Voltages used in "neighborhood" power distribution

Ah, double dipping Don? I have hacked it's domain as well....

The world is ruled by BS as it is, you are a slave to it, you started believing once in your Schooling days. Then you became a Warrior for it after you graduated, now, Who Cares! It's what all civilized people do :) ain't that a whoot '

The only way people and Other Humans:) will be happy together in an World of Electricity is if they can own & have a small atomic plant right in their backyard, and they fuel it from refuse from their daily consumption...lol

RR

I do this very carefully. you never know when any freaking contraption we devise is going to become a hazard, or turn against you.

It seems essential People All Over The World consort and abide in Common Belief., One Mishap and KKKKKKaPuuueee!

No! no' the truth is that That! can/shan't won't ever happen. Well, maybe......like in a Cinegraphic Reproduction for The Common People of The World.

Hahaha you're all insane;-)

RR

Back On Topic Most common or ordinary people are aware of the power behind the voltage when something goes wrong and the sparks and flames become a part of the scenario....on their block or news casts.

I am aware that it is in the 3-6-10k stepped down, then stepped up, and again stepped down to household and Commercially usable energy. The 3000 VAC being the nearest unlikely danger to local inhabitants., Good thing you got those oil filled tin cans to tackle that ;) RR

I am not sure what you imply or mean by "electrostatic stresses". I only know where stress cones are not used the "end impedance change" of the complete cable (sheild and conductor and semi-con)changes too rapidly and tends to break down. This implies, to me, a standing wave or similar is a problem. Theory behind it is not studied or understood by myself.

It sounds lke you think it would be from disturbances, consisting of higher frequencies from lightning or switching transients. Either way, I feel there will be a tuned resonant frequency of the cable and loads that will be the one to break down the termination as a weak spot.

Am I too far off base to your knowledge? Wasn't really my forte.

As to its effectiveness for frequency surges where the effects of an impedance change and reflections is important -possible but, in practice, it appears to be secondary or negligable with respect to the electrostatic situation. Possibly such grading in a double cone (as is the practice) may be beneficial but I have no knowledge of any analysis of this with respect to surges in a power cable situation. Do you? I would be interested.

I still wonder if there is some error in this.

Do the math.

- current at F.L = 2MVa / 240V = 8333 amperes (OK if three phase system wye then divide by 3) = 2777 amperes. - Cable size for 2777 amperes? Not knowledgabel about cable sizes that big in aluminum. Possibly 3000 MCM?? Who could even lift or pull that cable in a trench? (need wire charts and weight)

- Now think about all the small cables spliced to this monster going to house and the fault current available.

This whole thing doesn't sound logically feasible

Andrew: You sure about this engineering monster there? Any references/cites/ pictures? I would be interested in how this is done.

We typically would use a 2-5MVa transformer as a substation every few km and

"Don Kelly" wrote in message news:i2Hco.39137$ snipped-for-privacy@newsfe08.iad... There are many different voltage levels and schemes in effect. it depends on where you are, North America, UK or elsewhere as well as load density (roughly houses per km along the line) and single/three phase distribution. Different strokes for different people or conditions. Beyond the technical factors there are historic factors which which have generated a lot of inertia, attitude problems as well as economic problems in making changeover to a world wide uniform approach. Pity Japan with both 50 and 60 Hz systems -cheapest solution is an asynchronous back to back DC link. :)

I still wonder if there is some error in this.

Do the math.

- current at F.L = 2MVa / 240V = 8333 amperes (OK if three phase system wye then divide by 3) = 2777 amperes. - Cable size for 2777 amperes? Not knowledgabel about cable sizes that big in aluminum. Possibly 3000 MCM?? Who could even lift or pull that cable in a trench? (need wire charts and weight)

- Now think about all the small cables spliced to this monster going to house and the fault current available.

This whole thing doesn't sound logically feasible

Andrew: You sure about this engineering monster there? Any references/cites/ pictures? I would be interested in how this is done.

We typically would use a 2-5MVa transformer as a substation every few km and

"Don Kelly" wrote in message news:i2Hco.39137$ snipped-for-privacy@newsfe08.iad... There are many different voltage levels and schemes in effect. it depends on where you are, North America, UK or elsewhere as well as load density (roughly houses per km along the line) and single/three phase distribution. Different strokes for different people or conditions. Beyond the technical factors there are historic factors which which have generated a lot of inertia, attitude problems as well as economic problems in making changeover to a world wide uniform approach. Pity Japan with both 50 and 60 Hz systems -cheapest solution is an asynchronous back to back DC link. :)

----------------------- A bit far off base. Salmon Egg might have some comments which might help.

