Electronics > Beginners

Isolation transformer and electrons

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IanB:

--- Quote from: nForce on December 10, 2018, 05:34:09 pm ---If I reference to this picture. Because earth has everywhere potential of 0 V, how do electrons find the correct path through earth to the transformer? If we have a town with 20 transformers, and we create a short circuit somewhere on the grid, how do electrons know where to flow to that one transformer of the total 20?

--- End quote ---

Maybe a better answer:

If the electrical distribution transformers are connected to ground at the point of supply (as they normally are), then if you make another connection to ground somewhere else on the network (your short circuit), then there is a potential gradient through the ground between the two connection points. Current will flow in the direction of decreasing potential through the ground, which will take it back to the source.

This is like the direction of current flow in a lake. If there is an outfall at the edge of the lake then water will flow towards the outfall to maintain a level surface. You cannot see the potential gradient that causes the water to flow across the lake, but it is there.

Shock:

--- Quote from: nForce on December 10, 2018, 05:34:09 pm ---If I reference to this picture. Because earth has everywhere potential of 0 V, how do electrons find the correct path through earth to the transformer?

--- End quote ---

Firstly the analogy was just to give you an idea how current flows in a circuit. Don't read into it too much. I'd also get away from this earth is 0V idea, it's just earth a circuit path (not a great one either).

Not sure I understand your question, but if you are referring to mains distribution, it works like any other circuit, the least path of resistance will be taken as the return path. Which is going to be the neutral line over some ground/earthing rod in soil.

The earthing of the neutral throughout distribution systems from what I read a while back was to prevent dangerous DC voltages on the AC lines from lighting strikes and then added to homes then as a separate path to appliances.



--- Quote ---If we have a town with 20 transformers, and we create a short circuit somewhere on the grid, how do electrons know where to flow to that one transformer of the total 20?

--- End quote ---

Not sure I understand your question. If the one transformer had a completed secondary circuit allowing more current to flow it would cause the magnetic field (magnetic flux density) to increase which increases current flow on the primary.

There are ideal components and real components, if the transformers primary is connected it's actually using a small amount of current even if the secondary is open. So the first part of the answer is that current is already flowing.

When you have two conductors that have a voltage potential across them it does not need to find a pathway from the electrical station/substation/transformer/battery etc, it already has one and is sitting there waiting for the circuit to be completed. The magic happens straight away.

ArthurDent:
nforce – “If I reference to this picture. Because earth has everywhere potential of 0 V, how do electrons find the correct path through earth to the transformer? If we have a town with 20 transformers, and we create a short circuit somewhere on the grid, how do electrons know where to flow to that one transformer of the total 20?”

Once again, the language of science is math, not feelings, and electrons don’t willy-nilly choose to do or not do something, they follow the laws of science. First you have to understand how grounding works in a power distribution system. There are countless utility poles and many of them have grounding rods driven into the physical ground for system safety but in a single phase residential distribution system besides the hot line there is always a neutral wire that is connected to these ground rods at many poles and connected to neutral throughout the entire system without relying on ground except for safety. Here is a drawing showing how in a single MV (medium voltage-maybe 2500V) feeder circuit is grounded. Every so often there is a step-down transformer that has a 240V center-tapped winding and the center tap is also connected to ground, both at the pole and in any house it’s connected to.

The distribution system is the 2 feeder wires (hot and low) with as many loads (transformers) connected as needed up to the power rating of the distribution line. If there is a short at any transformer secondary, it has a primary fuse that will blow so it won’t take the entire distribution line down but all the houses fed from the transformer output will lose power.

If there is a short on the 2500V distribution line causing the distribution line fuse to blow at the sub-station then all the houses connected to all the transformers on that entire distribution line will lose power.

If there is a short on the sub-station feeding the distribution lines going to neighborhoods then all the houses connected to all the distribution lines connected to the power sub-station will lose power and earth ground could care less. 

nForce:
Hmm, but I want to know from electrons perspective. Maybe IanB gave a better answer.

So if I understand correctly: When we make a short circuit on the grid, those electrons flow from a transformer through wire into ground. Now the transformer has electron "holes" and the earth (our planet) has negative charge. Because of that the transformer tap, which is grounded suck electrons from the ground, to fill the "holes". Now everything is again balanced the earth and the transformer. Altough our planet is very large, so excess of electrons would be negligible.

Sorry but my question is pure physics, and not engineering.

Shock:
This is where the electrons are going in a completed AC circuit.





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