480 volt stepup

[ggreenx]

I have a project in which I need to provide a link between a 480 volt PV inverter, with a wye output, to a 480 volt utility service with a wye service (with loads connected on it). Due to the length of the proposed 480 volt feeder, we are going to stepup to 12kV, then stepdown back to 480 volt. I have a distribution manual that suggests I use a 480 volt delta to 12kV wye transformer for stepdown from the 480 volt inverter, and a 12kV wye to 480 volt delta stepup transfromer to get back to 480 volt and connect to the utility service bus. The manual suggests that delta wye stepup and wye delta stepdown is the best configuration.

In this scenario, I believe the 480 volt delta sides of the transformers are fine since the inverter and the utility service are both already grounded (neutral bonded to ground), and that each of the 12kV wye sides of the transformers would need to be grounded (neutral bonded to ground).

This proposed design seems to be fine with me based on what I know. Does anyone take exception to this proposed design or have any comments regarding this? Am I missing anything?

I had another engineer suggest to me that the transformers could each be delta delta but I am thinking that I need to have a ground reference for the 12kV feeder. I have also heard that it is always best to use delta wye transformers...something about zero sequence impedance.

 

[LarryFine]

GG, welcome to the forum! :smile:


I believe the best coniguration is delta-wye for both transformers, especially since your loads require a neutral.

There are many here who know better than I. I'm sure they'll respond soon.

 

[ggreenx]

480 volt stepup

Larry,

Thanks for your reply. Are you suggesting 480v wye to 12kv delta on one and 480v delta to 12kv wye on the other?

Gerry

 

[crossman gary]

At first thought, I think you need a 480 delta to 12KV delta from the inverter, and a 12 KV to 480 wye feeding to the service.

Is there any possibility of the inverter having a problem synchronizing with the service considering the xfmrs? Probably not, but I would check with the inverter manufacturer and see what they say.

 

[ggreenx]

At first thought, I think you need a 480 delta to 12KV delta from the inverter, and a 12 KV to 480 wye feeding to the service.

Is there any possibility of the inverter having a problem synchronizing with the service considering the xfmrs? Probably not, but I would check with the inverter manufacturer and see what they say.

I don't think there is a problem with that as it should only be a rotation item, although there will be a 30 degree shift due to the delta wye transformer. I am checking with the utility company to see what they have to say about it. Thanks for you feedback.
Gerry

 

[LarryFine]

Larry,

Thanks for your reply. Are you suggesting 480v wye to 12kv delta on one and 480v delta to 12kv wye on the other?

I am, with the delta toward the supply and the wye toward the load in all cases. That's based on stuff I've learned here. :smile:

 

[crossman gary]

At first thought, I think you need a 480 delta to 12KV delta from the inverter, and a 12 KV DELTA to 480 wye feeding to the service.

I just needed to be more explanatory in my opinion. I added DELTA after the 12KV above, just for clarification.

So the inverter is feeding a delta-delta 480 to 12kV xfmr, this xfmr feeding a delta-wye 12kV to 480 xfmr, and that xfmr feeding the service.

This opinion is different than Larry's. I am not sure which is better, because I am just guessing. I am interested in hearing more of the theory on this. Which installation would be most correct?

 

[ggreenx]

I am, with the delta toward the supply and the wye toward the load in all cases. That's based on stuff I've learned here. :smile:

Larry, your idea is actually what my first thought was for this system. I was thinking delta to wye in the direction of load flow as you have suggested. This would also be somewhat consistent with if I were to connect the inverter (which is a wye output) directly to the 480 volt service without the stepup stepdown scenario in that i would be connecting a 480 volt wye generator to the utility bus.

I had started to change my thinking on this based a paragraph in an electrical handbook that suggested the delta to wye and wye to delta configuration as being the best.

Anyway, load flow on the system theorically could flow either direction. With that in mind and if I were to look at either end of the system as a source point, then I would be inclined to make the 480 volt connection delta similar to what I would do if providing a 480 volt to 208 v load transformer. I have a call in to the utility company to see if they have a preference. i am currently leaning in the direction of what you had suggested. if i go that route, i will not bond the neutral of the 480 volt wye side where connecting the the utility load bus since it is already bonded at the service point. Any other thoughts?

 

[ggreenx]

I just needed to be more explanatory in my opinion. I added DELTA after the 12KV above, just for clarification.

So the inverter is feeding a delta-delta 480 to 12kV xfmr, this xfmr feeding a delta-wye 12kV to 480 xfmr, and that xfmr feeding the service.

This opinion is different than Larry's. I am not sure which is better, because I am just guessing. I am interested in hearing more of the theory on this. Which installation would be most correct?

crossman,

I think neither is more correct, but i have heard that it is best to have delta-ye or wye-delta vs delta-delta or wye-wye...not sure why though.

 

[crossman gary]

This is definitely an interesting topic and one that many of us will see in the future I suspect. Please keep us posted as to the outcome, or any other findings you have.

Anyway, load flow on the system theorically could flow either direction.

I'm not seeing it that way. When would energy ever flow from the service to the inverter? The only thing I see would be the minute quantity of energy required to tell the inverter that the service was energized, and to keep the inverter synched. When would the inverter ever act as a load?

go easy on me, I am just learning!

