Balancing 3Phase.....

[OnlQQker]

I was speaking to an Electrician a few years back as he was balancing out a 3 phase supply.

To me it sounded like he was trying to get close to each phase supplying roughly the same Kw's to both of the lines/live/hot.

Is this correct, if so how close do you have to be in this balancing stage and why?
Is this to do with sinewaves?

 

[littlespark]

It’s just a general rule that the loads on 3 phases be as close as possible to each other.
If there was no balancing, there could be an instance where, say L1 at the power station was twice what L2 was pulling.
It’s bound to cause some troubles… likely cable sizes not being up to it, or the generator itself being over tasked.

3ph motors will balance already, if nothing untoward, but it’s the single phase loads that can be off.

 

[James]

Phase balancing is really important for the generation and distribution people however it can be a cause for concern to the end user too.

for the end user
say you have a 3 phase supply

if your phase loads are perfectly balanced you can use all of the available power.

if you end up with one phase being loaded much more than the others then if you are close to the limit of the supply an additional load on that phase could blow your main fuse.
however it may be that you could have put a much larger extra load on one of the other phases and had no problems.

 

[Lucien Nunes]

There are a couple of separate benefits as mentioned above, the obvious one being to make use of all available power. No good having a 100A intake with 120A on one phase and 60A on another.

Next is if there are 3-phase motors on the system and any significant voltage drops, one wants all the drops as close as possible so that the motors see a symmetrical voltage. Suppose a long submain was feeding a board with 3-phase machinery and some heavy single-phase loads. If the single-phase part of the load is unbalanced then so will be the voltage drops on the three lines. Motors in the 3-phase machinery will then try to regenerate power into the lower phase(s) or at least draw less power from it, in an attempt to balance the drops, resulting in increased winding current and heat dissipation.

Finally there is a situation where in large industrial distribution, due to the generally balanced nature of machine loads, the neutrals are significantly reduced to save copper. These systems can only withstand a certain fraction of imbalance before the neutral current is exceeded. E.g. in a production hall with a 600A maximum line current, there might only be 50A of neutral current / imbalance to supply the 230V electronics. DNO's distribution cables sometimes have a reduced neutral as the diverse nature of a load made up of many customers spread across the phases makes for very low neutral current overall.

When systems with reduced neutrals get re-purposed they can come to grief. In 1995 a large feature film production moved into a disused wartime factory complex and connected the lighting distribution panels to the existing submains that had served production machinery. Film and theatre lighting can by nature be heavily unbalanced as any combination of lights might be turned on or off at any one time, and due to the 3n harmonics produced by phase-angle dimming the neutral current can be artificially high. The submains with reduced neutrals soon made themselves known.

I have an interesting story to tell about unbalanced 3-phase entertainment systems loads but this post is long enough already.

 

[Dustydazzler]

IIRC we spent at weeks at college working on the theory of 3 phase calculations of balanced loads and unbalanced loads and N currents etc

 

[Lucien Nunes]

It's worth a mention that higher-load domestic cooking appliances, in particular, often have multiple line terminals that normally have to be linked together for conventional UK single-phase installations. In European installations where 3-phase supplies are much more common, the incoming line current might be limited to say 25A. The total power available is still on a par with a UK service but the application of diversity is hampered by the fact that larger loads make up a much greater fraction of any one available line. Splitting a hob into two separate loads of half the power allows easier balancing and better diversity overall on the CU, despite each of the loads having less diversity in itself.

 

[OnlQQker]

Great explanation, many thanks.
@Lucien Nunes I would love to hear that story on the unbalanced 3 phase entertainment system.

To dig a bit deeper into this and looking at the supply from the power grid to the transformer and before it's changed to 230v I'm guessing this is the cable they use, though I find it interesting you can get it in copper or aluminium? 11kV Medium Voltage Power Cables | Eland Cables - https://www.elandcables.com/electrical-cable-and-accessories/cables-by-standard/11kv-cable

Am I right in thinking this is 2/line and one neutral.

Then neutral pinned to Earth along the way?

 

[SparkyChick]

I believe the 11kV grid is 3 phase conductors only with transformer windings wired in a delta configuration. The low voltage (240v) side if then a star winding arrangement with the star point earthed and providing the neutral.

 

[OnlQQker]

I believe the 11kV grid is 3 phase conductors only with transformer windings wired in a delta configuration. The low voltage (240v) side if then a star winding arrangement with the star point earthed and providing the neutral.

Hi SparkyChick,

I'm just looking at the 'Delta Configuration' What Is Wye And Delta? | Chapter 1 - Voltage, Current, Energy, and Power | Power Electronics Textbook - https://eepower.com/power-electronics-textbook/vol-i-electrical-power-systems-design/chapter-1-introduction-power/what-is-wye-and-delta/#

What would be the deciding factor in Delto/Wye networks? I can't seem to get much further than it's possible to convert a wye network into a functionally equivalent delta network and vice versa.

 

[pc1966]

What would be the deciding factor in Delto/Wye networks? I can't seem to get much further than it's possible to convert a wye network into a functionally equivalent delta network and vice versa.

Usually the step-down transformer is delta on the 11kV side, and star (wye) on the 400/230V LV side.

The advantage for the delta on the HV side is only 3 live wires, no neutral needed, so smaller distribution costs (approx 3/4 of star system cost). Less obvious is the delta winding shorts out 3rd harmonic currents so less flow back to the higher voltage grid.

On the LV side practically every installation needs neutral as most stuff is single phase L-N, and some is 3P L1-L2-L3, so using the star you get the neutral and L1-3.

The choice of N earthing is, of course, of course one of the key choices in system design. So TT if earthed only as sub-station, TN-S if earth run from star as well, or TN-C-S if N is combined with E.

 

[Lucien Nunes]

Delta is good for transmission as it uses the theoretical minimum number of wires and offers the greatest power transfer per unit line cost. Line plant economy is crucial when moving hundreds of megawatts over hundreds of miles.

Star is good for distribution, as it is more versatile. Single-phase loads can have one side returned to neutral and a single distribution system offers two different voltages (L-N and L-L). This closely matches the requirements of LV consumers; domestic systems can operate at 230V single phase while heavier machinery gets 400V 3-phase from the same set of conductors, saving on copper compared to 230V 3-phase. The cost of this flexibility is that of an extra conductor. This can be mitigated by reducing its CSA where the load is locally balanced, i..e neutral current circulates between adjacent unbalanced single-phase users and does not all have to return to the source.

It is possible to run distribution systems on delta, and this is done in some places and environments. A disadvantage of 3-wire delta in its star-grounded variety with the neutral not distributed, is that both wires of a single-phase load are lines. DP switching and protection are required everywhere on single-phase systems derived from delta. This has historically been used in Europe for example, and is one reason why some types of European domestic plug developed unpolarised - the two line conductors did not need to be distinguished from each other.

There is another delta variety where one phase is centre-tapped to earth. This so-called edge-grounded system is widely used in the USA etc. for residential areas where the single-phase load dominates over the 3-phase. It offers three different supplies (120V single, 240V single and 240V 3-phase) from four wires, and the lower voltage of the split-phase 3-wire service has an earthed neutral. But it doesn't offer the advantage of the higher voltage for 3-phase (compare European 230V single, 400V 3-phase), all the single-phase load is on two lines, and it tends to suffer from assymmetry especially in open-delta form. Often referred to as 'high-leg delta' because one line (the 'high leg') is further from neutral than the other two (208V in USA).

Oops, simulpost with @pc1966