What is the difference between Earth ground and circuit common?

[anorlunda]

<<<Moderator's Note This thread was split form https://www.physicsforums.com/threads/are-the-terminals-of-a-battery-neutral.939947/>> because the grounding posts are interesting but off-topic to that other thread.>>

The entire Earth (ground) might be charged plus or minus one million volts relative to infinity. It makes no difference. That definition using infinity is a useful abstraction to define the magnitude of one volt, but it has no practical application to real world devices like batteries. (Uh oh; now that I said that someone will prove me wrong with an application I never heard of. )

 

[Borek]

How can batteries have a positive and negative potential if the charges of their terminals are neutral?

I don't see how they can be perfectly neutral. Existence of the potential difference means there is an electric field between the terminals, as far as I can tell that requires charge separation. However, the charges involved are very, very small.

 

[rude man]

The entire Earth (ground) might be charged plus or minus one million volts relative to infinity. It makes no difference.

i never heard of that one before. The Earth is considered uncharged and therefore its potential w/r/t/ infinity is zero.

 

[sophiecentaur]

The Earth is considered uncharged

By whom?

 

[jim hardy]

By whom?

Certainly not by me. Earth has a field on order of a hundred volts per meter or so near its surface.

http://www.siencis-par-furlan.net/wp-content/uploads/Bressan.Ground.pdf

 

[anorlunda]

The Earth is considered uncharged

Yes, with emphasis on considered. The consideration is voluntary and a matter of convenience, not physics. It makes many things simpler to just define Earth potential as zero. But as I said in #2, if we defined Earth as some nonzero voltage, it would make no difference in our calculations.

The net charge of our planet must change with time, especially during bombardment by charged particles from solar prominences. The charge at infinity is yet to be measured. My point in #2 is that it doesn't matter.

Don't confuse this with particle physics, such as that of an atom being a net neutral particle.

 

[sophiecentaur]

Earth has a field on order of a hundred volts per meter or so near its surface.

I'm having a problem with that figure of 100V/m and what it actually means to the man in the street. According to the paper, it's not the value that you would get near the surface of a metal sphere in space. If it were, the overall absolute would be something like 3X10¹⁴V
I used the inverse law for potential so
V(1/r1 - 1/r2) = ΔV
where V is the absolute potential and ΔV is the volts per metre. r is about 6000km
How much net charge would this imply? The Capacitance of the Earth, treated as a conducting sphere, is around 700μF and Q=CV so
Q would be 3X10¹⁴X700X10^-6 = 3X10⁸ Coulombs. I think that would be negative (?)
That charge isn't a lot. It would represent, say 100A flowing for about 1000 hours .
Please check my maths.
But the paper is dealing with local field strength and the variation over time takes the value both positive and negative. Plus the change with height doesn't follow 1/r. So where does that lead us in justifying using Earth as 0V?

 

[anorlunda]

But the paper is dealing with local field strength and the variation over time takes the value both positive and negative. Plus the change with height doesn't follow 1/r. So where does that lead us in justifying using Earth as 0V?

Grounding is one of my least favorite subject, partially because it is so complicated and so mysterious.

Just part of the complications, consider the currents induced by those auroras and the constant bombardment of charged particles at the poles. Then think of relatively high resistance of the ground and you get appreciable voltages. @davenn might be able to illuminate us more on those numbers. Sometimes, the gradients are so high, they cause substantial currents in the power grid, and cause trips. A 1989 blackout in Quebec is notorious in that regard. There may also be ground currents induced when thunderclouds pass over, or when the day/night terminator passes by (I don't have any links, just speculating.)

In big buildings with lots of power hungry electronics, grounding science and grounding practices are non-trivial. Perhaps some member can tell us about modern server farm buildings.

