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Topic Title: Transmission & Distribution Voltages
Topic Summary: Reasoning behind the values?
Created On: 07 August 2013 04:00 PM
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 07 August 2013 04:00 PM
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I was just wondering if any could shed any light on the reasoning behind the voltages that are used for transmission and distribution?

For example, in the UK voltages of 400 kV / 275 kV / 132 kV / 33 kV / 11 kV / 6.6 kV / 3.3 kV are used, whilst in Ireland 110kV / 38 kV / 20 kV / 10 kV are used.

What is the reasoning for unsual values such as (for example) 132 kV or 38 kV being selected?
 08 August 2013 04:33 PM
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Someone PLEASE correct me if I'm wrong, but if I remember correctly its to do with the sine wave form factor of 1.1

And if I also remember correctly, the form factor should be the ratio between the rms of say, current between the average of current.
Same for voltage I believe.

Cant remember how you figure it out, but I'm sure someone more experienced will be able to enlighten you. I do on the other hand know it makes it a damn sight easier to construct transformers

Christopher Jones

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 11 August 2013 10:13 AM
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There is a rule of thumb that says 1KV per mile which makes some sense.
There are a great number of voltages used around Britain not just the ones you have mentioned recently in england I have worked on 10KV, 20KV and 150KV, if you go offshore to oil installations you often get american voltages.
 11 August 2013 06:22 PM
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Magic of the multiples of 11

Question: Why Transmission, distribution and utilization voltages are in multiples of 11, as in 110V, 220V, 440V, 1.1kV, 3.3kV, 6.6kV, 11kV, 22kV, 33kV, 66kV, 110kV, 132kV & 220kV?

Answer: It is true only with respect to AC power systems. The first known man-made source of electricity is a cell, which is DC in nature. But, after realizing the disadvantages of the DC electric equipments, AC Electricity generating machines were invented. When these AC Machines were developed, the power of these machines was to be compared with the already available DC electricity, as it is human tendency to compare anything new with the existing ones. As in the case of steam engines.

When steam engines were invented, the power of the steam engines was compared with that of the horses, which were the power sources before the invention of the steam engine.

So, a value called RMS Value for AC Electricity was derived which compared the effectiveness of the AC Electricity with that of the DC Electricity. This value is the Effective Value of AC Electricity. As we were more interested in knowing the effect of AC electricity, all measuring instruments were and are designed to measure only the RMS value of AC electricity - may it be Voltage, Current, Power, etc.

But, for the designer sitting in the design lab, more than the effective value, the average value over a period on one sinusoidal cycle of AC Electricity was important. So, he designed an AC electric Generator, which would produce, on an average, a voltage over a period of one cycle, of say, 10kV (10 is a round figure, you know).

But, when this machine was built to the design and put to operation and when the output voltage was measured, it was found to be 11kV, as the meter was measuring NOT the average value but the effective or the RMS value. This relation existed for any voltage. So, a factor was arrived at - relating the RMS value and the average value, called Form Factor, which is the ratio of RMS value to the Average value, which for a sinusoidal wave form was about 1.1. Then, when the voltage was to be transformed, it was easy to have a whole number for the turns ratio of the transformer and hence all subsequent AC voltages became multiples of 11.

 14 August 2013 11:07 AM
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There are several currents used around Britain not just the ones you have described lately in england I have conducted on 10KV, 20KV and 150KV, if you go worldwide to oil set ups you often get the united states currents.

trirated cables

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