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Author Topic: How is the electrical circuit completed between the powerstation and my home?  (Read 22040 times)

Offline chris

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We're always told that electricity flows in circuits, but the supplies to houses are from live lines and the neutral lines in the home are grounded to Earth. So how does the current flow back to the power station, does it go through the Earth?


 

Offline PmbPhy

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We're always told that electricity flows in circuits, but the supplies to houses are from live lines and the neutral lines in the home are grounded to Earth. So how does the current flow back to the power station, does it go through the Earth?
I think you're confused. There are three lines. Two are for power and one is a ground. The ground is the neutral wire that you're thinking about. That's only used when there is a high power device being used like a computer or an air conditioner etc. Didn't you ever wonder why some electrical plugs, like that from an air conditioner, have three prongs? That third prong, the thick round one, goes to ground. However sometimes the term "ground" has a different meaning. It can also mean a common power bus in a DC circuit where the negative wire goes to. That is connected to ground as well.
« Last Edit: 26/07/2015 21:57:29 by PmbPhy »
 

Offline evan_au

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In remote rural areas of Australia, I have seen electricity distribution using SWER (Single-Wire Earth Return); just one wire is fed to the property at a high voltage, with the current returning via the ground.  At the property, a transformer turns the high voltage into normal residential voltages on a pair of wires (230VAC in Australia vs 2x115V in some other countries). But this SWER system is inefficient, and the supply voltage is poorly regulated; it is a rare exception - it is only done because of the high cost of delivering two wires in remote areas.

In metropolitan areas, you will typically see 4 wires passing down the street. This consists of three phases of "Active", plus a "Neutral". You could imagine the Active carrying current "from" the power grid, and the Neutral carrying the current "back to" the power grid (even though the current flow is symmetrical).

As Tesla noticed, if the load in the street is well-balanced between the three Active phases, there will be no current flowing in the Neutral*. In practice, residential loads are always switching on and off on one phase or another, so the Neutral is often the same diameter as the Active wires.

These 4 wires would be sufficient if we were just interested in delivering electrical power. However, electricity is potentially lethal, so we also have to consider electrical safety. If the supply voltage were "floating", the voltage could rise to thousands of volts in the home, causing insulation breakdown and increasing the chance of electrocution.

This risk may be reduced by connecting one of the supply wires (the Neutral) to an Earth stake at each home or at the transformer (practices can vary, depending on the soil resistivity in each region). This way, the Active voltage won't rise above normal household voltages, and Neutral will be effectively the same as Earth potential. If everything is operating properly, there will be zero current flowing in the Earth stake.

A further safety feature is the "Residual Current Device", which compares the current flowing in the Active & Neutral wires. If they are not exactly equal and opposite, it suggests that an earthed person may have touched the Active wire, and it cuts off the electricity before a lethal shock is delivered.
 
* You will see that high-voltage transmission towers have a multiple of three wires (if you ignore the top lightning conductor, which is usually thinner). This is possible because high-voltage transformers are arranged to share the current equally between the three Active wires, rendering the Neutral effectively redundant.   
« Last Edit: 26/07/2015 23:35:27 by evan_au »
 

Offline alancalverd

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In the civilised world, all plugs have three prongs except for high power 3-phase plugs which have 5.

Just considering a single phase supply, the incoming cable consists of a "phase" wire whose potential varies at 50 Hz (60 Hz in the far-flung colonies), a "neutral" wire whose potential is very close to zero with respect to earth, and an "earth" wire which is literally pinned to the surface of the planet at various points along the way. The system is designed so that the working current flows between phase and neutral, but all "accessible conductive parts" (ACPs) of any machine (i.e. metal casings, screws etc) are connected to earth.

The reason for this complication is that the high voltage generating and transmission system produces 3 alternating voltages with a 120 degree phase difference between any pair, and the local mains voltage distribution network is established so that the current demand in each phase is pretty well balanced. If the phase currents are perfectly balanced there is no net return current along the transmission line neutral. You can thus eliminate the neutral conductor and send "bulk" electricity through 3 wires instead of 6, halving the infrastructrure cost.

