connecting a delta-wye transformer to a 12.47kV distribution system: pro's and cons

neusj

New User
Our utility has a grounded wye distribution system 12.47kV. We normally connect 12470GY/7200kV - xxx/xxxV transformers to our system to provide customers power.

We have a customer who wants to connect several 12.47/7.2kV delta primary transformers with grounded wye secondary voltages to our system. I would assume we would connect our system ground to their secondary ground and their primary would float.

I have not found any written information forbidding this, but our dispatch department is opposed to this.

I am aware that the delta traps harmonics. I do not have a complete picture of all the pro's and con's of hooking a delta primary transformer to a wye distribution system. What would ferro resonance do to the 12.47kV system? Usually the harmonics come from loads not the utility line. The object of a delta secondary is to keep the load harmonics of computers and variable speed drives, etc. from entering contaminating the utility line.

I am aware that there are ferro resonance issues with the delta side of a transformer. I am not completely certain I know what the ferro resonance does or how it affects the system upstream or downstream or how it affects the transformer.
 

AdrianWint

Senior Member
Location
Midlands, UK
I'm not familiar with your Alaskan codes (being a Brit), but....

In our distribution system the 11KV:400/230V transformers are all delta/star (usually Dyn11). We don't distribute a neutral at 11kV.

In terms of trapping the triplen harmonics it doesn't matter whether the delta is on the primary, the secondary or the tertiary the end result will be the same.

If the primary is delta then there wont be anywhere to land a ground on the primary side (other than the case), you just connect the 3 lines to the three corners of the delta, otherwise I cant really see any issues the transformer will work just fine.

On our British system we don't common the HV ground & the LV ground, they are kept separate.
 

kwired

Electron manager
Location
NE Nebraska
Our utility has a grounded wye distribution system 12.47kV. We normally connect 12470GY/7200kV - xxx/xxxV transformers to our system to provide customers power.

We have a customer who wants to connect several 12.47/7.2kV delta primary transformers with grounded wye secondary voltages to our system. I would assume we would connect our system ground to their secondary ground and their primary would float.

I have not found any written information forbidding this, but our dispatch department is opposed to this.

I am aware that the delta traps harmonics. I do not have a complete picture of all the pro's and con's of hooking a delta primary transformer to a wye distribution system. What would ferro resonance do to the 12.47kV system? Usually the harmonics come from loads not the utility line. The object of a delta secondary is to keep the load harmonics of computers and variable speed drives, etc. from entering contaminating the utility line.

I am aware that there are ferro resonance issues with the delta side of a transformer. I am not completely certain I know what the ferro resonance does or how it affects the system upstream or downstream or how it affects the transformer.
I'm a little confused with what you have, but from what I understand on such a distribution system most of your pole top type transformer banks would usually be connected wye on the primary side - mostly because they build the banks out of commonly stocked single phase transformers that are connected line to neutral on the primary side. But most (more like nearly all) pad mount or any other single unit transformers are typically going to have delta connected primary winding, and from my understanding if it is a single core unit it about has to be this way, or at least leave the neutral point of a wye primary floating.
 

meternerd

Senior Member
Location
Athol, ID
Occupation
retired water & electric utility electrician, meter/relay tech
I'm a little confused with what you have, but from what I understand on such a distribution system most of your pole top type transformer banks would usually be connected wye on the primary side - mostly because they build the banks out of commonly stocked single phase transformers that are connected line to neutral on the primary side. But most (more like nearly all) pad mount or any other single unit transformers are typically going to have delta connected primary winding, and from my understanding if it is a single core unit it about has to be this way, or at least leave the neutral point of a wye primary floating.
May be different where you are, but here in California (and in Alaska), we had a 12,470/7200 distribution system and we never used Delta primary padmounts. All were grounded Wye-Wye (Delta secondaries were rare) and the primary and secondary neutrals were connected together, bonded to the case and grounded to a driven ground rod. The neutral bushing bond is removable to allow for megger testing. Primaries were fed with concentric neutral underground primary cable with the shield being the neutral. Most of ours are "common core" windings (less expensive), which have single cores with three vertical "bars" and top and bottom horizontal bars. Primary and secondary wiring is wound together on the cores to increase the magnetic coupling.

If they are wired Delta and you lose a primary phase, won't their delta bank back feed the open phase? Sounds like a safety issue at least. Probably if it was our utility, we'd say no.
 
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GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
You can use a wye primary with a wye secondary, but if you try to use wye primary with a delta secondary you can get very high circulating currents if the primary voltages are at all out of balance. You will have a similar problem is you try to connect a delta primary of a transformer downstream of the first transformer bank's wye output.
To avoid the circulating current problem you can use wye to delta as long as you do not connect the common point of the input wye either to the input neutral or to ground. It does not matter what is done with the actual transmission line neutral in terms of grounding, etc. as long as it is not connected to the transformer(s) primary.
 

kwired

Electron manager
Location
NE Nebraska
You can use a wye primary with a wye secondary, but if you try to use wye primary with a delta secondary you can get very high circulating currents if the primary voltages are at all out of balance. You will have a similar problem is you try to connect a delta primary of a transformer downstream of the first transformer bank's wye output.
To avoid the circulating current problem you can use wye to delta as long as you do not connect the common point of the input wye either to the input neutral or to ground. It does not matter what is done with the actual transmission line neutral in terms of grounding, etc. as long as it is not connected to the transformer(s) primary.
My thoughts are pretty similar. Circulating current problem is not there if each coil is on a separate core - as is the case with multiple pole mounted units used to build a bank, but a padmount is generally all three phases on a single core, if the primary is wye configuration then the common point should not be connected to the neutral.

