Power Lines

[Electroenthusiast]

I was just wondering what happens if high voltage power lines get cut or broken, and fall down to ground. I had once seen this happen, but dont know grounding occured or whether the power-plants were automatically shut down.

Any idea what happens?
**broken link removed**

 

[JimB]

Most amusing

JimB

 

[ronsimpson]

There fuses/breakers all through the grid.

In power lines (where I live) there are 3 phases. The load is balanced so there is the same current on each wire. They will put 100 houses on phase 1, 100 houses on phase 2, and 100 houses on phase 3. This way there is nearly zero current in the neural wire (which is ground). If one phase of open or shorted to ground the balance is lost. This is much like how a ground fault breaker works. The power company knows when things are wrong.

 

[Electroenthusiast]

There fuses/breakers all through the grid.

In power lines (where I live) there are 3 phases. The load is balanced so there is the same current on each wire. They will put 100 houses on phase 1, 100 houses on phase 2, and 100 houses on phase 3. This way there is nearly zero current in the neural wire (which is ground). If one phase of open or shorted to ground the balance is lost. This is much like how a ground fault breaker works. The power company knows when things are wrong.

I didn't understand how a neutral wire will have a zero current at any condition. There has to be some current through it, uh? BTW, when you look at the above image, you see that there are only 6 wires, which is, 2 3-phase lines. There is no neutral wire here. Correct me if i'm wrong.

 

[ronsimpson]

Neutral is ground. The lines outside my house have a small neutral wire. Some times there is a small ground wire at the top for lightning.

In a balanced 3 phase system there is no (little) neutral current. A 3 phase motor often does not have a neutral wire. If you stand in the middle of three phases, and one is at the most positive part of the cycle, the other two phases are at -2/3 point of their cycle, and the average is zero.

It is common to have more than one wire per phase.

 

[Electroenthusiast]

consider the 3 phase wires (3 wires), now, all the three wires cut and fall down to ground. The reason maybe anything, like tower falls down, strong tornado or some natural dis@ster. Now, think that these three wires of a three phase system are broken. How will the power plant stop its working? Isn't there a possibility of highly-hazardous electric shock?

 

[JMW]

My understanding is the monitoring circuits can detect this, and power will be cut before the lines hit the ground.

 

[More Coffee]

Power lines don't always cut when they fall or get broken..
depending on the soil and many other factors they may stay on.

Generally however there is a three shot, the controller will attempt to establish a re-connection three times before the line is shut down and the big trucks show up.
The three shot is there just to clear debris that may fall across the lines,(tree limbs,bed sheets,etc)but the system doesn't always see fallen conductor.

A general rule of thumb around Hydro is to stay in the vehicle,move it clear if its still mobile,
If you heard the bang your still alive ,don't move and stay put ,probably the best advice.

until the electrical utility tells you the line is dead,permits and hold-off are in place ..the line is live and will kill you.Only the utility can authorise this..only the utility..not the fire department,police,no -one but the utility. very important that this is understood.

 

[Mikebits]

Once during a storm, a power transformer on a power pole had shorted. It arched and sparked with loud booms for quite a long time. Gave quite a show but was a bit scary.

 

[Sceadwian]

Neutral is ground

This is absolutely false. Neutral and ground are very frequently tied at common points making them effectively similar, this is not true under fault conditions, and even under heavy load conditions; neutral in a loaded system will always have a higher RMS voltage than a true unloaded ground line would.

A 3 phase motor often does not have a neutral wire

No but it will very frequently have a ground wire which is NOT the same as a neutral wire in a single phase system, it's for safety grounding in something like GFCI's and ground short situations. Much like double insulated single phase motors don't require a true ground, three phase also doesn't necesarily require a return to prevent floating voltages because of the additional isolation.

 

[Ian Rogers]

This is absolutely false. Neutral and ground are very frequently tied at common points making them effectively similar,

Not here in England.... Can't see why America is different.

