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    Backfeeding transformers with grid interactive micro-inverters

    This is my first post and I am hoping to get some insights on using a 150KVA - 480V(Delta) / 208V/120V (Wye) dry type transformer in a PV system using grid interactive inverters (specifically Enphase M250 micro-inverters) -

    The situation is this:
    Enphase M250 microinverters generate a 3-phase 208V output and connected to the low voltage (208V) side of a 150kva dry type transformer. These micro-inverters won’t power up until they sense the presence of the utility voltage which will need to come from the 480V side (primary) to the 208V (secondary) applying the 208V to the micro-inverters. In this specific application the 480V side will be the first side of the transformer to which a potential is applied which will in turn provide any inrush currents magnetically charging the transformer. Once the micro-inverters see the 208v potential on the secondary, they will then start producing an output which will be shared with the utility – through the transformer. So while power transfer will go back and forth between the 208V and 480V sides depending on the load sharing capability of the micro-inverters, the transformer will remain supplied from the 480V side, even when the micro inverters stop producing output after the sun sets. The primary (480V side) will remain the dominant source of power in the system.

    Problem:
    The transformer manufacturer specifies that when their transformer (Dry type 150KVA) is wired to be back fed or in a step up configuration the X0 connection should not be connected and not establish a neutral on the LV side. The reason given is:[COLOR=#ff0000](my commentary are provided in red)
    [/COLOR]
    1) the wye connection (120V) on the LV side is normally not needed when stepping up voltage ([COLOR=#ff0000]In my application the LV (208V) side needs to supply the 120V needed to power a Envoy communication gateway- which isn't possible without establishing a neutral on the LV side)[/COLOR]
    2) to avoid creating a direct path to ground and possibly generating parasitic currents on the case which may be harmful ([COLOR=#ff0000]not sure why potentials would be developed on the case of the transformer if it is properly grounded)[/COLOR]
    3) in the case of a short circuit or atmospheric discharge on the LV side affecting the HV side because the two sides not being isolated ([COLOR=#ff0000]again, not sure why there would be an issue with interaction between LV and HV sides if case is properly grounded - no neutral will be created or used on the 480V side)[/COLOR]

    I am hoping to get any insights on the hazards with connecting the LV side in a wye configuration with a neutral, that can help me to further understand the manufacturers concerns with connecting the X0 terminal of the transformer when stepping up voltage, or in my case, back feeding the transformer?

    Thanks,

    Badger

    #2
    Duplicate. Go [COLOR="#0000FF"]here[/COLOR] to reply.
    [COLOR="blue"]"Electricity is really just organized lightning." George Carlin


    Derek[/COLOR]

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