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  • PV backfeed into step-down transformer acceptable

    Hello,

    I wanted to inquire, whether it would be recommended to feed the Grid-Tied PV system back in the electrical panel which are either backed-up by emergency generator or panel receiving stepped down voltage from a higher voltage source.

    Our facility has 1200A, 3-phase, 575V, electrical system. Lighting and other standard 120/208V electrical services are provided via step-down transformer(s) further downstream in the electrical system. The facility also has a 70kW, 3-phase, 575V, emergency generator.

    Based on our evaluation, it may be feasible to install a 3 - 5 kW, 3-phase, 208V, 10-panel, Grid-Tied, micro-inverter based, PV System in the facility. We need to provide a 20A,3-Pole breaker in one of the 120/208V electrical panel which connects to the PV system. I have the following queries on the two (2) scenarios:

    1. Install 20A, 3-Pole breaker in a generator backed electrical panel

    - Is it recommended to install such a PV system in a generator backed panel?
    - During a power outage, the PV system should de-activate because it is mirco-inverter based. But if the panel is backed up by the generator and is energized during a power outage, would it be a safety concern as the PV system installation i.e. would be be re-energized?

    2. Install 20A, 3-Pole breaker in the 120/208V electrical panel which receives stepped-down voltage from 575V system

    - Is it recommended to install such a PV system in an electrical panel which recives stepped-down voltage?
    - Some step-down transformer manufacturers do not recommend feed back into the transformer, so would connecting a PV system be a safety issue or lead to void of any standards ratings?

    I'd appreciate your comments or suggestions on this issue.

    Thank you.

  • #2
    PV has to be connected to the Facility Line Side of the ATS so when the site is on generator, the Utility and PV are disconnected
    MSEE, PE

    Comment


    • #3
      There may be 2 things to consider
      Transformer size, it has to be large enough to allow the inverter to see a low impedance when it probes the AC line
      It's doubtful any standard generator will be stable enough 60.01 Hz to allow the inverter to qualify it. Inverter Generators ARE stable, and can get zapped by inverter backfeed
      Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
      || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
      || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

      solar: http://tinyurl.com/LMR-Solar
      gen: http://tinyurl.com/LMR-Lister

      Comment


      • #4
        Originally posted by katwalatapan View Post
        1. Install 20A, 3-Pole breaker in a generator backed electrical panel

        - Is it recommended to install such a PV system in a generator backed panel?
        - During a power outage, the PV system should de-activate because it is mirco-inverter based. But if the panel is backed up by the generator and is energized during a power outage, would it be a safety concern as the PV system installation i.e. would be be re-energized?
        The concern is damaging the generator. Expect damage if energy has nowhere to go but back into the generator. Size of PV compared to generator, building base load, and/or dump loads are things to think about.

        Originally posted by katwalatapan View Post
        2. Install 20A, 3-Pole breaker in the 120/208V electrical panel which receives stepped-down voltage from 575V system

        - Is it recommended to install such a PV system in an electrical panel which receives stepped-down voltage?
        - Some step-down transformer manufacturers do not recommend feed back into the transformer, so would connecting a PV system be a safety issue or lead to void of any standards ratings?
        PV power in relation to transformer size should influence your selection. Most step-down transformers are reported to run at about 30% loaded on average. If you match a PV system AC output with transformer size, the transformer better have been built to hand several hours near full load regularly. If not it can be expected to fail prematurely. Encapsulation may help reliability.

        Comment


        • #5
          Originally posted by adoublee View Post

          The concern is damaging the generator. Expect damage if energy has nowhere to go but back into the generator. Size of PV compared to generator, building base load, and/or dump loads are things to think about.



          PV power in relation to transformer size should influence your selection. Most step-down transformers are reported to run at about 30% loaded on average. If you match a PV system AC output with transformer size, the transformer better have been built to hand several hours near full load regularly. If not it can be expected to fail prematurely. Encapsulation may help reliability.
          Actually based on a lot of hands on data as well as engineering studies most step down (480v - 208/120v) distribution transformers are loaded below 17% and some below 10%.

          Crazy since the design for a dry type transformer shows the highest efficiency is based on 35% load.

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          • #6
            Thank you very much for your responses. I does provide a better understanding of alternatives.

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            • #7
              For e.g. if the 120/208V electrical panel is not able to use up the generated PV output and the power is back-fed into the 575V distribution equipment, then would the power be used by other panels or distribution equipment tapped into this 575V distribution equipment? or would it be fed back into the grid?

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              • #8
                It will take the path of least resistance, which would be expected to be the closest load (575V building distribution before grid transformer and grid).

                Comment


                • #9
                  Originally posted by katwalatapan View Post
                  For e.g. if the 120/208V electrical panel is not able to use up the generated PV output and the power is back-fed into the 575V distribution equipment, then would the power be used by other panels or distribution equipment tapped into this 575V distribution equipment? or would it be fed back into the grid?
                  You cannot really say.
                  You only know the sum of the currents, but you can approximate that higher currents will flow in the lower resistance paths.
                  SunnyBoy 3000 US, 18 BP Solar 175B panels.

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