Does a GFI actually trip on over current?

[eamarquardt]

Yer right, I didn't know about and haven't noticed the power company's neutral (I'll have to count the wires on the vertical drop into one of my houses). There must be three wires to carry electricity into your house (unless you have only 120 volt service not 240 volt but I don't know if 120 volt service even exists). Two 120 volt hots that are 180 out of phase and a neutral. If your load is unbalanced the unbalanced part will flow over the neutral. But, the neutral and ground are connected in the main panel of your house and the ground to earth. In some instances the power company, however, does use the earth as a conductor on some DC transmission services during emergencies. A friend works for a power plant on the Columbia River, they transmit DC here to So.Cal., and for a while they used the earth as the return. Apparently, they were eating up docks in lakes and all sorts of stuff due to the electrolysis. They, according to my friend, stopped this practice for normal operation. One might argue that if the path through ground/earth is better than the neutral, the electrons could flow that way, but it isn't likely or I guess desireable under normal circumstances.

Thanks for setting me straight.

Cheers,

Gus

[Chuck Craven]

Gus
How far is the Columbia River to Southern California? As far as I know it, there are no DC transmission lines in the U.S. and DC still needs a return path with the equivalent or lower conductor resistance. The best ground resistance it the country is along the seashore and its a few 1000 ohms pre inch. Ground resistance is way to high to transmit power. To send DC through a transmission line for a few city blocks you need copper wires 2” in diameter. We have buildings in down town Milwaukee that once were powered with DC. They had copper buss bars 2” by 6” running up through the buildings. All DC power was gone in the U.S. by 1930’s. Except for a few private own/run systems. Even the modern wind generators are AC power it’s just that the line frequency varies. So they use frequency stabilizers / converts on the generators. Some small wind generators are DC, only to charge batteries for private single use power systems.
DC transmission lines are too lousy to power houses or industry’s needs.

Chuck

[brian_bp]

DC power transmission is not bad... it's just expensive to get up to the high voltages which make any major transmission line practical; the fact that a sine wave can be stepped up and down readily with a simple transformer is the big reason for AC power transmission.

The high-voltage DC transmission systems I've heard of are typically used to connect AC systems which for some reason cannot be kept in synch. They are relatively rare (perhaps due to the expensive electronics to drive them): the only large examples I could readily find in North America are the Pacific DC Intertie, HVDC Vancouver Island, Nelson River Bipole, Intermountain, and Quebec - New England Transmission.

Using the ground as a conduction path would be a monopole and earth return system, as described in that Wikipedia page... clearly this is a viable configuration, at least in DC. I didn't know that anyone did that, and of the above list the Pacific DC Intertie may be the only one set up to run this way.

By the way, commercial wind turbines are AC, and run in synch with the grid, so they are at constant frequency and in phase. It's amusing to watch dozens of them (such as in southern Alberta) all turning slowly together like synchronized dancers. It's the first thing I noticed about them when I first saw them.


And now back to your regularly scheduled Teardrops & Tiny Travel Trailer programming...

[brian_bp]

...A friend works for a power plant on the Columbia River, they transmit DC here to So.Cal....

That would presumably be the Pacific DC Intertie.

[Larwyn]

Just Google "hvdc tie", over 18,000 results. Some even right here in Texas.

[eamarquardt]

Biran clearly knows more than I do and said it well. I have driven by the huge inverters near I-5 near the Northern fringes of Los Angeles.

Technology nowadaze is so diverse no one can keep up with it all. Who'd have thunk that they use DC to transmit lots of power long distances.

Some days I actually know what I'm talking about and make sense.

Let's keep learning.

Cheers,

Gus

[Chuck Craven]

Well now I see why electricity is so expensive in California. All that converting from AC to DC to AC. High voltage AC transmission lines are a lot cheaper, even with the booster stations. There has been a plan in building a 1 million-AC volt transmission line here in Wisconsin but they are having problems getting it approved. I have read some thing about zapping migrating flocks of geese and starting forest fires. The list is very small and only looks to be just two major players here in the states. With California as the only one fully state side. There are lots of privately owned small transmission lines here in the states and some may be DC but the major players of the electrical grid is AC. That fight has been going on since Edison and Tesla got in to it.

Chuck

[eamarquardt]

Ya know, one of my philosophies is "they can't be a dumb as they appear, I just don't have the big picture". Now sometimes this may not be true but there must be some good reasons to go through the ac to dc to ac conversions or else they'd stick with ac. Don't ferget about the skin effect.

Cheers,

Gus

[Larwyn]

Chuck, I'm far from an expert on DC transmission lines but I have read enough to know that what seems obvious is not necessarily so.