There is also the case of Sisters Creek in Marathon, Florida. Beside the creek is a Voice of America transmitter with lots of RF power. Boats anchored 200m away report radio & radar problems and LED lights that flicker even when turned off. I have observed sparks hopping between the links of my anchor chain when anchored out there at night. That leads me to suspect huge transient fluctuations in the ground potential at that location. But how would you go about measuring that?

 

[dlgoff]

I'm having a problem with that figure of 100V/m ...

Well, according to https://en.wikiversity.org/wiki/Natural_electric_field_of_the_Earth

The static fair-weather electric field in the atmosphere is ~150 volts per meter (V/m) near the Earth's surface, but it drops exponentially with height to under 1 V/m at 30 km altitude, as the conductivity of the atmosphere increases.

 

[jim hardy]

The entire Earth (ground) might be charged plus or minus one million volts relative to infinity. It makes no difference. That definition using infinity is a useful abstraction to define the magnitude of one volt, but it has no practical application to real world devices like batteries. (Uh oh;

Indeed. Since we can't get to infinity we'll never know what is Earth's absolute potential...

I'm having a problem with that figure of 100V/m and what it actually means to the man in the street. According to the paper, it's not the value that you would get near the surface of a metal sphere in space. If it were, the overall absolute would be something like 3X1014V

One could speculate that coulombic forces were at the root of Pioneer anomaly... but they've figured out it was something else.

I really should make a brief insights article based on this ancient post... it's on my bucket-list...
https://www.physicsforums.com/threa...s-on-an-empty-wire.903244/page-2#post-5688002
I was trying to answer a fellow's question about "voltage".. exaggeration is sometimes a useful teaching aid.

I think your trouble stems from definitions. I don't think your definition of voltage is correct.

This is close but not quite right
That's it's units allright
but it's actually a simpler concept than that.

And Rive is right, when we lazy engineers get accustomed to using it we broad brush past the details.

Here's how to think about it

Voltage is Potential Difference. Two words, not one. Difference is the easy one.
So what's potential?
Potential is the work required to bring a unit positive charge from infinity to wherever you're measuring potential.
That takes some thought.
Imagine yourself at Alpha Centauri(close enough to infinity for demonstration purposes)
with a one Coulomb sized bucket full of charge,
a force gage, and a ruler.
I grew up with dynes and centimeters and ergs but Newtons and meters and Joules are easier...

Now start walking toward earth, measuring the force in Newtons exerted on your bucket of charges and tabulating it at every meter along the way.
So as you move toward Earth you're going to tabulate the Newton-meters and keep a running sum.
When you've reached the top of your lightbulb you will write there what is that point's potential. That'd be its absolute potential.
Now repeat but this time walk to the bottom of your light bulb and again write its absolute potential.
The difference between those absolute potentials is the voltage across your light bulb.
When i grasped the concept was the day I imagined myself counting dyne-centimeters all the way from Alpha Centauri to my workbench in Miami Central High School's electronics lab, ca 1962 .

Now since we can't get to Alpha Centauri let alone infinity it's completely impractical to do that measurement,
and that's why we never know what is the absolute potential of anyplace.
So we just have to settle for the difference in absolute potentials between two places we can reach.
That's easily measured with a two wire voltmeter provided its leads are long enough to reach our two points of interest.
That difference in absolute potentials is "VOLTAGE" . The voltmeter reads that.

Whatever is the absolute potential at one end of a battery, it's different at the other end by whatever is the voltage of your battery. We can only measure that difference.

That's voltage. Forget about clouds of electrons.

Now, an electric field will cause charges to migrate along the field if they can. Inside a copper wire they migrate easily so a miniscule field will cause quite a bit of current . That's why the voltage between ends of a wire is miniscule, charges move equalizing local charge densities along its length..

This oversimplified layman's explanation should help you make sense of the concept. I don't mean to come across anti-academic; au contraire.
Don't just memorize formulas, understand what's happening and they'll become intuitive.
Looking up definitions is always a good idea - laying the foundation if you will. Then use your imagination to link them to your everyday experience. That's called "Memory Pegs" .

