Is 3 Phase Line Always Balanced?

Can you post the questions from the test directly to the forum so we can dispense with the pretense that you have experienced some offense at the realization that you have no sense....of the true purpose of the test?

Of course three phase lines are not fixed in balance. If they were always balanced then there would be no need for the adjective "balanced" to be used in this context.The real question here is can somebody be taught to think? I say no they cannot be taught to think. Sometimes one can be coerced into thinking with abuse, I prefer to use an argument today.

"... are all the 3 phases always balanced in terms of voltage?" NO

" ... what is typical %" Varies

Above 4% is range for overheating concerns. With freq drives, depends on circuit design (see manual).

3-phase lines are never truly balanced.

A 3-phase line connected only to a single phase load is about as unbalanced as it ever gets.

In the UK, what used to be the red phase always carries a higher current than the others, as the electrician always starts from the top left of the distribution board when adding new single phase circuits, and it is the former red at the top. The solution is to rewire some of the former red phase single phase loads onto the former yellow and the former blue.

The forum has dealt with this topic previously, as entering the words "balanced 3-phase" into the "Search all of CR4" box and clicking Go will reveal.

How about a 3 phase source driving two single phase load SWMPS that do not have power factor compensation circuitry.

Considering the OP has had a good answer in a thread on this very subject I find his question a little strange

As an aside. Whenever I've had to look in to loading on 3Ph supplies, red nearly always carries the heaviest load.

What do standards/codes like BS,NEC,VDE,IEC etc say,what is the maximum unbalance in utilitiy's voltage allowed?.

All on one phase and nothing on the other two, Boss. It doesn't need any code to do that, only a few switches.

In India I have been hearing about the three phases being unbalanced leading to heating of transformers. I also heard from someone that the thermal power generator is not stabilized and goes into oscillations whenever there are large load transients. Earlier- I used to listen to these passive as plain complaints. Now I am in a position to find a solution and have an opportunity to implement it.

First thought that came to my mind was - to check with CR4 whether this problem is a global phenomena or confined to India only.

This should answer various queries raised in this thread. I am happy to note that this seems to be a global phenomena and utilities need to find solution technologically and not be just making requests, cajoling , coaxing customers to ensure balanced load on all 3 phases all the time. I like taking up such challenges.

"Knowledge is the food of the soul." -- Plato

"Determine that the thing can and shall be done, and then we shall find the way." -- Abraham Lincoln

So why don't you make a start by spreading your own single phase loads around the distribution boards in a more balanced way, Mildred?

One of the reasons type "K" transformers are preferred now. The neutral is the same size as the three lines.

I often wonder how all the miles of 185mm² 3½c SWA in installed years ago is coping with imbalance and harmonics nowadays. It's rare to come across 3½c now, 4c is the cable of necessity.

From the generating stations almost upto 11KV it is just 3 wire system. Neutral only comes at distribution level. Imbalances at 11 or 33 KV level are transferred right up to the generator. Hence the problem needs need to be solved right at distribution / user level.

Thanks from me personally.

I guess that unless we design alternators to keep each separate phase voltage exactly the same in spite of load changes only affecting this phase, we are stuck with the problem.

Has anyone a better take on that idea?

You should have 3 single phase alternators each with a separate AVR. There should be 3 neutrals too. Any comments.

If an alternator (or the 3 phase supply after a transformer) stays with basically with the design s we have now, at some point the neutrals have to connected to each other.

In a domestic situation here in Germany where we have 3 phases coming into the house, they are linked in the house fuse box(es). 4 wire supply cable.

In the UK where usually only a single phase is supplied to a house generally, the links must be in some external distribution box where the 3 phases meet.

The only method I can see to keep neutrals separate is to only generate single phases per alternator....but what would industry do who need 3 phases?

Maybe 3 such alternators could be driven by a single prime mover, all on one shaft, and stay at 120° phase difference that way.....but any connection of three phases for say industry, except after a delta/wye transformer, would "link" them up again....

Most of the above is written when thinking, not always a good idea, so apologies if there are any major blunders......

So how do you stop one of them overtaking the other two, or vice-versa? Then I'd be worried about powering a regular 3-phase motor with the arrangement you suggest. That it isn't done would suggest it's a bad idea.

Due to voltage drop due to I & R voltage along the line will vary.

But if we somehow balance all 3 phases at distribution level say many kilometers away by modifying the transformers - will the generators remain balanced?

In theory, yes if the distribution is balanced then the generators will be balanced. In practice one can never perfectly balance all three phases. Different single phase loads get switched on and off at different times, mechanical loads on motors change suddenly. Sometimes the mechanical load changes so quickly that for a brief period the motor acts as a generator putting power onto the grid instead of taking power. Then there's the time delay between generation sources and changing loads that produce the reflected waves described by Oliver Heaviside's telegraphers equations for transmission lines. [It still amazes me that these equations were codified in 1880, long before any oscilloscopes existed.]

I do agree with you. I take pleasure in revisiting things mentioned in 1880. Thomas Alva Edison invented the incandescent bulb- but later scientists found gas discharge tubes give more light- without heat generation as in incandescent bulb. I have always believed that very little improvements have taken place on things developed by Edison, while the communication field has seen lot of innovative developments compared to what Graham Bell's invention. That also menas there is scope for development in power sector and it is a challenge- undoubtedly- not mere talking. While I write all this- I am also cautiously gathering information. Thanks.

Ah, the Harmonics of it all.

We should not tamper with generated voltage but try to adjust voltage by boosters.

I think you are from a Generating company.I wish to know- whether such imbalances on 3 phases lead to grid instability and wobbling/vibration in the generators- due to varying air gaps (back EMF effect), early wear and tear of bearings etc. Hence - I feel there is need to somehow solve the problem- not by utilities requesting, cajoling, coaxing customers to load all three phases equally- solution must be found through technology - not by prayers or hope!!!!!! Wish to know more from your practical experience.

"Success is not measured by what you accomplish but by the opposition you have encountered, and the courage with which you have maintained the struggle against overwhelming odds." -- Orison Swett Marden

I don't see it as a problem. Only a feature.

Do you work in sales?

Today it occurred to me that instead of modifying transformers all over- it is better that generators in hydro or thermal power stations be modified so that even single phase loads will equally load all three phases equally. So generator manufacturers need to pay attention. 3 phases can still be transmitted over 3 lines as at present and customers can use it as 3 different single phases or 3 phase- but generator end will always be equally loaded always!!!!!!!!!!!

"Perplexity is the beginning of knowledge." -- Kahlil Gibran

If alternator is designed with 3 AVRs for 3 phases there will be some balance.

The balance of 3 phase electrical power systems is ideal. In fact, this has never happened. However, utilities always strive to make their power systems appropriately balanced in operation. So, the percentage of the imbalance is quite small.