Out in the sticks, neutral is a big metal pole hammered into the ground...... The closer neutral is to ground the better.... At substations around towns they connect the neutral to the ground to ensure this...

 

[ronsimpson]

Not here in England.... Can't see why America is different.

Out in the sticks, neutral is a big metal pole hammered into the ground...... The closer neutral is to ground the better.... At substations around towns they connect the neutral to the ground to ensure this...

America is the same. Neutral and ground are connected together millions of times, at many places. Out doors there are effectively the same and only one may exist at some points.

In a building or house the two are connector together at one point. In a house there they could be a small voltage difference.

 

[Electroenthusiast]

from what i understand, Sceadwian, Ian Rogers & ronsimpson are talking same thing about neutral wires in different way. BTW, i didn't get the clear picture about what happens when these(above pic = 6 wires & no neutral there) high-voltage wires gets broken or fall to ground(even if not broken).

 

[JimB]

i didn't get the clear picture about what happens when these(above pic = 6 wires & no neutral there) high-voltage wires gets broken or fall to ground(even if not broken).

Maybe there is no one here with experience of high voltage power transmission lines who can give a clear answer, I have no experience in this area.

However when has lack of knowledge ever held anyone back here on ETO!

Some inspired guess work.

The voltages and currents will be monitored at varoius points in the network and transmitted back to the network control room via the telemetry system.
The guy in the control room will be keeping an eye on what is going on.
The network control computer will throw up an alarm if any voltage or current exceeds some predefined limit (high or low).

In the case of a complete break in a line, it will be obvious form the loss of volts at the end of the line which is broken.
What action will be taken as far as tripping and isolating the line is concerned, Who knows?

If a tower fails and the lines are on the ground but still passing power to the far end of the line, again there will be some telemetry from the remote end of the line which will indicate what? I guess it depends on the exact nature of the fault.

I don't know if overhead lines have an "Earth Leakage" measurement device, if they do, a sudden large increase in earth leakge current should be alarmed in the control room, where the operator will do? Who knows.

If the fallen line is reported by some member of the public who thinks that the world is about to end, again, who knows what the action would be?

I guess a first course of action would be to send the on-call line fault fixing guy to go and have a look to see what is really happening and report back to the network control centre where the engineer responsible for the network will make a plan to isolate and re-route around the faulty section (if possible) and send a repair crew to fix the problem.

Wow, for someone who knows nothing, I think I have just written half an operations manual for an electricity supply network!

So, did that rambling missive help your understanding?

JimB

 

[tvtech]

I really don't understand any of this.

But am willing to learn....considering I deal with a MAX of 30KV per day. Safe stuff. On TV's only.

OK. I will start again. Maybe I am really stupid...but I really don't get it when huge farms in the Karoo in South Africa have lines that run to the farm in the middle of nowhere...that have ONLY ONE Live line.

Neutral is apparently Earth. I really don't get it. As far as I know, AC needs a Live and Neutral to work..

I am humbled. Please teach me. Properly.

Regards,
tvtech

 

[JimB]

OK. I will start again. Maybe I am really stupid...but I really don't get it when huge farms in the Karoo in South Africa have lines that run to the farm in the middle of nowhere...that have ONLY ONE Live line.

Neutral is apparently Earth. I really don't get it. As far as I know, AC needs a Live and Neutral to work..

Yes!

This sort of system is called a Single Wire Earth Return system.
When I first saw one I though that someone was having a laugh. But this type of system is used in rural areas in various countries.
I first saw it in Iceland and was rather intrigued.
By using a high voltage, the current is low, and the relatively high resistance earth connection back to the main supply does not cause too much problems with the voltage regulation.

Have a look here for more information:
https://en.wikipedia.org/wiki/Single-wire_earth_return

JimB

 

[tvtech]

I am learning.....thanks Jim....still don't understand it though

Regards,
tvtech

 

[tvtech]

Not that I am the brightest crayon in the box....at least I try

tvtech

 

[tvtech]

Double post.

 

[tvtech]

Error in posting