Working inside circuits is different from electrostatics, we have simplifications like no field along a wire and V=IR neglecting magnetic induction.
Poynting Vectors and Magnetic Vector Potential come later, after you've got used to working inside circuits.

Good luck in your studies.

Apologies for being so basic in an academic forum, corrections to any of above are welcome.

old jim himself

Earth Ground is best thought of as just another wire whose absolute potential we do not know,
but since it goes most everywhere it is a mighty convenient spot to hook our voltmeter's black lead.

old jim

 

[sophiecentaur]

Earth Ground is best thought of as just another wire whose absolute potential we do not know,
but since it goes most everywhere it is a mighty convenient spot to hook our voltmeter's black lead.

old jim

Engineers are pragmatists and, when something works, they use it. A useable 'Ground' can be produced with a cross of just two wires, half wavelength long under a VHF monopole, on top of your chimney.
On a larger scale, I guess there must be some equilibrium between Gravitational and Electric forces which will affect the net charge that the Earth can acquire.

 

[Fisherman199]

Grounding is one of the most interesting electrical subjects, I think. The mystery of it all makes it so. I regret not having spent more time with the subject when I was in college and worked around several substation designers.

I just got an old version of an IEEE Green book in the mail. I'm reading through it as we speak (erm... type). The Emerald Book is next.

Despite the subject's difficulty I'm still constantly surprised how many people think current "flows to ground."

 

[jim hardy]

Despite the subject's difficulty I'm still constantly surprised how many people think current "flows to ground."

I feel every EE curriculum should include a one credit hour course on "The Green Book" (IEEE 142).

 

[essenmein]

Certainly not by me. Earth has a field on order of a hundred volts per meter or so near its surface.

http://www.siencis-par-furlan.net/wp-content/uploads/Bressan.Ground.pdf
View attachment 220749

I guess that depends on what you call "earth". To me the potential of the earth, that is the planet and its atmosphere as a single system within our solar system, I would expect to have the closest approximation of neutral or "0V" you can get. The constant blast of plasma from the sun would rapidly reduce any net charge to zero (ie integral of charge over all Earth = 0). plus E field and its forces are far stronger than gravity so any net charge would also affect orbits etc.

Now within the system of the Earth there are large potential differences, surface to outer atmosphere from memory is 100's of kV and depending on what we are doing with our power generation grounding I'd say there will also be large potential difference between two points on the ground. Then local events like wind storms would generate even larger voltages (lightning for example).

 

[Fisherman199]

Here's an interesting read. I wonder what he thinks about the Green Book? He seems to be saying Earth ground is a source and sink of charge.
http://www.gohz.com/why-current-flows-to-ground-earth-without-closed-path

 

[essenmein]

Here's an interesting read. I wonder what he thinks about the Green Book? He seems to be saying Earth ground is a source and sink of charge.
http://www.gohz.com/why-current-flows-to-ground-earth-without-closed-path

He sounds very confused.

 

[Fisherman199]

He sounds very confused.

Agreed, but I find many articles saying things along these lines. People have forgotten Kirchhoff's work.

 

[sophiecentaur]

He sounds very confused.

He seems not to know about Capacitance - or he is ignoring it.

 

[anorlunda]

Here's an interesting read. I wonder what he thinks about the Green Book? He seems to be saying Earth ground is a source and sink of charge.
http://www.gohz.com/why-current-flows-to-ground-earth-without-closed-path

I agree. gohz sounds confused. He begins with the following analysis which is completely wrong.

Let's pretend we have a 1,000 Volt DC source. Say the negative terminal is not connected to anything but we connect the positive terminal to a copper rod and bury it in the earth. All of a sudden, when we close the switch between the battery and the copper rod, charge will be allowed to diffuse from the positive terminal into the earth. The charge will continue to expand across the surface of the Earth until the Earth and the positive terminal of the source reach the same voltage.

