The competing systems

Edison's DC distribution system consisted of generating plants feeding heavy distribution conductors, with customer loads (lighting and motors) tapped off it. The system operated at the same voltage level throughout; for example, 100 volt lamps at the customer's location would be connected to a generator supplying 110 volts, to allow for some voltage drop in the wires between the generator and load. The voltage level was chosen for convenience in lamp manufacture; high-resistance carbon filament lamps could be constructed to withstand 100 volts, and to provide lighting performance economically competitive with gas lighting. At the time it was felt that 100 volts was not likely to present a severe hazard of electrocution.

To save on the cost of copper conductors, a three-wire distribution system was used. The three wires were at +110 volts, 0 volts and −110 volts relative potential. 100-volt lamps could be operated between either the +110 or −110 volt legs of the system and the 0-volt "neutral" conductor, which only carried the unbalanced current between the + and − sources. The resulting three-wire system used less copper wire for a given quantity of electric power transmitted, while still maintaining (relatively) low voltages.

However, even with this innovation, the voltage drop due to the resistance of the system conductors was so high that generating plants had to be located within a mile (1–2 km) or so of the load. Higher voltages could not so easily be used with the DC system because there was no efficient low-cost technology that would allow reduction of a high transmission voltage to a low utilization voltage.

In the alternating current system, a transformer was used between the (relatively) high voltage distribution system and the customer loads. Lamps and small motors could still be operated at some convenient low voltage. However, the transformer would allow power to be transmitted at much higher voltages, say, ten times that of the loads. For a given quantity of power transmitted, the wire size would be inversely proportional to the voltage used; or to put it another way, the allowable length of a circuit, given a wire size and allowable voltage drop, would increase approximately as the square of the distribution voltage. This had the practical significance that fewer, larger, generating plants could serve the load in a given area. Large loads, such as industrial motors or converters for electric railway power, could be served by the same distribution network that fed lighting, by using a transformer with a suitable secondary voltage.

Early transmission analysis

Edison's response to the DC system limitations was to generate power close to where it was consumed (today called, distributed generation) and install large conductors to handle the growing demand for electricity, but this solution proved to be costly (especially for rural areas which could not afford building a local station^[6] or paying for massive amounts of very thick copper wire), impractical (including, but not limited to, inefficient voltage conversion), and unmanageable. Edison and his company, though, would have profited extensively from the construction of the multitude of power plants required for introducing electricity to many communities.

Direct current could not easily be changed to higher or lower voltages. This meant that separate electrical lines had to be installed in order to supply power to appliances that used different voltages, for example, lighting and electric motors. This led to a greater number of wires to lay and maintain, wasting money and introducing unnecessary hazards. A number of deaths from the Great Blizzard of 1888 were attributed to collapsing overhead power lines in New York City.^[7][8]

Alternating current could be transmitted over long distances at high voltages, at lower current for lower voltage drops (thus with greater transmission efficiency), and then conveniently stepped down to low voltages for use in homes and factories. When Tesla introduced a system for alternating current generators, transformers, motors, wires and lights in November and December of 1887, it became clear that AC was the future of electric power distribution, although DC distribution was used in downtown metropolitan areas for decades thereafter.

Low frequency (50 - 60 Hz) alternating currents can be more dangerous than similar levels of DC since the alternating fluctuations can cause the heart to lose coordination, inducing ventricular fibrillation, which then rapidly leads to death within six to eight minutes from anoxia of the brain and medulla. ^[9]. High voltage DC power can be more dangerous than AC, however, since it tends to cause muscles to lock in position, stopping the victim from releasing the energised conductor once grasped. However, any practical distribution system will use voltage levels quite sufficient for a dangerous amount of current to flow, whether it uses alternating or direct current. Since the precautions against electrocution are similar, ultimately, the advantages of AC power transmission outweighed this theoretical risk, and it was eventually adopted as the standard worldwide.

Transmission loss

The advantage of AC for distributing power over a distance is due to the ease of changing voltages with a transformer. Power is the product of current x voltage (P = IV). For a given amount of power, a low voltage requires a higher current and a higher voltage requires a lower current. Since metal conducting wires have a certain resistance, some power will be wasted as heat in the wires. This power loss is given by P = I²R. Thus, if the overall transmitted power is the same, and given the constraints of practical conductor sizes, low-voltage, high-current transmissions will suffer a much greater power loss than high-voltage, low-current ones. This holds whether DC or AC is used.

However, it was very difficult to transform DC power to a high-voltage, low-current form efficiently, whereas with AC this can be done with a simple and efficient transformer.