I guess I really don't know if common utility padmounts around here are wye or delta primary, I always assumed they were delta for the reasons I have brought up here.

Recently had a situation where POCO had two fuses out on the top of pole where it fed the underground to a pad mount. If that transformer had wye primary I would have expected only power on one phase, instead I had some pretty odd voltages on all three phases, but sorry I don't recall just what those voltages were, I gave up on any further investigating as soon as I seen the POCO fuses were blown and instead called them and told them they had a couple fuses out supplying the underground and figured it was their problem to figure out why.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Actually wye to delta with primary neutral will not work with separate transformers either. The problem is the vector closure of the secondary delta. An open delta would be fine, although the voltages might not be well balanced.

For those who want to know more, wye can have voltage magnitudes such that the resulting delta voltages do not form a closed triangle with 60 degree angles. But the circuit connection forces that relationship. So the delta and wye voltages together have to change to satisfy the condition, and the only way to do that is via IR drop.
That means large currents.

Tapatalk!
 

meternerd

Senior Member
Location
Athol, ID
Occupation
retired water & electric utility electrician, meter/relay tech
You can use a wye primary with a wye secondary, but if you try to use wye primary with a delta secondary you can get very high circulating currents if the primary voltages are at all out of balance. You will have a similar problem is you try to connect a delta primary of a transformer downstream of the first transformer bank's wye output.
To avoid the circulating current problem you can use wye to delta as long as you do not connect the common point of the input wye either to the input neutral or to ground. It does not matter what is done with the actual transmission line neutral in terms of grounding, etc. as long as it is not connected to the transformer(s) primary.
I don't want to hijack this thread, but....we had at least six or seven floating neutral 12,470/7200 Wye primary/480 3W Delta ungrounded secondary overhead banks. The only problems we have ever had are when we do single phase switching or we lose a phase due to a blown fuse. In those cases, secondary phase to phase voltages can reach 500 to 600 volts, which have fried phase monitors and destroyed Kwh meters. In fact, meter manufacturers no longer list their meters as suitable for 480V 3W Delta services. To solve the problem, we tied down the neutral on the Wye primary. This solved the smoking secondary devices, but occasionally caused two or all three primary fuses to blow on close in primary faults. The other solution would have been to corner ground the 480 Delta, but I've posted before why I don't like that setup. I guess what I'm saying is that Delta seems never to be a good option. The cost of running a neutral to the service disconnect seems like a cheap way to save expensive headaches. 12,470/7200 grounded Wye primary and 277/480 grounded Wye secondary are what we use exclusively now whether overhead or underground. We still have a few 14,400 Wye with no neutral and 277/480 grounded Wye secondary. Getting rid of all of the Delta services as money allows. Been too long since I left college for me to try to figure out why things happen. Maybe it really is "Ohm's Theory!" Kinda glad I'm retired! Now all I have to do is remember where I parked the fifth wheel!:cool:
 
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GoldDigger

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Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
The odd voltages with only one wye winding driven were probably a combination of load currents from line to line loads or rotating three phase machinery or both.

Tapatalk!
 

kwired

Electron manager
Location
NE Nebraska
Actually wye to delta with primary neutral will not work with separate transformers either. The problem is the vector closure of the secondary delta. An open delta would be fine, although the voltages might not be well balanced.

For those who want to know more, wye can have voltage magnitudes such that the resulting delta voltages do not form a closed triangle with 60 degree angles. But the circuit connection forces that relationship. So the delta and wye voltages together have to change to satisfy the condition, and the only way to do that is via IR drop.
That means large currents.

Tapatalk!
I don't want to hijack this thread, but....we had at least six or seven floating neutral 12,470/7200 Wye primary/480 3W Delta ungrounded secondary overhead banks. The only problems we have ever had are when we do single phase switching or we lose a phase due to a blown fuse. In those cases, secondary phase to phase voltages can reach 500 to 600 volts, which have fried phase monitors and destroyed Kwh meters. In fact, meter manufacturers no longer list their meters as suitable for 480V 3W Delta services. To solve the problem, we tied down the neutral on the Wye primary. This solved the smoking secondary devices, but occasionally caused two or all three primary fuses to blow on close in primary faults. The other solution would have been to corner ground the 480 Delta, but I've posted before why I don't like that setup. I guess what I'm saying is that Delta seems never to be a good option. The cost of running a neutral to the service disconnect seems like a cheap way to save expensive headaches. 12,470/7200 grounded Wye primary and 277/480 grounded Wye secondary are what we use exclusively now whether overhead or underground. We still have a few 14,400 Wye with no neutral and 277/480 grounded Wye secondary. Getting rid of all of the Delta services as money allows. Been too long since I left college for me to try to figure out why things happen. Maybe it really is "Ohm's Theory!" Kinda glad I'm retired! Now all I have to do is remember where I parked the fifth wheel!:cool:
Not sure I am completely understanding either one of you, or possibly we are not talking about the same things.