 

[Fisherman199]

I agree. gohz sounds confused. He begins with the following analysis which is completely wrong.

It sounds to me like he needs that credit hour of the IEEE Green Book Jim was talking about.

 

[CWatters]

Is it totally wrong?... He refers to a voltage source, the terminals of which will have some small capacitance to earth. That capacitance maybe charged/discharged when a terminal is connected to Earth depending on its initial state.

 

[Fisherman199]

Is it totally wrong?... He refers to a voltage source, the terminals of which will have some small capacitance to earth. That capacitance maybe charged/discharged when a terminal is connected to Earth depending on its initial state.

I wouldn't say the article is totally wrong, but it makes many statements that are false to my mind. For instance:

In effect for all practical intents and purposes the Earth can absorb or give up an infinite number of electrons and remain electrically neutral

This seems to say Earth is both a source and a sink for charge. It isn't. Depending on how you look at the system it may or may not be electrically neutral. Relative to power systems, Earth ground is not "neutral." Perhaps he means there are "static" charge differences? That's true, but I'm unclear if that's what he means.
He touches on some truth is that differences in ground potential cause currents to flow in ground. It's not apparent to me how, if ground were "absorbing or giving charges," that any current should flow at all because of potential difference. His explanation just doesn't seem compatible.
It's also not apparent to me why one need concern themselves with the self-capacitance of the whole earth. It's interesting to the astrophysicists perhaps but I haven't run across the phrase in the IEEE Green Book as of yet.

 

[essenmein]

Regarding power systems, generally Earth is only used as a local reference and not for returning load current, dirt is not a very good conductor!

3 phase HV lines are generally delta connected and don't need a neutral, HV DC transmission have both pos and neg on the lines, Earth is only a reference for the converter/transformer/end user.

 

[Fisherman199]

Regarding power systems, generally Earth is only used as a local reference and not for returning load current, dirt is not a very good conductor!

3 phase HV lines are generally delta connected and don't need a neutral, HV DC transmission have both pos and neg on the lines, Earth is only a reference for the converter/transformer/end user.

Right. Perhaps I wasn't clear. I'm not saying Earth is conducting to the source. That's the neutral conductor's job, if there is one.

My point is that many electricians and lineman I talk to are convinced that, once something touches Earth reference, it becomes safe. That's a deadly misunderstanding.

 

[essenmein]

My point is that many electricians and lineman I talk to are convinced that, once something touches Earth reference, it becomes safe. That's a deadly misunderstanding.

I'm curious why this is a deadly misunderstanding? Aside from the working voltages (eg 120/230 etc) which will get you even if the ground is correctly connected, generally speaking if the thing you are worried about getting a shock from is grounded and you are also grounded then there is no potential difference between the thing and you to give you a shock?

 

[Babadag]

As usual, I'm late. However, I spent a lot of time looking for a 'true grounding theory', so I am quite intoxicated with usual information from NEC ,IEEE 142,IEEE 80, BS 7430, IEC 60364, IEC EN 50522 and other.
The Earth absolute potential it is not so important but only the voltage drop and the potential gradient since E[Electric Field Intensity]=-grad(V).
If an object is connected between two points of different potential the current passing through the resistance between these two points could be deadly.
The resistance between these 2 points depends on grounding resistance of the grounding electrode in each point [or grounding grid]- which is just a contact resistance between the electrode and the ground. The ground resistance between electrodes is considered 0 [somewhere I found this equation rE=π^2.freq/10^4 Ω/km].More important is the “depth” of this ground considered a conductor buried at this depth [according to Carson] in order to calculate the reactance from a wire to ground.
If we measure the ground resistivity the second spike could be anywhere in vicinity-10 fits. is good enough, for instance. So the infinite [where potential=0] it could be 10 ft. from the measuring point[!].

