Single-phase vs. Three-phase Power Systems

Thomas A. Edison’s work on electric light led to the concept of a centrally located power station with distributed electric power for lighting. Edison’s work on dc systems expanded with the development of three-wire 220 V dc systems but transmission distances and loads continued to grow, voltage problems were encountered but with the development of a commercially practical transformer in 1885 by William Stanley, alternating current became more attractive than direct current because of the ability to transmit power at high voltage with corresponding lower current and lower line-voltage drops.

Nikola Tesla’s work on electric machines in 1888 made clear, the advantages of polyphase over single-phase systems, infact the three-phase induction motor conceived by Tesla became the workforce of the industry.

One major drawback of the single-phase circuit is the oscillatory nature of the instantaneous power flow p(t). The consequent shaft vibration and noise in single-phase machinery are undesirable; whereas a three-phase circuit under balanced conditions has constant, non-pulsating (time invariant), instantaneous power; the pulsating strain on generating and load equipment is eliminated. Also, a balanced three-phase system delivers more watts per kilogram of conductor than an equivalent single-phase system; because of these reasons, almost all bulk electric power generation and consumption take place in three-phase systems.

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Majority of three-phase systems are four-wire wye-connected systems, in which a grounded neutral conductor is used. Some three-phase systems such as delta connected and three-wire wye-connected systems do not have a neutral conductor. Because the neutral current is nearly zero under normal operating conditions, neutral conductors for transmission lines are typically smaller in size and current carrying capacity than the phase conductors. Thus the cost of a neutral conductor is substantially less than that of a phase conductor. It is important to note that, the capital and operating costs of three-phase transmission and distribution systems with or without neutral conductors, are comparatively much less than those of separate single-phase systems.

Related: Single-phase and Three-phase Power Systems