I am talking about primary windings being wound an connected in a wye or in a delta fashion, what kind of source configuration is there only matters if there is a neutral involved from the source and of course could only be utilized when supplying a wye connected set of windings.

I am no expert on medium and high voltage stuff, but AFAIK most of what we are talking about is still the same issues if the voltage were lower. A lot of the basic concepts are still the same just the voltage variable in the formulas is bigger numbers.

GD I don't know what you are getting at or if they do something I am not aware of, but you will not find any three phase secondary from POCO around here that is coming from pole top transformers that doesn't use 7200 volt primary single phase transformers. They are all connected phase to neutral on the primary side of these pole top transformer banks.

Padmounts - I really don't know what is typical. I have always kind of assumed they mostly have delta primary windings regardless of what the secondary may be.
 
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meternerd

Senior Member
Location
Athol, ID
Occupation
retired water & electric utility electrician, meter/relay tech
GD I don't know what you are getting at or if they do something I am not aware of, but you will not find any three phase secondary from POCO around here that is coming from pole top transformers that doesn't use 7200 volt primary single phase transformers. They are all connected phase to neutral on the primary side of these pole top transformer banks.

Padmounts - I really don't know what is typical. I have always kind of assumed they mostly have delta primary windings regardless of what the secondary may be.
Good conversation......padmounts are typically wired the same as pole mounts. Primary line to neutral. Most padmounts use deadfront primary connections. These are "coaxial" concentric neutral cables with load break elbows that plug into primary bushings. The cable jacket it stripped back to expose the neutral "shield" wires and they are twisted together, lugged and connected to the primary neutral bushing. All I have seen are then bonded to the case and grounded to a ground rod. Since Delta primary would not use the neutral, I'm not sure how you would terminate them in a deadfront setup. Secondary connections are typically Wye also. We no longer offer 240V 4W Delta or 480V 3W Delta services. It's all 120/208 or 277/480. Keeps our transformer inventory lower. We have occasionally replaced 240 Delta overhead service transformers with larger 240, but we discourage it. Done the same here in CA and where I worked in Alaska. Maybe not typical other places.
 

kwired

Electron manager
Location
NE Nebraska
GD I don't know what you are getting at or if they do something I am not aware of, but you will not find any three phase secondary from POCO around here that is coming from pole top transformers that doesn't use 7200 volt primary single phase transformers. They are all connected phase to neutral on the primary side of these pole top transformer banks.

Padmounts - I really don't know what is typical. I have always kind of assumed they mostly have delta primary windings regardless of what the secondary may be.
Good conversation......padmounts are typically wired the same as pole mounts. Primary line to neutral. Most padmounts use deadfront primary connections. These are "coaxial" concentric neutral cables with load break elbows that plug into primary bushings. The cable jacket it stripped back to expose the neutral "shield" wires and they are twisted together, lugged and connected to the primary neutral bushing. All I have seen are then bonded to the case and grounded to a ground rod. Since Delta primary would not use the neutral, I'm not sure how you would terminate them in a deadfront setup. Secondary connections are typically Wye also. We no longer offer 240V 4W Delta or 480V 3W Delta services. It's all 120/208 or 277/480. Keeps our transformer inventory lower. We have occasionally replaced 240 Delta overhead service transformers with larger 240, but we discourage it. Done the same here in CA and where I worked in Alaska. Maybe not typical other places.
Again I don't do medium and high voltage stuff, but I do see what is done by POCO frequently as I am often there while they have things opened up. I can't recall ever seeing a neutral "bushing" on a pad mount transformer - even a single phase that operates line to neutral. That doesn't mean the neutral of the primary winding isn't bonded to the enclosure though as that needs done as well if the neutral is to be grounded - which it always is. I do see isolated secondary neutrals with bonding jumpers - I imagine that is to leave the option for ungrounded or even impedance grounded systems - though most utilities will only deliver grounded services - if you want ungrounded or impedance grounded the customer generally needs to separately derive that with their own equipment.

Add: I guess that optional bonding jumper also gives you the option to ground the system at the first disconnect instead of the source - again not something a POCO is likely to do but in the case of a facility that has service at medium voltage levels it allows to comply with NEC using same transformers.
 
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meternerd

Senior Member
Location
Athol, ID
Occupation
retired water & electric utility electrician, meter/relay tech
OK...my bad. I guess I'm just used to calling connection points "bushings". Most often, the primary neutral wires are connected to an isolated stud or a welded grounding stud below the phase bushings. The isolated stud may be in the secondary side if the transformer is wired with a common primary/secondary neutral. It is connected from the isolated neutral stud to the case with a flat copper jumper. When doing "megger" testing or TTR ratio testing, all grounds must be removed from the transformer windings. That's what the jumper is there for. You can order transformers which are internally bonded to the case, but we don't buy 'em because, as I said, there's no way to un-ground the windings for testing. I've only worked distribution in the Western US, so I'm sure there are lots of things I don't know about. Most of my power experience is substation stuff, but I do (did) train and help the linemen when we have transformer problems. What does all this have to do with the OP's question? Probably nothing!:p
 

mbrooke

Batteries Not Included
Location
United States
Occupation
*
Our utility has a grounded wye distribution system 12.47kV. We normally connect 12470GY/7200kV - xxx/xxxV transformers to our system to provide customers power.

