Some systems are required to be ungrounded such as some naval vessels and other industrial applications. I'm trying to understand the pros and cons of an ungrounded delta/delta vs an ungrounded delta/wye service transformer. To me, I can't see any real difference between the two form a functionality perspective. Does anyone have any special insight or a white paper that discusses this topic? Does one solution offer better stability/safety over the other?
Ungrounded Delta/Wye vs. Ungrounded Delta/Delta
- Thread starter strap89
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- Wisconsin
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- PE (Retired) - Power Systems
For all practical purposes there is no advantage to an ungrounded service for a general industrial installation, particularly with the way most power electronic equipment,like variable frequency drives, is designed nowadays.
The concept of being able to continue to operate during a 'first' fault to ground condition is usually overrated, as the second fault to ground can be just as catastrophic. The NEC allows ungrounded systems only where trained people are on site and can initiate appropriate troubleshooting and mitigation.
The concept of being able to continue to operate during a 'first' fault to ground condition is usually overrated, as the second fault to ground can be just as catastrophic. The NEC allows ungrounded systems only where trained people are on site and can initiate appropriate troubleshooting and mitigation.
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- Wisconsin
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- PE (Retired) - Power Systems
Naval ships are not subject to the NEC.
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- Bremerton, Washington
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- Master Electrician
Naval ships have ungrounded systems for continuity of power, have ground fault monitoring and electricians.
winnie
Senior Member
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- Springfield, MA, USA
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Ungrounded systems can be subject to extremely high voltages during re-striking fault conditions. Basically the inductance of the faulted circuit combined with the system capacitance to ground and the opening/closing fault can act like boost converter and generate voltages to ground that are higher than the line-line voltage.
A different type of system which can provide continuity of power with fewer issues than an ungrounded system is a 'high resistance grounded'.
These are wye systems with the neutral grounded through a resistance.
A wye system can also easily be solidly grounded if desired.
So if I had the choice between a delta-delta or a delta-wye for an ungrounded system, I'd go with the delta-wye to provide future options.
-Jon
A different type of system which can provide continuity of power with fewer issues than an ungrounded system is a 'high resistance grounded'.
These are wye systems with the neutral grounded through a resistance.
A wye system can also easily be solidly grounded if desired.
So if I had the choice between a delta-delta or a delta-wye for an ungrounded system, I'd go with the delta-wye to provide future options.
-Jon
Here's another question. I feel like I had a grasp back in school, but not today. How does the unbalanced load behave in ungrounded deltas and wye secondary?
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- Wisconsin
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- PE (Retired) - Power Systems
It depends on the severity of the unbalanced. Hypothetical questions often get hypothetical answers.
It is not worth describing unbalanced L-N loads as they cannot be supported by delta configurations. Grounded and ungrounded systems will have similar behaviour.
In general, unbalanced loads connected L-L will have similar effects on the line currents and phase voltages regardless of the transformer configuration. However there will be an affect on the individual transformer windings.
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