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Newbie here - need some high-voltage advice

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king nero

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Hello to all,

short intro: I'm from Belgium (so please forgive any spelling/grammar mistakes), I used to do fault-finding on cnc-machinery (both software as hardware), now I'm (again) into a whole new professional world, I like to get some change of environment every few years.

Currently I'm having a bit of a discussion with a friend, as described below:

Our (street-) power supply is from a transformer, secondary in Y, with the neutral (star-point) earthed to ground. Voltage is 3x400 V, or 230V for each neutral-phase voltage.

If you would hook up a large transformer (eg an industrial welder) between 2 phases (400V), Would it, or would it not, affect the grid?
Theoretical question, I know if any difference, it would be very small, but imagine quite a large transformer then.

More specific, if you would hook up a perfect symmetric 3phase resistor in Y between the phases from that grid, would you see a voltage difference between the starpoint of the Y-resistor (which is floating at this instant) and the starpoint of the grid (where it is asymetrically challenged due to the large welder)?

Now, both of us have a good understanding of electrics, and yet neither of us can deliver sufficient proof to the other one to help him convince for either situation.

I say yes, it would affect the grid, he says it wouldn't, but neither of us can really back it up.

I was pointed forward to here from another forum, If this isn't the right place to ask please let me know...

Many thanks in advance...
 
The easy one first. A perfect symmetric load on all 3 phase wires will have a starpoint voltage same as the power starpoint.

As for, "will it affect the grid"? Yes. All sources of power have some impedance. You might not be able to measure the difference, but it will happen. Consider proving there will be no effect. You would have to prove that the power transformers have no resistance, and neither does the wire to the power station, nor the generator at the power station. Earthlings have not invented superconducting transformers, outdoor cables, or generators, yet.
 
OK - I didn't know that one - that means, even if the supply is not symmetrical (unbalanced), hooking up a symm. Y-load will have the same earth-starpoint voltage as the power supply?

It seems I will have to eat humble pie (or how goes that expression...)
 
True. Try some math on it. Declare that one leg has 400 volts and the two other legs have 390 volts. Declare a 400 ohm resistor from each leg to a floating star point. The current in each resistor will be exactly enough to use up the voltage and calculate to zero at the powered starpoint.
 
It's likely that the transformer will have a voltage regulator on each so in practise the voltage per phase on the secondary won't change much.

The current taken by the primary will change and load down the distribution voltage a bit, but it's generally fairly low current 20A at a high voltage, typically 11kV for a small installation, by the way 400V is considered to be a low voltage.
 
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I wanted to make it clear in the topic title I was not after electronics but mains voltage advice... but thx for the tip anyway.

The point of the discussion was also more theoretically, I know it would be hard to hook up such a welding machine in a house (40A max! breaker, more likely 20 or 25A) so that it would have a significant effect on the grid...
 
Declare that one leg has 400 volts and the two other legs have 390 volts.

I could use some help with the maths: How would I start using the above?
They have ~400 V between them, so do you mean between P1 and P2 is 400 V, between P2 and P3, and between P1 and P3 there is 390V ?
 
Three phase WYE is what we call this in the US. There is an attempt made to keep the line current in all three lines balanced, but it never happens perfectly in practice. In a large warehouse, for example, the lighting loads (line to neutral) are distributed around the three phases, but there is no guarantee that all the lights are turned on, meaning that the line currents are grossly unbalanced.
 
Oops. Running around the 400 volt ends will not create a star point. Declare one voltage as 230 and the two other voltages as 220, then declare 220 ohm resistors configured to make a star point and calculate the current and voltage drop across the (3) 220 ohm resistors.

Doing the math is how you learn. It's the same thing as practicing piano. The more you do, the better you get, until you can do it in your head and you look like magic.
 
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