We have a customer who wants to connect several 12.47/7.2kV delta primary transformers with grounded wye secondary voltages to our system. I would assume we would connect our system ground to their secondary ground and their primary would float.

I have not found any written information forbidding this, but our dispatch department is opposed to this.

I am aware that the delta traps harmonics. I do not have a complete picture of all the pro's and con's of hooking a delta primary transformer to a wye distribution system. What would ferro resonance do to the 12.47kV system? Usually the harmonics come from loads not the utility line. The object of a delta secondary is to keep the load harmonics of computers and variable speed drives, etc. from entering contaminating the utility line.

I am aware that there are ferro resonance issues with the delta side of a transformer. I am not completely certain I know what the ferro resonance does or how it affects the system upstream or downstream or how it affects the transformer.
Delta is superior to wye when used as a primary in most cases. For one if a cut out opened, you don't have to worry about inductive tank heating as in a wye primary. Currents on the ground system are eliminated, and both the customer and the utility have better power quality.

Only reason why utilities go with a primary wye is a slight cost reduction in the transformer, and in higher voltage cases to reduce ferroresonance risk. Unless this transformer is fed by very, very long underground cables Ferroresonance is generally not a concern at 12.47kv. If you do fear it or it may be an issue you can always switch the transformer via a 3 phase gang switch or energize the unit with some heaters connected to the secondary. But again at 12.47kv its usually not a concern because its not likely to happen.



Outside of the utility world delta primaries are used almost exclusively regardless of how the supply neutral is grounded (ie ungrounded wye or delta, resistance grounded, solidly grounded). In California almost every 3 phase pad mount is delta wye regardless of what is going on in the substation.



As for the hook up, you would hook this up like a wye wye pad. Ground the tank and unless the customers specifically has a reason you would also connect the customers neutral to yours. Everything will be the same like on a wye wye pad. Only difference is internally inside the transformer you have a delta primary.

It will not negatively impact the system in any way.
 

mbrooke

Batteries Not Included
Location
United States
Occupation
*
I'm not familiar with your Alaskan codes (being a Brit), but....

In our distribution system the 11KV:400/230V transformers are all delta/star (usually Dyn11). We don't distribute a neutral at 11kV.

In terms of trapping the triplen harmonics it doesn't matter whether the delta is on the primary, the secondary or the tertiary the end result will be the same.

If the primary is delta then there wont be anywhere to land a ground on the primary side (other than the case), you just connect the 3 lines to the three corners of the delta, otherwise I cant really see any issues the transformer will work just fine.

On our British system we don't common the HV ground & the LV ground, they are kept separate.
In your system the 11kv is only 3 wires correct? No ground or neutral? When you ground the transformer tank and lightning arrestors I take it you don't jumper the secondary XO to that as well? Rather the secondary XO gets its own ground rod several feet or yards away from the ground rod that is connected to the tank and the lighting arrestors? Ie, the transformer tank has an independent grounding system as well as the secondary XO that way if the tank ever became energized at 11kv it wouldn't transfer over to the low voltage ground and neutral.


Hope Im explaining it right but wondering at the same time how its done on your side of the pond.













May be different where you are, but here in California (and in Alaska), we had a 12,470/7200 distribution system and we never used Delta primary padmounts. All were grounded Wye-Wye (Delta secondaries were rare) and the primary and secondary neutrals were connected together, bonded to the case and grounded to a driven ground rod. The neutral bushing bond is removable to allow for megger testing. Primaries were fed with concentric neutral underground primary cable with the shield being the neutral. Most of ours are "common core" windings (less expensive), which have single cores with three vertical "bars" and top and bottom horizontal bars. Primary and secondary wiring is wound together on the cores to increase the magnetic coupling.

If they are wired Delta and you lose a primary phase, won't their delta bank back feed the open phase? Sounds like a safety issue at least. Probably if it was our utility, we'd say no.



I think you would be saying no for the wrong reasons, but Id also imagine that a 3, 4 or 5 legged wye grounded primary core would back feed an open phase because the windings are all magnetically coupled to the same core? The 1 or two remaining windings inducing magnetic flux in the core would induce a voltage potential in the other winding connected to the disconnected phase? Your theory is right but I guess for single phase independent wye grounded wye grounded units, not shared common cores.




You can use a wye primary with a wye secondary, but if you try to use wye primary with a delta secondary you can get very high circulating currents if the primary voltages are at all out of balance. You will have a similar problem is you try to connect a delta primary of a transformer downstream of the first transformer bank's wye output.
To avoid the circulating current problem you can use wye to delta as long as you do not connect the common point of the input wye either to the input neutral or to ground. It does not matter what is done with the actual transmission line neutral in terms of grounding, etc. as long as it is not connected to the transformer(s) primary.
I agree. Well put. Usually when a wye delta is involved after energization the primary ground bond is removed leaving the wye floating. Most banks that are Y D often will have a solid blade cutout that will be opened once the last cut out is closed. If the bank needs to be de energized the cutout is closed grounding the primary as the fuses are pulled. Good way to prevent Ferroresonance during closing/opening cutouts yet reducing circulating currents in operation. However, around here when a delta secondary is involved a delta primary is attempted as much a possible. If Im right a delta primary is a lower ferroresonance risk over a wye ungrounded primary?




