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Shouldn't step-up transformers for 800kV be a monstrous beast?

11/30/2021 2:46 AM

I understand we need many secondary turns to step up to a high voltage. So N = 800,000/V-primary should be a very high turns ratio. But then, we cannot have very fine wires for the secondary winding. So what gauge (mm) copper wires are needed for the secondary winding?

Now we connect this secondary to the transmission cable that needs the lowest resistance to reduce I²R loss. So how is the size of this cable diameter compared to that of the transformer's secondary winding.

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#1

Re: Shouldn't step-up transformers for 800kV be a monstrous beast?

11/30/2021 3:41 AM
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#2

Re: Shouldn't step-up transformers for 800kV be a monstrous beast?

11/30/2021 7:13 AM

<...how is the size of this cable diameter compared to that of the transformer's secondary winding....>

That's a function of the current flowing, the distance between the transformer and the load and the method of installation. However both those diameters will exceed that of any fusible link installed between those places.

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#3

Re: Shouldn't step-up transformers for 800kV be a monstrous beast?

11/30/2021 7:18 AM

<...what gauge (mm) copper wires are needed for the secondary winding?...>

  • One could always dissect a scrap one, and have a look.
  • One could always look at the manufacturer's drawings and test documentation.
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#4

Re: Shouldn't step-up transformers for 800kV be a monstrous beast?

11/30/2021 7:24 AM

There are pictures of them on the internet; <...monstrous...> is an imprecise term.

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#5
In reply to #4

Re: Shouldn't step-up transformers for 800kV be a monstrous beast?

11/30/2021 7:59 AM

I am saying "monstrous" from my imagination. The winding ratio is 800,000/V-primary - a huge figure. To me, having those huge number of turns in the secondary is difficult to imagine. If the iron core is the diameter of a pencil, we could easily make many turns.

There is a picture of a step-up transformer - nothing intimidating; but it is not that from the Three-Gorges Dam.

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#6

Re: Shouldn't step-up transformers for 800kV be a monstrous beast?

11/30/2021 10:31 AM

OK. I have now a more reasonable picture. When I google for 800kV ac transformer, Mr. google gives me all about 800kV HVDC transformer - no picture of any monstrous equipment.

I finally google for typical voltage generate in a power stations and got the figure 15kV (6 - 40kV). I once read the voltage was "low", thinking in the 230V range! 15kV is already high; x 53 = 800kV. So a turns ration of 50 is manageable.

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#7

Re: Shouldn't step-up transformers for 800kV be a monstrous beast?

11/30/2021 2:22 PM

"But then, we cannot have very fine wires for the secondary winding." Why not? Wire size is determined with current, not voltage, unless there is a physical strain......such as a span. Have you ever seen high voltage experimental modules? 6 to 12 DC volts in......10....20...50...1000 kilo volts out. 10-20 bucks.

Disassemble an auto ignition coil. Those thin wires deliver a 250 millisecond current spark at 30-60 kilovolts.

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#8

Re: Shouldn't step-up transformers for 800kV be a monstrous beast?

11/30/2021 3:13 PM

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#9

Re: Shouldn't step-up transformers for 800kV be a monstrous beast?

12/01/2021 4:33 AM

Having spent time winding and rewinding power transformers one thing that often gets overlooked in discussions is the cross sectional area and permeability of the iron core.

When making 250V transformers we used some basic values to arrive at the primary winding. We divided 1200, 1600 or 2000 by the cross sectional area of the core to determine the number of turns. so a 2sq inch core would need 600, 800 or 1000 turns but if a 10 sq inch core was used 120, 160 or 200 turns were used.

Why the different values well it is a trade off, 2000 gives low iron losses and quiescent current, but higher copper losses and the converse happens.

Now large transformers can have very large core cross sections, like a 40inches by a 40inches so you end up with 1600 sq inches. So working on the mid point value for 250 volts 1 turn is needed so scale up to 800KV and it would need 3200 turns.

The HV coils in HV transformers are wound as wafers separated with an insulator and surrounded by dielectric oil to provide insulation and cooling. The LV coils are layer wound with interlayer spacers or wafer wound.

So transformers for HV do not scale up drastically in size, also by connecting them in star one end of the coil can be earthed at the star point so the HV end only requires the extra insulation. Mind you the highest voltage transformer windings I have worked on are 66KV to 132KV.

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#10
In reply to #9

Re: Shouldn't step-up transformers for 800kV be a monstrous beast?

12/01/2021 9:44 AM

Thanks. Informative comment. I originally though the voltage at power station was of the order 250V - it is 6 - 40kV!

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#11

Re: Shouldn't step-up transformers for 800kV be a monstrous beast?

12/01/2021 12:05 PM

The highest I have ever worked on was 113kv.

It fed a substation where it was stepped down to 19.2Kv.

The buyer stepped it down in stages according to their needs,from 19.2Kv to 7.2kv,to 4160v,to 600, 550,440,etc.

The local distribution overhead wires are usually about 19,200 volts to the pole pig (transformer) from there is is stepped down to single phase 240v.- center tapped to provide 120v for residential use.

A side note: Old analog color tv's generated 25KV for the picture tube,and the flyback was very small, due to the high frequency used(approx≈15750hz).

They also use voltage-doubler capacitor/diodes to increase the voltage.

High frequency transformers and motors can be much smaller for the same power,which is why most airport equipment is 400hz instead of 60hz.Motors can be much smaller and lighter.

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#12
In reply to #11

Re: Shouldn't step-up transformers for 800kV be a monstrous beast?

12/01/2021 11:21 PM

airport equipment airplane components

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#14
In reply to #12

Re: Shouldn't step-up transformers for 800kV be a monstrous beast?

12/02/2021 4:12 AM

The grid power is 50/60 hz,and is converted to 400hz for aircraft internal systems.

Their mobile power units are 400hz.

Plenty of 400hz surplus portable lighting is available,with 400hz generators,but good only for the lights installed.

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#13
In reply to #11

Re: Shouldn't step-up transformers for 800kV be a monstrous beast?

12/02/2021 1:13 AM

So our side of the pond uses different voltages to your system. Here we run 50Hz and have 240V to ground and neutral which is firmly grounded on the active wire, HOT in USA terms.

415/433V is the voltage phase to phase with 240/250V to neutral and ground used for consumers and industrial applications.

Overhead distribution lives can be 11KV between phases or for what we call SWER, single wire earth return 11KV to ground where the ground is used as the return line back to a SWER isolation transformer. For small single phase 11KV and SWER transformers they can be connected as 240/480V (2 phase)

There are also 22KV, 33KV, 66KV, 132KV, 275KV and 330KV for the higher voltages. Also some special cases use 3.3KV and 6.6KV, especially in mining applications for drills, draglines and electric shovels.

Power generators can generate at 11KV, 15KV, 22KV or whatever the manufacturer uses as the output voltage.

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