Ungrounded 240V System

[mls1]

We are working on an upgrade for an industrial customer with some very old portions of their system. The original distribution was installed in the 30s or 40s and was entirely 240V delta. A few years ago they changed the main service to 480V to feed some newer loads but retained the 240V system by adding a delta/delta 480 to 240V transformer. Much of the distribution is quite old and we will replace the main PDCs and some of the sub-panels but we are considering if anything should be done about the ungrounded system. They have not reported any surge voltage problems but there is that potential and the only ground detection they have is a set of passive ground lights. There are too many 240V, 3-Phase loads to replace the entire system and the transformer is pretty new. Should we consider adding a zig zag HRG to the 240V system or leave it alone as a grandfathered ungrounded system?

 

[waross]

I prefer a wye-delta HRG grounding bank. You can use off-the-shelf transformers and the delta voltage is unimportant.
Does anyone know any shortfalls of a wye/delta grounding system?

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

 

[HamburgerHelper]

You can't operate with a single ground fault. The only ungrounded system i worked on was a pump station that had to pump. The lead time on the motors became enough that the client no longer liked chewing up motor life to keep running so the project was to change the grounding.

 

[davidbeach]

Are you sure it's truly ungrounded? I'd expect that a 240V delta would be grounded at the midpoint of one winding. If you're in NEC land it has been a very long time since that configuration would have been permitted (if I'm remembering it correctly), if it ever was. I've seen ungrounded 480V buy not ungrounded 240V.

 

[mls1]

Yes, it's definitely ungrounded. Not high leg delta or corner grounded. We are in NEC land but this system definitely predates the codes restrictions on ungrounded systems. What baffles me most is why the previous engineering group that did the last renovations kept it ungrounded. I'd be most interested in hearing if there are any concerns if we derive a neutral and high resistance ground it. We've done this in some powerhouses that had ungrounded 480V systems and it revealed ground faults but so be it. At least they were found and fixed. I like the idea of putting a wye/delta on the system instead of a zig zag but at 240 a zig zag can be made out of three single phase PTs since it is LV to LV. Pretty low cost either way.

 

[waross]

You may use three single transformers for the wye/delta scheme also. They should be rated for line to line voltage on the primary, 240 Volts. The delta voltage is unimportant.

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

 

[MagneticFlux]

Curious, what loads are on the system that would require 240V Delta? Does a grounded system pose a higher risk to personnel safety or the facility? If it {the 240 delta] was installed over 75 years ago, what remains that has not been replaced over the many years? Being able to answering this may provide a simple solution to your question.

 

[jraef]

If you have anything with power electronics connected to that system, for example VFDs, they are not going to like the delta feed.



" We are all here on earth to help others; what on earth the others are here for I don't know.
" -- W. H. Auden

 

[mls1]

There are quite a few 240V three phase and single phase (240V line-to-line not 138V line-to neutral or 120V for high leg delta) such as compressors, heat trace systems, etc. Lighting systems are derived from 240 to 208/120 banks or 240 to 240/120 single phase transformers. There are too many existing loads to eliminate completely but we've opted to replace or, where dual rated, rewire all lighting transformers to 480V primary and move to a new 480V panelboard. The rest of the system we are going to add a zig zag or wye/delta grounding transformer with high resistance grounding to 5A. We think this is the best compromise between cost and bringing the system up to code/good practice. We do appreciate the comments and suggestions.

 

[waross]

That sounds like a good compromise.
Please share your findings on cost comparisons and availability of zig-zag vs. wye/delta grounding schemes.
Thanks.

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