My thoughts are pretty similar. Circulating current problem is not there if each coil is on a separate core - as is the case with multiple pole mounted units used to build a bank, but a padmount is generally all three phases on a single core, if the primary is wye configuration then the common point should not be connected to the neutral.

I guess I really don't know if common utility padmounts around here are wye or delta primary, I always assumed they were delta for the reasons I have brought up here.

Recently had a situation where POCO had two fuses out on the top of pole where it fed the underground to a pad mount. If that transformer had wye primary I would have expected only power on one phase, instead I had some pretty odd voltages on all three phases, but sorry I don't recall just what those voltages were, I gave up on any further investigating as soon as I seen the POCO fuses were blown and instead called them and told them they had a couple fuses out supplying the underground and figured it was their problem to figure out why.

Circulating currents will also occur in single phase banks connected grounded wye primary delta secondary. Any voltage imbalance (zero sequence currents) or phase angle distortion (such as voltage regulators attempting to even out a line) on the phases will cause currents to circulate in the delta secondary. The secondary will attempt to balance out the line so to speak. The same reason why a grounded wye primary broken delta secondary is so commonly used at detecting zero sequence currents in relaying. Believe it or not, where an artificial neutral is needed (such as when the source is fed via a delta generator or transformer secondary) an alternative to a zig zag is a wye grounded closed delta secondary transformer bank.


As for your pad mounts they may be all delta, all wye or a combination of the two. It varies widely between utilities. Some have a policy where all pad mounts must be wye primary others have a policy where they must be all delta primary. Others do it on a case by case bases depending on what needs to be met. Generally pocos will tend to gravitate toward wye primary because its a bit cheaper. Worth looking at the stock stickers on newer pads or asking the poco.




Actually wye to delta with primary neutral will not work with separate transformers either. The problem is the vector closure of the secondary delta. An open delta would be fine, although the voltages might not be well balanced.

For those who want to know more, wye can have voltage magnitudes such that the resulting delta voltages do not form a closed triangle with 60 degree angles. But the circuit connection forces that relationship. So the delta and wye voltages together have to change to satisfy the condition, and the only way to do that is via IR drop.
That means large currents.

Tapatalk!

Good explanation. I agree. :)







I don't want to hijack this thread, but....we had at least six or seven floating neutral 12,470/7200 Wye primary/480 3W Delta ungrounded secondary overhead banks. The only problems we have ever had are when we do single phase switching or we lose a phase due to a blown fuse. In those cases, secondary phase to phase voltages can reach 500 to 600 volts, which have fried phase monitors and destroyed Kwh meters. In fact, meter manufacturers no longer list their meters as suitable for 480V 3W Delta services. To solve the problem, we tied down the neutral on the Wye primary. This solved the smoking secondary devices, but occasionally caused two or all three primary fuses to blow on close in primary faults. The other solution would have been to corner ground the 480 Delta, but I've posted before why I don't like that setup. I guess what I'm saying is that Delta seems never to be a good option. The cost of running a neutral to the service disconnect seems like a cheap way to save expensive headaches. 12,470/7200 grounded Wye primary and 277/480 grounded Wye secondary are what we use exclusively now whether overhead or underground. We still have a few 14,400 Wye with no neutral and 277/480 grounded Wye secondary. Getting rid of all of the Delta services as money allows. Been too long since I left college for me to try to figure out why things happen. Maybe it really is "Ohm's Theory!" Kinda glad I'm retired! Now all I have to do is remember where I parked the fifth wheel!:cool:
Sounds like it could have been ferroresonance in action. Could this be caused by regular imbalances too or only ferroresonance?


"other solution would have been to corner ground the 480 Delta" :blink: Confused. Not sure if secondary grounding would make any difference.
 

meternerd

Senior Member
Location
Athol, ID
Occupation
retired water & electric utility electrician, meter/relay tech
Delta is superior to wye when used as a primary in most cases. For one if a cut out opened, you don't have to worry about inductive tank heating as in a wye primary. Currents on the ground system are eliminated, and both the customer and the utility have better power quality.

Only reason why utilities go with a primary wye is a slight cost reduction in the transformer, and in higher voltage cases to reduce ferroresonance risk. Unless this transformer is fed by very, very long underground cables Ferroresonance is generally not a concern at 12.47kv. If you do fear it or it may be an issue you can always switch the transformer via a 3 phase gang switch or energize the unit with some heaters connected to the secondary. But again at 12.47kv its usually not a concern because its not likely to happen.



Outside of the utility world delta primaries are used almost exclusively regardless of how the supply neutral is grounded (ie ungrounded wye or delta, resistance grounded, solidly grounded). In California almost every 3 phase pad mount is delta wye regardless of what is going on in the substation.



