Balancing 3 phase

[electromonkey]

Having served my apprenticeship working for sparks who predominately worked only on single phase I as a result only tend to work on single phase installations as I lack experience in the 3 phase game.

I have worked on and carried out some testing on 3 phase installations in the past but have never carried out a major works on one.

My question relates to balancing the board down it's 3 lines. I would say the majority of installations I have seen have not been perfectly balanced but seem to be working fine. What is the tipping point for a system percentage wise? i.e. you're never going to get 33.3% of a total load down the 3 lines at all times so at what point does it become imbalanced?

Also, what are the characteristics of a badly imbalanced installation?

Your input as always is appreciated.

Electromonkey.

 

[Darkwood]

Having a 3 phase supply coming in and having a good balanced load reduces neutral current and allows more for expansion, if you have:_
A-20amp
B-65amp
C-70amp.......... if you have 100amp front end fuses and you get a 3phase machine in you have effectively reduced the 3ph allowance because one ph is heavily loaded.

 

[Darkwood]

Just to add to this with the influx of switch-mode devices and other electronic device that create noise and harmonics then neutral sizing is starting to be a big issue so if you have a balanced 3ph load to the building then the neutral wont already be heavily loaded and may withstand unwanted harmonics.

 

[electromonkey]

So, having a poorly balanced installation only reduces the systems loading potential? Does it have any other effects?

 

[Darkwood]

check post 3 you may have missed but it can be financial as you may be paying for a given KVA allowance just because one ph is alot higher in demand but if balanced then it may be possible to reduce KVA allowance and the tarriff you pay, also inductive lighting that isn't high frequency will be best balanced over 3phases to reduce the possibilities of stroboscopic effect on moving machinery.

 

[Darkwood]

Just for future thought, with the advance of smart metering and online monitoring of energy demands its leaving open a possibility that energy companies could charge companies for imbalanced loading if its excessive as this has cost implimentations and nuiscence tripping issues where sub-station supplies are feed many units .... although they do tolerate this at the moment; i can see it been the new way of raising money to cover the network maintenance.

 

[Knobhead]

Neutral current
In=√(Ia²+Ib²+Ic²)-((Ia*Ib)+(Ia*Ic)+(Ib*Ic))

So if we use the figure Darkwood gave you would have 47.6A flowing in the neutral.

If the imbalance gets really bad then everything starts to suffer, cable, switchgear and transformers.