Unbalanced Load

Yes when a local tranformer blew up because of a fault in its windings. Big bang lots of smoke then sirens, more sirens then silence. several thousand went without power for several days.

We had a large air 3 phase air conditioning unit lose one phase. The unit ran faster but noisier. Everyone just stood around looking at it. They new something was wrong but did not know what, since to the observer, it was still operating. It even seemed to operate better (faster) than normal.

The extra vibration and heat would have eventually destroyed the unit if we had not shut it down. This is interesting since the protection ccts would not have protected it. The result again would have been a Bang, Smoke, sirens... as guest said in reply 1.

Generally a three-phase piece of electrical equipment (eg- a 3ph motor) will be designed to balance out the load over the three phases. Problems can occur when you have a system that uses 3ph and neutral, where part of it runs off 3ph (and is balanced) and another part runs of a single phase (using the neutral).

As a better example, consider a large factory powered by its own transformer and using mostly 3ph motors but also lots of large single phase loads connected across only one of the phases (transformer imbalance occurring, but not due to a fault). Generally thou the single phase loads would be spread over the three phases.

Another example of a unbalanced load occurring due to a fault is when a 3ph motor drops a phase. Not nearly as spectacular as a transformer failure due to a winding short thou (but that technically isn't due to imbalance, rather fault current).

it depends on what your loads are...if you have many single phase loads that are not evenly distributed between the 3 phases or if you have highly inductive or capacitive loads all in total your 3 phases will not balance well. what type of loads are you running? what does your power factor look like?

To my understanding a three-phase balanced system is that the phasor sum of the three line or phase voltages is zero, and the phasor sum of the three line or phase currents is zero.

Since you mention neutral, I assume you mean a Y connected system. The sum of the three phases PLUS neutral current add up to zero at every instant. Assuming your supply maintains a constant phase voltage for each phase with respect to neutral, the neutral will have current which is caused by the load imbalance.

In fact, unbalance will cause a negative sequence current.

This will occur in many conditions:

-when a conductor is broken and you are feeding a balanced load, such as a motor

-when a double phase fault occurs

-when a double phase to ground fault occurs

-when a single line to ground fault occurs

For such conditions the best way is to draw the symmetrical components sequence equivalent for the systems and it will be clear in what conditions this happens.

Regards

elbf2801

elbf2801,

I agree. Where you will be able to determine your Positive Sequence Components, Negative Sequence Components, and Zero Sequence Components with respect to the unbalance (fault) condition.

Your last sentence in the first paragraph: "The imbalance occurs when an open or short circuit appears at the load," isn't quite consistent with your previous statements, even though the last sentence is true as well as the previous statements.

It is very common to have imbalances in a 3 phase panel because all single phase loads aren't the same yet are distributed somewhat evenly among the phases, and I have recently run across a panel which fed a computer lab, lighting, and convenience outlets, which was so unbalanced that it didn't make sense. As it turned out, the transformer feeding the lab was undersized, and had been used for so long, that the transformer started breaking down. When a panel is fed by a poor ol' broken down transformer, it may be impossible to balance the loads, so we swapped it for a larger, younger model.

How would you like to have my experiences expressed?

In a normal delta star connected step down distribution transformers the balancing of load in each phase is a very difficult task when loading is combination of three phase power and single phase power and lighting loads.Hence the phase currents will not be uniform causing neutral shift .For a domestic power distribution lines the single phase loading is so uneven that there will always be unbalanced loading resulting large voltage variation in phase voltage and if the neutral earthing is not absolute there will exist neutral floating voltage which is harmful to electronic equipment. While designing LT distribution lines lot of care to be taken to balance the single phase loads. It is always better to use isolation transformers/ constant voltage transformers for computers and computerised controlled equipments in unbalanced loading systems.