 

[jim hardy]

If we measure the ground resistivity the second spike could be anywhere in vicinity-10 fits. is good enough, for instance. So the infinite [where potential=0] it could be 10 ft. from the measuring point[!].

That becomes your reference point and you're measuring potential difference with respect to it instead of with respect to infinity. .

If an object is connected between two points of different potential the current passing through the resistance between these two points could be deadly.

Hence the concept of "step potential".

http://www.esgroundingsolutions.com/what-is-step-and-touch-potential/

They design a switchyard grounding grid (network in that image) to hold voltage drop along 'ground' during a fault to a small enough number of volts per meter that it won't electrocute that unlucky fellow standing underneath the line. I don't know about transmission line towers..

3 phase HV lines are generally delta connected and don't need a neutral,

That's a misleading statement that i found posted on Stackexchange...
The lines themselves are just wires. It's the transformers at their ends that are connected wye or delta.
At my plant the main generator stepup transformer had a 24KV delta primary fed by the generator, and a 240 kv wye secondary feeding the switchyard.
So our HV lines were fed at their source end by a wye connected transformer.
Using a delta connected transformer at their load end would force neutral current to ~zero allowing neutral to serve as just a lightning rod under normal conditions. That was important in South Florida.. It also gives a return path via wire for fault current.

any help ?

old jim

 

[Svein]

So our HV lines were fed at their source end by a wye connected transformer.
Using a delta connected transformer at their load end would force neutral current to ~zero allowing neutral to serve as just a lightning rod under normal conditions. That was important in South Florida.. It also gives a return path via wire for fault current.

The main difference between wye and delta (seen from the viewpoint of the company supplying the power) is that a delta connection needs three wires and a wye connection needs four. Four wires cost more than three wires...

 

[Babadag]

Thank you for your remarks Jim and I agree with you.
About the essenmein post: usually HV Systems are Y connected as it is indicated in Stackexchange post.
However, a phase-to-ground short-circuit at HV appears as phase-to-phase short-circuit at 24 kV Delta. No neutral so no neutral current at generator side.
Usually, if the transmission line it is an overhead line then the overhead grounding wire it is also static wire protecting the live conductors and insulators against lightning strokes.
If lightning strikes an overhead ground wire, a traveling current wave will be set up which will
induce a traveling voltage wave. This voltage wave will generally increase in magnitude as it
travels down the wire, until it reaches a structure where the reflection of the traveling wave from
the ground prevents the voltage from further increasing. (The overhead ground wire is grounded
at every structure). If the traveling voltage wave at the structure is sufficiently high, a "back flashover" across the insulation from the structure ground wire or from the overhead ground wire
to the phase conductor will occur. The factors that determine if a back flashover will occur are:
the amount of insulation, the footing resistance (the higher the footing resistance, the higher the
voltage rise at the structure) and the span length.

 

[jim hardy]

The main difference between wye and delta (seen from the viewpoint of the company supplying the power) is that a delta connection needs three wires and a wye connection needs four. Four wires cost more than three wires...

Since the neutral need only carry the unbalance, which is small(or zero if the line feeds a delta transfomer load), it is often a much smaller wire that's run atop the towers to intercept lightning before it reaches the phase conductors. It'll be connected to the structure and to Earth at every tower. Since it doesn't need insulators it's pretty cheap to install. Power company can put a fiber optic cable inside it and earn revenue from a communication company...

Another consideration is that a Delta connected transformer winding nearly eliminates third harmonic distortion.

There's a long discussion at Stackexchange .

 

[Fisherman199]

I'm curious why this is a deadly misunderstanding? Aside from the working voltages (eg 120/230 etc) which will get you even if the ground is correctly connected, generally speaking if the thing you are worried about getting a shock from is grounded and you are also grounded then there is no potential difference between the thing and you to give you a shock?

