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Multiple in-phase AC sources to a load

  1. Oct 22, 2012 #1
    I'm trying to figure out how multiple AC sources which are in phase would be able to be used sequentially to power a load. This all started as I read some literature about solar PV panel systems which had a wiring diagram to tie output from the inverters directly into the electrical panel split phase poles. This being from a reputable PV inverter company. This system out puts 120 Vac split phase plus ground and neutral, it's a 4-wire output.

    I've attached a diagram to this post. I've simply represented the load as inductive for fun, no real point here.

    So here's the problem, wouldn't the MOST power that could be sourced from the non-grid AC be 50% of the load consumption?

    I'm not sure how these inverters work, but I'm assuming they will maintain 120 Vac up until they can't drive the current any more, and I would assume that they would then allow a voltage drop. I was thinking an op-amp circuit could be used for this difference to re-balance the voltage at 120 Vac.

    Otherwise, what I would think would happen is that the current from each source would be equal until the inverters couldn't drive any more current, at which point any additional current would be from the grid source.
     

    Attached Files:

  2. jcsd
  3. Oct 22, 2012 #2
    The inverter can put out as much power as the solar array can support withing the current limit of the inverter itself. - as long as the DC bus, fed by the array his high enough voltage it will export power. So you have the current limit of the system, by design say 10KW, and the power limit of the solar array based on the amount of energy it can convert.
    I do not understand your 50% comment however. If the inverter puts out AC Voltage - just slightly higher then the grid it will continue to export power. None of the voltages are exactly 120 and everything has impedances that ultimatly help to balance the system - or make it more stable.
     
  4. Oct 23, 2012 #3
    My point is that with two voltage sources, and load would pull current from Both sources, the grid and the solar inverter. They cannot be at different voltages otherwise there would be racing currents as the inverters output is directly tied to the split phase poles.

    This is just basic KCL. I have a junction, and V_grid and V_solar both 120∠0 are both at the junction. Additionally, my load is tied to the junction. Any current will come from both the grid and solar until solar cannot provide any more current, but grid will still provide additional current.

    If you add some resistor, any, between the Voltage sources and the junction, the voltage to the load is simply the average of the two voltages.
     
  5. Oct 23, 2012 #4
    The inverter will have higher voltage ( if it runs properly) at the point that they connect, and possibly slightly leading phase angle - to ensure it exports power. Keep in mind - nether the source or the inverter has zero impedance. Your diagram is over simplified the sources are nether identical nor ideal.

    Consider a Solar inverter tied to the grid - it would have to have higher voltage at the connection point to "push" current back to the grid as well. The Grid has impedance also.

    The utility parallels transformers - but must pay close attention to the transfromer's impedances - then need to match closely. If one has lower impedance it will carry most of the load, when the impedance are close enough, one will carry slightly more load but also heat up faster increasing it's impedance - until they balance.
     
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