 

[LarryFine]

We need the POCO pros to chime in here.

Charlie?

 

[dbuckley]

I'd have thought the 12KV section would be all delta. You dont functionally need a neutral.

As the 480 sides need a neutral both ways, they would both need to be wyes.

inverter -> (wye/delta) -> 12kv -> (delta/wye) -> poco

 

[crossman gary]

I'm not getting the need for the wye on the 480 from the inverter. Couldn't we just leave that wire out? What would we do if the inverter was feeding a 480 volt 3 phase load that did not have a neutral?

Uh oh.....

Important point....

if the inverter is feeding through those xfmrs, then that inverter just became a seperately derived system. It has no solid neutral connection to the service.

 

[wirenut1980]

Definately need to pay attention to the 30 degree phase shift through the transformations to make sure the phase angles match up with the utility supply.

IMO, I think your only two choices are as follows:

inverter -> (wye/delta) -> 12kv -> (delta/wye) -> poco

inverter -> (wye/wye) -> 12kv -> (wye/wye) -> poco

The reason is that I think you want a wye going to meet the utility 480 V. The only difference between the two choices in my opinion are how you will do over-voltage protection, if that is even being considered. I don't think there is any big difference between using delta and using wye on the 12 kV part. I would probably go with the second option.

I am not 100% sure on this, but that is my 2 cents.:smile:

 

[wasasparky]

Definately need to pay attention to the 30 degree phase shift through the transformations to make sure the phase angles match up with the utility supply.

I know nothing about these inverters, but I'll chime in anyway.

It seems to me that as long as the inverter sees a 480V-Y, you should be good. Since you are going through transformers, the utility isn't going to "care" anymore if there is a neutral...

I don't think phase shift is an issue at all either. The utility feeds the transformers and you get 480V-Y at the inverter, the inverter will see and sync to that, regardless if there is a phase difference with the utility 480V...

Once again, just guessing here...

 

[crossman gary]

I don't think phase shift is an issue at all either. The utility feeds the transformers and you get 480V-Y at the inverter, the inverter will see and sync to that, regardless if there is a phase difference with the utility 480V...Once again, just guessing here...

I think you are correct on that, just not 100% sure.

As I mentioned, my thought is:

inverter -> (delta/delta) -> 12kv -> (delta/wye) -> poco

I just don't see the need for the wye after the inverter.

As an example, if we have a 480Y/277 service, and we want to derive a 208Y/120 system, our xfmr is going to be delta/wye probably 99.974% of the time.

 

[wirenut1980]

As an example, if we have a 480Y/277 service, and we want to derive a 208Y/120 system, our xfmr is going to be delta/wye probably 99.974% of the time.

True, but what if you are tying into another 208Y/120 V supply that is fed by at 480Y - 208Y/120 V transformer? There is a difference in phase shift and the transition from one system to another would have to break before make since they would not phase together.

I know it's not a likely scenario, just trying to stay with the analogy.:smile:

 

[dbuckley]

Having reread the OP again, I'm changing my mind to:

inverter -> (delta/delta) -> 12kv -> (delta/wye) -> poco

There is no need for a neutral back to the inverter if your entire load is phase to phase, which is the case for the delta transformer winding.

I don't think phase shift is an issue at all either. The utility feeds the transformers and you get 480V-Y at the inverter, the inverter will see and sync to that, regardless if there is a phase difference with the utility 480V...

It pretty much has to be that way. If it didn't then the inverter couldn't sync! Also, the anti-islanding provisions required for a POCO backfed connection (some ANSI standard I can't recall the number of) requires that the inverter senses the incoming (from POCO) power so that if the POCO becomes disconnected the inverter can shut down to prevent dangerous backfeeding.

 

[ggreenx]

I think you are correct on that, just not 100% sure.

As I mentioned, my thought is:

inverter -> (delta/delta) -> 12kv -> (delta/wye) -> poco

I just don't see the need for the wye after the inverter.

As an example, if we have a 480Y/277 service, and we want to derive a 208Y/120 system, our xfmr is going to be delta/wye probably 99.974% of the time.

My investigation further with the utility company and with the inverter maker somewhat matches your scenario. the only difference i have which is more of a personal preference is to go inverter->(delta-wye)->12kv ->delta-wye. My logic here is that I would like to wye ground the 12kv circuit. Apparently the inverter can adjust to whatever the translated circuit becomes after transformation and the utility does not care if we give them wye or delta.

Gerry

 

[crossman gary]

Having reread the OP again, I'm changing my mind to:

inverter -> (delta/delta) -> 12kv -> (delta/wye) -> poco

There is no need for a neutral back to the inverter if your entire load is phase to phase, which is the case for the delta transformer winding.

There ya go...

Re: Synching with the service

It pretty much has to be that way. If it didn't then the inverter couldn't sync!

although I feel there will be no problem with the inverter synching, I was just thinking there may be some harmonics or maybe a phase shift due to power factor of the xfmrs under load versus no load conditions, perhaps something that would upset the electronics in the inverter.

Most likely no problem, but you never know until the manufacturer says "good" or you give it a try and it works.