Why are all the Transmission Line Voltages Multiple of 11kv? Not 10kV?
Obliviously, this is an interview type question for electrical engineers especially related to power engineering. Now, we are going to explain the simple logic behind the story.
First of all, It is not true that all the transmissions and distribution voltages are multiple of 11. In most case, they are multiple of 11 such as 11kV, 22,kV, 33kV, 66kV & 132kV. But 400kV, 765kV and 800kV etc are not multiple of 11 in case of electric power system (generation, transmission and distribution) in power lines from the generation station to the receiving point.
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It is not due to the Form Factor (1.11)
Most of you may opt for form factor (1.11) which is nothing to do related to this question. How? Lets see below:
What is Form Factor?
This way, the value of form factor is 1.11.
Now, we know that 400kV and 800kV are not the multiple of 11. Also,
- 10kV x 1.11 = 11.1kV
- 20kV x 1.11 = 22.2kV
- 30kV x 1.11 = 33.3kV
- 60kV x 1.11 = 66.6kV
- 120kV x 1.11 = 133.2kV (But the transmitted voltage are 132kV)
The above calculation clearly shows that the results are different then the general values i.e. in case of 132kV, with the from factor multiplication, it is 133.2kV.
In addition, We use 230V instead of 220V (Single Phase) and 400V instead of 440V (Three Phase).
Also, alternator and generator with terminal voltages are available i range of 10kv – 15kV which is not to do with multiple of 11.
If it is not related to the form Factor, What is it then?
Now back to the point, the voltage at the receiving end are generally:
- 120kV and so on…
Sending Voltage = 10kV x 10% = 11kV. While the receiving end voltage are 10kV due to voltage drop. More examples are given in below table.
|10kV x 10% = 11kV
|20kV x 10% = 22kV
|30kV x 10% = 33kV
|60kV x 10% = 66kV
|120kV x 10% = 132kV
|200kV x 10% = 220kV
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Well, this is not the end as some serious questions arise here. If this is the reason mentioned above, then:
1. Why the transformer and substation rating are mentioned in 33kV / 11kV , 11kV/440V and so on.
In case of no load, there may be negligible voltage drop and we may get 110% (i.e. Sending + 10% extra which is 11kV) voltage at the receiving point. For this reason, transformer and substation should be rated at full 110% which is 11kV, 22kV and so on.
2. Why only 10% extra ? why not more or less % ?
Valid point. But we dont know the exact value of voltage drop as it depends on the value of current where the current is not constant i.e. voltage drop may be less or more than the 10%. So the engineer pick the nearest and appropriate fig as 10% instead of 7 or 13 or so on. But for the updated and higher voltage level transmission (400kV – 800kV), the point of 10% is no more valid as voltage drop are compensated in these modern higher transmitted levels of voltage (due to capacitor banks for power factor improvement, voltage correction systems etc) as it is a separate topic that why the voltage drop are low in high voltage and low current transmission due to line losses (I²R loss). For the same rating of kVA, the higher the voltage and lower the current will reduce the line loss as well as voltage drop in the transmission line. This is the same case with corona i.e. for higher voltage level transmission, corona and voltage drop becomes insignificant.
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3. What about 800kV, 765kV, 400kV Voltage Levels (Which is not multiple of 11)
Same answer given for question number 2. i.e. the 10% is also not applicable for higher transmission voltage levels (400kV, 765kV, 800kV etc) due to compensation of voltage drop.
4. Why not 44kV, 55kV, 77kV and so on Voltage levels for transmission?
Yes, we can make it i.e any level of voltages, but Why?. why to make the system so complex. In the initial, engineers considered the 11kV, 22kV, 33kV, 66kV, 120kV etc based on the transmission distances (from generation point to the receiving end) and the system and these level were satisfactory for different transmission distance. So no need to make changes or make additional level of voltages due to high cost as new rating breakers and switchgears are needed to manage the new system even the whole pattern may need to change. So they want to keep is simple.
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5. Why we don’t generate power in 1kV, 2kV, 3kV or 33kV, 66kV etc instead of 11kV at generating station?
That is the valid point as well. Before the concept of distributed generation system, receiving end was far away from the generation station (e.g. Your home appliance or factory machines too far from the hydroelectric power plant). This is why it was needed to generate and transmit the power through transmission line and the voltage level were high as compared to the current where the power rating in kVA were same. The high voltage transmission has some advantages as compared to low voltage and high level of current transmission i.e. low power factor, corona loss, skin effect, l2R (Line losses), Heat etc. The only disadvantages of high power transmission is distance, insulation of the line and height of towers. Overall, High Level of Voltages transmission is more prefer over high current transmission while the rating of power in kVA is same. Due to this problem, We may not transmit very high level of voltage i.e 1MV or 10MV transmission is not possible due to insulation of the transmission lines, height of the towers and distance.
But in recent days, We have distributed generation system. This way, we may generate 1kV, 2kV, 3kVand so on instead of 11kV in form of wind power, solar power etc. So this rule is also eliminated.
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6. Why we transmit High Voltage levels, Why to not transmit the High Level of Current and low voltage while keeping the kVA rating of power is same?
Same answer mentioned for question number 5. If we transmit high current as compare to high voltage while the rating of power in kVA is same, We will have to need more size of conductors to handle the current. The more current, The more Line Losses (I2R), more heat, low power factor, skin effect, more distance between lines mean more materials for towers and poles and we would have need large number of rating machines like power and distribution transformer to handle this case. This is why we only transmit high level of voltage instead of high level of current while the power rating in kVA is same.
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