If there is no potential difference then there's no "shock", I agree. This does not necessarily mean if two things are grounded you're safe. Any voltage applied to Earth is going to create a gradient in the soil. When there's a voltage gradient there's a risk of step-potential. By my research as little as 60V AC from hand-to-foot can be lethal and current so low that the threshold of sensation isn't met can still cause serious arrhythmia of the heart. This means for someone with less optimal heart health, even 10V AC from hand-to-hand or hand-to-foot could be life threatening. These figures are well inside a threshold for possible step-potential for anyone working around live equipment. The best course of action is to assume anything not cut-off from the supply is deadly. Overkill? Probably. I'd rather be safe than sorry. Put on the rubber shoes and go home to your family.

There's a consideration to be made for the impedance of the ground material. Gradient will rely on this factor, mostly. Conductance of Earth is getting hairier the more I'm looking at it but generally you're right.

I'm arriving at a place where I think it's very difficult to explain through literature what grounding (or anything much more complicated than Ohm's Law) "is." Trusting the physics and creating pictures/animations might be the best option to come close to "getting it." This opinion may change if I can find or derive a good explanation. So far, all I do is wave my hands frantically and say "Kirchhoff and Conservation of energy!" This isn't going to help anyone, myself included.

 

[Fisherman199]

That becomes your reference point and you're measuring potential difference with respect to it instead of with respect to infinity. .

They design a switchyard grounding grid (network in that image) to hold voltage drop along 'ground' during a fault to a small enough number of volts per meter that it won't electrocute that unlucky fellow standing underneath the line. I don't know about transmission line towers..

old jim

If there is no potential difference then there's no "shock", I agree. This does not necessarily mean if two things are grounded you're safe. Any voltage applied to Earth is going to create a gradient in the soil. When there's a voltage gradient there's a risk of step-potential. By my research as little as 60V AC from hand-to-foot can be lethal and current so low that the threshold of sensation isn't met can still cause serious arrhythmia of the heart. This means for someone with less optimal heart health, even 10V AC from hand-to-hand or hand-to-foot could be life threatening. These figures are well inside a threshold for possible step-potential for anyone working around live equipment. The best course of action is to assume anything not cut-off from the supply is deadly. Overkill? Probably. I'd rather be safe than sorry. Put on the rubber shoes and go home to your family.

There's a consideration to be made for the impedance of the ground material. Gradient will rely on this factor, mostly. Conductance of Earth is getting hairier the more I'm looking at it but generally you're right.

I'm arriving at a place where I think it's very difficult to explain through literature what grounding (or anything much more complicated than Ohm's Law) "is." Trusting the physics and creating pictures/animations might be the best option to come close to "getting it." This opinion may change if I can find or derive a good explanation. So far, all I do is wave my hands frantically and say "Kirchhoff and Conservation of energy!" This isn't going to help anyone, myself included.

Here's an interesting thing relating likelihood of ventricular fibrillation to the time and magnitude of electric current exposure.

 

[jim hardy]

We usually think of "Ground" as conducting like a wire with no voltage drop. But Earth is actually a pretty poor conductor compared to copper. Since it's so big the current density we encounter from household wiring is low enough to not produce much voltage drop(potential difference) per meter. Utility lines and lightning bolts are another matter, though..

That's why i tell people "Ground is just another wire and it goes most everywhere."
Like any other wire it'll have voltage drop in proportion to the current flowing through it. More accurately, in proportion to the local current density .
That's why at the bottom of a lightning bolt there's a large number of volts per meter along the ground as charge flows outward from the strike. People or cows standing underneath a tree that's hit by lightning often get electrocuted from the ground voltage drop between their feet.
Myself i think that's why tropical wading birds sleep on one foot.

old jim

 

[Tom.G]

Additional fuzzy feathery info.
As @jim hardy said ...i think that's why tropical wading birds sleep on one foot.

Short version: https://www.allaboutbirds.org/why-do-some-birds-stand-on-one-foot/

Long version (click on "Full Transcript" button): https://birdnote.org/show/why-birds-stand-one-leg

 

[jim hardy]

Additional fuzzy feathery info.

