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For transmission systems with low power factor, losses are higher than for systems with high power factor. For a given amount of power, a higher voltage reduces the current and thus the resistive losses. In general, losses are estimated from the discrepancy between power produced (as reported by power plants) and power sold; the difference constitutes transmission and distribution losses, assuming no utility theft occurs. The voltage, power, frequency, load factor, and reliability capabilities of the transmission system are designed to provide cost effective performance. 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. 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. A DC link therefore stabilizes the AC grid at either end, since power flow and phase angle can then be controlled independently. 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. 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.

Single-wire earth return is also used for HVDC over submarine power cables. Not all cables perform the same application. 100, provided the same sized conductors are used in both cases. 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. The lossless line approximation is the least accurate; it is typically used on short lines where the inductance is much greater than the resistance. We are pretty much aware of what is optical fibre and its uses in a wide variety of applications. This monitoring solution uses passive optical fibers as temperature sensors, either inside a high-voltage cable or externally mounted on the cable insulation. Resistance increases with temperature. Understanding the temperature distribution along the cable route became possible with the introduction of distributed temperature sensing (DTS) systems that measure temperatures all along the cable. Without them maximum current was typically set as a compromise between understanding of operation conditions and risk minimization. The product of line length and maximum load is approximately proportional to the square of the system voltage.

The field strength was 1.6 kV/m, and became the interim maximum strength standard for new transmission lines in New York State. 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. In a residential setting, one study reported "limited evidence of carcinogenicity in humans and less than sufficient evidence for carcinogenicity in experimental animals", in particular, childhood leukemia, associated with average exposure to residential power-frequency magnetic field above 0.3 μT (3 mG) to 0.4 μT (4 mG). This is an everyday occurrence in AC systems, but one that can become disrupted when AC system components fail and place unexpected loads on the grid. 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. Typically, only larger substations connect with this high voltage. The terminal characteristics of the transmission line are the voltage and current at the sending (S) and receiving (R) ends.

For this approximation, the voltage and current are identical at the sending and receiving ends. The line is assumed to be a reciprocal, symmetrical network, meaning that the receiving and sending labels can be switched with no consequence. In such models, a capital letter such as R refers to the total quantity summed over the line and a lowercase letter such as c refers to the per-unit-length quantity. Spiraling, which refers to the way stranded conductors spiral about the center, also contributes to increases in conductor resistance. Factors that affect resistance and thus loss include temperature, spiraling, and the skin effect. Currents that flow solely in reaction to these properties, (which together with the resistance define the impedance) constitute reactive power flow, which transmits no power to the load. Since the power flow in a DC link is controlled independently of the phases of the AC networks that it connects, this phase angle limit does not exist, and a DC link is always able to transfer its full rated power.

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