Why did the designer Zig and Zag

[W@ttson]

Hello, I have a scenario where I am scratching my head as to why the original designer went through all of the complexity to require a Zig Zag transformer. Maybe some one here can give me a different perspective to some of the advantages that could have been the reason to do it in such a way. The following is the system setup:

There is a concrete pier in the middle of a river. This pier has many motor loads and some lighting loads. The pier is fed from one side of the river from one utility grid and from the other side of the river from another utility grid (redundant services, only one feeding at a time, switched between the two by an ATS at the pier). Both utility services are 208/120V but are stepped up to 480VAC 3W 3Phase. 3 phase conductors are sent to the pier, no neutral, no ground through a submarine cable (from each utility). The services terminate at an ATS on the pier which is considered the pier service entrance. At the ATS there is a connection to a grounding transformer (the zig zag transformer) where a ground is established. On the pier there is a separate 480V Delta to 208/120 Transformer for lighting loads. At the zig zag transformer a ground fault relay is installed to monitor the 3 phases to the zig zag. Each of the utility services are low, at the 208/120 service level, each are only 250A (again only one service feeding at a time).


Question: Why didn't the designer just send a neutral and a ground from each utility which is on shore? Then have the neutral be switched at the ATS if there was any concern of the two utilities being connected at all?

 

[ron]

Save on a regular 3 pole ATS vs 4 pole.

Save on copper.

Neutral load is likely small.

Less headache on SDS's and bonding / grounding.

 

[electrofelon]

Sounds like there is no need for a neutral, but they still need an egc (even if the system is ungrounded which you didn't specify). Sounds wacky.

 

[W@ttson]

Save on a regular 3 pole ATS vs 4 pole.

Save on copper.

Neutral load is likely small.

Less headache on SDS's and bonding / grounding.

Wouldn't the saving on 3 pole ATS vs 4 Pole ATS being nulled by the purchase of the zig zag transformer?

The saving on copper isn't likely since the submarine cables are custom products which the manufacturer makes to order. The manufacturer charges most of the money in equipment setup to begin making the cable, addition of 2 conductors would not be a great addition to the cost.

The lighting load at the pier is handled by a separate 480-208/120 transformer.

 

[W@ttson]

Sounds like there is no need for a neutral, but they still need an egc (even if the system is ungrounded which you didn't specify). Sounds wacky.

There is lighting load at the pier, but that is handled by a separate lighting transformer at the pier that is a 480-208/120 transformer. The submarine cables from each shore utility do not have a separate EGC within them for the service conductors, however, the armor for the cable is bonded to each services GEC system.

 

[xptpcrewx]

Hello, I have a scenario where I am scratching my head as to why the original designer went through all of the complexity to require a Zig Zag transformer. Maybe some one here can give me a different perspective to some of the advantages that could have been the reason to do it in such a way. The following is the system setup:

There is a concrete pier in the middle of a river. This pier has many motor loads and some lighting loads. The pier is fed from one side of the river from one utility grid and from the other side of the river from another utility grid (redundant services, only one feeding at a time, switched between the two by an ATS at the pier). Both utility services are 208/120V but are stepped up to 480VAC 3W 3Phase. 3 phase conductors are sent to the pier, no neutral, no ground through a submarine cable (from each utility). The services terminate at an ATS on the pier which is considered the pier service entrance. At the ATS there is a connection to a grounding transformer (the zig zag transformer) where a ground is established. On the pier there is a separate 480V Delta to 208/120 Transformer for lighting loads. At the zig zag transformer a ground fault relay is installed to monitor the 3 phases to the zig zag. Each of the utility services are low, at the 208/120 service level, each are only 250A (again only one service feeding at a time).


Question: Why didn't the designer just send a neutral and a ground from each utility which is on shore? Then have the neutral be switched at the ATS if there was any concern of the two utilities being connected at all?

Based on what you’ve said, the 3w feeders from each source are not considered services. They are separately derived systems. Also, because these are being stepped-up, I’m betting they have 480V delta secondaries with no neutral connection points, hence being ungrounded systems by nature.

Because ungrounded systems can be problematic and are typically not recommended, one way to turn this into a grounded system is by the use of a zig-zag transformer; which also happens to have favorable balancing/overvoltage-damping/harmonic-filtering advantages.

 

[W@ttson]

Based on what you’ve said, the 3w feeders from each source are not considered services. They are separately derived systems. Also, because these are being stepped-up, I’m betting they have 480V delta secondaries with no neutral connection points, hence being ungrounded systems by nature.

Because ungrounded systems can be problematic and are typically not recommended, one way to turn this into a grounded system is by the use of a zig-zag transformer; which also happens to have favorable balancing/overvoltage-damping/harmonic-filtering advantages.

Correct they are SDS's. Correct, it is actually a 208V Delta to 480V Delta transformer for each end. This is odd to me, its almost like doubling down on not having a neutral there. They could have easily established a neutral at that point. But I guess a benefit was that they didn't have to deal with that. Also the harmonic benefits might be the key here. There are drives associated with those motors and that could have been a concern on the pier.

So for now I will just assume that it was to have harmonic filtering benefits

 

[xptpcrewx]

This is odd to me, its almost like doubling down on not having a neutral there. They could have easily established a neutral at that point. But I guess a benefit was that they didn't have to deal with that.

