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These reactive currents, however, cause extra heating losses. Consider whether you’d benefit from extra features like automated rewinding or a built-in circuit breaker and can be used for space heater wattage. And since conductance is proportional to cross-sectional area, resistive power loss is only reduced proportionally with increasing cross-sectional area, providing a much smaller benefit than the squared reduction provided by multiplying the voltage. While power loss can also be reduced by increasing the wire's conductance (by increasing its cross-sectional area), larger conductors are heavier and more expensive. Current flowing through transmission lines induces a magnetic field that surrounds the lines of each phase and affects the inductance of the surrounding conductors of other phases. For this approximation, the voltage and current are identical at the sending and receiving ends. Hydro and wind sources cannot be moved closer to big cities, and solar costs are lowest in remote areas where local power needs are nominal. The 287.5 kV (Hoover Dam to Los Angeles line, via Victorville) and 345 kV (Arizona Public Service (APS) line) are local standards, both of which were implemented before 500 kV became practical. Local sources (even if more expensive and infrequently used) can protect the power supply from weather and other disasters that can disconnect distant suppliers.

The effect is more pronounced the higher the frequency, so DC cables do not suffer from this limitation. The skin effect causes the effective resistance to increase at higher AC frequencies. Factors that affect resistance and thus loss include temperature, spiraling, and the skin effect. Specific resistance is the electrical resistance of a specific size of a material, and it is shown by the Greek letter ρ. At power stations, power is produced at a relatively low voltage between about 2.3 kV and 30 kV, depending on the size of the unit. In the United States, power transmission is, variously, 230 kV to 500 kV, with less than 230 kV or more than 500 kV as exceptions. For a long transmission line, these lower losses (and reduced construction cost of a DC line) can offset the cost of the required converter stations at each end. At higher voltages, where more than 2,000 kV exists between conductor and ground, corona discharge losses are so large that they can offset the lower resistive losses in the line conductors. Substation transformers reduce the voltage to a lower level for distribution to customers. Transmitting electricity at high voltage reduces the fraction of energy lost to Joule heating, which varies by conductor type, the current, and the transmission distance.

Submarine HVDC systems are often used to interconnect the electricity grids of islands, for example, between Great Britain and continental Europe, between Great Britain and Ireland, between Tasmania and the Australian mainland, between the North and South Islands of New Zealand, between New Jersey and New York City, and between New Jersey and Long Island. As an example, to adjust the flow of AC power on a hypothetical line between Seattle and Boston would require adjustment of the relative phase of the two regional electrical grids. This causes the voltages induced by a magnetic field between the shield and the core conductor to consist of two nearly equal magnitudes which cancel each other. Energy is stored in an electric and/or magnetic field. Finally, at the point of use, the energy is transformed to end-user voltage (100 to 4160 volts). Throughout most of history, energy was harvested locally, primarily in the form of wood. To accomplish this, line position is swapped at specially designed transposition towers at regular intervals along the line using various transposition schemes.

Lowering line sag at high temperatures can prevent wildfires from starting when power lines touch dry vegetation. It is uneconomical to connect all distribution substations to the high main transmission voltage, because that equipment is larger and more expensive. Loops can be normally closed, where loss of one circuit should result in no interruption, or normally open where substations can switch to a backup supply. Voltage is stepped down before the current is sent to smaller substations. The same is true for the electrical current. These grids do not all operate in phase, and in Europe in particular, they often do not operate at the same frequency either. 100, provided the same sized conductors are used in both cases. Since all the circuit conductors required can be installed in a cable at one time, installation labor is saved compared to certain other wiring methods. Why is it taking decades to replace old cable? Phones are really very robust, and the basic design of the system is over 100 years old. On a much larger scale, over a third of the 22 GW produced by the Three Gorges Dam in China is transported to East and South China by way of 500 kV DC cables.

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