Electric power distribution from powerplant to homes

[Babadag]

By the way, even overhead line presents insulation: the insulators and the air. However, if the heat is evacuated by conduction in cables in the overhead line it is convection and radiation the evacuation way. The conductor temperature is limited by metal mechanical properties since annealing aluminium produces elongation of the conductor and so permanent sag increase. For short-circuit the limit may be the fusing temperature of the conductor. So, the melting point may be the limit.

 

[Hop-AC8NS]

I am trying to understand how transferring electric from the powerplant to my house is more effective using high voltage.

...

What am I missing?

You are missing WHY the power plant is more effective using high voltage. It is not for YOUR benefit, but is for the benefit of the utility providing your house with electrical energy.

The reasoning is simple: for a given voltage at the power plant, the amount of power it can deliver is the product of its voltage multiplied by the current consuming that power. For any given amount of power, some will be lost as heat created in the transmission wire resistance, R. The amount of power lost is I²R.

Clearly a reduction in I will cause a reduction in power lost. Since this lost power is not billed to the customer, it benefits the utility to minimize it by making the transmission line voltage as large as possible.

Drive-by posters are the bane of serious discussion groups, but at least you don't have to worry about their responses.

 

[sophiecentaur]

It is not for YOUR benefit, but is for the benefit of the utility providing your house with electrical energy.

That comment is a bit pessimistic. Reducing losses will help both supplier and customer. You'd be paying for losses anyway. But I'm an engineer, basically and a total innocent where money is concerned.

Drive-by posters are the bane of serious discussion groups,

I have to agree. One chooses the words of a reply very carefully, with a mixture of helpfulness and smugness and the visitor never lets you know just how impressed he is. (I say "he" because they are all 'he's. 'She's would all be polite enough to give a reply.

 

[Hop-AC8NS]

That comment is a bit pessimistic.

It wasn't intended to be. I believe the fuel cost per kilowatt-hour to generate electrical power is much less than the continuing, and increasing, cost to build and maintain the infrastructure that supports electrical power transmission over long distances for an increasing demand, year after year. But the money the electric utility bills to its customers is based on how much energy (kilowatt-hours) the customer uses. That rate has to include fuel costs as well as all the other costs involved in getting a few thousand watts to your home and a few thousand megawatts to the rest of the customer base.

Amazing that it still works. Here in west-central Florida there are three-phase power lines everywhere. These were originally supported (at lower voltages) on wooden poles with glass or ceramic insulators, but because of hurricanes those wood poles are slowly being replaced with concrete poles, and a lot of the wires are going underground, too. A few years ago, FPL promised to move my power distribution underground "real soon now" but this has been stalled, probably by my neighbors saying "not in my back yard" for the placement of the ground-installed transformer.

I suffer a few days of FPL power loss, almost every year, because a hurricane has knocked down tree branches, that then has taken down power lines and sometimes wooden utility poles. My house, and a few others, are presently serviced by a 7kV "high line" that powers a single-phase "pole pig" transformer and some street lights in the neighborhood. The pole-mounted transformer distributes 120-0-120 vac power on three overhead wires to most of the houses, so those wires are subject to damage too.

My house, and two of my adjacent neighbors, have our power distribution buried underground from the pole pig to the house. The previous owners of our custom-built house started this, but the two previously vacant lots next door recently had houses built on them and their power is also underground. There are several houses still sipping their energy from overhead distribution, but I don't know how many. FPL has said they will pay to move those wires underground, but nothing has happened yet.

I have thirty-eight LG photovoltaic solar panels, with Enphase micro-inverters on each panel, and net-zero metering. After the panels were installed, I have not received a bill for energy delivered to my house. The panels (during the day) send energy back to the grid. This more than makes up for energy used at night when the sun don't shine, or it is too cloudy, foggy, rainy, or whatever. I pay about twenty-five bux a month to Florida Power & Light for the privilege of sending power back to their grid.

It's worked out fine (electrically) so far, but it's been a financial loser: I pay more in monthly principle and interest on the solar loan from Corning Credit Union than I used to pay FPL for their electricity. That could change in the future if the cost of electrical energy increases faster than the solar panels degrade with age. Plus, with inflation, I am paying with "cheaper" dollars every year. I don't see that changing any time soon. It's a retirement experiment, not an investment. And it has nothing to do with "saving the planet" which is doing fine also, last time I looked. And, yeah, I am an electrical engineer with no financial acumen, too.

The reason that three-phase alternating current prevailed over direct current back in Edison's day was because it had two strong poles in its tent: (1) three-phase rotating magnetic fields allowed simple induction motors of almost unlimited size to be built, sold, and used to jump-start American manufacturing and (2) alternating current is easily stepped up and down in voltage, as needed, using transformers which is essential for efficient distribution of electrical power with minimal losses.

