Unbalanced Power Systems

Balanced load in 3 phase system is a condition where all three phases (lines) carry same magnitude of current, with evenly spaced phase difference. If the load is star connected, with neutral as return path, this neutral will carry no current. This is so since vector sum of all three phase currents meeting at common neutral point is zero.

Unbalanced load makes the lines / phases to carry different current magnitudes, and sum total of these at neutral point is not zero. Load in each phase is different, carrying ts own current. Neutral in this case carries the net unbalanced current .

If there are reactive impedance components in load, there will be phase difference in line currents from respective voltages, and even if magnitude becomes same, the phase difference will result in unbalanced current in neutral.

In practice, with so many single phase loads, or some two phase equipment in operation, currents in 3 phase system are never fully balanced, despite best efforts to achieve load balancing.

You may have individual balanced loads in many applications like motors and 3 phase ovens, but other loads (for example lighting, single phase motors or single phase household supplies) make the system unbalanced.

The term “balancing” comes from how much percentage of load you have on each phase with respect to the total load on the system. Think of a three way teeter totter with different sized people sitting in each of the three seats. The size of each person is the amperage for their respective phase, with phases designated as A, B, or C.

If you have 1/3 of the total amps going through each phase, you are said to have a perfectly balanced system. For example, you have nine 10A loads, and you install three on phase A, three on B, then three on phase C, and the result would be a perfectly balanced system (with all loads running at the same time of course). However, this system doesn't exist in the real world. It is an ideal model we electrical engineers design for. We try to “balance” the system by moving circuits around in a panel to even them out (balance them) to the best of our ability, with the loads we are given.

Balanced load means sum of the three phase current equal to current passes through neutral wire. And unbalances load is opposite of above.

i.e IR +IY +IB - IN =0

IR = Current in R phase

IY= Current in Y phase

IB=current in B phase

IN current in neutral phase

This is very fundamental but essential protection to be provided to any feeder supply. You can measure the unbalanced current by simply using clamp meter as shown below.