Ideal xfmr(s) for connecting 480/277Y inverters (60kW total) to 12470/7200 grid

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1 It looks like jaggedben has been injecting useful knowledge into this thread while I have been off-line.
you are asking interesting questions that are making people think outside of the box in useful ways.

2 In any case, there are many things that are _possible_ but which are very unlikely. Some approaches to a particular problem might be clever and elegant in a specific situation, but simply non-standard, and not used because you couldn't easily get support.

3 It _is_ possible to put transformers in parallel, and in theory a pair of 50kva single phase transformers could be connected to a single primary phase with secondaries in parallel, but this is unlikely. There are lots of reasons for this, but I suspect that if the power company puts multiple transformers on the pole, then they will be arranged to provide 3 phase power.

4 It _is_ possible to set 3 single phase 7200:120/240V transformers in a 3 phase bank, and then join the neutrals of these transformers, giving 'hexaphase' (mentioned earlier), and with custom (read expensive) transformers you could connect your three phase inverters to this non-standard power system. In addition to custom transformers, you would also need custom circuit breaker assemblies to properly protect the transformers, and it might not be possible to implement the required protection in a code compliant fashion. (This is a physical possibility, but so 'out to lunch' that it isn't worth exploring the details of the code involved.)

5 It is possible to take 3 phase power and convert it to single phase power;

6 If you get a single phase '600A' service, it is _very_ likely that the power company will simply set a single 50kVA transformer, possibly even using the existing 25kVA transformer and simply giving you fatter wires to your service drop. The power company likes to load things as much as possible, know that transformers can take substantial overloading for considerable time, and know that a '600A' service will _never_ be used at the full 600A for more than moments at a time.


1 Yes, hes sure has, and thanks.

2 That's part of the idea- someone (other than myself) might think of something that someone else might not.

3 That weird setup, from page 209 there, appears to be 2 xfmrs, each on it's own overhead line (3rd overhead not used) running into "120/240 open delta" or some such thing, which I'm thinking is two 25kVA xfmrs put together to give 400A 120V service, so the 400A isn't drawn from just one overhead. I could of course be wrong there.

4 I'm thinking of three 277 to 120/240 single phase connected that way, Y from inverter to delta for load- if you don't connect the neutrals on the load side, you have a delta, and can go with high leg for the load- the inverters can feed Y to delta xfmr(s). Not trying to get anywhere near hexaphase.

... one 12.47 to 480/277 delta to wye model to put the PV straight into the grid.
Inverters going out at 277V because the N is connected (leading out) on the LV side.
Then three 277 to 120/240 xfmrs for the load
...

5 Arrgh. And there are more than a few ways.
Scott T Transformers

This is another popular method of generating a single phase supply from a three phase supply this time using two single phase transformers, The “Main Transformer” and the “Teaser Transformer” The main transformer is connected to 2 of the 3 supply lines of the 3 phase system. The teaser transformer primary is connected between a center tap on the primary of the main transformer and the third supply line.

6 Even if 600A is 75kVA at 120V...and the 72A of the PV system = 525A at 120V if I have that right...so a 50kVA=400A at 120V xfmr would be overloaded 30% for a few hours on most sunny days bfrom the PV power?
 
me-And they offer 277/480 single phase service at 100A also. Arrgh.

No, they don't. 480/277 is always three phase.

Well, I did say 277/480, which is this, from their manual.
Commercial 277/480 --Phases:1 --Amps: 100 --Wires: 3 -- Meter Terminals: 5 --Sequence:Cold -- Bypass req.: Yes --See diagram: 7.3-4

Single phase inverters can be connected to either single phase or three phase services, but three phase inverters can only be connected to three phase services.**

Unless you do something totally overcomplicated and pointless, as winnie seems to have said. Got it.

I do not mean to sound unkind, but the reason that all this is confusing to you is that you do not understand the fundamentals of single phase and three phase power.

Dude, I also discuss gun control and genetic engineering with strangers on the internet- you sound like Mother Teresa, trust me!
I am in the process of filling in huge gaps in my knowledge, and xfmrs are probably a terrible place to start, sure...
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
Thanks for hanging in here, ben.

2 However, it could be two services with separate PV systems on each, and those PV panels could all be in the same spot, just wired to different inverter(s) for each service. Does the NEC address that anyhwere, anything about physical separation of panels/inverters as opposed to electrical separation?