However as we get closer to the consumer the phase currents will become less balanced as different consumers switch on various appliances, so there will be some net return current and you need a neutral conductor. Since the neutral conductor will have a finite impedance, its potential at the consumer end will not always be zero, so we provide an additional "protective earth conductor" (PEC) that normally carries no current: its function is to ground the ACPs so that any fault that would otherwise make the casing live, simply blows the protective fuses or trips.     

You can get away without a PEC if the consumer device is "double insulated", i.e. constructed so there is no possibility of anyone coming into contact with the live or neutral. 
 

Offline Colin2B

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We're always told that electricity flows in circuits, but the supplies to houses are from live lines and the neutral lines in the home are grounded to Earth. So how does the current flow back to the power station, does it go through the Earth?
I think you are in UK. PmbPhy is answering with US system which I believe provides 110v for low wattage appliances and 220v for higher.
In UK we have 230v supplied on 2wires Line (L, brown) and Neutral (N, blue). The L wire is what most people would consider to be the live part carrying power into the appliances, the N is the return path to the supply system. Earthing arrangements depend on your particular installation and can be one of 5 types, some of which are pretty well obsolete. This can be confusing as the most common system for new housing combines N and E in a single conductor which is separated out at the supply box/meter. Because of the resistance of the supply cable it is possible for the N wire to rise above 0 so other protective measures are provided including bonding the earth wire to metalwork eg pipes to prevent dangerous voltages in the case of a fault. Further confusion is due to older systems where there was an earth connection to a plate in the ground, this was never intended as a return path for the current.
Because the current in the L and N wires is the same there are devices which detect any imbalance (RCD - Residual Current Device) due to a fault and disconnect the supply. Such an imbalance could be a fault causing current to flow to earth or someone touching the circuit and again causing current to flow to earth.

Edit: just noticed that Alan C has replied while I was typing, his is a more complete answer than my top level summary.
« Last Edit: 27/07/2015 00:04:11 by Colin2B »
 

Offline chris

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Thank you for the comprehensive answers everyone.

But I do not follow Tesla's observation "...if the load in the street is well-balanced between the three Active phases, there will be no current flowing in the Neutral."

Can you please explain how this comes about?
 

Offline Colin2B

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..if the load in the street is well-balanced between the three Active phases, there will be no current flowing in the Neutral."

Can you please explain how this comes about?
The 3 phases are 120 apart so if they are equal the sum is zero (think of it as a vector sum with 3 vectors 120 apart). The loads would have to be carefully balanced as any inductive load would give an additional phase shift.
 

Offline alancalverd

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This accounts for the difference between the high and low domestic mains voltage in the USA: the voltage between live phases is  nominally 208V rms, but each phase is nominally 110V to the common neutral. There is a higher voltage industrial standard in the USA giving 480/277 volts.

In Europe the single standard is generally 400/230V. It is common for all three phases to be delivered to domestic users in parts of Germany and Scandinavia as this makes the supply to and balancing of large fixed domestic loads such as cookers and washing machines a lot easier, but ordinary plug sockets all run on one phase per house and you need a professional to install your fixed appliances (if you swap two phases, all the motors run backwards!)
 

Offline Bored chemist

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Even if the loads are not well balanced it's cheaper to have a single neutral wire the same gauge as the live phases than to have 3 return wires that gauge.
Much of the "return" current rns up and down that wire between users, rather than back to the power station.
 

Offline evan_au

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Quote
Tesla's observation "...if the load in the street is well-balanced between the three Active phases, there will be no current flowing in the Neutral."
Can you please explain how this comes about?

Tesla invented the three-phase AC electrical distribution system, competing with Edison's more established DC electrical distribution system.

The AC system has three Active wires, each carrying a sine wave with the same amplitude of AC voltage (eg 230V rms = 325V peak) at the same frequency, but with a phase that is separated by 120. Plus the Neutral wire, which is close to the Earth voltage (lets call it 0V; in practice it should be < 1V for safety reasons).