As for the hook up, you would hook this up like a wye wye pad. Ground the tank and unless the customers specifically has a reason you would also connect the customers neutral to yours. Everything will be the same like on a wye wye pad. Only difference is internally inside the transformer you have a delta primary.

It will not negatively impact the system in any way.
I know this is way off the OP post, but since we seem to be going there.....my understanding is that in California, the tanks of overhead transformers are never grounded. GO95 or some such regulation. The thinking is that there is less potential for incidental contact with ground when working on live lines in the vicinity of the transformer primaries. That's why you see two bushing transformers instead of single bushing. Also, with an ungrounded Delta primary, how would any ground faults trip adjacent reclosers or substation breakers. No ground fault current, since there's no reference to ground. In mountainous country like ours, trees in lines due to snow load is a common problem, and phase to ground faults happen all the time. Am I missing something? Alaska was different, in that they do ground the cans, but still use grounded 7200 Wye for much of their distribution. Not sure this is useful info for most, but it's been my world for over 40 years, so I find it interesting. Sorry if others are bored spitless. As far as padmounts, I have yet to see a Delta/Wye transformer. I'm only familiar with our utility, so I can't speak for all of California. We are grounded 12,470/7200 exclusively, so that's the only transformers we stock. Ditto for the parts of Alaska I worked in (Anchorage and the Matanuska Valley). Substations, on the other hand, are all wired Delta/Wye because of the three wire transmission systems. Is there a more appropriate forum to discuss this kind of stuff? Don't want to get blacklisted for hijacking threads.:p
 
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kwired

Electron manager
Location
NE Nebraska
I don't see that we are hijacking the thread when the title/topic includes "pros and cons". Most of this recent conversation certainly is associated with pros and cons of either method.




If you lose an input phase of a wye primary/delta secondary aren't you left with basically the same thing as if you had designed an open delta bank?

Output phase and voltage would remain unchanged, but the KVA capacity is reduced with the lost primary coil.
 

mbrooke

Batteries Not Included
Location
United States
Occupation
*
I know this is way off the OP post, but since we seem to be going there.....my understanding is that in California, the tanks of overhead transformers are never grounded. GO95 or some such regulation. The thinking is that there is less potential for incidental contact with ground when working on live lines in the vicinity of the transformer primaries. That's why you see two bushing transformers instead of single bushing. Is there a more appropriate forum to discuss this kind of stuff? Don't want to get blacklisted for hijacking threads.:p

Transformer tank grounding varies in California. There are indeed floating tanks in California employed by some utlities. Some one told me on here its done to help prevent flashover and make servicing lines safer. An equal potential method is employed.

The double bushing transformers are actually required for different reasons. In California PUC 95 applies to line construction which also has rules on EMFs and limiting stray currents on grounding systems much like other countries. As a result all transformers must be connected phase to phase or if connected in wye phase to an isolated neutral kept up on insulators and minimally grounded. Hence 3 wire is the dominant design and where a neutral is present its propt up on insulators. A 4th or 5th ground wire may be run but is only for grounding and bonding, no neutral load current.


Most (not all [I will explain latter]) 3 wire lines in California are actually uni grounded wye (supplied via a wye secondary substation transformer with the neutral solidly grounded to the substation grounding mat but often not run along with the poles). Just because all loads are connected in delta out on a line doesn't mean the substation supply transformers is also delta on the secondary. Common misconception. When a uni grounded line does ground down current does flow. The amount is dependent on the resistance of the fault, the soil resistance and the distance from the supply substation. Generally faults close to the substation are much higher in current magnitude then those furthest since less soil impedance is present. Running an EGC along the line drastically helps to increase the current since its no longer dependent on soil resistance. If faults don't generate a high current (they don't reach the breakers or fuses normal trip curves) they are cleared via GFI logic in the recloser or substation breaker. Because no phase to neutral loads are connected to the grounding system like in a multi grounded neutral system, its possible to sum the readings across the 3 phases. Exact same concept that applies to GFCIs, RCDs, and GFI logic. One could have 459, 429 and 487 amps for example on each phase from all the delta connected loads. If no ground faults exist the zero sequence sum would equal to zero. However, if a 15 amper fault took place on phase A, when the new values 474, 429 and 487 are sumned in the recloser's or breaker's ground fault logic relay it will come out as +15, indicating a ground fault exists out on the line. If the ground fault persists long enough over the established time current curve it will open the breaker or recloser.



"Also, with an ungrounded Delta primary, how would any ground faults trip adjacent reclosers or substation breakers. No ground fault current, since there's no reference to ground."