What I looked at as the basic consideration of the use of stress cones is not due to any impedance changes. What is of concern is that where there is an abrupt change in the geometry there will be a change from a uniform E field to a non-uniform field, near the point of change, which can produce excess stress on part of the insulation. This can lead to internal breakdown of the insulation or surface flashover-both not desirable. The reference I gave shows distributions of stress as contours (and also in colour gradients) where equipotential contours that are close together indicate higher E field stress (as in maps-close contours indicate steep slopes).

This is present in the DC situation as well as AC so standing waves are not a concern. Hence "tuned resonance" is really not of concern.

Where I may have confused you is my going on to consideration of surges. In the case of no "flaring" there may well be a change in characteristic impedance at the junction and this could be a problem with reflections. Flaring would affect this characteristic impedance in the transition region. However, I suggest, and I haven't analyzed this, that the dimensions involved are such a small part of a wave length of any incoming surge that the flaring is not going to be effective in reducing the overvoltages due to reflections. This could be an interesting analytical study for a grad student -considerable related information is available in references on the behaviour of exponential cones in audio systems. I suspect that the end result of such a study might lead to a conclusion that, for surges, it isn't of importance. Note also the flaring typically means a conic section rising to a maximum diameter at some point and then decreasing to a smaller diameter. This makes sense in consideration of the electrostatic field stresses but not in terms of impedance matching (an analog would be using a transformer to step up from 20 to 40V and then using another transformer to step down from 40 to 30V rather than stepping directly from 20 to 30V. Ideally the same but practically of no purpose).

Impedance should be in Henry, why now in %?

% of what?

Of voltage?

I read the book, it talks about preventing ground fault, you have to have higher impedance 7% if voltage is over 1000kv.

That would be 550 Henry for 1100kv. A lot of coil winding. This makes no sense. If you have access to a large transformer, could you please check its premary and secondary resistance for me?

Thank you in advance.

To tell the truth, your post makes little sense. Electrical impedance is given in ohms not henries, Henries are used to measure inductance,

Because much of electrical equipment can be scaled in size, "per unit" descriptions are often used. For example, a transformer will pretty much have the same characteristics irrespective of the turns ratios. the reference impedance would be the design voltage divided by rated current. Other impedances, such as leakage impedance can be given as a fraction of the reference impedance for a whole class of transform differing in the number of turns on a winding.

Bill

Edmond could search for "per unit impedance". One source from this newsgroup is

Hahaha....You really think Edmond is a real person., you get more twisted into that fantasyland in your brains with every post you respond to... we hoped better from you. Go see a shrink, dude that thinks I am Roy Quijanus...you fruitcake.

RR

Look Edmond... Just Shut Up! And go have an O-Henry candy bar on the group. You yank hating, post changing braggart. RR

Hugh? Transformer is not part of inductance? Ok I Ok using Ohm to measure impedance, I do that all the time any way, but still 5% of impedance of what? This is stupid and non-sense for an Electrical engineer to write a book and use vague term, 5% or 7% to signify impedance.

Really?

Yes, the plot is normally about half the size of a house plot. Sometimes the transformer and switchgear are out in the open, sometimes in a brick building which looks like a large garage.

Several 240/415V circuits, heading off down different streets, separately fused. It's quite common that an individual circuit is what's called a "ring main", i.e. it leaves the distribution panel, runs up a street and then loops back down the other side (or another street) back to the same terminals. This provides some redundancy if the cable breaks (providing it breaks into an open circuit), and the repair can be scheduled as a non-emergency.

It's not really - you care about the 120V regulation, or you have light bulbs flickering. We're 240/415V, but again it's the 240V regulation which matters, not just 415V.

(Actually, we're notionally 230/400V now with EU harmonisation, but the tollerance has been changed so we actually stay on 240/415V as measured.)

Most UK houses are 100A max, and splice only one of the three phases. If you want 3-phase, you can have it, and if you want more than 100A, then you _have_ to take a 3-phase supply (and a 3-phase supply is not limited to 100A).

