Doubts about 3-phase transformer bank 1

[vlargueta]

Is there some criteria for 3 phase tansformer bank connections?
specifically

Can I connect three transformers of different KVA rating in delta?
Can I connect, let´s suppose 2 Transformers of Additive polarity and 1 of substractive polarity?

 

[stevenal]

It’s common to provide a higher kVA unit as “lighter” to serve single phase loads in addition to its share of three phase. I see no problem with combining additive and subtractive as long as connections are to the proper bushings.

 

[waross]

A large transformer for lighting and a smaller transformer for three phase loads is a good solution on OPEN DELTA.
On a three transformer delta secondary, all three transformers contribute to single phase loads and the unbalanced portion of unbalanced three phase loads.
With equal transformers, each of the three transformers contributes one half of the load KVA.
Not a typo. The sum of the transformer KVA loadings will be 150% of the load KVA.
If one transformer is undersized, it may be overloaded by a single phase load on another phase.
However, if the %imp of all three transformers is equal, two equal, smaller transformers may be used on the off phases.
The two smaller transformers will still support part of the single phase load, but will do so in the inverse ratio of their KVA ratings.
Wye delta connections should be avoided on distribution circuits.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[stevenal]

Like Waross, I agree the full wye-delta should be avoided. Unfortunately my predecessors did not avoid them, and existing customers are averse to change. Some of the sins of the wye-delta are avoided by floating the wye point.

 

[waross]

The full wye-delta with a floating star point also has an issue.
During energization, there may be switching over-voltage transients.
I have seen one mitigation for this.
There were four fused cutouts at the transformer bank, the forth cutout for the star connection.
When a crew was working on the bank locally, they would instal a fuse to ground the wye point.
Then they could energize, phase by phase without transient over-voltages.
When the bank was energized, the neutral fuse would be removed and taken away with the crew.
I saw one such arrangement about 12 years ago.
I still regret not taking a picture.
Another case.
In a micro grid, (about 5000 subscribers) there were several wye-delta banks.
The burn-out of customers refrigerators was frequent.
When I got involved, it took several years but I eventually had the wye-delta banks converted to wye-wye.
I had been away, but I was back in the area and the System Manager asked me to do a walk-around of the system.
Low and behold, a new wye-delta bank.
"I thought that we had agreed that that connection would never be used again."
"Yes but that customer is owned by the National Government and they are too big for us to argue with."
"How long has it been since a customer has complained about a burned out refrigerator?"
"Not for years now."
"Do you want to go back to those days?"
"No way. That was terrible."
"Well we have to do something."
Solution:
Transformer bank capacity: 150 KVA.
Three phase load: 17 KVA.
Single phase load: Less than 17 KVA.
For those who may not be familiar with fused cut-outs;
The fuse holder has a tiggle on the bottom end.
The toggle is held in position by the replaceable fuse link.
When the fuse blows it releases the toggle and when the toggle collapses the fuse holder swings down 180 degrees and is readily visible as a blown fuse.
We removed the fuse link from a fuse holded and tack welded the toggle in position.
When we installed the dummy fuse holder, it looked like a full delta but was actually an open delta.
The customer never knew.


Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[stevenal]

We generally use a personal protective ground to tie the wye point to the system neutral during energization, then remove it.

 

[waross]

That will be just as good, stevenal.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!