Replacing Delta/Wye with Autotransformer

[nuckythompson]

Hi there, I am wondering if it is OK to replace a delta wye transformer (600-480V) with a wye autotransformer (600-480V).

With no neutral available on the primary side, should I ground this point? Likewise, should I connect the available neutral on the secondary side? Explanation is much appreciated!

Thanks!

 

[winnie]

I am not sure if a wye autotransformer will work properly without a supply neutral, unless you have something like a delta winding (not connected to anything, just magnetically coupled to the wye) to maintain the neutral balance. A stand-alone wye doesn't create a low impedance neutral and can't be used to derive the neutral. I think that a wye autotransformer with a floating neutral might work for powering balanced three phase loads, but have not analyzed this in depth.

You would not want to ground the wye point. If the 600V supply is ungrounded, then you are unintentionally grounding the system. If the 600V supply is grounded then you would expect current flow from the neutral point on the autotransformer to the ground terminal on the source.

If you need a 480V grounded wye system without a neutral from the 600V source, then you need an isolating transformer such as a delta-wye.

-Jon

 

[synchro]

With a few exceptions, autotransformers require a neutral on both the line and load sides:

215.11 Circuits Derived From Autotransformers

Feeders shall not be derived from autotransformers unless the system supplied has a grounded conductor that is electrically connected to a grounded conductor of the system supplying the autotransformer.
...
Exception No. 2: In industrial occupancies, where conditions of maintenance and supervision ensure that only qualified persons service the installation, autotransformers shall be permitted to supply nominal 600-volt loads from nominal 480-volt systems, and 480-volt loads from nominal 600-volt systems, without the connection to a similar grounded conductor.

In your case, Exception No. 2 might possibly apply. However, for the reasons winnie mentioned, an autotransformer may be a bad idea without a neutral. If the 600V system is ungrounded but the 480V system was designed to be grounded, having an autotransformer could be a nonstarter for numerous reasons. These include its impact on fault protection designed for low impedance grounding, L-G voltage tolerance of loads, a lack of isolation with autotransformers, etc. If you don't know all the details about the existing system and why a delta-wye was used, I would not replace it with an autotransformer.