three cables for three phases or three wires from three cables

[Kataeb]

Kindly, we connected electrical power, 3x380 vac, between a supply station and an electric cabinet, away 180 meters.
The calculation software proposed to use 3 aluminum cables, each 3x240 mm2, but did not mention how to practically connect them.

We tried first to use each cable for 1 phase, by connecting its three wires together.
But we noticed that when we put the load on, the voltage drops by more then 15%, at the electric cabinet side.
Then, we tried to connect 1 color wire from each cable, together with the same color wires from the other cables, to 1 phase, and the problem was solved.

Can you explain why this happened ?
What is the difference between using a complete cable for each phase, or a wire from each cable ?

 

[lfe]

If you use 3x240mm2, the current that circulates is quite large. Thousands of amps, I guess.

If 15% of the voltage is lost, a lot of heat would be generated somewhere. Surely that 15% drop was measured correctly? Are you sure that in the first case all 3 wires were used and not just one?

Theoretically the voltage drop should be very similar in both cases.

 

[Corsair]

Inductive Heating

1000 amp feed.

First Case:

If you have 9 conductors feeding through 3 conduits (3 wires per conduit) from one panel to another - and if you run Phase A through the first conduit, Phase B through the second, and so on - the conduits will get hot.

Second Case:

If you run one Phase A, one Phase B, and one Phase C conductor through each conduit the conduits will not get hot.

In both cases if you put an amp clamp around an individual conductor you would read a theoretical 333 amps. In the second case if you put a (big) amp clamp around all three conductors you would read 1000 amps. In the first case if you put an amp clamp around all three conductors you would read zero amps. The net current flowing though the conduit is zero - what is coming in equals what is coming out and they cancel - when you view all three phases.

Any responsible industrial electrician would always wire using the second case.

You may object - 'I'm using cable, not conduit.' The same thing applies to some extent when you run that cable through a knockout in the side of the box.

 

[I_Automation]

Think of the 3 conductors in one cable wired together as one long electromagnet


The current flowing through now has to get through the magnetic field.


Wired individually the reverse current in another wire counters the magnetic field and the current flows much easier.

 

[Kataeb]

The drop happens immediately when we put the load on, while the cables are still cold.
So, it is probably a magnetism issue.
The wires inside the power cables come twisted together, as manufactured, maybe to reduce the magnet fields?

 

[Corsair]

Cancellation from twist - severely off-topic today

When I did a term on the city council we completed a new power feed to the city. Came from a substation maybe 20 miles away. The 10 miles closest to us was on a railroad right of way. I forgot the exact voltage - over 100 kv.

I suspect the railroad required the 'twists' to cancel induced voltage on the rails. As you drive down the road if you pay attention to the poles you can see 4-5 changes in which phase is closest to the tracks in the 10 miles.

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I've heard of farmers that have power lines across their property talking about stringing an electric fence to catch free power. I don't know if anybody has tried it or if it is practical. I suspect there are safety and voltage regulation issues.

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A college professor friend talked about some theoretical discussion of a across the county power line using multi-megavolt DC. The use of DC takes care of induction, impedance, and related issues.

According to him the switching to get it to AC at the end would be done with vacuum tubes. They're capable of much higher voltage ratings than semiconductor devices since you increase the physical spacing to get the rating. Those of us that worked with vacuum tubes are interested in how some things come back.

I figured the current limits from the electron emission of the cathode would be an issue but he says there is some sort of cold cathode thing that can be used.

This is a long way away from the plug-in replaceable electromechanical buzzer 'choppers' that I remember in car radios and electric fencers. And I also remember the low-speed thermal buzzers that were used as turn signal flashers on cars. The time constant changed when you hooked up a trailer.

Lots of technology history discussion in my PLC classes. Maybe you need to know something about where we came from for a basis to understand where we are going.

Maybe Edison had some valid points concerning DC?

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[blkfalc4]

Here in the US it's against the NEC to execute it in your first scenario.

 

[I_Automation]

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I've heard of farmers that have power lines across their property talking about stringing an electric fence to catch free power. I don't know if anybody has tried it or if it is practical. I suspect there are safety and voltage regulation issues.

- - - - - - -

North of Detroit there was a subdivision of pricey homes running along a high tension corridor.
Detroit Edison had monitors every 2 miles on the corridor and that length had a noticeable power drain. The checked and replaced insulators and tested for leaks for a couple of years.


Then they finally went house to house in the subdivision checking meters to see if something was off there.


It was after a few checks that someone noticed the list of electric meters by address skipped a house. The house had lights and outside outlets, but no power wires going to it, and this house was directly on the border with the corridor.


When they checked the homeowner said he was pulling his power off the magnetic field of the Earth with a huge coil he had installed in his attic {on the power corridor side of his house} and connected to a AC inverter.


They failed suing him but had the court order him that it was their power and ordered him to stop and get his house wired properly with a meter.


EDIT: Edison's DC power lost to Tesla's AC power because DC power can not be transmitted over long distances, and long then was 2 or 3 miles. AC can travel further and the voltage kicked back up with intermittent substations


EDIT2: Regarding utility phasing along railroad tracks in the Options for Mitigating Risk subheading

 

[NetNathan]

Anytime you run multiple phase cables in more than 1 conduit (or wireway... don't forget) the phases MUST be mixed in the conduits. The EMF generated will cause inductive heating of the metal conduits. The induced current will also cause voltage drop.
Obviously, the more current pulled by the load.... the hotter the conduits will get (and any metal near the conduits).

Plus also the additional side effects.
You will also generate a large amount of current to ground.
The induced current will also cause voltage drop.

Having more than one phase, in the metal wire run, greatly reduces the EMF, because each "phase cycle" being 120 degrees apart is canceling out this type of generated EMF.

 

[Squarewave]

The drop happens immediately when we put the load on, while the cables are still cold.
So, it is probably a magnetism issue.
The wires inside the power cables come twisted together, as manufactured, maybe to reduce the magnet fields?

If the voltage drops like that as soon as the load is on, the cables aren't big enough for the load you're pulling. Also, the cable grouping comments above are to be taken into account.

I've seen and done all sorts to stop eddy currents forming. Cables in trifoil, or cut split along where the cable entries are, or use a non ferrous metal or a paxolin gland plate.

Did you work to any calculations?

 

[Kataeb]

Reactance is a magnetic effect, the changing magnetic field produced by the alternating current induces an EMF which opposes the original current flow.
When all the “legs” of the 3 phase supply are in the same duct, there is approximately zero current, therefore zero magnetic fields and so virtually no reactance.
The single conductor by contrast does have a current which produces a magnetic field and hence there may be a significant reactance.

https://www.quora.com/When-we-pass-...separate-duct-why-does-the-reactance-increase

 

[I_Automation]

To see a physical demonstration of what is happening take a 12mm diameter permanent magnet and a 18mm copper pipe that's 1M length.


Copper is not magnetic and it seems should then have no effect on the magnet. Dropping the magnet through the pipe the magnet should hit the floor in 1.2 seconds just as if you just dropped the magnet on the floor.



However, if you hold one end of the copper pipe and drop the magnet through it the magnet in the electrically conductive copper pipe causes the eddy field and the magnet can take 3 or 4 seconds to drop out of the bottom of the pipe.


Change the magnet to 50Hz AC and that fight is happening every 1/100th a second whether it is in a conduit or just the magnetic field around a cable, but the conduit would have a stronger effect.