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Superconducting cables are particularly suited to high load density areas such as the business district of large cities, where purchase of an easement for cables is costly. The ratio of real power transmitted to the load to apparent power (the product of a circuit's voltage and current, without reference to phase angle) is the power factor. These were induction motors running on polyphase current, independently invented by Galileo Ferraris and Nikola Tesla. Practical three-phase motors were designed by Mikhail Dolivo-Dobrovolsky and Charles Eugene Lancelot Brown. Most North American transmission lines are high-voltage three-phase AC, although single phase AC is sometimes used in railway electrification systems. For this approximation, the voltage and current are identical at the sending and receiving ends. Approximately, the power flowing over an AC line is proportional to the cosine of the phase angle of the voltage and current at the ends. It is often used as a transmission line between an antenna and a receiver (e.g., TV and radio). If demand exceeds supply, the imbalance can cause generation plant(s) and transmission equipment to automatically disconnect or shut down to prevent damage. A sophisticated control system is required to ensure that power generation closely matches demand.

Thus, generators were sited near their loads, a practice that later became known as distributed generation using large numbers of small generators. Corona and resistive losses can be estimated using a mathematical model. Local sources (even if more expensive and infrequently used) can protect the power supply from weather and other disasters that can disconnect distant suppliers. By 1914, fifty-five transmission systems operating at more than 70 kV were in service. Coaxial cable - used for radio frequency signals, for example in cable television distribution systems. No fixed cutoff separates subtransmission and transmission, or subtransmission and distribution. As power systems evolved, voltages formerly used for transmission were used for subtransmission, and subtransmission voltages became distribution voltages. Lower voltages, such as 66 kV and 33 kV, are usually considered subtransmission voltages, what are electric cables but are occasionally used on long lines with light loads. It was powered by a 2 kV, 130 Hz Siemens & Halske alternator and featured several Gaulard transformers with primary windings connected in series, which fed incandescent lamps. 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. Adverse weather conditions, such as high winds and low temperatures, interrupt transmission.

The first solution to these problems is to keep cable lengths in buildings short since pick up and transmission are essentially proportional to the length of the cable. Where the interfering signal has a wavelength that is long compared to the pitch of the twisted pair, alternate lengths of wires develop opposing voltages, tending to cancel the effect of the interference. Ribbon cable - Useful when many wires are required. Underground lines are limited by their thermal capacity, which permits less overload or re-rating lines. Mainstream scientific evidence suggests that low-power, low-frequency, electromagnetic radiation associated with household currents and high transmission power lines does not constitute a short- or long-term health hazard. This reduces the losses produced by strong currents. Long underground AC cables have significant capacitance, which reduces their ability to provide useful power beyond 50 miles (80 kilometres). Copper was sometimes used for overhead transmission, but aluminum is lighter, reduces yields only marginally and costs much less. The voltage is stepped up for transmission, then reduced for local distribution. HVDC links stabilize power distribution networks where sudden new loads, or blackouts, in one part of a network might otherwise result in synchronization problems and cascading failures. Norway and Sweden also use this frequency but use conversion from the 50 Hz public supply; Sweden has a 16 2/3 Hz traction grid but only for part of the system.

Transmission companies determine the maximum reliable capacity of each line (ordinarily less than its physical or thermal limit) to ensure that spare capacity is available in the event of a failure in another part of the network. Such a system could be less prone to failure if parts of it were suddenly shut down. Spain's transmission system is interconnected with those of France, Portugal, and Morocco. In 1882, DC voltage could not easily be increased for long-distance transmission. Commercial electric power was initially transmitted at the same voltage used by lighting and mechanical loads. High-voltage direct current (HVDC) is used to transmit large amounts of power over long distances or for interconnections between asynchronous grids. All wires running inside this shielding layer will be to a large extent decoupled from external electrical fields, particularly if the shield is connected to a point of constant voltage, such as earth or ground.