Hi,
I've searched the topic (first in NEC 2017, and then scoured the web) but haven't come to a satisfactory understanding of my options for a project we are currently designing out. Sorry I am probably telling you more than you need to know but I figured thorough is better than you getting frustrated with only part of what you need to help me out here.
It is a PV installation from a 3000A service, 480V, 3PH, 4WY fed from a 1500kVA transformer, we are installing a 1MW AC system. The interconnecting breaker is shunt trip enabled and we will have a relay on-site monitoring for islanding conditions, as well as neutral current from zig-zag transformer to ensure effective grounding (as required by the EDC), and tripping the interconnection breaker if any of these conditions exist.
Here is area I have some questions as to the best value engineering (not trying to be cheap just trying to not be frivolous as MV gear adds up quick and this system size isn't that big to justify not being budget conscious): we have a long distance to run and in order to combat losses and v-drop we are stepping up to 4800V delta and stepping back down to 480V wye at the array combiner gear (1600A CB or fuse) (additionally, the facility is has constraints that we must trench new conduit in concrete and the concrete has be remain glass smooth so all trenching is done with horizontal drilling, so also expensive and instead stepping up to 4800V we can use existing spare conduits). These transformers are in series with no load in the middle, technically article 690 (my 2017 model is in my office so not sure exact reference on the top of my head) tells me to examine the system with both sides of PV transformers as primary to see which way is the safest to run protection. The transformers will be manufactured with the primary winding towards the utility - they will be reverse fed by the solar but because they are soaked from the utility before the PV kicks on, inrush will be in the direction of utility so we want that direction to be the primary to minimize inrush. For what it's worth (I know for some here its not worth much in discussions I've seen) the site will be supervised and as I pointed out there will be SCADA and automated trip functions on the interconnected circuit. We haven't specified a %Z yet and I'm not worried about guidance on that but would like to know if your suggestions have any tie to a certain %Z of course.
I think I painted the picture but for summary we have, in order, utility main, utility metering cabinet, a split through a 4000A bus gutter to the existing 3000A main gear/loads, and a 1600A interconnection feeder on the other lug of the bus gutter on opposite end from utility (can be larger if need be to accommodate inrush). The main on the interconnection feeder is shunt trip enabled with supervisory control from an SEL utility grade relay. The main of that cabinet goes to a CT cabinet for production metering as well as CTs/PTs for the relay cabinet, then to a fused disconnect that is the EDS disconnect and also would function as the primary protection of the transformer (480v primary - could be delta or wye if that helps, 4.8kV delta secondary), also lugged off the bus is a 25kVA station service tap for PV related loads (for instance the relay, ST power, and communication equipment). Out of that transformer on the 4.8kV side there is a conductor that terminates solely at another 4.8kV transformer, but this time with a 4800V primary and 480V secondary which terminates solely in the PV combiner gear. Before anyone asks, yes, we asked the EDC to receive primary service on-site @ 13.8kV and they said no.
So the questions really revolve around primary and secondary protection here and what is required/desired. Mostly we aren't clear as to what is considered the primary or secondary side of each of the two transformers.
1)Inrush concerns feeding two transformers at the same time in series?
1a) one consideration is (even if no secondary protection on the first transformer) to have a disconnect near the main gear to be able to sequence soaking on black start to avoid nuisance tripping. If this is a silly thought then just place the disconnect by the transformer in the field as the service disconnect (it just occurred to me, I didn't is there any difference with MV service disconnects as far as the rule about line of site or placard plus lockable disconnect? this site will have lockable disconnect and electronic means of lock-out, tag-out on the entire PV feeder).
1a1) Any concerns of damage to the first transformer providing inrush to soak the second? Is there any merit to the thought that the first transformer while soaking would actually act somewhat as a soft-start for the second transformer actually reducing inrush? I can see this plausible even though its counter-intuitive in my brain because usually I think lower voltage, higher current...
2)What protection would you say is required in between the two transformers (secondary on the step up from the utility and primary on the step down from the solar? I'd venture to say that from a fundamental perspective you have generation on one side and the utility on the other side of the MV system therefore the entire MV system is secondary and both transformers are technically step up (utility is a voltage source primary in the direction from the utility to the transformers, the solar is a current source primarily flowing from the inverter to the transformer).
3) This is maybe a wacky thought, could CT's on the MV line function as secondary protection if overcurrent on the MV conductor was to shunt trip the interconnection breaker, via the relay on site. (we probably need to monitor the MV side anyway to ensure no islanding on loss of phase with a Delta-Wye Transformer)
3a) Same question but if shunt trip was moved to the primary protection device of the transformer does it make a difference
3b) if yes would you worry about an islanding condition even though there is no load between the transformers...would your concern be different if we used WYE configured transformers.
3c) any reason we'd want a specific location for the zig-zag relative to the primary OCPD of the first transformer (utility side)?
I think that gets it all. Sorry for the long winded post, just trying to give all the details I can to get some guidance on this. Usually we just go straight to utility voltage for projects large enough to warrant it and get primary metering on-site but this project, being relatively small, is a unique situation to deal with. Hopefully, I am not asking something that should have been right in front of my face somewhere, but as one would expect we need the project yesterday so many of these longer term things to think about and research has to be figured out relatively quickly...winter is coming.
Thanks in advance,
Steve
P.S. Not sure if I mentioned in but the MV run from xfmr to xfmr is about 1200 conductor feet.
