Padmounted Floating Wye-Delta

[jwaugh2014]

Referring to thread238-431498. Can anyone tell me if their utility has a method of temporarily grounding a 3-phase wye-delta pad-mount transformer other than using an H0 bushing well? Also, I have heard of utilities only offering the 208Y/120 or 480Y/277 secondary variety from their pad-mounted transformers. Has anyone else adopted this practice? I ask because there is an existing transclosure with 3 overhead pots in this configuration and ordering a 3-phase pad-mounted transformer with an H0 bushing well is an additional cost.

 

[stevenal]

I'm not sure how you can switch the neutral without access. I suppose you could specify an internal contact with an exterior handle, but I'm sure that would be more expensive than an added well. Until recently we provided only wye or open delta secondaries from padmounts. We'll be ordering some wye-delta transformers soon for exactly the purpose you speak of for existing services.

 

[jwaugh2014]

What about if the tx has an H0 bushing and a full sized neutral was raised to an overhead switch (ie the riser pole). Is there anything wrong with switching the neutral at the riser during energization?

 

[waross]

Old school, pole mounted transformers.
There were four cutouts on the cross-arm.
When a bank was taken out of service locally and was to be re-energized, a fuse would be installed in the neutral cutout.
With the neutral connected, the three phase line fuses would be installed.
Then the fuse would be removed from the neutral cutout and taken away in the line truck.

Steve, are you floating the primary wye points?

We had a customer In Canada with a 480 Volt service.
They were a branch of an Ontario company where 600 Volts was the norm.
We got a lot of 600 Volt equipment to install.
We had small open delta auto-transformers installed beside the 575(600) Volt motors.
Then they moved to a new plant.
600 volt service was now the utility standard.
The power was incoming to a power distribution center at 600 Volts. On the end of the PDC was a section with a pair of open delta auto-transformers stepping down to 480 Volts.
Have you considered suggesting to your customers a 120/208 Volt wye system with a pair of auto-transformers feeding a 240 Volt three phase equipment panel?
Then you don't have to worry about the issues with a lost phase on a wye/delta bank.


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

 

[jghrist]

We have client who is replacing three single phase OH transformers connected wye - delta in transclosures with two padmount transformers connected open wye - delta. Most customer load is single phase 120/240 V, with a small amount of three phase 240 V. One transformer is 167 kVA and one is 25 kVA. This maintains the existing customer services, uses standard padmount transformers, and avoids having to switch neutral grounds.

 

[stevenal]

Bill,

Four bushing wells on the primary. Neutral/ground elbow will be parked during normal operation allowing the primary wye point to float, but can be connected during single phase switching similar to your four cutout system.

jwaugh2014,
I'm not sure what advantage the overhead switch would provide. A 200 A load break elbow is all that's needed.

 

[waross]

Thank you Steve.

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

 

[jwaugh2014]

Thank you, Steve!

 

[cuky2000]

Reading an IEEE article on "Ferroresonance in Low-Loss Distribution Transformers" by R. A. Walling
I'm having a hard time to reconcile the above statement with the discussed practice of their utility of temporarily grounding a 3-phase wye-delta pad-mount transformer.

It is understood that pole-mounted transformers are less susceptible to ferroresonance. However, this is not the case for pad-mounted units.

It should be mentioned that the main reason for the utility industry shifted from the delta – grounded wye to the grounded wye – grounded wye was to reduce the random event of ferroresonance. More recent research indicates that even wye-wye connections in pad-mounted transformer are prone to ferroresonance.

Not sure the benefit why a utility can be self exposed to take a risk on this type of operating procedure. It is suggested in this forum to take a fresh look at this issue and advise if there is a better and safer recommended practice to de-energize pad-mounted distribution transformers.