Most EU countries are similar rules, except the 100A single phase limit varies. In some countries, it's as low as 20A, so almost everyone has to have a 3-phase supply.

OK to understand this better.

You use 240v/415v secondaries in a substation type building with some switchgear or fusing to down limit fault current damages into multiple neighbourhood feeds, using a ring main bus, mostly. This would allow smaller conductor sizes to neighbourhood feeds. It still sounds liek some long runs for the low voltage in far properties.

In my area(in Canada) we do this same thing at the 8/13.8kV level or

Then we have the "street transformer" that step the 8kV or 16.1kV(rural O/H) down to the 120/240volt residential voltage. No voltage regulation is typically done past the transformer output of the 8kV. None is typically needed. The "street transformers". would feed up to about 13 homes at about

With electrically heated home areas the distribution is less, say about 4-8 homes. The voltage drops would be too high for any length and not everybody wants a transformer on the front of their lawn. It devalues the property somewhat and may attract children.

So in the end we both use the same voltage distribution to most residences (240v), although different source style behind them. Most of our major loads use 240v and do not use the 120v, much...clothes dryers, electric stoves, hot tubs with heaters and 10hp pumps, etc... Lighting, small fans and recepticals typically are all 120v.

Nice to compare systems with you!

Really?

Yes, the plot is normally about half the size of a house plot. Sometimes the transformer and switchgear are out in the open, sometimes in a brick building which looks like a large garage.

Several 240/415V circuits, heading off down different streets, separately fused. It's quite common that an individual circuit is what's called a "ring main", i.e. it leaves the distribution panel, runs up a street and then loops back down the other side (or another street) back to the same terminals. This provides some redundancy if the cable breaks (providing it breaks into an open circuit), and the repair can be scheduled as a non-emergency.

It's not really - you care about the 120V regulation, or you have light bulbs flickering. We're 240/415V, but again it's the 240V regulation which matters, not just 415V.

(Actually, we're notionally 230/400V now with EU harmonisation, but the tollerance has been changed so we actually stay on 240/415V as measured.)

Most UK houses are 100A max, and splice only one of the three phases. If you want 3-phase, you can have it, and if you want more than 100A, then you _have_ to take a 3-phase supply (and a 3-phase supply is not limited to 100A).

Most EU countries are similar rules, except the 100A single phase limit varies. In some countries, it's as low as 20A, so almost everyone has to have a 3-phase supply.

How far is it when it is raining?

Sorry ;-)

------------------------------ I have a quibble here. Core size(Laminations) at 50Hz twice the size of that for 60Hz.? Are you assuming the same voltage, turns and max flux density in both cases? If so, I disagree on the basis of fundamental principles. I agree that Canadian usage in residential areas is not 1-2MVA transformers with 240V secondaries -partially because the typical load densities mean that that is prohibitively expensive and complex as you indicate. In high density areas- , I could see a 1-2MVA sub within a specific apartment/condo unit.

As for center vs centre- (US vs UK) both are valid according to the Canadian Oxford Dictionary- so being neither a Brit or a Yank, my choice is dependent on the colour/.color of my mood.

In terms of electrical supply as with language details - what exists in different regions is a rational approach (mostly) based on historical choices (right or wrong- the conversion in the latter case becomes irrationally prohibitive).

Cheers

Typical (if there is such a thing in the US electric utility industry) distribution voltages range from 4kV (line to line) to 35kV (line to line) with 15kV being most common. Of course, no one actually uses exactly 15kV, that is just a term for a "class" of voltages that range from 11kV up to

Charles Perry P.E.

------------------------------------------------- Actually it is very useful to use % impedance, or voltage or whatever in power system analysis. While it is not particularly useful for a single transformer, for a network with many transformers etc, it is nice to normalize the data so that a transformer, for calculation purposes becomes just an impedance and normalized voltages, currents etc are used in calculations. At one point in the system the normalized voltage may be 500KV but at another point it may be 120V =both of which are 100% or 1 per unit Note that: If a transformer has 5% impedance the maximum fault current is 1/0.05 =20 times the rated current if the source is at rated voltage.

As to the basis for this, take an example of a transformer rated 2000/200V

We can also define base impedance HV: 100% impedance =2000/5=400 ohms (essentially inductive as the resistance is generally