting of a positive sequence voltage (hopefully the dominant sequence), a negative sequence and a zero sequence voltage.

The more unbalanced the system voltage is, the higher the negative sequence and zero sequence components will be.

Figure 1 – Voltage Phase Sequence Components

Common Problems Associated with Unbalanced Voltages

Voltage unbalance is another power quality phenomenon that generally goes unnoticed, primarily because most load is single phase and as such is not affected by unbalanced voltages.  The equipment primarily affected by voltage unbalance is 3-phase rotating plant such as induction motors.

A negative sequence voltage rotates in the opposite direction, so when you have a three-phase motor fed from unbalanced voltages containing both positive and negative sequence voltages, there are two rotating fields set up; one in the direction that the motor turns (assuming that the positive sequence voltage is the dominant sequence component) and one in the opposite direction.  The result of this is that a pulsating (juddering) torque is applied to the motor which reduces motor performance and can cause mechanical damage to motors, often in the form of excessive wear of the bearings.

In addition to premature bearing damage, voltage unbalance can produce motor overcurrent, which ultimately leads to: –

  • Electrical overload resulting in overheating
  • Reduced potential output of motor
  • Reduced life of motor

How PureSine Can Help

Using advanced power quality analysis equipment, PureSine can identify unbalanced voltage conditions, provide mitigation recommendations and project manage the installation of any proposed solutions.

We have numerous solutions we can offer for voltage unbalance issues, including:-

  • Voltage Conditioners
  • Dynamic Voltage Optimisation equipment (if high voltage levels are also an issue)
  • Three-phase power factor correction to balance VAr flow amongst the phases
  • Inverter drives where only certain items of plant are affected.

 Voltage unbalance is one of the many power quality parameters covered in PureSine’s Power Quality Health Check – a service designed to be one step ahead of power quality issues by monitoring on a regular basis or as part of large electrical projects.

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