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One voltage cycle of a three-phase system. A polyphase system (the term coined by Silvanus Thompson) is a means of distributing alternating-current (AC) electrical power that utilizes more than one AC phase, which refers to the phase offset value (in degrees) between AC in multiple conducting wires; phases may also refer to the corresponding terminals and conductors, as in color codes.
The return path for the current in any phase conductor is the other two phase conductors. Constant power transfer is possible with any number of phases greater than one. However, two-phase systems do not have neutral-current cancellation and thus use conductors less efficiently, and more than three phases complicates infrastructure unnecessarily.
In a three-phase system, at least two transducers are required to measure power when there is no neutral, or three transducers when there is a neutral. [5] Blondel's theorem states that the number of measurement elements required is one less than the number of current-carrying conductors. [6]
For example, if the imbalance is limited to 25% of the total load (half of one half) rather than the absolute worst-case 50%, then conductors 3/8 of the single-phase size will guarantee the same maximum voltage drop, totalling 9/8 of one single-phase conductor, 56% of the copper of the two single-phase conductors.
A typical one-line diagram with annotated power flows. Red boxes represent circuit breakers, grey lines represent three-phase bus and interconnecting conductors, the orange circle represents an electric generator, the green spiral is an inductor, and the three overlapping blue circles represent a double-wound transformer with a tertiary winding.
These reactive currents, however, cause extra heating losses. 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. As reactive current increases, reactive power increases and power factor decreases.
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Angle notation can easily describe leading and lagging current: . [1] In this equation, the value of theta is the important factor for leading and lagging current. As mentioned in the introduction above, leading or lagging current represents a time shift between the current and voltage sine curves, which is represented by the angle by which the curve is ahead or behind of where it would be ...