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Thread: Step-Up to Step-Down Transformer Winding Configurations

  1. #1
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    Step-Up to Step-Down Transformer Winding Configurations

    Hi,

    Long-time lurker, first-time poster here with a configuration I would like to get the hive opinion on.

    I'm designing a 480kW, 480/277V PV system w/ 60kW Solectria string inverters. Although it's interconnecting into a 480/277V switchgear, the interconnect is located approximately 2,000' away from the array. I wanted to avoid stepping the voltage up, but it's just not practical to run 480V for this distance. I'm planning to step up to either 4,160V or 12,470V (most likely 4,160 but it will be a simple economic analysis) to transmit the power to the interconnection before stepping back down to 480V.

    My proposed configuration is to step from 480/277V G-Wye to 4.16/2.4kV G-Wye at the array, then from 4.16/2.4kV G-Wye to 480V delta at the interconnect. The 480V side of each transformer would be protected by a 3-pole breaker, the 4.16kV side of each transformer would be protected by built-in expulsion and current limiting fuses.

    Here's the thought process/justification:
    On the utility side, it is prudent to specify an un-grounded configuration to avoid excessive neutral currents in the event of voltage imbalances. A delta configuration allows zero-sequence current to circulate, preventing zero-sequence harmonics generated by the PV inverters from passing through and providing an effective ground for the 4.16kV neutral. The 4.16kV system would be G-wye to allow the fuses to operate in the event of a ground fault on the MV system.

    The inverters require a 4-wire system, so this constrains the primary (inverter) side of the step-up transformer to a Wye-G config. Given that the utility is isolated at the step-down transformer and there won't be any voltage/current monitoring on the MV system, the G-Wye 4.16kV configuration would minimize any chances that a lost-phase on the MV system would go un-detected by the inverters. Although a Delta winding would work, I believe this introduces the possibility of the inverters continuing to produce power even if a phase is lost on the MV system due to the single phase fusing used for protection.

    If there is a phase lost on the interconnection side than this may not be detected by the inverters due to the delta winding. Unlike the MV feeder, there will be a revenue meter on the interconnection side so the voltages can be monitored and addressed if there is an issue. Also the facility would notice this phase loss as well.

    This appears to be a relatively straight-forward design but I like to get alternate opinions, any thoughts?

  2. #2
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    Perhaps this is relevant, just found it in an inverter manual and thought of your post:

    If the neutral on the Utility Side is grounded, the transformer core
    structure must be 4 or 5 limbs to detect an open phase condition on
    the Inverter Side of the transformer. Special detection configurations
    are required to implement Inverter shut-down on loss of utility phase
    if the transformer core is a 3-limb construction.
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

  3. #3
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    60kW @ 480 volts is about 72 amps.

    I've run many 100 HP irrigation well motors with supply conductors in the 1500 foot range - and a time or two maybe closer to 1900 feet. Those typically are 118 FLA on the nameplate so even more kVA then you have. I've increased conductor size for voltage drop, I don't see using MV cable, associated gear plus two transformers costing less then increasing size of conductors, especially aluminum conductor like we usually are using for those irrigation feeds. 250 kcmil aluminum is what I always have run to 100 hp at 1300-1500 length, longer then that I do some calculating to see what I have, Significantly shorter I might do some calculating also.

  4. #4
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    Quote Originally Posted by kwired View Post
    60kW @ 480 volts is about 72 amps.

    ...
    Yes but evidently there are eight inverters (or perhaps six or seven), and I'm sure he would be combining the output before any step-up. Would you still make the same comments at 500 amps?

  5. #5
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    The best deal without stepping up would be to send the dc strings back, then you would be working with ~700 volts. PV wire is available in Aluminum. Combining 1 inverters worth of strings, if one used 350MCM, that would be in the ballpark. Now would be the time to reread that article in solarpro from a while back about the economics of not going crazy to keep voltage drop low Last I checked 350 AL THHN was 1.05 per foot, but we would need PV wire. WAG that it is $1.50/ft, that would be $6000 per inverter, with no EGC. Multiply that times 8. A 500KVA transformer is around 13K IIRC. 15KV primary cable is 2 something a foot. $2.50? Need 6000 feet = 15k. Wow its actually kinda close. Might be worth a closer look. One thing you can usually expect with MV is you will pay a lot for it if you cant do it in house. Dont forget your transformer losses. .5% each 24/7 plus load losses.