HVDC transmission lines are actually less expensive to build (fewer conductors), the inverter/converter along with the lack of need for var control offsets the cost to the point that it is very much a wash as far as expense is concerned. Especially where long distance transmission is concerned as with HVDC the only concern is pure DC resistance as opposed to the capacitive/inductive losses with AC transmission. It is the loss concerns as well as system isolation that makes the HVDC an attractive alternative for long distance ties, as well as ties to other systems which cannot be syncronized and even in some cases between two systems operating at different a different frequency (between two countries).

If you were to research it a bit there is some quite interesting reading on the net about HVDC transmission. I worked with transmission/generation for over 20 years and never had any direct dealings with HVDC transmission, most of what I know about it, I have read on the net tonight due to this thread. I just knew that Brazos Electric, one of the utilities with which we had ties also have a well know DC tie here within the ERCOT system.

[Larwyn]

Ya know, one of my philosophies is "they can't be a dumb as they appear, I just don't have the big picture". Now sometimes this may not be true but there must be some good reasons to go through the ac to dc to ac conversions or else they'd stick with ac. Don't ferget about the skin effect.

Cheers,

Gus

You are on to something there Gus.

HVDC losses are much less on long distance transmission lines as compared to AC at or near the same peak voltage. (Based on what I have learned so far)...

[eamarquardt]

Thanks Larwin, it's not often someone agrees with me. Especially if my wife and two boys are in the room.

Cheers,

Gus

[Chuck Craven]

I can see the 60 to 50-cycle conversion from DC now days with high power electronic devices. But in the past it was done with dynamotors. As for DC being more efficient it is not true. Tesla proved that a 100 years a go when Edison lost the DC debate in Buffalo New York. It’s probably one of those things that got so much money into it and can’t stop doing it…. The only other thing that comes to mind is: The path of the transmission line is over the mountains and it was worth the losses not to have and maintain sub stations. Which is probably the real reason. That also would explain short distance lines for a small power transmission company. The sub stations have to be maintained or the transmission lines were established before AC lines were developed in that aria and the cost did not justify converting. Wisconsin has some small dam DC power stations but Edison built them in 1900’s. There are only a couple left still running today more as a museum piece but do produce power for a small arias. They convert DC to AC at the plant.


Chuck

[Alphacarina]

Yer right, I didn't know about and haven't noticed the power company's neutral (I'll have to count the wires on the vertical drop into one of my houses). There must be three wires to carry electricity into your house (unless you have only 120 volt service not 240 volt but I don't know if 120 volt service even exists). Two 120 volt hots that are 180 out of phase and a neutral. If your load is unbalanced the unbalanced part will flow over the neutral. But, the neutral and ground are connected in the main panel of your house and the ground to earth

Gus,
You still have some 'misconceptions'

What you have on the power pole is a hot 7,200 volt wire and a grounded neutral - That's it

Your house isn't directly connected to the power company at all. The hot 7,200 volt and neutral are applied to the primary of a transformer ('pole pig') and the secondary is a centertapped 240 volt winding. What runs to your home is the secondary of that transformer - 2 wire 240 volts, single phase with a centertapped 'neutral' and you get 120 volts between either of the 240 volt lines and the neutral wire. If the 240 volt wires were out of phase with each other as you suggest, you would only have the 120 volts between one of them and the neutral . . . . across the 240 volts you would have nothing if one wire was 180 out of phase with the other wire

The neutral wire has nothing to do with balancing the load either - Your entire home can run off of one of the 240 wires and the neutral with nothing on the other 240 wire and it's prefectly fine. Older homes were wired that way for many years until they needed 240 for something

Proof that the 'earth' has nothing to do with it is when you plug in a 240 volt heater which runs on just those 2 hot wires . . . . no neutral or earth connection used or needed . . . . well, maybe to 'ground' the case of the heater, if it's a metal one

Don

[SteveH]

Don,

Sorry, but Gus is right. The two 120 volt lines are 180 degrees out of phase with each other. When one is positive, the other is negative. If they were not, you would measure 0 volts between the two.

[Larwyn]

Don,

If the distribution in Gus's area is 12,470 Line to Line, then yes 7,200 Line to Ground would be correct.

The two legs of the 240 single phase actually must be 180 degrees out of phase with each other. Otherwise the voltage would be less, right down to zero volts line to line if they were in phase with each other. they would still be 120 line to ground/neutral regardless of the phase relationship.

Any imbalance between the two legs of the 240 volt ckt must flow in the neutral if used in the circuit or ground if by ground fault, it will not simply go away. The neutral also provides the return path for all the 120 volt circuits.