In order to justify the second service under the NEC it would have to have ONLY pv connected to it. The first service could have pv and loads. And no, generally speaking, there are no restrictions in the NEC regarding physical separation. You can connect panels on the same array to multiple inverters. Some jurisdictions might require you to run the wiring in separate conduits, which is not too big a deal.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
3 That weird setup, from page 209 there, appears to be 2 xfmrs, each on it's own overhead line (3rd overhead not used) running into "120/240 open delta" or some such thing, which I'm thinking is two 25kVA xfmrs put together to give 400A 120V service, so the 400A isn't drawn from just one overhead. I could of course be wrong there.

A _delta_ is a type of 3 phase supply. A 3 phase supply may be used to feed single phase loads; you simply use only 2 of the supply terminals.

The type of supply shown above is essentially a hybrid of 3 phase supply and single phase supply. It's _single phase_ capability is set by the rating of the mid point tapped transformer. The other transformer (called the 'stinger') is used only to supply 3 phase loads.

This setup does not _convert_ three phase power to single phase power; it simply _supplies_ single phase power from one of the 3 phase supply legs, and three phase from 2 of the primary supply legs (plus the neutral!). It is specifically used where the major loads are single phase, but there is a small three phase load.

4 I'm thinking of three 277 to 120/240 single phase connected that way, Y from inverter to delta for load- if you don't connect the neutrals on the load side, you have a delta, and can go with high leg for the load- the inverters can feed Y to delta xfmr(s). Not trying to get anywhere near hexaphase.
Okay, this would not give you _single phase_; this would give you 240V 3 phase delta with 120V line-neutral available between 2 of the legs. You could put single phase 240V loads between any of the legs (A-B, B-C, or C-A) and you can put 120V loads on A-N or C-N. This is a normal arrangement of the secondary, but has a really big risk: leg B-N has 208V; put a circuit breaker in the wrong place and you damage equipment.

It is also far more common to make the primary _delta_ as well, so you would have 3 480V to 120/240V transformers, connecting only one of the neutrals. Again, this is a _3_ phase delta set up to supply both 3 phase 240V loads, single phase 240V loads and single phase 120V loads. It does _not_ convert three phase to single phase.

_Much_ more relevant to your situation is that these systems are designed to supply single phase loads from _three_ phase supplies. They can never be used in the opposite direction! They will not generate 3 phases from a single phase source. PV inverters are funny beasties. They _generate_ power, but they match the voltage that is supplied to them. To use a 3 phase inverter you will need a 3 phase supply.

5 Arrgh. And there are more than a few ways.
Scott T Transformers

This is another popular method of generating a single phase supply from a three phase supply this time using two single phase transformers, The “Main Transformer” and the “Teaser Transformer” The main transformer is connected to 2 of the 3 supply lines of the 3 phase system. The teaser transformer primary is connected between a center tap on the primary of the main transformer and the third supply line.
Actually the Scott T connection is used to convert between 3 phase and 2 phase systems, not 3 phase to single phase. 2 phase systems are pretty archaic, but they are a true polyphase system which delivers power continuously.

6 Even if 600A is 75kVA at 120V...and the 72A of the PV system = 525A at 120V if I have that right...so a 50kVA=400A at 120V xfmr would be overloaded 30% for a few hours on most sunny days bfrom the PV power?
[/QUOTE]

Hey, you have a 200A service. That is 200 *240V (_not_ 120V!) = 48kVA...being supplied by a 25kVA transformer...and in most areas you would have _several_ residences each with a 200A service supplied by a single 25kVA transformer. The utility likes to keep things loaded.

me-And they offer 277/480 single phase service at 100A also. Arrgh.
Well, I did say 277/480, which is this, from their manual.
Commercial 277/480 --Phases:1 --Amps: 100 --Wires: 3 -- Meter Terminals: 5 --Sequence:Cold -- Bypass req.: Yes --See diagram: 7.3-4
This is 2 legs of a 3 phase supply. Almost certainly used by someone who needs 480V single phase for something like lighting.

-Jon
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Dude, I also discuss gun control and genetic engineering with strangers on the internet- you sound like Mother Teresa, trust me!
I am in the process of filling in huge gaps in my knowledge, and xfmrs are probably a terrible place to start, sure...
I am glad to hear that I haven't offended you. Three phase power is strange to everyone at first; I have a BSEE and it was mindbending to me when I first had to deal with it.