The voltage on the three Active wires keeps chasing after each other - a pattern that is extremely useful if you want to power an electric motor, as the magnetic field from 3 coils continually sweeps out a circle, spinning the machine's rotor.

Lets call the phases black, red and blue. So if the black phase is +325V relative to Neutral at some instant in time, the  blue and red phases will have a voltage of half this amplitude, but negative, or -162V. So even though all the voltages are continually changing, the sum of the voltages at the Neutral wire is always zero if the loads are balanced.

With a resistive, capacitive or inductive load, the currents in the three wires will also be sine waves, with a 120 phase difference. Similar logic applies so that if the current is 100A in each wire, the sum of the currents in the Neutral wire is always zero, even though the currents in each Active wire are always changing.

Electrical engineers describe the 3-phase electrical supply with a phasor diagram: the three radiating arrows represent voltages or currents with the same amplitude and frequency, but a 120 phase difference. The time-based oscilloscope trace of three sine waves is an equivalent representation that may be familiar to more people.
« Last Edit: 27/07/2015 15:04:39 by evan_au »
 

Offline alancalverd

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The more I learn about Tesla, the more I recognise him as a genius - as well as a naive fool! If he hadn't come up with 3-phase AC supplies, I wonder what state the whole of industry would be in today?
 

Offline chris

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Thanks everyone for patiently explaining this. But can you provide clarification on one aspect of the points above:

The zero in the neutral line, is that relative to the powerstation, or the local substation and Earth?
« Last Edit: 29/07/2015 08:10:47 by chris »
 

Offline Colin2B

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The zero in the neutral line, is that relative to the powerstation, or the local substation and Earth?
It is relative to itself.
The neutral conductor is the zero reference point, whether it is connected to earth or not, and all the line phase voltages are measured relative to that neutral.
In addition the neutral is connected to earth at various point for safety eg generator, substation, consumer exact arrangement depend on the country and supply system. So at those connection points neutral and earth would have the same zero voltage.
However, when you say "The zero in the neutral line" are you talking about the current? This is not measured by taking a reference point. Because current flows in/through a wire we can measure it in a number of ways:
- measuring the voltage drop across the load and using the known impedance of the load to calculate the current
- putting a sensor in the line - low resistance, coil etc and using that to drive a meter
- measuring the magnetic field around the wire eg clamp meter.
 

Offline evan_au

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The electrical transmission between your home and the power station is built in several fairly independent segments, with transformers in between (yet another advantage of Tesla's 3-phase AC power system!). The decision about whether to have a Neutral wire or not, and where to earth it can be made on a segment-by-segment basis.

The first segment is between the power station and a high voltage substation; this segment may run at a voltage like 250kV-750kV AC, and the high voltage means low current, so the power can travel for hundreds of km with little loss. The current and voltage is very well balanced here.
  • The generator is arranged so that when one active wire is positive, the other two will have voltages that are negative or near zero, so that the average voltage at the Neutral will be zero.
  • Similarly, when the current in one wire is positive, the current in the other wires will be negative, carrying all of the current supplied by the positive wire, ie the total current in the Neutral wire is zero.
  • The zero current in the Neutral is logical - if you had a physical wire, and the voltage across it is zero, then the current in the wire will also be zero, and you may as well discard the wire! Long-distance transmission lines save a lot of cost by omitting the neutral wire.
In high voltage transmission systems, the transformer wires are sometimes connected in a "Δ" (Delta) configuration. You can't measure the voltage at the neutral wire, or measure the current in the neutral wire, because there isn't a place to connect a neutral wire!

After several more segments, at successively lower voltages (each of which are designed to balance the current between phases)...

The last segment is between your home and a transformer in the street, converting 11kV to 230V or 110V to supply household appliances. Because the voltage is so low, the current is high, and the losses are high, so this segment is kept as short as possible. The voltage is fairly well balanced between phases.
  • However, most appliances and many homes are attached to just one phase, so the current is not very well balanced in this segment.
  • The positive current from one phase will only be partially cancelled by the negative currents in the other phases; the difference is carried in the neutral wire.
  • The current in the neutral for this segment may be half of the current in any one phase, so a physical Neutral wire is needed in this segment.
     
