Unbalanced 3 phase power - what happens without a neutral wire?

[Pawlal]

Hi

I've recently started learning about electrical three phase theory, and therefore got some questions.

Imagine the following:
Countries like Albania, Norway ++, have IT earthing configuration in their distribution grids, i.e. the neutral point is not solidly grounded, however "grounded" through the system capacitance.

So, I imagine that its hard to balance each transformer and some unbalance current exist?
How can this exist, if there is no neutral, does all this unbalanced current return back via the capacitance?Thanks for any help for this fundamental question.

 

[jim hardy]

So, I imagine that its hard to balance each transformer and some unbalance current exist?
How can this exist, if there is no neutral, does all this unbalanced current return back via the capacitance?

No,
the line currents become unequal and their phase difference shifts away from 120 degrees.
They still sum to zero (or to whatever small amount can return through system capacitance)

so your current phasors no longer make an equilateral triangle.
It becomes an exercise in geometry to construct a triangle with sides proportional to your actual line currents.

 

[cabraham]

In 3 phase distribution, unbalanced loads are supported without the need for a neutral wire. No load is truly balanced, but system phase & line voltages stay balanced even with severe load unbalance.
It was learned over a century ago, that transformers connected in Y-Y, without a delta tertiary, cannot support unbalanced load with only 3 wires, a neutral wire is needed for Y-Y units.
But that defeats the purpose of using 3 phase. Using 3 wires with 3 phase requires only 75% the conductor mass as other numbers of phases.
So the power company avoids Y-Y xfmrs. They use Y-delta, delta-Y, delta-delta, & V-V (open delta). These xfmr configurations support unbalanced loads with only 3 wires.
If it is necessary for primary & secondary to both be Y connected, a third winding, a tertiary, connected in delta, is used. With Y-Y-delta, unbalanced loads are supported with just 3 wires.
The grid is a 3 wire system, but 4th wires are used for lightning protection & safety grounding. The lighter gauge wires above the 3 power wires are for lightning protection. Xfmr neutrals are grounded to keep system close to ground potential. Otherwise the potential to ground is determined by capacitance. The 240 volt side of a xfmr could be near 2,400 volts with respect to ground via capacitive coupling, if ungrounded.
Neutral connections are not relied upon to carry unbalance in current.

Claude Abraham
PhD student in EE now
40 year EE

 

[cnh1995]

No load is truly balanced, but system phase & line voltages stay balanced even with severe load unbalance.

Hi @cabraham, this is interesting!
Could you please elaborate this point? How is this achieved in an HV/EHV/UHV 3-wire transmission network?

 

[cabraham]

Hi @cabraham, this is interesting!
Could you please elaborate this point? How is this achieved in an HV/EHV/UHV 3-wire transmission network?

Power has been studied for over a century. Transformers are used to step generated voltage up (current gets stepped down), transmit long distance, then xfmr steps down voltage at load end.
3 phase xfmr configurations include Y-Y, delta-Y, delta-delta, Y-delta, & V-V (open delta).
Only the Y-Y connection requires 4 wires to stay balanced when load is unbalanced. Suppose the load end xfmr is Y-delta, with unbalanced load at secondary, delta side.
A delta is a closed loop, & currents may circulate inside delta without leaving the closed loop.
If the B-C leg of the delta is loaded, with phases A-B & C-A open, what happens? The current in B-C liad is sourced by 2 parallel paths, the B-C secondary winding of the delta, in parallel with the series combination of A-B & C-A windings of the delta secondary. The current divides as follows. Leg B-C sources 2/3 of load current. Leg A-B-C-A sources 1/3 load current.
The 3 primary windings of the Y reflect these currents. Since the B-C leg of secondary delta has 2/3 load current, the corresponding Y winding in primary has 2/3 the amp turns of the load as well. The remaining 2 windings have 1/3.
Kirchhoff current law is met as 2/3 enters neutral & 2 times 1/3 leaves neutral.
Remember this, 3 phase systems stay balanced with only 3 wires, except Y-Y, because unbalanced current in one phase can return via the other two phases.
In the case above, if B-C winding in delta secondary couples n-c winding in Y primary, the neutral to phase C winding has unbalanced current, 2/3 value. The return path for phase C excess current is phases A & B.
I will attach sketches later.

Claude Abraham
PhD student in EE
EE 41 years