I think if you were to study the fundamentals of three phase power the rest of this would be much clearer to you. At the grossest level transformers are pretty simple devices. You trade voltage for current at the rate of the turns ratio, and where the grounding point is establishes the voltage to ground of the rest of the nodes.
 
1 A _delta_ is a type of 3 phase supply. A 3 phase supply may be used to feed single phase loads; you simply use only 2 of the supply terminals.

2 The type of supply shown above is essentially a hybrid of 3 phase supply and single phase supply. It's _single phase_ capability is set by the rating of the mid point tapped transformer. The other transformer (called the 'stinger') is used only to supply 3 phase loads.
This setup does not _convert_ three phase power to single phase power; it simply _supplies_ single phase power from one of the 3 phase supply legs, and three phase from 2 of the primary supply legs (plus the neutral!). It is specifically used where the major loads are single phase, but there is a small three phase load.


3 Okay, this would not give you _single phase_; this would give you 240V 3 phase delta with 120V line-neutral available between 2 of the legs. You could put single phase 240V loads between any of the legs (A-B, B-C, or C-A) and you can put 120V loads on A-N or C-N. This is a normal arrangement of the secondary, but has a really big risk: leg B-N has 208V; put a circuit breaker in the wrong place and you damage equipment.

4 _Much_ more relevant to your situation is that these systems are designed to supply single phase loads from _three_ phase supplies. They can never be used in the opposite direction! They will not generate 3 phases from a single phase source. PV inverters are funny beasties. They _generate_ power, but they match the voltage that is supplied to them. To use a 3 phase inverter you will need a 3 phase supply.


5 Actually the Scott T connection is used to convert between 3 phase and 2 phase systems, not 3 phase to single phase. 2 phase systems are pretty archaic, but they are a true polyphase system which delivers power continuously.

6 Hey, you have a 200A service. That is 200 *240V (_not_ 120V!) = 48kVA...being supplied by a 25kVA transformer...and in most areas you would have _several_ residences each with a 200A service supplied by a single 25kVA transformer. The utility likes to keep things loaded.


7 This is 2 legs of a 3 phase supply. Almost certainly used by someone who needs 480V single phase for something like lighting.

1 I've got that. It's a way of wiring either type of 3ph supply. The radial feeder here is 12470/7200 Y (not sure where the Y is supposed to go there), meaning L-L is 12470 and L-N is 7200.
Assuming that I'm correct about the POCO "keeping the N on their side", meaning the lines will be connected to N (and grounded?) the three xfmrs for 480/277 service would be 7200Y primary to 480/277 secondary.
The thing is, with 480/277 service and 480/277 inverters, there is no xfmr. (Or the inverters are the xfmrs?)
So..how does that fact affect this below? (I might have had the wrong dang state before with the bullet points....)
Kinda sounds like the 2nd one here is saying "ok to attach the Tripowers directly, since they're the equivalent of the grounded Y secondary of a xfmr."
The "interface" here means "your xmfr changing the PV voltage to grid voltage."
But there is none!

Effectively Grounded, Four-wire Multi-grounded 3-phase Wye EPS:
The Company requires that all interface transformers be configured to have a
wye connected primary winding with a fully insulated neutral, and the secondary
winding to have a delta connection.
• If infeasible, an alternative that is subject to Company acceptance review is a
primary wye grounded - secondary wye grounded transformer requiring a
grounded source permitted under specified conditions.
• Any DG or aggregate DG below 500kW in a Customer’s facility may be
permitted to utilize a primary delta - secondary wye grounded transformer.


2 Right- I was just saying that's an example of single phase (and 3ph) service coming from two overheads.

3 Yep, I think I have that too- that's why the "stinger leg" has to be orange by code and such.

4 OK. Check. Not backwards there, makes sense, thanks.

5 There's an example of something which might in fact work, but a lot of people might have...forgotten about?

6 I'm sorry- I really don't get that- it's 120/240 split phase, so if it was 200A/240V service, that would be two legs of 120V @ 200A, which would be 400A service.
It's two legs of 120V, 100A each, so 200A total, and only 100A at 240V.
Why would a basic residential service be 400A of 120?
The reason the MDP trips under heavy 240V load is the dozen or so 6-10 amp motors are pulling all or close enough to 100A- that's the theory.
Don't yell at me if that's 100% wrong, I thought we went over it a while ago.

