There are two types of imbalance being distribution and curve.
With distribution imbalance it is simply the unequal allocation of loads on the 3 phases. It is common knowledge that electricians see red when opening a distribution board to connect in a new circuit, and so this is the phase that is burdened with whatever needs to be added to the network.
There is no apparent reason for this unexplained behaviour. Maybe it's perceived that red has more power than yellow or blue. C'mon! Red is hot and blue is cold! Yellow sits somewhere in between. Don't laugh. Next time you open a distribution board measure the currents of all 3 phases and see what you come up with. You'll then know why major electricity suppliers keep rotating the phases from building to building!
With rural lines this imbalance takes on a new flavour. Here it is the yellow phase that is found to have the least load while red and blue are 'weighed down' with red being the most burdened. Again, red is seen as hot but having blue more burdened than yellow, at first, appears odd.
What takes place is the typical layout of the lines is triangular with red and blue at each corner of the base, and yellow at the apex. When installation teams come to install pole mounted transformers they have a tendency to connect to the red and blue phases as the yellow is regarded as out of reach and/or dangerous to connect to (as it has to feed through the middle of red and blue).
Whatever the reason for the imbalance the issue is it causes useless current to flow in 3-phase devices such as transformers within the network and therefore increases the losses associated with this. Motors are especially affected by imbalance which causes them to overheat. Power factor correction capacitors too cannot be optimally switched in as they will over correct on unloaded phases while still not achieving suitable correction on the overloaded phase.
In older installations the Neutral was also smaller than the phases and this is now having to carry a large load which leads to higher losses, and in extreme cases heat damage. This loss (voltage drop) will also raise the voltages on the other phases leading to over-voltage issues on them.
Now imagine arriving at a distribution point with a view to correcting any distribution imbalance. The Neutral is measured and found to have current on it. Ascertaining that there is a current imbalance the 3 phases are then measured so as to ascertain which is carrying more and which less. This done so as to plan what loads to migrate to which phase. But surprisingly the phases are nearly in keeping with one another, and by no means the value as measured on the Neutral.
This is as a result of non-linear loads being present on the phases and more than likely hi-tech which only draws current on the peaks. This distortion was discussed in the section on "waveform purity". What is being measured on the Neutral is the current which, more than likely, has a high harmonic content from the distorted current curves of the phases.
This method of defining the 'unequalness' of a 3-phase supply, i.e. that the RMS of the three voltages are not equal and/or the angle between each is not 120°, is defined as three components referred to as 'Positive Phase Sequence' (PPS), 'Negative Phase Sequence' (NPS), and 'Zero Phase Sequence' (ZPS).
The following is a very simplistic definition of each, but according to a highly acclaimed electrical engineer (my Dad) it is perfect.
Two parallel analogies are presented in the hope of successfully demonstrating these effects. The first is a three blade propeller simulating the three phases of a supply with the blade lengths representing the voltages, and the angle between the blades representing the angle between the phases.
Coupled with this is an analogy are the effects on a 3-phase star-wound motor with a 4-wire connect (i.e. Neutral is also connected) when subjected to an impure source.
Positive Phase Sequence:
This is what would be defined as the component that would turn the motor in the normal fashion (and in the required direction). In a perfectly balanced supply then this would be the component that would deliver the full torque from the motor.
Negative Phase Sequence:
Similarly, during an imbalance the motor does not operate at full efficiency and one could again say there is an invisible brake slowing the motor down (or simply stealing some torque).
In both of these analogies the braking effect can be equated to an energy source that is said to want to turn the propeller/motor in the opposite direction. This 'braking effect' is therefore the NPS component.
Zero Phase Sequence:
The blades that are 'smaller' do not exert as much centrifugal force on the shaft as does any 'longer' blade and therefore can be seen to be giving energy at the same time and in the same direction as the 'longer' blade.
If subjected to an unbalanced supply (especially angular), the armature of our motor will also want to wobble with the forces from the three phases placed on it in equal phase (i.e. no phase angle between them).
This 'wobble' effect is therefore the ZPS component. An interesting fact about ZPS is it always manifests as that which add on the Neutral.