Voltages used in "neighborhood" power distribution

snipped-for-privacy@cucumber.demon.co.uk (Andrew Gabriel) writes:


In Finland the most important distribution voltages to the transformer are 20 kV and 10 kV (three phase power, voltage between phases). 20 kV is the most commonly used voltage for both overhead lines and cables on modern installations. 10 kV is used on some older installations (especially on the center of big cities like Helsinki).
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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
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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.
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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
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Salmon Egg wrote:

Edmond could search for "per unit impedance". One source from this newsgroup is http://tinyurl.com/2fntofq "Per unit" makes calculations simpler.
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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
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wrote:

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.
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ohms (essentially inductive as the resistance is generally << than the inductive reactance) LV: 100% impedance 0/50 =4 ohms
Now suppose that the impedance is listed as 5% The actual impedance seen from the HV side is 0.05*400 ohms and from the LV side it is 0.2 ohms Short circuit current HV 00/20 0 A which is 20 times the rated current Short circuit current LV is 200/0.2 00A which is 20 time rated current.
Suppose the HV tap is changed to 2200V - the actual impedance changes but the % impedance based on 2200/200 V is unchanged.
I can give more examples but for now look at
http://en.wikipedia.org/wiki/Per-unit_system
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ohms (essentially inductive as the

Hey Don,
Finally someone explains it, thanks so much Don, so the percentage is a number assigned by factory right? You can't calculate it from input parameter can you? If you can let's see if you can calculate this real data:
a.. 3 Phase a.. 60 Hertz a.. Class: OA a.. KVA Rating @ 5,000 Ft. Altitude: a.. 2,500 Continuous 55C Self-Cooled b.. 2,800 Continuous 65C Self-Cooled c.. 3,500 Continuous 65C Future Forced-Air a.. Primary Voltage: 24,900Y / 14,400 Volts a.. Secondary Voltage: 4,160Y / 2,400 Volts a.. HV Winding: Copper a.. LV Winding: Copper a.. Connection: Wye/Wye a.. Impedance: ?? % @ Rated Volts @ 2.5 MVA
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ohms (essentially inductive as the

All that I have done is give an example of what Salmon Egg stated.
The % value is determined by actual factory test measurements of the impedance. I can't calculate it from the data given. All I can do is what Salmon Egg mentioned- in a given range, transformer impedances will all be close to a typical values .This one falls in about 6%.
http://www.arcadvisor.com/reference.html
Note that the winding resistance is much lower than the reactance so X/R  to 12 is typical in this size and resistance can be ignored in such things as short circuit studies. If the transformer was boosted to 2800KVA the actual impedance would not change significantly so the % impedance would change to about 5.4 % from 6%
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wrote:

Look Edmond... Just Shut Up! And go have an O-Henry candy bar on the group. You yank hating, post changing braggart. RR
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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 13.8kV (again, line to line).
Charles Perry P.E.

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On Sun, 29 Aug 2010 21:16:11 -0400, "Charles Perry"

FPL in SW Florida uses 13kv L/N wye distribution. This is the label on a typical pole pig.
http://gfretwell.com/electrical/50_kva_label.jpg
http://gfretwell.com/electrical/50_kva_transformer.jpg
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Yup, we designed for 13.8kV an attempted to ship 14.2kV from ou stations.

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