7 Isn't 277 for lighting, not 480? So 277/480 would be for mostly lights and a few small 480V loads? Instead of the 3ph 480/277, which is for mostly 480 motor loads and some 277 lighting?

In order to justify the second service under the NEC it would have to have ONLY pv connected to it. The first service could have pv and loads. And no, generally speaking, there are no restrictions in the NEC regarding physical separation. You can connect panels on the same array to multiple inverters. Some jurisdictions might require you to run the wiring in separate conduits, which is not too big a deal.

Great, cool. I'll just assume that's separate CONDUITS, and not trenches, unless you correct me.

I am glad to hear that I haven't offended you. Three phase power is strange to everyone at first; I have a BSEE and it was mindbending to me when I first had to deal with it.
I think if you were to study the fundamentals of three phase power the rest of this would be much clearer to you. At the grossest level transformers are pretty simple devices. You trade voltage for current at the rate of the turns ratio, and where the grounding point is establishes the voltage to ground of the rest of the nodes.

I'm thinking about going for at least an associate's degree- not sure what type yet. Maybe electrical/industrial automation or something.
Yep, they're simple (kinda)- it's the 3 bazillion possible combinations that are the hard part!
 
Hey, you have a 200A service. That is 200 *240V (_not_ 120V!) = 48kVA...being supplied by a 25kVA transformer...and in most areas you would have _several_ residences each with a 200A service supplied by a single 25kVA transformer. The utility likes to keep things loaded.

If the service was via 240 delta, 200A center tapped, having the weird 208 leg, (last diagram in link) that would twice what it is now.
When it's 120/240, the "split" is referring to the single overhead line of 7200V coming out of the single pole xfmr as two legs of 120V, which you combine for your 240V.
http://www.programmablepower.com/support/FAQs/DF_AC_Distribution.pdf

I mean, you can overload the xfmrs, but several services of 200A, if 200A = 48kVA and several is...4...that's 192kVA on a 25kVA xfmr...that doesn't sound possible.

I really think I'm right here, possibly for the first time in this 14 page thread! :huh:
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
I'm thinking about going for at least an associate's degree- not sure what type yet. Maybe electrical/industrial automation or something.
Yep, they're simple (kinda)- it's the 3 bazillion possible combinations that are the hard part!
With a better understanding of three phase power you'll see that there really aren't all that many unique variations.

With a three phase 4 wire 480/277 service you would combine the inverters' outputs in a three phase AC combiner and interconnect with the service. No transformers. This is the best way to go with Tripowers.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
Assuming that I'm correct about the POCO "keeping the N on their side", meaning the lines will be connected to N (and grounded?) the three xfmrs for 480/277 service would be 7200Y primary to 480/277 secondary.
Not quite.
If you have 3 single phase transformers, then they would have 277V secondaries. The three of them would be connected together in a wye bank to give 480/277V.

The thing is, with 480/277 service and 480/277 inverters, there is no xfmr. (Or the inverters are the xfmrs?)
There will still be the utility transformers.

So..how does that fact affect this below?
[...]
Effectively Grounded[...]
• Any DG or aggregate DG below 500kW in a Customer’s facility may be
permitted to utilize a primary delta - secondary wye grounded transformer.
The details of 'effective grounding' as far as the utility is concerned are totally different than what electricians normally deal with on the customer side of things. This has to do with how distributed generation interacts with the POCO systems in the event that one of the POCO wires faults. I don't know any more than the reason for why the requirements exist, not how to properly implement them. As you learn more about this particular topic, writing about it would be a great way to return knowledge to the forum.

6 I'm sorry- I really don't get that- it's 120/240 split phase, so if it was 200A/240V service, that would be two legs of 120V @ 200A, which would be 400A service.
It's two legs of 120V, 100A each, so 200A total, and only 100A at 240V.
Why would a basic residential service be 400A of 120?

This is just the standard way of naming services. 200A is the 'trip rating' of the main breaker on the service. Breakers are rated in terms of the _per leg_ trip rating, not some sort of 'total' trip rating. (I won't get into the meaning of the trip rating, because it isn't as simple as 'don't trip at 199A, do trip at 200A.) This means that a 2 pole 200A breaker is essentially composed of two single pole 200A breakers, connected together and arranged for 'internal common trip'. Yes, a 200A 240V service delivers the same VA as a 400A 120V service...but you never call that 200A 240V service a 400A service. It is a 200A 240V service. Similarly a 200A 480V service is _not_ an 800A 120V service!