Now, Ungrounded lines. Current, believe it or not does flow during a ground fault :happyyes:There are genuine ungrounded delta lines in California where the supply substation transformer is indeed delta secondary. Actually a lot more common in California then in most other places. When a phase does ground down current does flow believe it or not. Not much but it does due to the fact a capacitive potential does exist between the phases and earth. The longer the line is the more capacitive potential and thus the more current. One average, assuming an average sized system that current is about an amp or two. During the fault the current also rises to the phase to phase voltage on the none faulted phases so lines are built with fully rated insulator, bushings and surge suppressors. (More expensive) These systems have the advantage that they can run with a faulted phase as well as fewer momentary interruptions (reclosing and voltage sags from single phase to ground faults) since a squirrel bridging a bushing or tree touching a single phase wont cause a violent phase to ground flash over or massive currents to flow to ground. Better service continuity and far less thermal/magnetic stress on the system over time as long as the first fault is fixed before a second one occurs latter on. (2 faults on different phases cause massive current flows). There is one major disadvantage in that an arcing fault can cause voltage stress above the phase to phase voltage due to a type of "resonant" effect. This of course can be deterred by making an artificial neutral point or using the supply transfomer's XO (if it was previously a floating wye secondary) and grounding that neutral point or XO either through a properly sized reactor (Peterson coil earthing) or via a resistor that that will pull current slightly higher over what would normally flow during a solid phase to ground fault. IE, capacitive phase to ground reactance causes 1 amp to flow during a ground fault; the resistor's ohm value is sized to allow 1.5 or 2 amps to flow. It causes more current to flow during a fault yes (still a few amps and can be continues if the resistor is rated for it), but this "shunts" the resonant effect of an arcing ground fault on an ungrounded system. At lower voltages such as 240 its usually not a concern but at 16kv an arcing ground fault can cause significant voltage spikes.
FWIW this also holds true for ungrounded industrial systems. Hence why most newer systems are high resistance grounded. The factory can still run on a faulted phase, but the resistor prevents an arcing fault from destroying every MOV and puncturing cable insulation. A 1.73 rise still happens during a fault just not 6, 8 or 12 times voltage surges:eek::thumbsup:



Now, even though current does flow its not enough to trip a standard recloser or breaker. Standard RCD logic is hard to pull of to due to the nature of it all, both the small amount (often under 5 amps) and the need to do complex differential current flows. You could add sensitive enough CTs on all the breakers and use complex logic (it does exist) to monitor currents going in and out of the feeders and phases, but its complicated an expensive. Setting up the relays to differentiate between a none faulted feeder and faulted one is difficult as well as accuracy under so little current. Generally the best thing to do is just add a 15 amp draw (continues rated if you want to have the benefit of running the feeder until the fault is found) resistor and use standard RCD logic. Makes things a whole lot easier.


However, there is an easy way to indicate a ground fault exists. California utilities that operate genuine ungrounded systems will take 3 25kva pole pigs and wire them up grounded wye primary broken delta secondary. The secondary is wired across a relay. The primary neutral is connected to the substation's ground mat. This is the only wye grounded load in the system. Under normal conditions not much voltage is present across the relay because the primary voltages are fairly even between phase to ground, but when a phase to ground fault occurs the delta secondary will attempt to circulate currents to balance the phases out. Because its a broken delta and it can't (relay is across it) a voltage rise takes place enough to close the relay. The relay limits the current flow but the voltage remains high enough to keep it closed as long as the ground fault exists. If the relay was not present (closed delta) a massive amount of current would flow eventually blowing the protective fuses. If those transformers were large enough, say 6MVA each, low impedance and had a closed delta secondary any ground fault out on the line would result in thousands of ampers of current flow! The system would behave as a solidly grounded wye, since it now has a neutral point!:cool: An artificial neutral yes, but none the less one that will behave like a normal one. The current would blow regular fuses and trip recloser on regular time current curves since the bank now gives a place for neutral current to flow. This is exactly what happens in a wye grounded delta banks of any size. They are basically an artificial neutral that tries to balance voltage discrepancies out. Your not hijacking this thread. Just giving a detailed account of why a Y delta bank wont work for the op in his predicament:p:lol:



Anyway, as for the poco once the relay closes it alarms them a ground fault exists. Its then up to them to do the hard part and track it down. Adding resistors in parallel with the relay can increase the current to the point regular RCD logic can pick it up, so it helps to pinpoint what circuit is in trouble.


Hope I didn't throw you off. Grounding and bonding complex topic with a lot of dynamic parts to it. Let me know if confused you on anything :)
 

meternerd

Senior Member
Location
Athol, ID
Occupation
retired water & electric utility electrician, meter/relay tech
Not to worry...I'm easily confused. Where do I start? First off, we use two bushing transformers because our transformer cases are not grounded. A single bushing 7200V transformer has one side of the primary winding internally bonded to the common neutral of the secondary. That is in turn bonded to the case. That's a no-no, at least here. The primary neutral bushing is connected to the system neutral on the pole.

Anyway...maybe we are talking different stuff about substation connections. What I'm talking about is that we wire our step down transformers A-C, B-C, C-A in the subs. To me, that's Delta. As a relay tech, we used to wire our differential relay primary CT's Wye and secondary CT's Delta to compensate for the phase shift. As I mentioned above, we do have a small section of 14,400 3W Wye grounded at the sub, with no neutral brought out to the system. I know that a ground on this system produces fault current, because we burned down a pole and about a hundred feet of phone wire when a tree broke off a cross arm, dropping a phase onto the phone wires. The circuit never tripped. Fire was called in by a passerby. With a truly ungrounded system (I'm not familiar with Delta distribution), I don't see how there could be any ground fault current since there is no ground reference back to the sub. Only phase to phase would cause any real current. Where I worked, we have trees and mucho snow, so down wire, trees, etc. are a constant problem. Seems to me the simplest way to clear a ground fault quickly is to blow a cutout, operate a recloser or trip a breaker relay. That all takes fault current. To get that, we try to have a solidly grounded neutral all they way back to the sub. If a wire is down and is buried in snow or underbrush and an innocent skier or hiker steps on it, couldn't that prove fatal even on an ungrounded system? And in the summer, the fire danger is huge. Quick tripping and setting the reclosers to "one-shot" is one way we try to minimize the hazards. I'm pretty sure an ungrounded medium voltage system could still produce an arc to ground. Am I wrong?
 

