Current Flow in Single Phase Neutral but Not Three Phase

The 3 phase distribution is high voltage, which goes through a low voltage isolation transformer. It's this low voltage that is connected to the house. The neutral is not connected back to the primary side, and the primary side does not have a neutral on the high volt distribution. So it's the center tap on the secondary that handles any neutral currents, and it's resolved there.

You are assuming that its a 60 Hz home installation in say the USA or similar. Even some of the 60Hz installations around the world may not follow the system you mentioned in quite the same way....

The OP needs to confirm where he is an what system he is using, but it sounds like a European system at a guess, which of course may be completely wrong. He does appear to know its 3 phase......

The USA uses these single phase transformers and the neutral connection goes to the middle of the secondary for 2 x 110 volts output.

It is not 3 phase. Its single phase. There is technically no phase difference between the two outputs (except that as they are usually measured relative to neutral, there appears to be a 180 degrees phase difference.....)

Single phase step down transformers are occasionally used here (no central neutral) for outlying farms and the like, but usually even those are usually 3 phase nowadays...

The system in question is just an average UK street which I believe is fed by 3 phases with individual streets getting a single phase each.

...and assuming the phases are balanced which they never are, the load presented by the identical three houses' toasters in this hypothetical estate shares the neutral conductor between the houses and the neutral current flows between the houses. There is no net current in the neutral back to the star point of the distribution transformer LV side. As far as the distribution transformer is concerned, the loads are balanced and there is no current in the neutral between that place and the estate.

Once one of the toasters has ejected its toast, the loads are no longer balanced and the neutral cconductor from the estate back to the distribution transformer LV star point carries the vector sum of the phase currents to the other two toasters back to the star point of the distribution transformer. That is its job.

Simples.ζ

In a center-tapped single-phase system, if the loads are balance on each side of the center, then the combined neutral carries zero current. It's just like a 2-point star vs. a 3-point star.

Dobidust,

It sounds like you are dealing with three dwelling units, each one supplied by two "hot" wires and the common neutral from a three-phase four-wire star system. Now, at each dwelling you are connecting a "toaster" that has two identical heating elements connected from the neutral to each of the two hot wires. For dwelling #1, the connected phases are A & B, with the unbalanced current on the neutral pointed as a vector in the direction of C. In dwelling #2, the connected phases are B & C, with the unbalanced current pointed in the direction of A. Similarly, in #3, the connected phases are C & A, with the unbalance in the direction of B.

At any point "upstream" of the three dwellings, the unbalanced current is pointing equally in the direction of A, B, and C. Since they are 120-degrees apart, the vector sum of the three is zero. Hence, you have no net neutral current drawn from the transformer, even though each dwelling has a net neutral current on its individual connection to the power line.

--JMM

I am guessing the the principle is the same for UK households which each receive a single phase and neutral, only that the net neutral current is cancelled out by vector sum back at the LV substation at the star point ? So would we would in deed be able to measure a current on the neutral coming out of the toaster and also at the consumer unit ? Is this correct thinking on my part ?

Yes.

Each household neutral wire will have current flowing in them. But the vector angle of each one is different, (phase angle), and if balanced, 120 deg appart.

the resulting vector of the 3 neutral current will be zero if they are absolutely equal in amplitude, when they meet at the star point.

If you add three equal amplitude sine waves that are 120 degrees offset, they add to zero. Thus, the three neutral currents from the three phases can add to zero.

That having been said, the neutral currents will only cancel if the loads are equal and are linear resistive loads. A good example of a non-resistive load is fluorescent lighting. Because the conduction is higher at peak voltages, the current waveform has higher harmonic content, i.e., it is not sinusoidal. In this case you loads may be equal but you could have considerable third harmonic and higher currents flowing in the neutral.

My question is that in a UK house with a single phase supply the phase current matches the neutral current, when does it become a vector sum ? is it back at the substation or out on the street ?

I am assuming the you have a Y connected primary (three phases + neutral)

A three phase transformer can be thought of as 3 single phase transformers. The voltage on the primary windings is higher than the secondary and the current is lower on the primary side by whatever factor the step-down ratio is. The primary currents, proportional to the secondary currents (by 1/step-down-ratio) combine to cancel out on the neutral on the primary side of the transformer.

I hope this was helpful.

In a balanced circuit, the current in your neutral will reach the star point (which is at zero volts) and split two ways to flow back via the other two phases. Because at that instant of time the voltage of the other two phases will be less than zero (negative) so no current will flow through the common neutral.

If the phase loads are unbalanced then the star point will not be at zero volts and some current will flow through the neutral.

Hi there

Normally the Distribution transformers in the area got a vector group of Dy1 or Dy11...it all depends on the manufacturers...

If you are a 380V three phase power user....you are know the only one taking power from that transformer...all other equipment shut off.......only taking power for a 380V 3 phase motor being used..."NO" current flow in the neutral of the transformer

You also want to take power to boil a kettle in the kitchen...that is now 220V single phase power drawn from the transformer.....know there is current flow in the neutral of that transformer through the starpoint (380/sq rt3=220V)....for all 220V equipment being used there is current flow in the neutral of that transformer.......................

May be you understand it better know.........................

This is not helpful at all.

You might have confused the issue, unless I did not understand your last paragraph on the "... kettle...".

If the Neutral is not available because it is cut off at the particular pole, then he will not have any current returning by the neutral point. All his loads will be between any 2 phases, via other possible loads, meaning that the voltage accross any switched load, will be anywhere between 0V (if no other load is switched on at another phase), and near 380V (possibly 300V + ) when some other load is switched on another phase).