Yes, I've not many takers on my theory as yet.

 

[cabraham]

The main difference between wye and delta (seen from the viewpoint of the company supplying the power) is that a delta connection needs three wires and a wye connection needs four. Four wires cost more than three wires...

Wye connection does not necessarily need 4 wires. A transformer connected in wye om the primary

 

[cabraham]

The main difference between wye and delta (seen from the viewpoint of the company supplying the power) is that a delta connection needs three wires and a wye connection needs four. Four wires cost more than three wires...

Wye connection does not necessarily require 4 wires. If a transformer has a wye primary, with delta secondary, the primary feed only need be 3 wires. Triple harmonic current circulates in the delta secondary, as well as zero sequence currents due to secondary load unbalance. The wye neutral is connected to Earth for safety reasons, but 3 wires carry full load current balances or unbalanced.
If the transformer has both primary & secondary wye connected, without a 3 legged E core, then a 4th wire is needed. To avoid 4 wires, power company avoids Y-Y connection. A tertiary winding is added, connected in delta. Or a core type construction is used.
Claude
PhD student
EE 39 years

 

[essenmein]

That's a misleading statement that i found posted on Stackexchange...
The lines themselves are just wires.
old jim

Correct, poor choice of words on my part.

 

[essenmein]

Wye connection does not necessarily need 4 wires. A transformer connected in wye om the primary

This is only true if you have a balanced load. For example all the machines I've worked with do not have a neutral connection from the winding star point when they are Y, the stator (should) be balanced. An unbalanced Y system with no neutral connection will unbalance the Line to neutral voltage on the phases. A Y-delta transformer does this balance with the flux in the core. If its a Y-Y transformer then the construction needs to be considered, this can be coupled core (Eg E), or literally three mechanically separated single phase transformers, where the flux cannot help to balance EMF.

 

[cabraham]

This is only true if you have a balanced load. For example all the machines I've worked with do not have a neutral connection from the winding star point when they are Y, the stator (should) be balanced. An unbalanced Y system with no neutral connection will unbalance the Line to neutral voltage on the phases. A Y-delta transformer does this balance with the flux in the core. If its a Y-Y transformer then the construction needs to be considered, this can be coupled core (Eg E), or literally three mechanically separated single phase transformers, where the flux cannot help to balance EMF.

If a xfmr has a wye primary, with delta secondary, the primary needs only 3 wires even if secondary load is unbalanced. The reason for avoiding wye-wye xfmr is the desire to use only 3 wires.
Do you agree or disagree with the following:

A wye-delta xfmr needs only 3 wires on the primary side & will support an unbalanced secondary load with 3 wires.

 

[essenmein]

If a xfmr has a wye primary, with delta secondary, the primary needs only 3 wires even if secondary load is unbalanced. The reason for avoiding wye-wye xfmr is the desire to use only 3 wires.
Do you agree or disagree with the following:

A wye-delta xfmr needs only 3 wires on the primary side & will support an unbalanced secondary load with 3 wires.

I believe I agree with this already:
" A Y-delta transformer does this balance with the flux in the core. "

 

[cabraham]

I believe I agree with this already:
" A Y-delta transformer does this balance with the flux in the core. "

I did not say that. If a wye-delta or delta-wye is built by combining 3 single phase xfmrs, then the core flux does not maintain balance.
A 3 phase core type xfmr does that regardless of wye or delta connection. With 3 phase core construction, a Y-Y connection stays balanced with unbalanced loads due to core flux. Likewise for other connections.
Claude

 

[essenmein]

I did not say that. If a wye-delta or delta-wye is built by combining 3 single phase xfmrs, then the core flux does not maintain balance.
A 3 phase core type xfmr does that regardless of wye or delta connection. With 3 phase core construction, a Y-Y connection stays balanced with unbalanced loads due to core flux. Likewise for other connections.
Claude

Had to think about that, don't normally deal with 3ph transformers. With three single phase transformers with one side in Y the other in delta the EMFs still force the other phases to balance via the transformers, I kind of loosely look at it from the perspective that on the Y side you have two windings in series line to line, on the delta you have one winding line to line, so any line to line voltage change on the Y, forces the same EMF change on two windings in the delta side. So indirectly the transformer flux is still doing the work via the linked EMFs.

