Open Delta

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Re: Open Delta

If the neutral only carries unbalanced currents, it does not have to be counted as a current carrying conductor. If it feeds loads that put harmonics in the neutral, such as computers, electronic ballasted lighting, etc., it may need to be counted.
Jim T
 
Re: Open Delta

If the neutral only carries unbalanced currents, it doesn't have to be counted. If it serves loads that impart harmonics on the neutral, such as computers or electronic ballasts in lighting, it may need to be counted.
Jim T
 
Re: Open Delta

About the Harmonics, computer loads etc. ???
The only place that I can find concerning harmonics and the need to count the neutral is Article 310-15,b,4,c. This Article references a 4-Wire/3-Phase WYE Connected system.
We have an Open-Delta.
Therefore, only Article 310-15,b,4,a applies and the neutral is not counted.
Do you see differently?
 
Re: Open Delta

Any harmonics would not be an issue in a center tapped (grounded) winding that you would have in a delta, be it opened or closed.

Roger

[ February 28, 2005, 12:53 PM: Message edited by: roger ]
 
Re: Open Delta

Do you know of any information out-there that talks about why Harmonics are a factor in a WYE connected system and not in a center-tapped (grounded) winding. I would just like to know more about this.
 
Re: Open Delta

Dave, actually my post was a little misleading, harmonics can cause trouble in a Delta set of windings but the neutral heating is not as much of a concern as it would be in a Wye configuration.

Maybe Ed or someone here might have a good easy to understand explanation. I know Ed has posted a diagram in the past that shows how odd number harmonics will add in the neutral of a Wye.

This is basically due the the 120 degree offset of the phases (difference in time)in the system. In a center tapped single winding everything is happening at the same time only on the positive and negative sides of zero.

This is probably enough to really confuse you due to a very poor explanation, hopefully some else will come forward. :(

Roger
 
Re: Open Delta

When a system serves loads that create harmonics, the harmonic waveform combines with the fundamental (60hz) to create a distorted waveform.

Harmonics3.gif


In this sketch, (below) the waveforms of the three phases, and the 3rd harmonic content of each, are purposely shown separately (un-combined) to illustrate that the 3rd harmonic content of the three phases are actually in phase with each other and therefore add, instead of cancelling, and must be carried by the neutral conductor.

Harmonics4.gif


Ed
 
Re: Open Delta

Doug, for what little I offered you are welcome, and I apologize for the name slip. Doh! :eek:

Roger
 
Re: Open Delta

Is the primary difference between open/closed delta not a current issue. Closed delta will produce 57% more ampacity than open delta. When three x-formers are reduced to two, the voltage
stays the same, the ampacity is compromised. Please explain if I'm wrong or patially correct.
 
Re: Open Delta

Not exactly. The open delta will deliver 57.7% of the power that the closed delta will.

The full load current capacity of the individual transformer winding is the limiting factor.

Ed

Trans13.gif
 
Re: Open Delta

Thanks Ed for the clarification. My memory is not
what it used to be, but my original point is valid. The unbalanced/neutral load is not an issue here. The system must have a ground reference before ground is considered fault.
3-wire closed delta with 1 phase finding ground,
becomes delta corner grounded.Now there is a reference or potential to ground. OCP now has a chance.If I'm wrong, I'm listening!!
 
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