Load Balancing in a Three-Phase System

Thanks for your practical technical info. we presently monitor each phase line current.Hope we can start to transfer some of the loads to A & B. Maybe, we need to identify first the non & contin. loads. God bless.

There will always be some imbalance as, by their very nature, single phase loads are usually of an intermittent and variable nature.

Good point. GA.

Mr. Guru, we experienced frequent trippings in our 3 Phase elevator motors. Thank you.

I have a basic agreement wit all said before. But, think for a moment. If one phase is 300 Amps, the other is 100Amps, which will burn out first?!?

Your good guess will earn browny points.

In case of Y connected secondary with the star point earthed i.e in a three phase four wire system, the high unbalance current in the three phases will increase the neutral current. Ideally if I_A= I_B = I_C then the vector sum of all the phase currents will be zero, hence neutral current will be zero. High unbalance current may cause heating of the neutral conductor if the size of the neutral conductor is half the size of the phase conductor.

check this

Practical Load Balancing:

Ideally you want to try and balance out the loads. When juggling single phase loads at best you can get close. In a building a hot air hand dryer or local zip boiler can throw the loads around and imbalance the system.

The generating station likes a nice even load - it smooths out the mechanical imbalances on the generating machines.

As for billing its unlikely affect it.

Be aware of the need to monitor and if neccessary increase the size of the neutral in a TP&N cable if there are lots of single phase loads around and I'm thinking here single phase loads with the ability to introduce harmonics.

The supply authority has a remit to supply within certain tolerances so I can't see over running a motor being an issue.

I would like to view the problem from the point of view of the generating equipment namely the generator. Each generator has limited capability of supplying the unbalanced load. This is known as I2squared t (sorry I could not write 2 as subscript of I). Here I2 is negative sequence current and t is the time for which is I 2 could be permitted. Any unbalanced load can be considered to be made of three components, positive sequence, negative sequence and zero sequence. The negative sequence current produces magnetic field which revolves at synchronous speed but in reverse direction to that produced by positive sequence (balanced) load. So the relative speed between magnetic field created by the negative sequence and the rotor is twice the synchronous speed. This results in induction of double frequency voltage and the resulting current in the rotor body which is normally not designed to carry such current. So the rotor gets heated and may get damaged if such current is allowed to pass. The rotor has inverse time characteristics for with standing this current. To protect the generators from negative sequence current, protection relays (negative sequence protection relay) are provided. So many power providing companies impose a penalty for unbalanced loads on bulk consumers. I as a generator designer, would request each consumer to understand the effect of unbalanced load and try to minimize the unbalance load.