Single-phase inverter connected to 3-phase distribution board design

minabenyamin
minabenyamin Solar Expert Posts: 45 ✭
in Grid Tie and Grid Interactive Systems #1
Hello dear friends,

I want to discuss something with you. For now I've a GT system with a 5kW single phase inverter. This inverter would supply a around-35kW-load workshop. The mains of the workshop which is feeding the main distribution board - which the inverter would be connected to- is a 3-phase supply from the utility.
My question is how I will connect my inverter, I mean to which phase. or I can connect the inverter output to any phase of them.

Another question relating to the inverter output. As far as I know, the inverter makes some kind of synchronization with the utility, and thus is done by monitoring the wave power coming from the utility and when it's same as the wave power that the inverter will generate, it starts to pump its power. if my info is not right pls tell me. my question is that, the inverter has only one AC terminal which is output and there's no AC inputs at all, so how the inverter can read ( detect) the wave power coming from the utility to monitor it and check if it's same as its wave or not to start pumping power or not?

many Thanks for you all

Mina
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #2
    Re: Single-phase inverter connected to 3-phase distribution board design
    minabenyamin wrote: »
    Hello dear friends,

    I want to discuss something with you. For now I've a GT system with a 5kW single phase inverter. This inverter would supply a around-35kW-load workshop. The mains of the workshop which is feeding the main distribution board - which the inverter would be connected to- is a 3-phase supply from the utility.
    My question is how I will connect my inverter, I mean to which phase. or I can connect the inverter output to any phase of them.

    Another question relating to the inverter output. As far as I know, the inverter makes some kind of synchronization with the utility, and thus is done by monitoring the wave power coming from the utility and when it's same as the wave power that the inverter will generate, it starts to pump its power. if my info is not right pls tell me. my question is that, the inverter has only one AC terminal which is output and there's no AC inputs at all, so how the inverter can read ( detect) the wave power coming from the utility to monitor it and check if it's same as its wave or not to start pumping power or not?

    many Thanks for you all

    Mina
    You can connect the GTI to any one of the three phases (line to neutral, often 120V) or between two phases (line to line, often 208V).
    The GTI does not wait until the power line waveform matches what it will produce. It waits until the POCO waveform is somewhere in the range that it can handle and then matches (synchronizes to) that.
    There is no point in monitoring the POCO waveform at a different point than the GTI is injecting power into, since they would all be in parallel anyway.
    The GTI can see the voltage from POCO and controls the current it feeds in in response. One of the features of a solid grid connection is that the current drawn or fed in will not affect the voltage much.
    SMA SB 3000, old BP panels.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,623 admin
    #3
    Re: Single-phase inverter connected to 3-phase distribution board design

    Mina is in Egypt--So his standard voltages are different--But same idea.

    There is no "cheap and efficient" way to convert from 1 phase to 3 phase power for feeding power bank in the utility.

    In the US, we are (generally) allowed to feed up to 10kWatt into one phase of a three phase power panel... However, they utility engineer has the final say.

    As far as the utility meter is concerned--It does not care--It adds all the power (+/-) on the three phases into to one billing number (and it should be accurate to within 2% or so). The meter does not "need" exactly balanced 3 phase loads for accurate billing/credits.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • minabenyamin
    minabenyamin Solar Expert Posts: 45 ✭
    #4
    Re: Single-phase inverter connected to 3-phase distribution board design

    Many thanks dear
    may I know what does POCO refer to?

    Also how the GTI will monitor the wave coming from the utility, the GTI has no AC input??

    Thanks

    Mina
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  • stephendv
    stephendv Solar Expert Posts: 1,571 ✭✭
    #5
    Re: Single-phase inverter connected to 3-phase distribution board design
    minabenyamin wrote: »
    Many thanks dear
    may I know what does POCO refer to?

    I think it means POwer COmpany. i.e. the energy supplier.
    minabenyamin wrote: »
    Also how the GTI will monitor the wave coming from the utility, the GTI has no AC input??

    It measures it on its output.
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  • minabenyamin
    minabenyamin Solar Expert Posts: 45 ✭
    #6
    Re: Single-phase inverter connected to 3-phase distribution board design

    thanks dear for your reply,

    so the GTI AC output acts as input in the same time??
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #7
    Re: Single-phase inverter connected to 3-phase distribution board design
    minabenyamin wrote: »
    thanks dear for your reply,

    so the GTI AC output acts as input in the same time??

    Not exactly an input. The GTI monitors the power on its connection to the grid. It is looking for Voltage and frequency. In effect two superimposed sine waves: one from the grid and one from the inverter. If they are not in sync the inverter adjusts its output to match. Lack of synchronization 'appears' as incorrect frequency (double Voltage peaks within an expected cycle time) and/or excess harmonic distortion (Voltage curve outside of the norm 'looks' like >5% THD).

