240/120 3ph.

[kingpb]

Don't ever try to understand the logic of a utility company. It is layers and layers of bureaucracy and different departments that don't talk to each other.

 

[kwired]

I don't know if it *is* open delta; I'm going to reach out to their transformer expert & see what he says.

Of course, if it's closed delta, that just furthers my confusion as to why a wye installation is Special Facilities but a delta is not.

One reason why a delta could be seen as not special facilities is the fact that they could supply other customers with 120/240 single phase services from this transformer. 120/240 single phase services for them is like carrying currency in $20 or less denominations.

Now if it is located near owners building instead of near roads, alleys or other areas where they already have easement that makes it harder to supply other customers.

 

[kwired]

Don't ever try to understand the logic of a utility company. It is layers and layers of bureaucracy and different departments that don't talk to each other.

That all depends on the size of the company. Around here electric utilities are still publicly owned and that seems to make some difference also. All the rural cooperatives are usually fairly easy to work with. Lately they have been charging customers for service they used to do for free but that is understandable with rising costs. Some of the largest ones do seem to have more political and bureaucratical (is that a word?) crap to put up with.

 

[ggunn]

Don't ever try to understand the logic of a utility company. It is layers and layers of bureaucracy and different departments that don't talk to each other.

It could be something as simple as equipment they have lying around that they want to get rid of vs. gear they have to special order and buy.

 

[kwired]

It could be something as simple as equipment they have lying around that they want to get rid of vs. gear they have to special order and buy.

exactly.

120/240 single phase transformers are the most common ones they have in stock. They can easily be used to build a delta transformer bank, both open and closed delta. You can easily add one to an existing setup to gain a third phase.

 

[ggunn]

exactly.

120/240 single phase transformers are the most common ones they have in stock. They can easily be used to build a delta transformer bank, both open and closed delta. You can easily add one to an existing setup to gain a third phase.

Are you sure? I thought you had to use some sort of rotating mechanical device to make three phase out of split phase. Or is there already three phase power to the site?

 

[broadgage]

Are you sure? I thought you had to use some sort of rotating mechanical device to make three phase out of split phase. Or is there already three phase power to the site?

If by split phase, you mean what what otherwise be called single phase 3 wire, then yes mechanical rotating machinery, or electronic converting plant is required to produce 3 phase.

In this case though it appears safe to asume that the utility has 3 phase at MV available.
Therefore 3 phase open delta via 2 transformers, or 3 phase closed delta via 3 transformers is a simple matter.

3 phase delta service at 120/240 volt is often regarded as obsolete or old fashioned, but has its merits.
Standard transformers all with a center tap on the secondary are used, but obviously only one center tap may be grounded.

 

[Open Neutral]

First off, this transformer [?bank] will be for my clients house & pool period. It's a 1400 ft run {their route} from their pole lines [3ph 21KV w/Neutral]. He's paying {seemingly insane amounts} for the service because of that. I have no idea if they will spec 2 pad xfmrs or one 3-ph; but they will be pad mount; it's underground service.

The Greenbook Table 9 lists a 25/10KVA transformer #261547 with 240/120 out. Does that mean the stinger leg is 25KVA and the other two are 10, maybe?

 

[texie]

From the POCO's standpoint it's just simple economics. For example, Florida Power and Light will only give you a 208Y service if you can guarantee a minimum of 150 KVA demand and thats assuming that they have the facilities available (essentially right at the property). They have to be sure that the revenue from a given service will give them a reasonble return on thier investment. As a stock holder I see the their logic. I'd bet that they wouldn't give you plain old single phase 120/240 volt for free either due to the distance.
In the OP's case it is an extreme situation due to the distance plus they want 3 phase. The open delta will be cheaper for them to install as they only need 2 xformer and 2 primaries so they want less money than the 208Y which will require a 3 phase xformer and 3 primaries.
The open delta will be fine, you just have design your end accordingly.

 

[jim dungar]

... you just have design your end accordingly.

Yep.
The issue of capacity with closed versus open delta, should be of no concern to the customer. Tell the POCO what loading you need them to feed, based on their service agreement they will decide on the transformer size and connection just like they do for any other service.

 

[Open Neutral]

Yes/no/maybe

First, I'm trying to picture the situation. The primary is 20780VGrnY/12000V.

[BTW: the single-phase quote is $29K; 208 wye $60K]

I don't know if they will run open delta or not. Assume they do; so they use both shields {as I'd call it with coax cable..} in parallel for the neutral. Is that also the neutral on the centertapped secondary; or is that just a local creation with a ground grid?

Assuming they don't; they bring in all three phases & don't need a neutral on the transformer primary.

The complications of this will be from the solar and appliance aspects. I'd assume we'd want to push bigger 240v loads to non-centertap sides. That means we have to get "really 240" not 240/120 range top, oven, dryer, sauna, heat pumps. More research.

