Connection in Three-Phase System Assignment Help

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Connection in Three-Phase System:

The three elements that means three phases of 3-phase source or three impedances of 3-phase load may be inter-connected in two different ways called

1. Star connection, and

2. Delta connection.

In star connection similar ends the 3-phases are joined together to compose the neutral as shown in Figure (a), star connection is mention by Y and the common or neutral point is denoted by n, it is also called star point.

1532_Connection in Three-Phase System.jpg

 (a) Star Connection    (b) Delta Connection

In delta connection three-phases are connected in series, end-to-end to compose a closed mesh as illustrated in Figure (b). Two dissimilar ends are joined together. Delta connection is mentioned by Δ.

It may be shown that for 3-phase balanced system, the sum of instantaneous values of currents or voltages is always zero. Let instantaneous 3-phase voltages of a balance system are represented by :

Va  = Vsin ωt , Vb  = Vm  sin (ω - 120o ) , Vc  = Vm  sin (ω - 240o )

then, at any instant,

va  + vb  + vc  = Vm  sin ωt - Vm  sin (ωt - 120o ) + Vm  sin (ωt - 240o )

= Vm [sin ωt + sin (ωt - 120o) + sin (ωt - 240o)]

= Vm [sin ωt + 2 sin (ωt - 180o) cos 60o ]

= Vm [sin ωt - 2 sin ωt cos 60o]

= 0

Likewise, for 3-phase currents of a balance system, we can show that

ia + ib  + ic = 0

It is the important property of 3-phase balanced system which has made possible the star and delta connection of three phases, without any circulating currents.

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