This was the key to the success of the AC system. Modern transmission grids regularly use AC voltages up to 765,000 volts. ^[10]

Alternating current transmission lines do have other losses not observed with direct current. Due to the skin effect, a conductor will have a higher resistance to alternating current than to direct current; the effect is measurable and of practical significance for large conductors carrying on the order of thousands of amperes. The increased resistance due to skin effect can be offset by changing the shape of conductors.

more: 

see wikipedia's article on war of the currents

News

Power transmission

Where the wind blows

A grandiose plan to link Europe's electricity grids may recast wind power from its current role as a walk-on extra to being the star of the show

The question of whether the world would be powered by direct current (DC), in which electrons flow in one direction around a circuit, or by alternating current (AC), in which they jiggle back and forth, was decided in the 1880s. Thomas Edison backed DC. George Westinghouse backed AC. Westinghouse won.

The reason was that over the short distances spanned by early power grids, AC transmission suffers lower losses than DC. It thus became the industry standard. Some people, however, question that standard because over long distances high-voltage DC lines suffer lower losses than AC. Not only does that make them better in their own right, but employing them would allow electricity grids to be restructured in ways that would make wind power more attractive. That would reduce the need for new conventional (and polluting) power stations.

AC/DC/PC

Wind power has two problems. You don't always get it where you want it and you don't always get it when you want it. According to Jürgen Schmid, the head of ISET, an alternative-energy institute at the University of Kassel, in Germany, continent-wide power distribution systems in a place like Europe would deal with both of these points.

The question of where the wind is blowing would no longer matter because it is almost always blowing somewhere. If it were windy in Spain but not in Ireland, current would flow in one direction. On a blustery day in the Emerald Isle it would flow in the other.

Dealing with when the wind blows is a subtler issue. In this context, an important part of Dr Schmid's continental grid is the branch to Norway. It is not that Norway is a huge consumer. Rather, the country is well supplied with hydroelectric plants. These are one of the few ways (but not the only way, see article) that energy from transient sources like the wind can be stored in grid-filling quantities. The power is used to pump water up into the reservoirs that feed the hydroelectric turbines. That way it is on tap when needed. The capacity of Norway's reservoirs is so large, according to Dr Schmid, that should the wind drop all over Europe—which does happen on rare occasions—the hydro plants could spring into action and fill in the gap for up to four weeks.

Put like this, a Europe-wide grid seems an obvious idea. That it has not yet been built is because AC power lines would lose too much power over such large distances. Hence the renewed interest in DC.

Westinghouse won the battle of the currents in the 1880s because it is easier to transform the voltage of an AC current than of a DC current. High voltage is the best way to transmit power (the higher the voltage, the smaller the loss), but high voltage is not usually what the user wants. Power is therefore transmitted along high-tension AC lines and then “stepped down” to usable voltages in local sub-stations.

Edison was right, however, to argue that DC is the best way to transmit electricity of any given voltage. That is because the shifting current of AC runs to earth more easily than DC does. To avoid this earthing, AC lines have to be built a long way from the ground—and the higher the voltage, the farther away they need to be. At 400 kilovolts, a standard value for long-distance transmission, an alternating current 30 metres (100 feet) from the ground has a fortieth of the loss of a similar cable at ground level. But even at this height an overhead DC line will beat an AC line at distances more than 1,000km (600 miles), while ground-level DC will beat AC at distances as short as 30km.

Dr Schmid calculates that a DC grid of the sort he envisages would allow wind to supply at least 30% of the power needed in Europe. Moreover, it could do so reliably—and that means wind power could be used for what is known in the jargon as base-load power supply.

Base-load power is the minimum required to keep things ticking over—the demands of three o'clock in the morning, or thereabouts. At the moment, this is supplied by traditional power stations. These either burn fossil fuel and thus contribute to global warming, or use uranium, which brings problems such as how to get rid of the waste, as well as political opposition.

Though wind power has its opponents, too, its environmental virtues might be enough to swing things in its favour if it were also reliable. Indeed, a group of Norwegian companies have already started building high-voltage DC lines between Scandinavia, the Netherlands and Germany, though these are intended as much to sell the country's power as to accumulate other people's. And Airtricity—an Irish wind-power company—plans even more of them. It proposes what it calls a Supergrid. This would link offshore wind farms in the Atlantic ocean and the Irish, North and Baltic seas with customers throughout northern Europe.

Airtricity reckons that the first stage of this project, a 2,000 turbine-strong farm in the North Sea, would cost about €2 billion ($2.7 billion). That farm would generate 10 gigawatts. An equivalent amount of coal-fired capacity would cost around $2.3 billion so, adding in the environmental benefits, the project seems worth examining. Such offshore farms certainly work. Airtricity already operates one in the Atlantic, and though it currently has a capacity of only 25 megawatts, increasing that merely means adding more turbines.

Nor is this the limit of some people's vision. The Global Energy Network Institute, based in San Diego, California, reckons high-voltage DC lines could be used to bring solar energy to market from places such as the Sahara. Wind and geothermal power could be gathered from as far afield as South America and Siberia. Such a globalised market has its attractions. Whether the world is ready for the Organisation of Electricity Exporting Countries to take over from OPEC, though, remains to be seen.

source: economist