 

[cabraham]

Had to think about that, don't normally deal with 3ph transformers. With three single phase transformers with one side in Y the other in delta the EMFs still force the other phases to balance via the transformers, I kind of loosely look at it from the perspective that on the Y side you have two windings in series line to line, on the delta you have one winding line to line, so any line to line voltage change on the Y, forces the same EMF change on two windings in the delta side. So indirectly the transformer flux is still doing the work via the linked EMFs.

The linked emf's is correct, but the fluxes are NOT linked. It so happens that flux is related to emf per Faraday's Law. A delta forces the 3 emf's to sum to zero. A 3 legged E core pair for es the 3 flutes to sum to zero. Different means, but same end result.

Claude

 

[essenmein]

It so happens that flux is related to emf per Faraday's Law.

Claude

Almost like one can't happen without the other :D

 

[anorlunda]

My point is that many electricians and lineman I talk to are convinced that, once something touches Earth reference, it becomes safe. That's a deadly misunderstanding.

No it is mandatory standard practice. We use grounding straps on power lines to protect linemen when they are working on them. The power should be shut off before starting work, but just in case the power is mistakenly turned on when the men are working it assures that the current will go to ground through the strap rather than through the workers.

By the way, in power system analysis, exactly that mistake, closing the breaker on a line grounded by straps close to the power plant is the worst case scenario for short circuits. It results in higher short circuit currents than any other scenario.

 

[Svein]

We usually think of "Ground" as conducting like a wire with no voltage drop. But Earth is actually a pretty poor conductor compared to copper.

And at that, the conductance of "Ground" varies very much with the composition of the ground in question. If you live in the mountains and the only ground available is bare rock, you will be hard pressed to measure any conductance at all. This used to be a real problem with short-wave transmitters in Norway, because there are just three places in the whole country where the "Ground" conductance is good enough for that kind of appliance. That is also why the traditional power grid is delta-connected with "protective ground" being a local responsibility.

 

[jim hardy]

That is also why the traditional power grid is delta-connected with "protective ground" being a local responsibility.

Be circumspect of that statement. As discussed earlier the transmission lines are usually connected at one end to a wye transformer and at the other end to a delta transformer.

 

[Svein]

Be circumspect of that statement. As discussed earlier the transmission lines are usually connected at one end to a wye transformer and at the other end to a delta transformer.

Yes, and the center of the wye is connected to "Ground". Assuming that the ground resistance (between the wye transformer and a power consumer) is ≈100kΩ, a ground fault current of 10mA will try to create "ground" potential difference of 1000V.

It sounds crazy, but I once worked in a research institute where one of the guys measured the three delta voltages and phases relative to "ground" and concluded that "ground" was outside the delta...

 

[jim hardy]

Assuming that the ground resistance (between the wye transformer and a power consumer) is ≈100kΩ, a ground fault current of 10mA will try to create "ground" potential difference of 1000V.

Between the wye and the consumer is a transformer.
Fault current is constrained to one side of that transformer.On the transmission side of most systems the neutral that runs along the top of the towers provides a metal path through which fault current can get back to its wye source (if there is one, and usually there is).
Annotating the previous illustration

On the consumer's side IEEE 142 and NEC require a metal path for fault current back to the transformer or generator winding from whence it came.

Not arguing there's no such thing as a "high impedance grounded system" like you describe
just they're the exception not the rule.

old jim