Advantages of three phase four wire system over three phase three wire system?
A four wire system allows you to connect equipment "phase to neutral".
I don't know if you're referring to an electric utility distribution system or the electric service to a commercial / industrial customer.
From a utility perspective, the majority of customers require single phase service for residential applications. Having a four wire system allows you to tap one phase and the neutral to provide service.
While you COULD provide single phase service with a delta connection, that would require two fuses, and a more expensive transformer.
From a commerical customer perspective, the common US voltage of 208Y/120 V allows 120 volt equipment connected from any one phase to the common neutral. Three phase equipment (motors) are rated for 208 volts and are connected phase-to-phase.
The fourth wire is ground. Every electrical system needs one.
great answer Thomas, may i add
only big advantage of 3 wire, one less wire = 25% savings in metals. In distribution we see delta wiring.
at the point of consumption the US standard is a Y connection grounded at the center. larger home appliances un the us (and all in europe) connect to a single phase 220 line, in both countries, this is generally center tapped to provide two120 volt circuits, in the us this is the Neutral (white) wire in a typical outlet. but in Europe, this grounded tap is not used to transmit power..
a more efficient DC transmission line brings all new design problems into the equation. Leakage path formation starts to be a big problem in DC systems at any voltage, and at the supervoltages of modern long haul lines, insulators start to cost as much as a car.
even today, most analogue video and audio signals are AC coupled to prevent electrolytic salt formation at connection points.
as far as the renewable collection/storage market, what i hear on the street is that 24 volts is the best convergence of efficiency and leakage/corrosion
and, please, negative ground. you don't want yer car short circuiting on the windmill.
i lived near a DC power station in long island that made DC out of 6 - phase AC.
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