mbrooke

Batteries Not Included
Location
United States
Occupation
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Not to worry...I'm easily confused. Where do I start? First off, we use two bushing transformers because our transformer cases are not grounded. A single bushing 7200V transformer has one side of the primary winding internally bonded to the common neutral of the secondary. That is in turn bonded to the case. That's a no-no, at least here. The primary neutral bushing is connected to the system neutral on the pole.

Anyway...maybe we are talking different stuff about substation connections. What I'm talking about is that we wire our step down transformers A-C, B-C, C-A in the subs. To me, that's Delta.

Is this the primary connection such as the 66kv? I was mainly referring to the MV secondary windings (in say a 66kv to 12kv 40MVA step down transformer) that supplies the distribution system. There are several ways to ground the secondary XO. The most common is Delta wye with the XO solidly grounded (bonded) to the substation mat. The second most common is delta delta which gives a genuine ungrounded system on the MV.


As a relay tech, we used to wire our differential relay primary CT's Wye and secondary CT's Delta to compensate for the phase shift. As I mentioned above, we do have a small section of 14,400 3W Wye grounded at the sub, with no neutral brought out to the system. I know that a ground on this system produces fault current, because we burned down a pole and about a hundred feet of phone wire when a tree broke off a cross arm, dropping a phase onto the phone wires. The circuit never tripped.

What are your ground pick up values set at in the breakers/reclosers? If this feeder has only phase phase connected loads the pick up values should be set as low as possible, preferably with a definite time current curve but as long as its set to operate quick it doesn't matter much.


Fire was called in by a passerby. With a truly ungrounded system (I'm not familiar with Delta distribution), I don't see how there could be any ground fault current since there is no ground reference back to the sub.

You don't need a ground reference. The system already has one from the capacitive potential that exists between phases and earth. The fault current is usually under a few amps, but still high enough to start a fire or kill. Ordinary CTs and relaying will not catch this.


Only phase to phase would cause any real current.

Phase to phase would cause major currents to flow only limited by the fault impedance and that between the 2 grounded phases. Far more than capacitive coupling.


Where I worked, we have trees and mucho snow, so down wire, trees, etc. are a constant problem. Seems to me the simplest way to clear a ground fault quickly is to blow a cutout, operate a recloser or trip a breaker relay. That all takes fault current.

Either high fault current, an arcing signature, high zero sequence values, ect ect anything that's programmed as abnormal. The best way to clear a fault to open all 3 phases with a recloser or breaker. Cut outs help but are limited in protection ability over a micro processor recloser or breaker relay. Cutouts also can leave one phase energized while another is down.




To get that, we try to have a solidly grounded neutral all they way back to the sub.

Not necessarily. Its the easiest way, but if zero sequence differential relaying is involved a resistor can be inserted to limit the fault current. Limiting the fault current has many benefits (limitations on thermal and magnetic stress, arc flash reduction, less fault carnage ect) and if summing type relaying is involved a fault current of 50 amps will trip a breaker just as fast as 8000.




If a wire is down and is buried in snow or underbrush and an innocent skier or hiker steps on it, couldn't that prove fatal even on an ungrounded system? And in the summer, the fire danger is huge. Quick tripping and setting the reclosers to "one-shot" is one way we try to minimize the hazards. I'm pretty sure an ungrounded medium voltage system could still produce an arc to ground. Am I wrong?

Your correct that an ungrounded system can leave a phase energized, down and appear dead. Most ungrounded systems are not relayed to trip on such faults because that's either the intention or its just to complicated to pull of.

Ungrounded delta systems and Multi grounded neutral systems are the most dangerous when it comes to downed conductors since both are very, very likely to leave a downed conductor energized. Most ungrounded systems don't trip for single phase to earth faults, and most MGN systems rely on massive fault currents in the order of hundreds or thousands of amperes to activate a protective relay. Most faults don't hit the grounded neutral but rather land on asphalt, concrete or dirt where many times the fault current isn't enough to clear a device. The conductor sparks and arcs for some time. The reason is that reclaoser and breakers have very high ground trip values to allow for phase to neutral load imbalances. Ground pick up values are usually between 1/3 to 2/3 the phase value. Systems that have all loads connected in delta out on the line have the advantage that ground pick up values can bet set to a few amps, clearing the fault very fast as well as when high impedances are involved.

The best systems at clearing faults are those that have an EGC run along the poles with multiple ground rods, all tanks and equipment are bonded to it, the substation neutral is solidly grounded, all loads are connected phase to phase and protective relays employ differential RCD and arc fault wave form analysis, in addition to standard inverse time curves.
 
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