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A few reasons, this should enlighten some.
http://www.electricaltechnology.org/2014/09/comparison-between-star-and-delta-connections.html
M.
http://www.electricaltechnology.org/2014/09/comparison-between-star-and-delta-connections.html
M.
hevans1944
Hop - AC8NS
Where did you get this "information"?
Here in the USA three-phase power is transmitted to sub-stations with just three high-tension wires: a delta connection. At the sub-station it is transformed to a lower voltage for distribution as a three-phase "Y" or wye. At the end-use point, one or two of the three phases may connect to a single transformer for single-phase distribution.
The secondary of a single-phase distribution transformer has one terminal connected to earth ground and to the "neutral" wire feeding the load. The other one or two (if center-tapped secondary is used) secondary terminals provide line power to the load. The line voltages are single phase with respect to the neutral wire, and for a center-tapped secondary, the line voltages are 180 degrees out of phase with respect to neutral and with respect to each other. This means the line-to-line voltage is twice the line-to-neutral voltage for a center-tapped transformer. This is typical of home power distribution in the United States.
If power demands are high, three transformers (or a single transformer with three primary windings) are used to transform the three-phase distribution to a suitably lower voltage three-phase distribution. The distribution on the secondary windings can be either delta or wye, depending on customer requirements. If a three-phase distribution has a balanced load, there is no current in the "neutral" wire of the three-phase wye distribution. However, there is no way to guarantee that the load will be balanced, so the neutral wire must be sized as if it were to carry the full current of any of the phase wires.
With respect to power distribution, it is the interconnection of two or more power sources to multiple loads attached to the grid.
Even if the 3-phase load is severely unbalanced, there should be very little current in the neutral. The purpose for the neutral is to provide a connection to earth, as well as lightning protection. On a 3-phase cross-country tower, the neutral(s) is (are) placed at the top, shielding the phase wires from a lightning stroke.
One of the reasons that each transformer utilizes at least 1 winding connected in delta, is so that unbalanced loads can be supported without a 4th wire. Take a 3-phase transformer whose primary is wye with a secondary in delta. If the secondary load is severely unbalanced, one phase at 100% load current, the other 2 phases at 0% load (open), the 3 phase voltages will remain very well balanced within a few percent, with only 3 wires. The neutral is not needed.
The neutral is, however, connected to earth for safety, as well as carried on the tops of the towers for lightning protection. The neutral plays an important role, but not for carrying load current. Balanced or severely unbalanced,
a delta winding assures that unbalanced loads can be supported without current in the neutral. In fact, if both primary & secondary are very high voltage windings, sometimes they are both connected in wye. A 3rd winding, known as a "tertiary", is used to maintain balance. The tertiary is delta connected, & is not connected to the external terminals, as it drives no load. It merely provides the needed ampere-turns to maintain balanced voltages on the 3 phases even when the 3 phase currents are severely unbalanced. The amps needed to balance out the unbalanced load amps circulate in the closed loop of the delta.
I just thought I should point that out. Thanks, BR.
Claude
hevans1944
Hop - AC8NS
Gee, I must have missed something... are you saying that if there is an unbalanced load on a wye distribution there is no current in the neutral?
It depends. There are several possibilities.
1) Primary in wye, secondary in delta: unbalanced 3 phase load on delta secondary, - negligible neutral current. The primary can carry 3 or 4 wires, & stay balanced. Neutral not needed even if load is severely unbalanced.
2) Primary in wye, secondary in wye, tertiary in delta: unbalanced 3 phase load on wye secondary, - negligible current in primary neutral. The neutral wire can be opened & the 3 phase voltages stay balanced even with severe load current unbalance.
3) Primary in wye, secondary in wye, 3 single phase units ganged for 3 phase operation or a single 3 phase shell type construction (core), unbalanced 3 phase load on wye secondary, - neutral current is substantial, & neutral wire is necessary for the 3 phase voltages to remain balanced when currents are unbalanced. This is the only configuration where a 4th wire, i.e. a neutral, in the wye primary is mandatory for phase balance.
4) Primary in wye, secondary in wye, single 3 phase core type construction (3-legged E core with windings on each leg), unbalanced 3 phase load on wye secondary, - negligible neutral current. The 3 legged E core requires no primary neutral to stay balanced. A Y-Y configuration without a tertiary or any delta whatsoever, stays balanced without a primary neutral even when secondary load current is severely unbalanced.
The only "problem" configuration which presents balance issues, is the Y-Y, no tertiary, built with 3 single transformer units interconnected for 3 phase operation, as well as a 3 phase shell type unit. To maintain balance, the neutral must carry significant current.
With a 3 phase "core type" transformer, a Y-Y without a tertiary can support unbalanced secondary load currents without a primary neutral.
With any type of construction, a delta secondary, or a Y-Y with delta tertiary assures balanced 3 phase voltages even with very unbalanced secondary load currents without the need for a primary neutral.
Hopefully I have explained this clearly. I can elaborate if needed. BR.
Claude
hevans1944
Hop - AC8NS
Thanks, Claude! Now I gotta go back and study what I must have missed in my power class. About the only thing I remember from that course was the concept of a constant amplitude rotating magnetic field created by three-phase excitation.
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