It is this last segment which comes closest to the public, so safety is most critical for this segment. The street transformer has a "Y" (Wye) configuration, with a central point to connect a Neutral wire. This central point is also attached to earth at the transformer.
  • 4 wires run down the street: 3 phases+neutral
  • Most customer appliances are connected between one phase and the neutral wire, leading to a current imbalance between the phases. The neutral wire is connected to Earth at the customer premises.
  • If the current is unbalanced, the current returns to the transformer via the Neutral wire; because dirt has a much higher resistance than a thick copper wire, almost no current flows through the Earth.

If we measure voltages relative to the earth stake at the street transformer:
  • The voltage at the transformer Neutral will be <0.1V, because these two points are connected by a thick wire, which normally carries no current.
  • The voltage at the neutral pin at a home wall socket will be <1V, because it is connected back to the transformer neutral by a thick wire, carrying a moderate current.
  • The voltage at the earth stake at each home will be <1V, because it is connected to Neutral in the home by a thick wire which normally carries no current.
  • The voltage at the earth pin at a wall socket will be <1V, because it is connected back to the home earth stake by a thick wire which normally carries no current.
  • The thickness of the earth wire and the neutral wire is determined by the worst-case fault current; in case of a short-circuit, they have to carry the full current of one phase for long enough to blow the fuse. During this period, the voltage of any human-accessible part must not reach dangerous levels (>32V AC is considered potentially dangerous).   
Normally, there is not a continuous neutral connection all the way from home to power station (which could be hundreds of km away). The reason is that a Coronal Mass Ejection from the Sun can induce high voltages in the body of the Earth, causing very high currents in the neutral wire. The frequency of these voltages is very low, and this can cause transformer overload and breakdown, such as happened in Quebec in 1989.

In the absence of magnetic field disturbances or power line faults, there should be
  • <1V between the earth stake at your home & the earth stake at the transformer in the street (for safety).
  • Only a few volts between the earth stake at the transformer in your street, and the earth stake at the power station. 

Sorry about the essay...
 

Offline chris

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Thanks, again. This is turning into such a fascinating and educational discussion for me!

If I may, then: at the first step down between power station and downstream grid, where the neutral line is omitted, is the "neutral" side of the transformer winding connected to the other two supply phases so that, as you highlighted above, these act as the sink for the current when the input line is "high"; in turn, this line is the sink for the inputs from the other phases then they are high?

Chris
 

Offline scotty stull

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Most portable generators are not neutral bonded, meaning that the ground is not connected to the neutral side of the generators out put windings. Notice i said "windings" were two coils are connected electrically, this is the neutral bond. If a power line that is hot falls to the ground it will create a conduction path through the ground-neutral connection back to the H. V. transformer.
 

Offline Colin2B

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If I may, then: at the first step down between power station and downstream grid, where the neutral line is omitted, is the "neutral" side of the transformer winding connected to the other two supply phases so that, as you highlighted above, these act as the sink for the current when the input line is "high"; in turn, this line is the sink for the inputs from the other phases then they are high?

Chris
I might be misunderstanding your question so a somewhat long-winded answer.
On the downstream side the transformer is wired delta and there is no neutral connection. Each of the 3 transformer secondary windings are connected together in a triangle and the phase of the windings arranged so that line end of one winding acts as the reference (or neutral if you want to think of it that way) for the next, so around the triangle. The transmission system is then 3 wire with one connection to each of the points of the triangle. So across any of the windings the 2 connections effectively run as an output and a neutral, or sink, for one phase. So each of the connections are indeed fulfilling 2 functions at the same time.
Does that make sense, or have I completely misunderstood your question?
 

Offline chris

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No, Colin, that is EXACTLY what I was asking and your answer was spot on!

So, the "neutral" from my house to the substation is actually an Earthed line connected to one of the other two incoming phases (ie one of the other two points on the triangle as you put it)?

