Question about 3 phase 4 wire distribution systems

[Kartracer087]

Hi and as a side note its my first post!

Getting to the meat of the matter, I am wondering how do they establish a neutral point on the 14.4KV (or 30 something KV) distribution lines that power homes and businesses. I have researched a lot and I noticed that most distribution substations (69kv/125KV Primary to 35kv secondary) only have three wires (A, B, C phase) connected to the utility poles. So this leaves me wondering, how do they get a neutral without going to the ground? Do they, in fact, safely go to the ground somehow? If so, I'm sort of skeptical because connecting directly to the ground would create a short circuit effect, right? I know that many single phase transformers are commonly connected to the neutral wire and a phase wire on the utility poles, so it would be helpful to understand where the neutral comes from since it seems like a delta wye doesn't exist on the distribution transformer since I only see three phase wires hooked up to the utility poles. I also know that most poles are grounded and the ground is connected to the neutral, so it gets me thinking about some kind of a ground return system. By the way, I'm in the US, so I'm not talking about Single Wire Earth Return (at least I don't think thats what we have here).

Thanks!

 

[gobblerhuntr]

We have a 7200/12470 system and the neutral starts at the substation. There is a ground mat under the station that is configured in a checkerboard manner and the system neutral taps off of that. Hope that answers your question.

 

[George Stolz]

My experience is limited to 480V or less.

You do not need a neutral on the line side of a transformer. For instance, when creating 208Y/120 from a 480V source, only the A, B, and C phases are used on the 480 side, no neutral.

On the 208V side, a lower voltage A, B, C, and N are created. The neutral for that system starts there.

Welcome to the forum.

 

[Kartracer087]

Yeah,

Then I would think the transformer secondarys would be wye connected and the neutral point (center of the Y) would then be connected to the ground mat to establish a zero reference.

 

[72.5kv]

Typical distribution station transformer are connect Delta/Wye. As mention earlier the neutral is connected to a grounding mat that is below the substation. Depending on the availiable fault current at the transformer a current limiting reactor is placed in series with the neutral to limit fault current. The neutral for each feeder is tied to the grounding mat. Typical size for the main feeder neutral is 4/0 al.

 

[steve66]

Going to ground does not create a short circuit effect. Normally, a delta to wye transformer is used to step the voltage down to 208/120 or whatever. The center of the wye is connected to ground with a ground rod (or water pipe, or something similar).

Since the secondary of the transformer is completely isolated from the primary, all the current that leaves the secondary windings must return to the secondary windings (Kirchoffs current law - what goes out, must come back or Iout = Iin). There is no way for the current to leave a secondary winding, and take a shortcut back to the powerplant or back to the HV side of the transformer.

And even if the current could flow back through some other path, since the voltages are balanced very closely, there would be very little current taking this path.
Steve

 

[wirenut1980]

Hi and as a side note its my first post!

Getting to the meat of the matter, I am wondering how do they establish a neutral point on the 14.4KV (or 30 something KV) distribution lines that power homes and businesses. I have researched a lot and I noticed that most distribution substations (69kv/125KV Primary to 35kv secondary) only have three wires (A, B, C phase) connected to the utility poles. So this leaves me wondering, how do they get a neutral without going to the ground? Do they, in fact, safely go to the ground somehow? If so, I'm sort of skeptical because connecting directly to the ground would create a short circuit effect, right? I know that many single phase transformers are commonly connected to the neutral wire and a phase wire on the utility poles, so it would be helpful to understand where the neutral comes from since it seems like a delta wye doesn't exist on the distribution transformer since I only see three phase wires hooked up to the utility poles. I also know that most poles are grounded and the ground is connected to the neutral, so it gets me thinking about some kind of a ground return system. By the way, I'm in the US, so I'm not talking about Single Wire Earth Return (at least I don't think thats what we have here).

Thanks!

Here in Indiana, we have some 34.5 kV distribution systems that used to be sub-transmission systems. These are connected in delta with no neutral, or it has an old steel static neutral that is about as good as having no neutral.