In this case, having a 3w system, as Ron pointed out, would be cheaper and simplify the ATS installation.

Also the harmonic benefits might be the key here.

I’d say this is probably the lesser point for requiring the zig-zag and not the main reason.

 

[oldsparky52]

I'm not an engineer nor very educated, but .... with no reference to the neutral or grounding of the supplying sources, doesn't that help reduce the problems of stray currents caused by parallel paths on the multiple grounding of the PoCo distribution system? The OP did say they are out on a river.

 

[xptpcrewx]

I'm not an engineer nor very educated, but .... with no reference to the neutral or grounding of the supplying sources, doesn't that help reduce the problems of stray currents caused by parallel paths on the multiple grounding of the PoCo distribution system? The OP did say they are out on a river.

There is a neutral/ground reference once you establish the zig-zag. What stray currents/parallel-paths/multiple-grounding are you referring to?

 

[winnie]

I'm not an engineer nor very educated, but .... with no reference to the neutral or grounding of the supplying sources, doesn't that help reduce the problems of stray currents caused by parallel paths on the multiple grounding of the PoCo distribution system? The OP did say they are out on a river.

I am only guessing here, but you might be on to something.

The _loads_ being fed are 3 phase only, no neutral.

If your supplies were _grounded_ at the location of the supply transformers, then you would have grounding electrodes on both sides of the river, with grounded wires of some sort crossing to the pier. Perhaps this would be a path for stray current between the supplies on opposite sides of the river.

By using the zig-zag, you create a normal grounded system, but now the two systems share a single earth ground point.

The sketchy part is the 480V feeders without an EGC. Even true ungrounded systems require bonding of enclosures....

-Jon

 

[oldsparky52]

There is a neutral/ground reference once you establish the zig-zag. What stray currents/parallel-paths/multiple-grounding are you referring to?

The closest substation is very well grounded (earthed, whatever the correct term is), and the river is most likely a better path for current than the earth. If you have a grounded conductor from the distribution systems tied into the local power supply transformer then any stray current on the water could seek a path back to the substation on the ground tied into the PoCo distribution, especially if there is a deteriorating neutral conductor farther down the line.

I don't know if I explained this very well.

 

[xptpcrewx]

The closest substation is very well grounded (earthed, whatever the correct term is), and the river is most likely a better path for current than the earth. If you have a grounded conductor from the distribution systems tied into the local power supply transformer then any stray current on the water could seek a path back to the substation on the ground tied into the PoCo distribution, especially if there is a deteriorating neutral conductor farther down the line.

I don't know if I explained this very well.

How the substation is grounded is not relevant; what we care about here is how/if the SDS where the feeder is supplied from is grounded. I’m still not understanding what you mean by parallel paths and why you think the water would have stray currents.

 

[oldsparky52]

How the substation is grounded is not relevant; what we care about here is how/if the SDS where the feeder is supplied from is grounded. I’m still not understanding what you mean by parallel paths and why you think the water would have stray currents.

Since the PoCo grounds their distribution system neutral all along the distribution system, the earth (and the river) are parallel paths back to the source, always. The low impedance connection of the neutral conductor (and the difficulty in getting a really low impedance connection to earth) keeps nearly all of the neutral current on the neutral conductor. Getting a low impedance connection at the river is much easier than to earth. The hard wired ground connection back to the PoCo distribution system makes a earthing connection to the river a much easier path for parallel current.

It makes sense in my head.

 

[xptpcrewx]

Since the PoCo grounds their distribution system neutral all along the distribution system, the earth (and the river) are parallel paths back to the source, always.

I don’t believe the PoCo is doing multipoint neutral grounding on the 208Y/120-V level. I think you missed the part about the feeder being transformed to 480V.

 

[oldsparky52]

I don’t believe the PoCo is doing multipoint neutral grounding on the 208Y/120-V level. I think you missed the part about the feeder being transformed to 480V.

Maybe I'm not understanding what the PoCo does with their distribution system. I was of the understanding that they continuously earth the neutral on both the primary and secondary of all of the systems under their control. That would mean that the neutral conductor from the PoCo has a reference to all of their systems.

I came across a marina that even though the service was grounded on shore, the ground conductor at the dock had a small voltage to the waterway. Turn off the main, voltage was still there. The only way to get rid of the voltage was lift the service neutral from the meterbase.

We can ground the neutral to the waterway and get rid of the potential difference but that just raises the voltage at the water. So that current is flowing through the water to the source (the well grounded substation most likely). Let a neutral connection in the system start deteriorating and more current will flow through the waterway.

What am I misunderstanding?

 

[xptpcrewx]

What am I misunderstanding?

That this isn’t a service. It’s a SDS. There is no utility involved.

 

[oldsparky52]

That this isn’t a service. It’s a SDS. There is no utility involved.

I was offering a suggestion as to why it was done that way.

 

[winnie]

That this isn’t a service. It’s a SDS. There is no utility involved.

Yes. But there are _two_ SDSs on opposite banks of a river.

If each is grounded, and the grounded conductor brought from each bank to the pier, then the grounded conductor would end up tying the two banks together.

As described, there are no grounded conductors and no EGCs brought with these feeders.

-Jon

 

[xptpcrewx]

I was offering a suggestion as to why it was done that way.

Right but it would still be a SDS if it was a 4w feeder. The service is 208Y/120-V but they need 480-V.