    Disclaimer: super quick and dirty analysis, take with a grain of salt.

    Edit: Maybe send the 480 back. Could skip the expensive PV wire, but would need three conductors (dont need a neutral for the solectria inverters, see my other post) works out about the same and it would be less of a bastard system and avoid the combiners and hassle of changing up and down to 350 PV wire.
    Last edited by electrofelon; 12-28-17 at 11:03 PM.
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

  6. #6
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    Quote Originally Posted by jaggedben View Post
    Yes but evidently there are eight inverters (or perhaps six or seven), and I'm sure he would be combining the output before any step-up. Would you still make the same comments at 500 amps?
    I missed that there were multiple units.

    Now that changes my thoughts to if you are going to have medium voltage anyhow to look into either having POCO run a service to the PV site or subscribe to MV as your service voltage.

  7. #7
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    Quote Originally Posted by kwired View Post
    60kW @ 480 volts is about 72 amps.
    Per the OP, it is a 480kW system.
    Still don't think I would go for the step up/step down option. Too much extra complication and additional points of potential failure. Not to mention the additional skills/qualifications/authorisation required.
    Si hoc legere scis nimium eruditionis habes.

  8. #8
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    Quote Originally Posted by Besoeker View Post
    Per the OP, it is a 480kW system.
    Still don't think I would go for the step up/step down option. Too much extra complication and additional points of potential failure. Not to mention the additional skills/qualifications/authorisation required.
    Quick and dirty comparison of "trading" transformer no load losses for more voltage drop. I get you could have 4% MORE VD without tranny than with to break even. And that is just no load losses.
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

  9. #9
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    Quote Originally Posted by Besoeker View Post
    Per the OP, it is a 480kW system.
    Still don't think I would go for the step up/step down option. Too much extra complication and additional points of potential failure. Not to mention the additional skills/qualifications/authorisation required.
    Certainly something to consider, I initially had blinders on and only saw 60 kW - definitely don't think the step up/step down would be worth the investment for that kW level.

    I know a fair amount of things associated with over 600-1000 volts applications - but would never install such equipment without additional training first. I'd want more $$ for such skill set if I had it also, not to mention I would likely have some equipment that goes along with doing such work.

  10. #10
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    Thanks for the input.

    The system consists of (8) 60kW inverters, which at 480V is approximately 584 amps. (6) 500 Cu conductors/phase gives me approximately a 2.3% voltage drop at full load. This would be in addition to the voltage drop from the inverter to the AC combiner box (one run is up to 400'). The interconnect is to a customer switchgear, so I can't go directly into the POCO's 12.27kV system.

    Regarding AC vs DC, moving the inverters to the interconnect would require DC combiner boxes at the arrays (the Solectrias have direct fused inputs) to the tune of $800 or so each. On the conductor side, it would need 500 Cu as well to keep voltage drop around 2%. The advantage would be only (16) conductors are required instead of (18), but it would be more expensive PV wire. In my experience 480V AC vs 1000V DC tends to be pretty similar in terms of line losses/cost and this is no exception.

    Good point on the neutral, on the Solectria I can leave off the neutral and just use a jumper from the EGC to the neutral terminal. I will have some small single phase loads (under 1kVA) at the array for a DAS system. I could use a 480V L-L xfrmer and then there will not be any L-N loads.

    I think the next step is to look at my hourly plant output over the year and compare losses and equipment costs. The MV transformers will run 12-19k each, plus concrete pads and additional installation costs. If it's looking pretty close, there are certainly advantages to avoiding the transformers and simplifying the design and install. Liquid fill transformers also have significant lead times. From a design perspective I'm comfortable w/ MV equipment, but there's a lot to consider with respect to winding configuration (hence the questions).

    I'll report back with some numbers after I get lost in excel for a few hours.

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