    If the quality of power from the grid is particularly bad (large variations in its frequency, distortion, or Voltage within the qualifying period), the GTI will not be able to sync and will shut down. Then it will retry after a certain amount of time.
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  • solar_dave
    solar_dave Solar Expert Posts: 2,397 ✭✭✭✭
    #8
    Re: Single-phase inverter connected to 3-phase distribution board design
    Cariboocoot wrote: »
    Not exactly an input. The GTI monitors the power on its connection to the grid. It is looking for Voltage and frequency. In effect two superimposed sine waves: one from the grid and one from the inverter. If they are not in sync the inverter adjusts its output to match. Lack of synchronization 'appears' as incorrect frequency (double Voltage peaks within an expected cycle time) and/or excess harmonic distortion (Voltage curve outside of the norm 'looks' like >5% THD).

    If the quality of power from the grid is particularly bad (large variations in its frequency, distortion, or Voltage within the qualifying period), the GTI will not be able to sync and will shut down. Then it will retry after a certain amount of time.


    Coot that is about the best description of the conditions I have seen here! Nice one!
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  • minabenyamin
    minabenyamin Solar Expert Posts: 45 ✭
    #9
    Re: Single-phase inverter connected to 3-phase distribution board design
    Cariboocoot wrote: »
    Not exactly an input. The GTI monitors the power on its connection to the grid. It is looking for Voltage and frequency. In effect two superimposed sine waves: one from the grid and one from the inverter. If they are not in sync the inverter adjusts its output to match. Lack of synchronization 'appears' as incorrect frequency (double Voltage peaks within an expected cycle time) and/or excess harmonic distortion (Voltage curve outside of the norm 'looks' like >5% THD).

    If the quality of power from the grid is particularly bad (large variations in its frequency, distortion, or Voltage within the qualifying period), the GTI will not be able to sync and will shut down. Then it will retry after a certain amount of time.

    your reply is amazing!

    my question is that "how the GTI knows the result of superimposing the two waves?". For me if there's an AC input to the GTI, I was to consider that the result of superimposing the waves would be fed using this input but actually it's just the output connected to the pass-bar!!

    I hope you got my poin

    Thanks

    Mina
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,623 admin
    #10
    Re: Single-phase inverter connected to 3-phase distribution board design

    The Utility grid itself has essentially near zero resistance (impedance) as far as anything we can connect to the grid would see... The wiring for the shop/house may have a fraction of an Ohm or so.

    So--For the first approximation, the Voltage wave form is the same "everywhere" around the shop/house/out to the pole mount utility transformer.

    The GT Inverter feeds current to the wire--following the AC voltage wave form. About the only "distortion" of the sine wave is the current from the GT inverter and the resistance of the wire from the GT controller to the utility pole/street transformer. This will raise the AC voltage at the GT inverter by a few volts (typically).

    And, the amount of voltage change is just ~2-5 volts or so (out of ~240 VAC in North America)--So, the effect is not very large.

    There are installations where the high voltage exceeds the GT inverter's limit (say line voltage is approaching ~260 volts and there is a 5 volt "rise" due to current*resistance of the wiring)... In the US, the high line trip point for the GT inverter is typically somewhere around 260-264 VAC--And it will cause the GT inverter to "fault" and time out for ~5 minutes after the line voltage falls back into range. This can happen in regions where the electrical mains voltage is near the high limit or not well regulated (remote farms, sometimes on islands with large GT Solar Power stations).

    And, there have been posters here that have contacted their utility and asked them to adjust the line voltage down to closer to nominal voltage. Many times the utility will do that (high voltage can damage lighting/equipment). And there has been a few times where the utility did not (for various reasons).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #11
    Re: Single-phase inverter connected to 3-phase distribution board design
    minabenyamin wrote: »
    your reply is amazing!

    my question is that "how the GTI knows the result of superimposing the two waves?". For me if there's an AC input to the GTI, I was to consider that the result of superimposing the waves would be fed using this input but actually it's just the output connected to the pass-bar!!

    I hope you got my poin

    Thanks

    Mina

    All the GTI has to do is look at the power on the wires, much the same as you would view it with an oscilloscope only not visually.

    Within a time frame such as one second it would expect to count 60 Voltage peaks: that's 60 Hz frequency. If it counts more than that then the frequency is out of sync so the GTI adjusts its frequency until there is only 60 peaks per second.

    Likewise it can measure the Voltage of the AC line and adjust its output to match. It will do this qualifying before 'turning on the power' and then periodically check to make sure it stays in sync.
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