Why do utilities like wye-wye transformers

[rockman7892]

I overheard a conversation yesterday with someone trying to explain why utilities like or prefer wye-wye distribution transformers for supplying power to a facility. His explanation was basically saying that with a wye-wye transformer they didn't need a ground grid on the primary, and also explained that they tied the priamry and secondary neutrals together on these transformers.

Is there any truth to this or can someone please explain or give a reference why the utilities would prefer a wye-wye transformer for grounding purposes?

I understand that with a wye primary the utility can pass zero sequence current. Also from what I have read, wye-wye transformers are traditionally cheaper and also may lead to ferroresonace issues.

 

[rbulsara]

Primarily cost, I would think.

Rafiq Bulsara

http://www.srengineersct.com

 

[jghrist]

Grd wye - grd wye transformers are much less susceptible to ferroresonance that delta-wye tranformers. US utilities often have single phase overcurrent protection and switching that can result in ferroresonance of delta-wye transformers connected by underground cables.

 

[waross]

Single phase transformers connected line to neutral need only one lightning arrestor and one fused disconnect. Remote single phase distribution lines may use small cheap insulators or no insulators or even a ground return for the neutral. My rural home uses a ground return on the primary. Typically the primary neutral will be run for one span and grounded at each pole and that's it.
So, the great majority of their single transformers are rated for line to neutral voltage. For three phase banks, use what is in stock.

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

 

[wcarter]

Cheaper. US supply is in a WYE configuration, and load can be a delta or wye. Utilities are required to keep at least one of that type transformer on hand in case of failure. Do, it's cheaper to have wye-wye's rather than wye-delta AND wye-wye. One instead of two.

As for grounding the primaries, there is a ground at every pole connected to the transmission neutral line, though I still tend to see one in the transformer. Though in the cases where I have seen, the neutrals were not tied together.

 

[m3ntosan]

@jghrist

it works at distribution level to suppress ferroresonance by using wye-wye, but not at the transmission level

May you grow up to be righteous, may you grow up to be true...

 

[waross]

Grounding and multiple grounding is a safety issue.
Connecting the neutral to the primary system neutral is required to stabilize the secondary voltages on most wye:wye transformers.
An example of the difference between grounding and connecting the neutral to the source neutral.
In many distribution systems the neutral is carried with the phase wires and is multiple grounded. A connection to the neutral conductor simultaneously grounds the transformer and connects it to the system neutral.
Now consider a hypothetical delta distribution system. A wye:wye transformer bank may have the primary neutral grounded but that will not necessarily stabilize the voltages on the secondary.
Note: the voltages may also be stabilized by a delta tertiary or to some extent by a three legged core. (phantom delta)


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

 

[sonic02]

Normally for a Y-Y connected system, the high-voltage neutral is connected internally to the low-voltage neutral, with the provisions of testing. At times, the high-voltage neutral is grounded externally.