How these solar inverters self-start?

I was attending a PV design and install class and from there I realized that actually solar inverters required a min.voltage to start-up. And usually, this start-up voltage is very near the MPPT range voltage.

I understand that before it start up, the driver and control circuit need some (though small) power to get start.
What I am very curious is before the start-up, there wouldn't be any mppt tracking.
So? It can really start-up with the mppt tracking? The power is really enough?
Or some kind of battery must be needed.

Hopefully, someone really knows to help me unlock this problem. Thanks.

Comments

  • BB.BB. Super Moderators, Administrators Posts: 31,425 admin
    Re: How these solar inverters self-start?

    Not quite sure I understand the question...

    1. Inverter have an operating range... I am assuming you are asking about Grid Tied inverters (Solar DC in, AC out to grid/utility feed). They have a minimum and maximum input voltage range. And the minimum is where there is enough voltage for them to generate the applicable AC voltage out.

    The vendor will list the MPPT tracking range... For Xantrex high voltage inverters, the MPPT tracking range is around 200-550 volts... From 550 volts to 600 volts, MPPT is not operational (should not design array to operate this close to the maximum 600 volt limit anyway--easy to damage inverter with Voc.

    2. There are low voltage input GT inverters too... Those designed to operate with a 24 or 48 volt battery bank which can operate in Grid Tied mode (feed the utility lines with power from the solar array or operate off-grid for when the utility power has failed--such as in a storm). For example, a GT Inverter would be set to start GT operation when the battery is around 52 volts DC. The higher the battery bank voltage above 52 volts DC, the more power the GT inverter will output to the grid.

    When running from a battery bus--it would not be correct to call this an MPPT type operation--As the point of this GT inverter is to adjust its output current based on the current state of charge of the battery bank--not to optimize/maximize current/voltage draw from the battery bank (not Maximum power point tracking).

    In this case, the MPPT function is done by the solar charge controller (which is connected from the solar array, to the charge controller, to the battery bank). The charge controller, once there is a minimum amount of sunlight on the panels--will adjust the Pmax=Vmp*Imp equation to take maximum power from the solar array.

    In both cases (1. for the GT inverter, and 2. for the MPPT solar charge controller)... You will have a mix of series and parallel connected solar panels such that Vmp (panels hot) and Voc (panels cold) fall within the min/max design range of the solar controller.

    I will stop here--in case I have completely misunderstood your question.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2System2 Posts: 6,290 admin
    Re: How these solar inverters self-start?

    Thanks for your reply. That's nice of you.
    Yup, I realize that I didn't post my queries clear enough.

    I am aware what MPPT means and that it will be given in the datasheet. The problem is I realize that not all inverters put the min.starting voltage of the inverter in the datasheet clearly. I read a datasheet from sunnyboy. It is stated it that the min.Pdc(dc power) for start up is 13W. (dun understand why they need such high power to start up) And for some inverters, it will stated the min.start-up voltage as e.g 200V or 100V. Some are worst in the sense that it is not very obvious in the datasheet. My lecturer (without much PV experience) mention to us that he did not realize this issuse when he order the inverter. End up, he had to tied more PV in series just to get enough voltage to start the inverter up.

    Ok, enough of this. What I am curious about is without MPPT tracking at the start, will there be enough power to start-up? Do they use some kind of battery to provide power to start-up?

    Thanks. Hope I have made the problem clearly.
  • BB.BB. Super Moderators, Administrators Posts: 31,425 admin
    Re: How these solar inverters self-start?

    Joy,

    Here is a thread with some links to various articles and a thesis paper...

    If you are interested in the inverter from an engineering/design perspective--that would be a good place to start.

    Otherwise, if you are looking more for a high level summary of why things are what they are--just ask.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: How these solar inverters self-start?

    that sounds like a straight gt inverter and no batteries are used in that type of an inverter. if you have found an inverter that fails to give the minimum voltage needed to start, but gives the wattage then you may wish to confront the manufacturer with an email inquiry as to what voltage it needs so as to properly design a pv system for it.
  • System2System2 Posts: 6,290 admin
    Re: How these solar inverters self-start?

    Thanks for the replies.

    Is there any resources with regard to how to design the start-up control circuitry of the grid-tied solar inverters?

    I suppose there is need for a 15V and/or 5V source to power the driver circuitry and microprocessor if any and the control circuitry before the inverter can get start up. I read up a lot on solar inverters and the converter topologies. Many focuses on the different MPPT designs rather than the design of the start-up circuitry.

    Is it because it is too simple or what? Can a buck converter do the job? In additional, how can one determine/estimate the required power of the converter?
    Thanks.
  • boBboB Solar Expert Posts: 983 ✭✭✭✭
    Re: How these solar inverters self-start?
    Joy_spore wrote: »
    I suppose there is need for a 15V and/or 5V source to power the driver circuitry and microprocessor if any and the control circuitry before the inverter can get start up. I read up a lot on solar inverters and the converter topologies. Many focuses on the different MPPT designs rather than the design of the start-up circuitry.

    This is one of the big reasons that the old Xantrex Sun-Tie had problems. Sometimes when the sun-tie would sweep the PV, it would reset itself because the power supply was not using the grid as well as the PV so it would "crash" the supply. Then you'd have to wait the 5 minutes for it to come back on cuz the processor was reset. They always said it would "crash the array", but it was really the auxilliary supply that crashed.
    Mr. Guppy was part of the team that fixed that, although it was a bit too late for that product to continue.

    Startup auxilliary supplies do have to be thought out well in advance.

    boB
  • Solar GuppySolar Guppy Solar Expert Posts: 1,963 ✭✭✭
    Re: How these solar inverters self-start?

    The Suntie was issues were mostly software ( Written by Tim E's, at magnum now I have heard ), the array collapse was due to the software not dialing down the AC load fast enough when the vmp point was passed. It never actual went to zero, it would bottom out at about 40 volts at which point the secondary HV supply was lower than the AC line, it can't draw it any lower as it can't sink to the grid at that point.

    This was all documented in a white paper in 2003 , the internal supply's were all powered from the AC side with a isolated flyback design

    If your interested ... STXR-V5 white paper
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