Four wire system 2

[pukar12345]

Hi all,

In North America I saw four wire Transmission/distribution system for upto 27.6kV system.

What is main purpose of neutral in such high voltage as all loads are three phase only in this voltage range.


Thanks in advance for reply.

 

[jghrist]

Loads are not three phase only in this voltage range in North America. The neutral also increases Ø-grd fault currents making coordination (grading) easier.

 

[magoo2]

We operate a 34.5 kV 4-wire distribution system. Residential loads are served from single phase transformers connected from phase to neutral. The 4-wire system is the more common design for distribution circuits in North America. You'll find some 3-wire distribution circuits in California and you also find 4-wire distribution circuits at the same utilities.

Transmission systems don't need neutrals because they serve three phase load or substations with delta grd-wye transformers. Some utilities have used 34.5 kV as a subtransmission feed to stepdown stations to 12.5 or 13.2 kV. The 34.5 kV feed is often supplied without a neutral.

If it's functional a transmission circuit, it won't typically have a neutral. If it is a distribution circuit, I'd guess more than 85% of them will be 4-wire.

 

[pwrtran]

27.6 and 13.8 feed to the a lot end users (110V) via single phase step down distribution transformers, therefore a neutral is required.

 

[ScottyUK]

Do economics prevent using a single phase transformer strung across two lines rather than line-neutral? A single phase application would not know the difference.


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If we learn from our mistakes I'm getting a great education!

 

[waross]

Two lightning arresters, two fused disconnect switches and higher voltage transformers required.
Or, one lightning arrester, one fused disconnect switch and a transformer voltage rating about 58% of line to line voltages.
Also the neutral conductor works well with the multiple ground points common on our distribution systems.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jghrist]

With 3-wire distribution, you need sensitive ground fault protection at the substation. In North America, taps with fuses or line reclosers are common on the primary distribution system. Isolating ground faults on taps is difficult with sensitive ground fault protection at the substations.

 

[ScottyUK]

Thanks guys... I'll stick to making the MW's and let the experts get them to the customers.


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If we learn from our mistakes I'm getting a great education!