Also, we can only have a backup inverter on the 240/120 leg. While the gridtie inverter are happy with stinger systems, the backup inverters are 120-only in the US.

THAT means, during an outage, only a third of the PV arrays are working. [The gridtie inverters will 'follow' the PoCo line OR when up, the backup inverter.} That's a loss, as we hope PV input will keep propane usage by the generator to a minimum.

 

[kwired]

If by split phase, you mean what what otherwise be called single phase 3 wire, then yes mechanical rotating machinery, or electronic converting plant is required to produce 3 phase.

In this case though it appears safe to asume that the utility has 3 phase at MV available.
Therefore 3 phase open delta via 2 transformers, or 3 phase closed delta via 3 transformers is a simple matter.

3 phase delta service at 120/240 volt is often regarded as obsolete or old fashioned, but has its merits.
Standard transformers all with a center tap on the secondary are used, but obviously only one center tap may be grounded.

If by split phase he means standard 120/240 single winding with center tap then there is no three phase available without conversion equipment. This system only has two wire input on the high side. An open delta input will have 3 wire input - usually 2 phase conductors and neutral. But it must have three wire input or you will never have three phase output.

First off, this transformer [?bank] will be for my clients house & pool period. It's a 1400 ft run {their route} from their pole lines [3ph 21KV w/Neutral]. He's paying {seemingly insane amounts} for the service because of that. I have no idea if they will spec 2 pad xfmrs or one 3-ph; but they will be pad mount; it's underground service.

The Greenbook Table 9 lists a 25/10KVA transformer #261547 with 240/120 out. Does that mean the stinger leg is 25KVA and the other two are 10, maybe?

25 KVA is the coil with the center tap that is used as the neutral. They are expecting it to be higher loaded with 120 volt loads. If you were expecting to have majority of load as 240 volt load they will make all three the same size, and you should try to balance all three phases in this case.

Yes/no/maybe

First, I'm trying to picture the situation. The primary is 20780VGrnY/12000V.

[BTW: the single-phase quote is $29K; 208 wye $60K]

I don't know if they will run open delta or not. Assume they do; so they use both shields {as I'd call it with coax cable..} in parallel for the neutral. Is that also the neutral on the centertapped secondary; or is that just a local creation with a ground grid?

Assuming they don't; they bring in all three phases & don't need a neutral on the transformer primary.

The complications of this will be from the solar and appliance aspects. I'd assume we'd want to push bigger 240v loads to non-centertap sides. That means we have to get "really 240" not 240/120 range top, oven, dryer, sauna, heat pumps. More research.

Also, we can only have a backup inverter on the 240/120 leg. While the gridtie inverter are happy with stinger systems, the backup inverters are 120-only in the US.

THAT means, during an outage, only a third of the PV arrays are working. [The gridtie inverters will 'follow' the PoCo line OR when up, the backup inverter.} That's a loss, as we hope PV input will keep propane usage by the generator to a minimum.

Either way they will use the neutral and ground the transformer case to it. If open delta they likely only use two phases and neutral for supply. If closed delta they likely use all three phases and use neutral to ground the transformer case, it will be bonded to secondary grounded conductor in either case.

PV complicates things but there are likely more than one possibility with as many differences in cost as there are possibilities. Ranges and dryers is best to keep away from wild leg - just keeps from something getting changed down the road from creating a problem. Other 240 appliances usually is no problem as they usually have no neutral connection. Those are the ones to put on wild leg if you want to try to distribute load more evenly across phases. Keep in mind the wild leg may be derived from a smaller transformer winding than the main 120/240 winding.

 

[Open Neutral]

Heard back from my new best friend, a senior PGE engineer.

Yes, they supply open delta in cases such as this. Poles, they hang two single ∅ units; on pad-mounts, their term is a "duplex" transformer. Using my

  B

A g C

where AgC and BC windings exist, but BC is open; they call AgC the "lighter" secondary as it supplies most of the load. The B terminal is the "stinger"; I had that backwards.

I understand the issues re: future range replacements, but setting that aside; do I strive to put the 240V loads only on A-B; or also on B-C? [B is the "stinger terminal" but are both A-B & B-C "stinger legs" or not?

How concerned should I be about balance on the 3-phase loads; and what do we do if they are not? Rotate connections, etc. but what's above that?

What about harmonics? We depend on deltas for suppression; here we lose that… or do we?

 

[kwired]

I would use single phase panels for most of the lighting/general loads. If you use three phase panels here you will have a lot of open spaces where the wild leg is.

I would only run three phase to areas where you will use it and also run some of the HVAC, water heater, well pump and other similar loads that can utilize the wild leg to a three phase panel.

 

[Smart $]

Heard back from my new best friend, a senior PGE engineer.