I've searched the topic (first in NEC 2017, and then scoured the web) but haven't come to a satisfactory understanding of my options for a project we are currently designing out. Sorry I am probably telling you more than you need to know but I figured thorough is better than you getting frustrated with only part of what you need to help me out here.
It is a PV installation from a 3000A service, 480V, 3PH, 4WY fed from a 1500kVA transformer, we are installing a 1MW AC system. The interconnecting breaker is shunt trip enabled and we will have a relay on-site monitoring for islanding conditions, as well as neutral current from zig-zag transformer to ensure effective grounding (as required by the EDC), and tripping the interconnection breaker if any of these conditions exist.
Here is area I have some questions as to the best value engineering (not trying to be cheap just trying to not be frivolous as MV gear adds up quick and this system size isn't that big to justify not being budget conscious): we have a long distance to run and in order to combat losses and v-drop we are stepping up to 4800V delta and stepping back down to 480V wye at the array combiner gear (1600A CB or fuse) (additionally, the facility is has constraints that we must trench new conduit in concrete and the concrete has be remain glass smooth so all trenching is done with horizontal drilling, so also expensive and instead stepping up to 4800V we can use existing spare conduits). These transformers are in series with no load in the middle, technically article 690 (my 2017 model is in my office so not sure exact reference on the top of my head) tells me to examine the system with both sides of PV transformers as primary to see which way is the safest to run protection. The transformers will be manufactured with the primary winding towards the utility - they will be reverse fed by the solar but because they are soaked from the utility before the PV kicks on, inrush will be in the direction of utility so we want that direction to be the primary to minimize inrush. For what it's worth (I know for some here its not worth much in discussions I've seen) the site will be supervised and as I pointed out there will be SCADA and automated trip functions on the interconnected circuit. We haven't specified a %Z yet and I'm not worried about guidance on that but would like to know if your suggestions have any tie to a certain %Z of course.
I think I painted the picture but for summary we have, in order, utility main, utility metering cabinet, a split through a 4000A bus gutter to the existing 3000A main gear/loads, and a 1600A interconnection feeder on the other lug of the bus gutter on opposite end from utility (can be larger if need be to accommodate inrush). The main on the interconnection feeder is shunt trip enabled with supervisory control from an SEL utility grade relay. The main of that cabinet goes to a CT cabinet for production metering as well as CTs/PTs for the relay cabinet, then to a fused disconnect that is the EDS disconnect and also would function as the primary protection of the transformer (480v primary - could be delta or wye if that helps, 4.8kV delta secondary), also lugged off the bus is a 25kVA station service tap for PV related loads (for instance the relay, ST power, and communication equipment). Out of that transformer on the 4.8kV side there is a conductor that terminates solely at another 4.8kV transformer, but this time with a 4800V primary and 480V secondary which terminates solely in the PV combiner gear. Before anyone asks, yes, we asked the EDC to receive primary service on-site @ 13.8kV and they said no.
So the questions really revolve around primary and secondary protection here and what is required/desired. Mostly we aren't clear as to what is considered the primary or secondary side of each of the two transformers.
1)Inrush concerns feeding two transformers at the same time in series?
1a) one consideration is (even if no secondary protection on the first transformer) to have a disconnect near the main gear to be able to sequence soaking on black start to avoid nuisance tripping. If this is a silly thought then just place the disconnect by the transformer in the field as the service disconnect (it just occurred to me, I didn't is there any difference with MV service disconnects as far as the rule about line of site or placard plus lockable disconnect? this site will have lockable disconnect and electronic means of lock-out, tag-out on the entire PV feeder).
1a1) Any concerns of damage to the first transformer providing inrush to soak the second? Is there any merit to the thought that the first transformer while soaking would actually act somewhat as a soft-start for the second transformer actually reducing inrush? I can see this plausible even though its counter-intuitive in my brain because usually I think lower voltage, higher current...
2)What protection would you say is required in between the two transformers (secondary on the step up from the utility and primary on the step down from the solar? I'd venture to say that from a fundamental perspective you have generation on one side and the utility on the other side of the MV system therefore the entire MV system is secondary and both transformers are technically step up (utility is a voltage source primary in the direction from the utility to the transformers, the solar is a current source primarily flowing from the inverter to the transformer).
3) This is maybe a wacky thought, could CT's on the MV line function as secondary protection if overcurrent on the MV conductor was to shunt trip the interconnection breaker, via the relay on site. (we probably need to monitor the MV side anyway to ensure no islanding on loss of phase with a Delta-Wye Transformer)
3a) Same question but if shunt trip was moved to the primary protection device of the transformer does it make a difference
3b) if yes would you worry about an islanding condition even though there is no load between the transformers...would your concern be different if we used WYE configured transformers.
3c) any reason we'd want a specific location for the zig-zag relative to the primary OCPD of the first transformer (utility side)?
I think that gets it all. Sorry for the long winded post, just trying to give all the details I can to get some guidance on this. Usually we just go straight to utility voltage for projects large enough to warrant it and get primary metering on-site but this project, being relatively small, is a unique situation to deal with. Hopefully, I am not asking something that should have been right in front of my face somewhere, but as one would expect we need the project yesterday so many of these longer term things to think about and research has to be figured out relatively quickly...winter is coming.
Thanks in advance,
Steve
P.S. Not sure if I mentioned in but the MV run from xfmr to xfmr is about 1200 conductor feet.