winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
In thread http://forums.mikeholt.com/showthread.php?t=87981 I hijacked a useful thread with a theory discussion. The original poster was looking to run some lighting with a 480V three wire feed, and had asked about deriving the neutral.
Vern responded that you can't pull a neutral out of thin air, and I threw in a comment about 'zig-zag' transformers. Rather than hiding answers which are actually useful to the original poster, I figured that I should direct any further discussion of this approach to a different thread.
In doing more reading, I've learned that there are _two_ different things called zig-zag transformers. One type is a regular transformer with separate primary and secondary, but where each secondary phase is made from a pair of coils driven by two primary phases. This phase shifts the output, and is useful for some harmonic mitigation situations.
The other type of 'zig-zag' transformers is essentially an autotransformer; there are no separate primary and secondary circuits. Each phase has two coils, electrically in series. The two coils are placed on different legs of the core. See http://www.electrical-contractor.ne...flat/Forum/15/topic/000072/Number/0/site_id/1
This second type of transformer is used to derive a neutral in a delta system. The most common use is to derive a neutral that is used to ground an otherwise ungrounded delta, generally with a rather _undersized_ zig-zag transformer and a method of high resistance grounding. Such small 'grounding transformer' is not sized to the full load of the supply delta; it is just large enough to provide the neutral and to tolerate fault current when it occurs; it is not sized to carry load on a regular basis.
There are other types of grounding transformers; I don't understand the reason for selecting one or another type.
In the situation that I was describing, a zig-zag autotransformer is sized to carry neutral loads in a system that uses a neutral. Because this transformer creates a neutral, and the neutral is being used as a current carrying conductor, the system is required to be grounded. Since this is an autotransformer, with no isolation between primary and secondary, you can't simply bond the secondary and ignore the primary. You have a single system (not a second separately derived system) and you have to figure out where to ground it.
This is just me musing on how such a transformer would be used. I see three scenarios:
1) Ungrounded delta. Zig-zig added to the transformer to create a grounded system. Treat like a transformer bank which provides a wye secondary.
2) Ungrounded delta, zig-zag somewhere down a feeder. Can't make this work legally. You have created a system that needs to be grounded, but don't have a good location for the bond.
3) Grounded supply, zig-zag somewhere down a feeder. Now you have a system which needs to be grounded, and which is grounded, but you also have a derived neutral which cannot be connected to ground, and is thus an ungrounded conductor even though it is at 0V.
-Jon
Vern responded that you can't pull a neutral out of thin air, and I threw in a comment about 'zig-zag' transformers. Rather than hiding answers which are actually useful to the original poster, I figured that I should direct any further discussion of this approach to a different thread.
In doing more reading, I've learned that there are _two_ different things called zig-zag transformers. One type is a regular transformer with separate primary and secondary, but where each secondary phase is made from a pair of coils driven by two primary phases. This phase shifts the output, and is useful for some harmonic mitigation situations.
The other type of 'zig-zag' transformers is essentially an autotransformer; there are no separate primary and secondary circuits. Each phase has two coils, electrically in series. The two coils are placed on different legs of the core. See http://www.electrical-contractor.ne...flat/Forum/15/topic/000072/Number/0/site_id/1
This second type of transformer is used to derive a neutral in a delta system. The most common use is to derive a neutral that is used to ground an otherwise ungrounded delta, generally with a rather _undersized_ zig-zag transformer and a method of high resistance grounding. Such small 'grounding transformer' is not sized to the full load of the supply delta; it is just large enough to provide the neutral and to tolerate fault current when it occurs; it is not sized to carry load on a regular basis.
There are other types of grounding transformers; I don't understand the reason for selecting one or another type.
Cody K said:
In the situation that I was describing, a zig-zag autotransformer is sized to carry neutral loads in a system that uses a neutral. Because this transformer creates a neutral, and the neutral is being used as a current carrying conductor, the system is required to be grounded. Since this is an autotransformer, with no isolation between primary and secondary, you can't simply bond the secondary and ignore the primary. You have a single system (not a second separately derived system) and you have to figure out where to ground it.
This is just me musing on how such a transformer would be used. I see three scenarios:
1) Ungrounded delta. Zig-zig added to the transformer to create a grounded system. Treat like a transformer bank which provides a wye secondary.
2) Ungrounded delta, zig-zag somewhere down a feeder. Can't make this work legally. You have created a system that needs to be grounded, but don't have a good location for the bond.
3) Grounded supply, zig-zag somewhere down a feeder. Now you have a system which needs to be grounded, and which is grounded, but you also have a derived neutral which cannot be connected to ground, and is thus an ungrounded conductor even though it is at 0V.
-Jon