Yes, they supply open delta in cases such as this. Poles, they hang two single ∅ units; on pad-mounts, their term is a "duplex" transformer. Using my

  B

A g C

where AgC and BC windings exist, but BC is open; they call AgC the "lighter" secondary as it supplies most of the load. The B terminal is the "stinger"; I had that backwards.

I understand the issues re: future range replacements, but setting that aside; do I strive to put the 240V loads only on A-B; or also on B-C? [B is the "stinger terminal" but are both A-B & B-C "stinger legs" or not?

How concerned should I be about balance on the 3-phase loads; and what do we do if they are not? Rotate connections, etc. but what's above that?

What about harmonics? We depend on deltas for suppression; here we lose that? or do we?

The A-N-C secondary got the term "lighter" early in adoption of open-delta services because the lighting load was only L-N. Not sure how "stinger" was coined.

An open delta system has a greater capacity for 1? loads connected across the stinger winding. This is because current of 1? loads connected across the open terminals has to travel through both the lighter and the stinger windings. Capacity is calculated for equal loading A-B and B-C.

Regardless, as stated earlier, you provide loading numbers and the POCO determines the capacity of the transformers they install. Balance your system as best possible for all the loads... but if you want to help the system perform better longer, put 240V 1? loads that operate longer than others across the stinger terminals.

I don't know enough about harmonics on these systems to answer your questions with any certainty... but I don't believe harmonics are an issue to be concerned unless you have some extremely abnormal conditions of use.

 

[Smart $]

I would use single phase panels for most of the lighting/general loads. If you use three phase panels here you will have a lot of open spaces where the wild leg is.

I would only run three phase to areas where you will use it and also run some of the HVAC, water heater, well pump and other similar loads that can utilize the wild leg to a three phase panel.

Good point.

 

[Open Neutral]

An open delta system has a greater capacity for 1? loads connected across the stinger winding. This is because current of 1? loads connected across the open terminals has to travel through both the lighter and the stinger windings. Capacity is calculated for equal loading A-B and B-C.

B is the stinger terminal. But is the "stinger winding" A-B here? I assume so.

And do you mean "greater than B-C" or greater than A-C? Clearly A-C has the most capacity; and I assume B-C is the least, because of the [in effect] bucking windings.

Should we totally avoid any B-C loads?

The panel inventory is a bigger question. We'll have

1. Inverter fed {Priority-1 loads}
2. Generator {P2}
3. Grid-only {P3}

And then add the single vs 3ph issue....as 2 & 3 will have 3 ph. loads.

 

[walkerj]

Can one of you smart guys post the picture of the open delta secondary and explain this via that picture so us not-so-smart guys can learn something?

I am confused which winding you are referring to as the 'stinger' winding.

I know and have serviced this type of system, but I want the low down on it as far as the capacities of each phase.

Thanks

 

[gar]

110722-2114 EDT

In our area open deltas are quite common. This usually means the A-N-C secondary (120-0-120) single phase supply is the larger transformer. I am currently powered from such a system as I type. The A-N-C transformer is of moderate size, maybe 100 KVA or more, The A-B transformer is smaller.

The only purpose of the A-B secondary to to supply 3 phase loads. All single phase loads 240 or 120 are on the A-N-C secondary.

In our case the primaries are supplied from a Y supply. One could connect the primaries as either Y or delta and do it with three wires. If you wanted a full Y system with three transformers and primary neutral, then 4 wires are required.

A side subject. A my home the poles carry 3 phase on 3 wires. Thus, for loading you would consider it a delta source, and I do not really care how the substation is wired. Most likely it is a Y source at the substation. There is no primary neutral, and this I like. I do not have major ground currents and voltages from bad primary neutrals. The only neighbors connected to my 240/120 neutral are those on my transformer, and in my case that is only one neighbor. The primary of my transformer has two primary posts and these connect to two of the 3 phase lines. I suspect if I want three phase there would be no great cost. Just one added transformer and I would have an open delta.

Back to the building where I am presently. We have 5 CNC machines, all rated 20 HP, a scroll and reciprocating air compressors, MIG and TIG arc welding, and some more moderate size machines. If we got the power company transformers too hot they would just increase the sizes. But there is a weight limit to what they can hang on the pole.

In my opinion there is nothing functionally wrong with an open delta system.

.

 

[Smart $]

Can one of you smart guys post the picture of the open delta secondary and explain this via that picture so us not-so-smart guys can learn something?

I am confused which winding you are referring to as the 'stinger' winding.

I know and have serviced this type of system, but I want the low down on it as far as the capacities of each phase.

Thanks

See page 9 of this Cooper publication:

http://www.cooperindustries.com/con...urces/library/201_1phTransformers/R201902.PDF

Note the symbollic diagram shows the higher-voltage-to-ground terminal as "a", which it can be on the POCO side of the service point. On the load side of the service point, the NEC requires it be connected as B [408.3(E)]