 

Offline scotty stull

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Wye windings is the more common set up , the neutral is connected to the center tap of all three legs which makes for a perfectly symmetrical load balance. Some wind generators have both a wye and delta winding,  when the wind is weak electronics will switch to the delta windings. Delta windings generate to much voltage when the wind is to strong.
 

Offline syhprum

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When I first encountered it I found the USA system rather confusing they use a delta three phase system with one output winding centre tapped and grounded to supply either 117 or 234 volts for domestic use.
the result is if you check the voltage of the delta lines relative to ground you find two of 117v and one of about 160v which rather puzzled me !
 

Offline Colin2B

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No, Colin, that is EXACTLY what I was asking and your answer was spot on!

So, the "neutral" from my house to the substation is actually an Earthed line connected to one of the other two incoming phases (ie one of the other two points on the triangle as you put it)?
I had assumed you meant the first step down from generator to grid. You are meaning the consumer end.
The delta system is only used for transmission between the intermediate transformers. In the grid system. So in the UK the generators are outputting Star or Y configuration into the transformer to be converted to delta for long distance transmission (where the currents can be better balanced) and then back to star for local service. So your neutral goes back to the centre tap of the star/Y.
Your most likely earth arrangement, unless your property is very old, will be what is called Protective Multiple Earthing (coded TN-C-S) where the earth from your nearest transformer is carried on the neutral conductor to your home.
It can seem quite complicated, but I think we are there.


 

Offline evan_au

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Quote from: Chris
So, the "neutral" from my house to the substation is actually an Earthed line connected to one of the other two incoming phases (ie one of the other two points on the triangle as you put it)?
In 230V countries*, for safety reasons, residential street supplies use a "Y" substation configuration, with an Earthed neutral.

If all houses on the street just had 3-phase appliances, the current and voltage would be balanced in all 3 phases (red, blue, black). No current flows in the neutral or the earth, because all 3 currents cancel each other.

If a house has a 1-phase appliance (and all do), it draws current from just 1 phase (red, in this example). The current returns to the substation via the neutral wire. Almost no current flows in the earth wire.

Houses in the street are wired so that different single-phase houses are connected to different phases, so although the neutral currents may not cancel within the house, they are mostly cancelled in the street neutral (because the neutral current from one house is partially cancelled by the neutral current from the adjacent houses, on a different phase).

The street transformer side facing the power station generator may be wired in a Delta configuration, as this shares the unequal currents on the subscriber side more equally across the three phases. But this side can have no Earth connection. An Earth connection for this segment could be provided at the next substation, if it were wired as a Wye.

*110V countries have some different arrangements, one of which sounds like what Chris & Syphrum describe.
 

Offline chris

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Thanks everyone; I have learned so much from this discussion. While I had a notion of what was going on previously, now I actually understand properly how it works.

This interaction has made me think that, if only we had more resources, we could set up a system for summarising the salient points of relevant or resolved threads to produce a distilled, high-quality synopsis. Maybe something to think about...

Many thanks all...
 

Offline syhprum

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When servicing TV,s many years ago I found in one house the line voltage had dropped to about 130 volts and varying wildly which I diagnosed as a disconnection between the neutral connection of the local distribution transformer and the neutral line.
connecting the neutral line to ground at the house made a temporary fix so that I could check the TV
 

Offline evan_au

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Two applications where the return path really did occur through the body of the Earth:
  • Early Morse-code telegraph lines used a single wire, powered by batteries, with the return through the Earth. Unfortunately, DC currents tend to corrode the earth stake, so you need to use a "sacrificial anode".
  • Early Radio receivers used an antenna wire plus an earth connection to provide a strong signal. The Earth acts as a large capacitor, and can absorb the high-frequency currents picked up by the antenna.
In both of these applications, the voltages and currents are far less than those involved in today's household power supplies, and don't create a health risk to those walking near the earth stake. But high-resistance rocks (eg granite) can make it harder to get a good circuit.
 

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