Technology today allows huge amount of power to be transmitted over hundreds of miles with low energy loss using direct current at megavolt potentials. This can only occur when devices exist to synchronously rectify high-voltage alternating current at the transmission origin and then invert it back to alternating current for distribution at the load end.

 

[Baluncore]

The economics of transmission lines is a problem for the power distribution company. The decisions will be based on current interest rates, and the length of the lines being installed or upgraded.

With higher voltages, the transmission lines cost less for copper, but the insulators and towers cost more. The voltage conversion equipment also costs more for higher voltages, but those transformers are only needed at the ends of the transmission line.

Thicker copper wires have less loss, but cost more up front. The distribution company will trade future energy losses, against immediate financial investment. That does not strictly benefit the consumer, but more the distribution company shareholders, who must support the investment.

If your power company makes unwise or unlucky decisions, then you can expect regional power costs to rise faster, than if they had correctly guessed future interest rates. The rising tide lifts all boats.

 

[Hop-AC8NS]

The power loss in transmission lines is unavoidable, but when alternating current is involved there is also the problem of "skin effect" causing most of the current to flow in a narrow layer near the surface of the wire. This is avoided with direct current transmission. See, for example, the Pacific DC Intertie that brings power from cheap hydroelectric generating facilities in the northern coastal states of America to southern California. It took awhile for technology to be developed that enabled this to happen.

In the future we might be getting electrical power generated in orbit from massive arrays of photo-voltaics that transmit the energy via microwaves to rectifying antenna arrays on Earth. Whether that energy is then transported to the end user as AC or DC is an economic decision. If I were betting on this, I would bet on high-temperature DC superconductors moving electricity from where it is generated on the ground to where the end users are. Or perhaps use fiber-optics to transmit power, as is currently done for some isolated and/or remote Internet applications. Not sure how this could ever be scaled to industrial levels of power, but who knows what will eventually be possible? Rising tides, indeed!

BTW, almost no power is transmitted with copper wire because it is too weak to support itself when installed between the long spans between support towers. Notice that the wires are NEVER stretched taut between support towers because the "sag" is essential. Copper (even hard-drawn copper) does not hold up well in the outdoor environment. Steel and aluminum are the "go to" metals for massive power transmission.

 

[sophiecentaur]

when alternating current is involved there is also the problem of "skin effect" causing most of the current to flow in a narrow layer near the surface of the wire.

And don't forget the phase problem over long cables and the need for synchronisation between multiple generators. Power factor always needs to be corrected when high powers are involved

we might be getting electrical power generated in orbit from massive arrays of photo-voltaics that transmit the energy via microwaves to rectifying antenna arrays on Earth.

That's a popular idea amongst futurists but terrestrial transmission is controllable and safe. and most of the Earth's surface gets a useful daily dose of solar radiation and surface engineering is incredibly much cheaper than space engineering.

 

[Baluncore]

BTW, almost no power is transmitted with copper wire because it is too weak to support itself when installed between the long spans between support towers.

The core wire of an aluminium or copper catenary cable is usually a high tensile steel wire. Skin effect prevents high currents and losses in the steel core.

Copper is the better conductor, followed by aluminium. Aluminium weighs less for the same ampacity, so you might think that the support structure would cost less, but the greatest forces on catenary cables are not due to gravity, but wind pressure, especially when the cable is cold, taught, and covered with a rime of ice. The wind pressure on copper is less than on aluminium cables, because copper can have a smaller diameter for the same ampacity.

Power factor always needs to be corrected when high powers are involved

Copper (ohmic) losses in transmission lines are not always wasted. Losses are sometimes needed to prevent ice formation, or to shed ice in winter. If insufficient current is flowing to remove ice, a reactive current can be encouraged, to heat the wire further.

 

[Hop-AC8NS]

In TARC we have a retired member, San Yoder, who spent his career involved with electrical power transmission. The next time I see him I will ask how high-tension lines in the USA are made. I am not familiar with how it's done in Australia, nor do I feel qualified to comment on the copper versus aluminum conductor trade-offs. Plus, I have no experience as a line man. Might be exciting to work on a "live" 300kV line from an insulated bucket... I have seen videos of that! Copper has become quite expensive compared to aluminum, but "down under" you may have better access to less expensive copper for power transmission, so I will just take your word for it that copper is still being used.

Florida has a huge source of bauxite used to make aluminum, and the Tennessee Valley Authority (TVA) has the hydroelectric power necessary to electrolytically refine it. Alcoa TN was founded on it. The TVA lakes support thousands of recreational boaters and fishermen every year.