Ungrounded Electrical Service

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Shoe

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Currently engineering an ungrounded secondary 480V electrical system to serve a Navy shipyard. Medium voltage on the primary-side is customer-owned. Because ungrounded systems are required to serve shorepower to ships, is there a best option for transformer configuration to serve the ungrounded system (i.e. wye-delta, delta-delta, etc.) from the primary distribution? I know that many of these will "work", but curious to hear your thoughts as to what is the "ideal" in this scenario.
 
I would have thought that the Navy would have detailed specs on this. My thought would be delta/delta but maybe others more experienced with this will have better feedback. Of course you will need ground detection on the 480 side. And is the transformer customer owned and fed by a feeder owned by the customer?
 
Correct - transformer and primary is both customer-owned. Navy's only specification is that it is an ungrounded services. (Yes, we still need the equipment ground but no neutral) Ground fault indication is a must at the secondary gear (per code).
 
I could be forgetting something obvious that would eliminate this answer, but it may be best to just go with the typical delta-wye but leave the X0 floating. That type of transformer might be the most readily available and cheapest due to popularity.
 
Moderator Charlie B was an electrical guy on the Nimitz and he may know, although I suspect the Nimitz class may use a higher shore power voltage.
 
Ideally you want a delta-wye for a lot of good reasons but since you already have an established ground on the primary side you are stuck with a delta primary and since you need an ungrounded system then it’s delta-delta.

Be aware that the biggest problem with ungrounded systems is “unlimited” transients. Insulation coordination is critical.
 
paulengr said:
Ideally you want a delta-wye for a lot of good reasons but since you already have an established ground on the primary side you are stuck with a delta primary and since you need an ungrounded system then it’s delta-delta.
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Paul,. Can you elaborate,. In particular why does pretty much every POCO disagree that delta-wye is the way to go?

Also, is there any reason a wye secondary couldn't be used? You could float the XO.
 
Keep in mind that the utility is not providing the transformer, so the a grounded-wye primary is not a must, but I would be curious to know why utilities are in favor of it. It's a fine solution if the customer is looking for a grounded-wye on the secondary, but I don't think a grounded-wye primary and ungrounded-wye secondary is a good solution here.
 
Shoe said:
Keep in mind that the utility is not providing the transformer, so the a grounded-wye primary is not a must, but I would be curious to know why utilities are in favor of it. It's a fine solution if the customer is looking for a grounded-wye on the secondary, but I don't think a grounded-wye primary and ungrounded-wye secondary is a good solution here.
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I believe it is due to ferroresonance.
 
My understanding is that utilities like grounded wye primary systems as part of their whole 'multi-earth-neutral' system to make sure that the neutral conductor remains close to local earth potential, and make sure that the phase conductors remain at their appropriate voltage relative to the neutral. This whole setup means that the earth becomes a parallel conductor for the neutral current. Such a setup is prohibited for installs governed by the NEC; the utility _desired_ earth connection causes what the NEC calls 'objectionable current'.

-Jon
 
So I'm at the point where I'm trying to decide between a delta-delta ungrounded or a delta-wye (XO float) ungrounded. It's kind of where I've been sitting when I started this thread, so just curious about pros/cons for each. Delta-wye may be a bit more common and have the option to add a grounded system in the future if need-be... just wasn't sure if there are drawbacks to this.
 
winnie said:
Such a setup is prohibited for installs governed by the NEC; the utility _desired_ earth connection causes what the NEC calls 'objectionable current'.
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Not true if over 1000V, which is also where the utility is doing it. 250.184(C)
 
What's the advantage to having a wye secondary if the X-0 will be floated? D-D seems much more efficient.
 
Jraef said:
If the ships use 277V for lighting, you need the Wye secondary. If not, I would use Delta.
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277/480 can not be used ungrounded per NEC. 277/480 that doesn't supply any neutral loads possibly could leave the wye point floating, and utilize as three wire supply system. Think that may be done more with standby sources on something that is delta secondary from the normal source though. It could be impedance grounded, but then you couldn't supply 277 volt loads with it either they would have a resistor in the neutral before the source.
 
No 277V systems on ships. They will use a step-down transformer in the ships distribution system to provide single-phase loads.
 
Apparently floating-wye/delta transformers have been commonly used to supply ungrounded systems according to the following article. The center of the wye primary must of course be floating to avoid having circulating currents in the delta secondary when there's any imbalance between the line input voltages.

pacbasics.org

A Study On Intermittent Ground Faults On Floating Wye Delta Connected Transformer Banks – PAC Basics

A MATLab simulation study on high transient peak voltages resulting from intermittent ground faults in floating wye – delta transformer banks.
pacbasics.org pacbasics.org

The article below mentions :
"In actual practice totally ungrounded systems do not exist. As soon as a fault detector is applied using one or three voltage transformers, the system is grounded through the high impedance of these devices. The resistance of the relays and associated ballast resistors help in limiting the transient overvoltages, so that very few cases of overvoltage essentially exist."

electrical-engineering-portal.com

Ground faults in ungrounded systems (risks & detection) | EEP

Ungrounded systems are power systems with no intentionally applied grounding. However, they are grounded by the natural capacitance of the system to ground.
electrical-engineering-portal.com electrical-engineering-portal.com
 
Thanks everyone for your input. I also reached out to a company that provides portable substations for these shorepower applications. They typically provide delta-delta configuration on their substation transformers.
 
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