Ok Vector group Dy11--for example only

lets try this....On the High voltage side 22kV(HV is Delta connected)

On the Low voltage side 380V (phase-phase) LV is star connected--the transformer connection "HV three windings" and "LV three windings"

On the LV side the starpoint is earthed(at the Customer house his neutral must also be earthed .....then for him to use the kettle(220v) single phase there is current flow in the neutral conductor and also in the live conductor...either R or W or B ...it all depends on which phase being used to supply the 220v for that house for then using a 220V kettle

Hope it makes sense know

greetings

If you are #14 contributor, Thanks for the clarification.

Now you are saying that the earth return is used as Neutral return. This is not a safe and sound situation since the Voltage drop will depend on the earth resistance and the total current flowing down the earth return. Your 220V will be lower as more current is flowing: ex: if R=1 Ohm, and Current is 10A, you will drop 10 volts, to 210V. For 1 kettle, this is maybe OK, but imagine more single phase loads at the same time, and if R = 2 or 3 or 4 Ohms.

The described case (by OP) is a case where the Neutral to the house is cut, and the loads are now, mainly between phases (the earth return will be contributing in the faulty situation, but will be less influential if other loads are on different phases).

If the load is only on one of the phases, then the case is as you described, with the voltage drops due to earth resistance (the earth connection will exhibit some voltage).

What you say is true and I agree, but if you look around the world, such methods as using the earth, also as a "return" is almost "standard"......but are intentional, not accidental. That is a two wire distribution. It saves a wire.

There are some CR4 members who know more about this than I do for example who could chime in if needed.

Some of the more remote parts of South Africa & Australia spring to mind for example.

Where I live in Germany we have the neutral & earth linked in the house distribution box because we have 3 phase and neutral coming into the house, but no earth.

The neutral & earth link is not made in the local substation.

In the UK for example, the link is in the substation I believe, but like the USA, no 3 phase into the houses, only a single phase, neutral and earth.

I agree that the earth could be used as a Neutral return for economy. This will be a single wire distribytion system, if for single phase, and not a 2 wire system.

But this method can only be used for small loads (Very small) and the earth rods need to be properly checked for near zero resistance.

Any 2 wire supply, for single phase system, will mean that a Neutral is there or if the main supply is 3 phase 115V and 2 phases are used to supply 220V for a house, then the one of the 2 phases is considered as a neutral while the Starpoint at the transformer is not grounded. This will make it possible to ground the 2nd phase (as Neutral), at the house, if that is necessary for protective devices to operate (?).

You have got it wrong on both 115vac US house systems and single wire earth return systems, sorry to be negative, but it is always best to get it right.

115vac US house systems.

Only one phase is supplied into a house but at 220vac. The supply is center tapped for 115vac neutral to phase and this is also grounded as safety ground, which is most likely also connected to an earth spike and any metal water pipes in the house.

I would guess that the transformer is a delta output at the substation. (citation needed)

You can see this well on a scope as the two "ends at 220vac" show as being 180° out of phase, whereas on a true 3 phase system, it will show any two phases as being 120° out of phase to each other, an obvious difference....

Single wire earth return systems.

The single wire earth returns systems use I believe a huge grounding plane buried in the ground, but to get the levels of power needed, they send a very high voltage, that is transformed down locally to the user to achieve normal mains voltage. (Citation required). What actual high voltage is sent I could not say.

This achieves useful amounts of power for the local home/farm user, not "very small" (Though no definition was supplied for very small, but I am talking about a house or farm supply) as you appear to believe......that would hardly make it worthwhile running wires for miles if you can only run a single light bulb at the end of it!!!

Thanks for the US supply clarification.

Hi Dobidust,

In a star you have a Neutral is the Key item---in typical European residential systems the different phases are phase to neutral not phase to phase 3ph, except in the water heater and sometimes the stove or oven, so most of your current will be from a phase through the neutral.

Expat 07

I do not understand your thinking.

I am going to stick my neck out here as it is very many years since I studied this stuff, but here goes:-

In a European (not a UK or a US one as far as I am aware) home with 3 phases and a neutral coming into the house, all the single phase current will flow in the separate neutral used to supply sockets and other loads, via a 3 wire cable (Phase, Neutral and Earth), back to the switchboard/fuse box in that house.

At the SWB/FSBX, all the neutrals will be tied together and also tied to the neutral coming in from the street.

If all phases are only connected to single phase loads, just for an example, and if all the single phase loads are all the same, perfectly symmetrical and identical, there will be no current on the neutral going back to the local substation. It will only be seen on the individual neutrals leaving that point and going into the house...

At that time, measuring on the neutral point to ground will show 0 volts. A condition that in practice will probably never occur.....due to unbalanced loads, capacitive and and inductive and well as load currents. It is next to impossible to balance loads exactly in practice.

Neutral & Ground will be tied together at some point depending upon local Electrical laws, it could be in the house and it could be at the substation. I am personally not aware of any other possibilities for tying earth and neutral in Europe domestically, but I bet there are some!!

All the current/power will be now passing only via the 3 phase connections to the substation.

You need to draw it out and look at it carefully before it is clear. Using 3 wire and 5 wire cables in the correct places.....

I do expect the "electricians" among you all to tell me I am wrong (as usual!!), but I expect most of the good EEs to agree with me fully....

OK?