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Help:Achieving Mains Sync with Solar inverter

Discussion in 'Renewable Energy' started by abicash, Jun 29, 2014.

  1. abicash

    abicash Member

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    Hello

    I have already posted it here
    I will explain it again here..

    I am running a H-bridge inverter with sine weighted PWM with a microcontroller.The DC voltage is from Solar PV.
    The inverter generated sine frequency is 50 Hz.
    I now take the Mains/grid rectified wave to an input capture channel of the MCU.
    The inverter is indexed to zero at every rising edge of the input capture event.

    Now with this scheme i have a problem of understanding the phase of the input wave, so sometimes the generated wave is 180 deg shifted to i/p mains.
    For this i took a half wave rectifier of the mains, so i have confirmed positive wave of the mains sine.
    But with this , sometimes a half cycle misses sync.

    Is there a simpler way of doing this?
    Rather , what is the best method of doing this

    All help appreciated
     
  2. alec_t

    alec_t Well-Known Member Most Helpful Member

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    Unless you are hoping to pump power back into the mains to get payment from your utility company (in which case you'd have to have an officially approved setup anyway) does it really matter if there is exact sync with mains at all times? What will you be powering with this?
     
    • Agree Agree x 1
  3. abicash

    abicash Member

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    Hello alec_t

    Thanks for replying
    This would be used in a sine inverter.
    A relay connects the i/p AC wave to the o/p of the inverter.

    I normally will check for sync, and when that happens, transfer the relay and then i switch off inverter PWM
    With this the load does not see an off cycle.
    Am i on the right track here?
     
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  4. dave

    Dave New Member

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  5. alec_t

    alec_t Well-Known Member Most Helpful Member

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    Understood. How fast can your relay change over the connections (bearing in mind the mains cycle lasts <= 20ms)?
     
  6. abicash

    abicash Member

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    close to 5 ms
     
  7. alec_t

    alec_t Well-Known Member Most Helpful Member

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    I don't see how you can guarantee that :(. Suppose the mains fails immediately after a rising edge? That will go undetected until the following cycle, and then there will be a 5mS interval before the inverter connects to the load. In addition, the start-up time of the inverter needs to be factored in.
     
  8. abicash

    abicash Member

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    I will explain another scenario where it is worthwhile

    Mains was absent and inverter was ON.
    Mains resumes , yet i haven't transferred the relay.Now i lock both phases.Once locked, i transfer the relay.
    Once Mains fails again, do not wait for anything, start the inverter right away.
    Does this make any sense? or am out of line
     
  9. abicash

    abicash Member

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    sync inverter.jpg I see a bit of phase shift (yellow channel). I have filtered the PWM channel with an RC.
    Blue channel is mains from a stepped down transformer.
    I am a bit surprised right now, since i see not a single half cycle missing sync , which was happening earlier, which prompted me to post this thread.
     
  10. alec_t

    alec_t Well-Known Member Most Helpful Member

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    I don't see how that is different? As I explained in post #6, there will inevitably be delay in detecting mains absence, relay switching and inverter start-up (reaching target voltage from zero).
    Perhaps the missing half cycles were due to mains transients.
     
  11. savvej

    savvej Member

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    Hi abicash and altec,
    Just wondering as to how much adverse effect can it have on the PV inverter or the load, in case if there is non-synchronized abrupt switch-over?
    Is it really that important for the PV inverter to be in sync with the mains while switching over?
    Regards,
    SAV
     

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