As to why a residential service might need 48kVA of service? Dunno. The _average_ consumption of an average home in the US is about 1-2 kW. But peak loads can be higher.

If the service was via 240 delta, 200A center tapped, having the weird 208 leg, (last diagram in link) that would twice what it is now.
When it's 120/240, the "split" is referring to the single overhead line of 7200V coming out of the single pole xfmr as two legs of 120V, which you combine for your 240V.

Yes, if the POCO adds a second 240V 25kVA transformer to the current one, in an 'open delta' configuration, then you will have a 3 phase source that is roughly double the capacity of your current single phase system. (Actually 25 * 2 * 86.6% because it is an incomplete 3 phase system)

I mean, you can overload the xfmrs, but several services of 200A, if 200A = 48kVA and several is...4...that's 192kVA on a 25kVA xfmr...that doesn't sound possible.

So if several 200A 240V residential services really meant a significant portion of 48kVA being used by each for significant periods of time, then it would not be possible. But as I said, the average consumer on average only uses some 1-2kW.

-Jon
 
Assuming that I'm correct about the POCO "keeping the N on their side", meaning the lines will be connected to N (and grounded?) the three xfmrs for 480/277 service would be 7200Y primary to 480/277 secondary.

Not quite.
If you have 3 single phase transformers, then they would have 277V secondaries. The three of them would be connected together in a wye bank to give 480/277V.
---

The details of 'effective grounding' as far as the utility is concerned are totally different than what electricians normally deal with on the customer side of things. This has to do with how distributed generation interacts with the POCO systems in the event that one of the POCO wires faults. I don't know any more than the reason for why the requirements exist, not how to properly implement them. As you learn more about this particular topic, writing about it would be a great way to return knowledge to the forum.
---

This is just the standard way of naming services. 200A is the 'trip rating' of the main breaker on the service. Breakers are rated in terms of the _per leg_ trip rating, not some sort of 'total' trip rating. (I won't get into the meaning of the trip rating, because it isn't as simple as 'don't trip at 199A, do trip at 200A.) This means that a 2 pole 200A breaker is essentially composed of two single pole 200A breakers, connected together and arranged for 'internal common trip'. Yes, a 200A 240V service delivers the same VA as a 400A 120V service...but you never call that 200A 240V service a 400A service. It is a 200A 240V service. Similarly a 200A 480V service is _not_ an 800A 120V service!

As to why a residential service might need 48kVA of service? Dunno. The _average_ consumption of an average home in the US is about 1-2 kW. But peak loads can be higher.
Yes, if the POCO adds a second 240V 25kVA transformer to the current one, in an 'open delta' configuration, then you will have a 3 phase source that is roughly double the capacity of your current single phase system. (Actually 25 * 2 * 86.6% because it is an incomplete 3 phase system)

So if several 200A 240V residential services really meant a significant portion of 48kVA being used by each for significant periods of time, then it would not be possible. But as I said, the average consumer on average only uses some 1-2kW.

1 Assuming the 3 POCO xfmrs are 7200V L-N on their side, couldn't they be wired either 480 L-L OR 277 L-N on the inverter side? But SMA does NOT recommend the delta approach.
These are in reverse order- going by these, it would have to be either 12470V delta on the POCO side, (3rd one) or most probably the 4th, 7200V to 277Y but not sure what the difference between "outward" and "out" is...but it would seem the Tripowers only connect at 277 L-N...is L-L ever even used with Tripowers then? Hmm.

Sunny Tripower TLUS needs a neutral conductor, so transformers with delta topology in the low voltage side are not
compatible for direct connection of the inverter (see figure below e.g. YNd1).

SMA only restrictively recommends "Yyn_" transformers with a star connection on the medium-voltage side and a star
connection on the low-voltage side with a neutral point that leads outward (see figure below e.g. Yyn0).

SMA recommends "Dyn_" transformers with a delta connection on the medium-voltage side and a star connection on the
low-voltage side with a neutral point that leads outward (see figure below e.g. Dyn1).

