Wye would that be?

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Open Neutral

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I was at the Orlando Fairgrounds recently. There's some newish rule that the carnival rides visiting periodically must run off POCO, not Diesel, power. Hence, there are a lot of poles with 3 unlittle pigs on each. At their base is a locked panel that the rides will be hardwired to; as well as duplex outlets.

They won't run out of capacity; there's a major substation adjacent to the property.

  1. Why would two of the pigs be bigger than others? I saw that twice. I'm aware of open neutral configurations but this isn't.
  2. As far as I could tell, the transformers were wired wye-wye. That makes little sense to me, who was taught that you need delta somewhere to keep the 3rd harmonics in check. I don't know if the pictures are good enough to see same, but I felt sure I saw a bare jumper on at least 2 poles, joining 3 primary terminals.

Comments?
 
Open Neutral said:
  1. Why would two of the pigs be bigger than others? I saw that twice. I'm aware of open neutral configurations but this isn't.
  2. As far as I could tell, the transformers were wired wye-wye. That makes little sense to me, who was taught that you need delta somewhere to keep the 3rd harmonics in check. I don't know if the pictures are good enough to see same, but I felt sure I saw a bare jumper on at least 2 poles, joining 3 primary terminals.
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1. They use what they have lying around. Although different size units do usually indicate a delta connected secondary.
2. Wye-wye connections are acceptable for utilities, especially when individual transformer coils are involved. Customer owned transformers should almost never by connected Yg-Yg.
 
Open Neutral said:
Why would two of the pigs be bigger than others?
Click to expand...
One larger kVA and two smaller kVA units usually means a delta secondary where the larger unit is connected to serve single-phase loads as well as its share of the three-phase loads.

Two physically bigger units probably is just a mix of different styles or ages. The two bigger units may have the same kVA rating as the smaller unit as the sizes can vary. Older units were typically much bigger.

Open Neutral said:
As far as I could tell, the transformers were wired wye-wye. That makes little sense to me, who was taught that you need delta somewhere to keep the 3rd harmonics in check.
Click to expand...
Personally, I like delta and the zero sequence blocking and the ability to have 120/240 instead of 120/208 for single-phase loads.
 
Open Neutral said:
I was at the Orlando Fairgrounds recently. There's some newish rule that the carnival rides visiting periodically must run off POCO, not Diesel, power. Hence, there are a lot of poles with 3 unlittle pigs on each. At their base is a locked panel that the rides will be hardwired to; as well as duplex outlets.

They won't run out of capacity; there's a major substation adjacent to the property.

  1. Why would two of the pigs be bigger than others? I saw that twice. I'm aware of open neutral configurations but this isn't.
  2. As far as I could tell, the transformers were wired wye-wye. That makes little sense to me, who was taught that you need delta somewhere to keep the 3rd harmonics in check. I don't know if the pictures are good enough to see same, but I felt sure I saw a bare jumper on at least 2 poles, joining 3 primary terminals.

Comments?
Click to expand...

1. Open delta or even a closed delta with a large amounts of single phase, little 3 phase often ends up with a large main pot and 1 or 2 smaller pots. Also the can size can vary from manufacturer to manufacturer compared to the same kva, or features such as dual voltage primary taps can increase the size. The size difference could be from age, an older can was changed to a newer one that happens to be smaller or larger.

2. In the US and Canada wye primaries are almost exclusive. Not the best connection but it will work ok and do so on billions of poles. One less primary bushing can be used and ferroresonance is a none issue.
 
One reason to use WYE on the primary of a HV transformer is that it allows the designer to cost save by grading the insulation.

Consider a winding which is intended to be connected as part of a delta .... all points of that winding will be at the same potential with respect to ground.... so the insulation of the entire winding will be under the same stress with respect to ground.

Now consider a winding intended to be used as part of a WYE bank. The voltage with respect to ground will diminish as it gets closer to the star point. It therefore follows that you can have a lighter insulation as the winding approaches the star point since it will not be under the same stress. This can lead to a saving in size & cost.

The problems of having a WYE/WYE connected transformer can be mitigated by having a delta connected tertiary winding. This gives the advantages of both DELTA/wye & WYE/wye condfigurations.

Adrian
 
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