Ungrounded Distribution System

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synergy08

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Hi All,

This is my first time posting here, and I would like to apologize in advance if I am asking a stupid question here. I am a co-op student with a power company, but eager to learn as much as possible regarding power engineering.
This week an engineer was telling me about a project to add a neutral wire to an ungrounded wye distribution circuit on the system. He stated that special transformers have to be ordered because this is the only spot on the system that is ungrounded, and that the voltage to the customers served from these transformers is unacceptable. He mentioned that the transformers cannot be connected properly due to the absence of a neutral wire. After speaking to him and doing some research, I am not sure that I understand; it isn't completely uncommon for systems to be ungrounded, is it?
Also, he stated that this could be a safety issue since there is no return path for fault currents. Does anyone have any experience with ungrounded distribution systems? Does what the engineer told me make sense? Thanks in advance for the comments!
 
Zig-Zag transformers are typically used with a delta secondary in order to establish a wye point for grounding.
 
Ungrounded system means none of the transformers (A, B, or C phase) are connected to case frame and grounded to earth. Because of this there is no reference to equipment frames, conductive items and earth, there?s no potential.

Ungrounded systems are used mainly in industrial and for critical equipment. This equipment can actually have a short to frame and not open the circuit just "indicate" or "annunciate" the fault giving the electrician time to locate and repair the fault while in operation.

Ungrounded systems will allow one short but if two short happen the overcurrent protection will open because this is actually a phase short between two phases.
 
Thanks all for the replies!
In my research, I have learned that the neutral is not designed to carry fault current in the event of a fault; this is what grounding is for. Now I'm scratching my head even more :-?
 
In premises wiring (in past the utility service) it?s all the more important to bond and equipment ground as per 250 parts V & VI, even with these done the system is ?ungrounded? without any of the phase transformers bonded to case frame and earth. This enables fault path to source so in the event of the second short the overcurrent protection will open.
 
synergy08 said:
Thanks all for the replies!
In my research, I have learned that the neutral is not designed to carry fault current in the event of a fault; this is what grounding is for. Now I'm scratching my head even more :-?
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Stop scratching, or it'll spread. :smile: You're correct that fault current is supposed to travel on grounding conductors, but that's only within the premises, or to be more specific, on the load side of the main disconnect.

The grounding conductors begin at the main disconnect, where they connect to the supply system neutral. On the line side of the main, the neutral is depended upon to complete the pathway to the utility transformers.
 
Either the engineer you are talking to is not a power engineer, he is calling himself an engineer and is not an engineer, or you do not completely understand the situation. Ungrounded systems are fairly common but some electric utilities do not offer ungrounded systems, IPL is one of those, but we do have some on our system.

Adding the MGN to an ungrounded service does nothing except providing a connection between two different grounding systems. It provides no safety or a return path for fault currents and, if not connected to the building grounding system, can set up a shock hazard!

An ungrounded system has nothing to do with the system voltage except that there is no reference to the MGN or the building grounding system. There is nothing in the delta configuration that requires a ground reference unless it is a 120/240 volt 3? system. If the service is a wye configuration, the mid-point must be connected to the MGN and the customer must take the grounded conductor.

Please verify that the information is valid. :smile:
 
Thanks again everyone for the replies.

I've been thinking more about ungrounded systems - are they safe? How do you maintain safety on an ungrounded system? I still don't think I understand how an ungrounded system works.
 
synergy08 said:
Thanks again everyone for the replies.

I've been thinking more about ungrounded systems - are they safe? How do you maintain safety on an ungrounded system? I still don't think I understand how an ungrounded system works.
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Less safe but more relaible, I actually have more experience on ungrounded systems than I do on grounded systems. The big advantage is relibility, you can have 1 phase ground out and still run the system, all 3 phase to phase voltages stay the same and the 3 phase lloads are unafected, this is why ungrounded is used in high value prodection lines (Used to be more common than it is today, today HRG systems are the in thing) and on naval combat vessels.
 
synergy08 said:
  1. . . . are they safe?
  2. How do you maintain safety on an ungrounded system?
  3. I still don't think I understand how an ungrounded system works.
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  1. Yes, to a point. The ungrounded phases can ride on impressed static since there is no ground reference.
  2. Follow the Code, use ground detectors, and use appropriate PPE.
  3. One thing to remember, you may ground the secondary of any 1? or 3? transformer one time and one time only, anywhere. The second time will give you a phase to phase fault. It does not matter if it is a phase or a neutral point. :smile:
 
charlie said:
[*]Follow the Code, use ground detectors, and use appropriate PPE.
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And when you have a ground, do something about it, most plants just ignore these devices, after you get 1 ground the other 2 phase voltages increase (In reference to ground) and stresses the insulation on the other phases, another ground and boom, you get a fault.
 
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