SMA recommends "YNyn_" transformers with a star connection on the medium-voltage side with a neutral point that leads
out
and a star connection on the low-voltage side with a neutral point that leads outward (see figure below e.g. YNyn0).

http://files.sma.de/dl/7418/STP24-US_MV_Trafo-TI-en-10.pdf
---

I will definitely let you know about POCO grounding...when I get a clue!
I can theorize that, just like they want your loads to be balanced, they want the PV output to "not interfere".
Even clouds going across the panels can mess with grid voltage, so I THINK that, since neutral is only supposed to go to ground at one point ONLY (right?), they want their neutral doing that on their side, and yours on your side, so "harmonic imbalances" don't get transferred., among other things.
---

And all I can say is the POCO guy told me on the phone that "their records indicate" that it's a 25kVA transformer, might be 30 or 40 years old, might be less.
I'd just think it would have blown already running at 100% overloaded quite often for that many years.
So that's 208 amps at 120V and 104A at 240V.
This is how they list it:
Commercial 120/240 Phases:1 Amps:100 / 200

That has to mean 200A at 120 and 100A at 240.

My electric water heater (the new efficient one) pulls like 25A at 240V, so it does make sense that a 48kVA (50?) could run a bunch of houses- that's 200A, so eight water heaters all at once, not gonna happen every day...not everyone does their laundry at the same time...but in the middle of summer when everybody turns on the AC at the same time, they don't want a xfmr to go kablooey, so...they have to pay some sort of attention to the actual loads?
But the load in this case is more like 4 or 5 houses all by itself- one 5hp motor and another 10 or 12 smaller, maybe 2hp average. So if 1hp is 7-hundred-something watts, but you're supposed to call it more like 1000w, 25hp of motors is 25kVA.
 
The details of 'effective grounding' as far as the utility is concerned are totally different than what electricians normally deal with on the customer side of things. This has to do with how distributed generation interacts with the POCO systems in the event that one of the POCO wires faults. I don't know any more than the reason for why the requirements exist, not how to properly implement them. As you learn more about this particular topic, writing about it would be a great way to return knowledge to the forum.

I can tell you that they don't want to desensitize the relays upstream.
What that actually means is totally up to you!
Trick: open the pdf, hit ctrl-f, and type in the word ground.

The Company may require that grounding impedance be limited to the highest value
suitable for neutral stabilization or to limit generator ground fault contributions.
Contribution to the faults on the Company’s four-wire distribution EPS can
desensitize the relays upstream of the Customer-Generator’s interconnection.
Hence, it is required that the Customer-Generator provide a means to install a
grounding reactor within their facility to limit the ground fault current to three times
the generator rating and, where possible, to limit the rise of ground fault current at
the point on the high voltage (primary) level nearest the proposed service point by no
more than 10%. The grounding reactor shall not violate the effectively grounded
system requirements. The size of the grounding reactor will be specified by the
Company. If the 10% criteria is not met with the grounding reactor, other methods,
upon Company acceptance may be required by the Customer-Generator to mitigate
the increase in fault current.
• If the Customer-Generator is permitted to interconnect through an un-grounded
source, a “zero-sequence” voltage or “3V0” scheme will be required on the primary
side of the approved delta primary wound transformer (see Section 5.5.3) supplying
the DG system. In cases where the Company’s EPS is an ungrounded circuit, the
3V0 scheme may be waived at the Company’s discretion on a case by case basis.
Refer to Figure 4 for additional information.

5.7.8 Overcurrent Relays
See Sections 5.3.4 and 5.5 for transformer requirements. Overcurrent protection is
required on the high side of the DG Customer’s interface transformer to detect faults on
the Company’s EPS. Voltage controlled overcurrent elements (51C) are required for
both phase and ground. These relays shall utilize voltage sensing via the Yg-Yg VTs on
the DG Customer’s transformer secondary delta winding to detect the single line-toground
faults. The 51C elements shall trip the high side interrupting device.

The DG Customer is responsible for over
voltage detection and the detection of line-to-ground faults on the primary and
secondary sides of the step-up transformer as well as the Company’s EPS. For
wye reactively grounded primary, delta secondary transformers, the DG
Customer is required to install Yg-Yg VTs on their transformer secondary delta
winding which will have the dual purpose to detect the voltage depression on the
faulted phase on low voltage line-to-ground faults to enable the operation of the
51C controlled overcurrent elements when faults occur on the Company EPS, as
well as, detect the over voltage on the unfaulted phases for single phase-toground
faults on the delta side of the step-up transformer. The use of Yg-Open
Delta VTs will be considered by the Company on a case-by-case basis.
http://www.nationalgridus.com/non_html/shared_constr_esb756.pdf
 
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