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  • Buck Boost Transformer Help

    So, I got a fantastic deal on a bunch of the now bankrupt tenK Solar's equipment. The problem is all they could get me was 277v (650watt) microinverters for the (200 half cell) 500W Panels tenK solar produced.
    I did a test with a buck boost transformer to get 272v volts from 240v with 6 panels & 6 microinverters and the inverters seemed to work ok. So I took the leap and installed a 12KW system at my house (24 microinverters/panels) along with (4x) .5KVA buck boost transformers.

    1 - buck boost transformer per 6 inverters - feed from/to a 20amp double pole breaker (grid tie in) -4x

    The system does work, but there is something definitely wrong with the microinverters in this configuration - They are very loudly chirping with a high frequency static with not much load and the chirping gets louder and louder on a sunny day till the inverters reset and throw a "AC fast over current" flash code.
    I put some inline volt meters on the system and the 240v bounces around quite a bit from 238-252v depending on the output from the inverters -@252v - boosted to 284v the inverters sound like they are going to explode.

    If I shut down 3 of the 4 buck boost (just have 1 running with 6 inverters) the voltage remains constant and inverters are relatively quiet.

    The 277 inverters have a black, pink, and green wire -normally the entire 277v would be on the black and the pink as the neutral. The buck boost needs 2 hot legs to get 277. One just passes through @ 120v, the other is boosted 32v to 152v = 272v..
    I tried boosting both ways the black and then the pink wire and it made no difference.

    Buck Boost - 500sv46b - boost house voltage 32v
    Lead Solar - LS650-277 Inverters (246v-303v working range)
    Apex 500W Panels

    Am I missing some logical reason using multiple buck boost transformers on voltage varying inverters wont work? Any ideas for using voltage regulators or anything else?

    I'll try anything at this point. Thanks



  • #2
    Originally posted by scegg View Post
    Am I missing some logical reason using multiple buck boost transformers on voltage varying inverters wont work? Any ideas for using voltage regulators or anything else?
    Possibilities:

    The inverters are seeing a large inductance (i.e. the transformer) between their output and the grid. This distorts the VI waveform and turns a poor power factor into a very distorted voltage waveform, which the inverters don't like. The transformer also has a fairly high impedance*, such that the current from the inverters pushes the voltage above safe operating levels and they shut down.

    (* - both from the inductance (complex impedance) and from winding resistance.)

    I'd try a different transformer; probably a larger one with lower inductance and lower overall impedance. Also, use one transformer, and make sure it goes 240-277V (i.e. adds 37 volts via the additional windings.) Two 120 to 144 volt transformers will also work.
    Last edited by jflorey2; 10-31-2018, 06:26 PM.

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    • #3
      Seems like the manufactures dont readily list the inductance and impedance. I got a 2KVA on order.
      I forgot to mention that i reconfigured the buck boost for just +16v - 256v output and the inverters quieted down a bit but they still tripp on "AC fast overcurrent" even with the reduced voltage.
      Thanks for the suggestion.

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      • #4
        Originally posted by scegg View Post
        I forgot to mention that i reconfigured the buck boost for just +16v - 256v output and the
        inverters quieted down a bit but they still tripp on "AC fast overcurrent" even with the reduced voltage.
        I think you are going in the right direction, connecting the inverters as near to directly as possible. I also think you
        have some power factor issues. Measurements might reveal what is going on, a few caps in the right place might
        help a lot. I would test putting some across that 16V winding. good luck, Bruce Roe

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        • #5
          Originally posted by scegg View Post
          I forgot to mention that i reconfigured the buck boost for just +16v - 256v output and the inverters quieted down a bit
          That's the right way to do it; use an autotransformer to give as little boost as possible. That way the transformer is doing as little "work" as possible, and you're not losing a lot of power to transformer losses. A 277 volt inverter will generally work down to 250 volts, so getting the voltage to some margin above that (~255 volts?) does two things:

          1) Gets the inverter working with minimal power transferred through the transformer
          2) Leaves as much margin on the topside as possible for voltage rise due to impedance.

          but they still tripp on "AC fast overcurrent" even with the reduced voltage.
          Thanks for the suggestion.
          That sounds like too much inductance still, which is effectively a power factor problem. Agree that caps may help, but be careful - adding capacitors to the stray inductance of a transformer secondary can also give you unexpected voltages due to resonance. Take it slowly, start with small values and use good capacitors (film caps rated for across-the-line use, often called "X class" caps, are a good choice here.)

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          • #6
            Ive been trying to determine the power factor so that I can properly size the capacitors but its beyond my ability. I'm also having a hard time finding any x class caps that have any kind of substantial micro farad rating. Can you give me a starting value? 5uf? Also i'm familiar with how to wire capacitors to motors and compressors but for this application would I just break the boosted hot wire with the cap?

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            • #7
              Originally posted by scegg View Post
              Ive been trying to determine the power factor so that I can properly size the capacitors but its beyond my ability. I'm also having a hard time finding any x class caps that have any kind of substantial micro farad rating. Can you give me a starting value? 5uf? Also i'm familiar with how to wire capacitors to motors and compressors but for this application would I just break the boosted hot wire with the cap?
              No, cap goes across the entire phase (i.e. across the output terminals of the inverter.) The goal is to compensate the complex impedance of the transformer with additional capacitance. On a smith chart inductance will tend to make the impedance point go up (+j) you want to bring it back down with capacitance (-j.)

              To do it right I'd use a network analyzer looking into the grid (with appropriate DC block) but that's expensive. Fluke makes some good power quality analyzers but they are very $$$ so you'd have to borrow one. You could do it with a scope and a good (phase accurate) current probe. Measure current and voltage, and add capacitance until the current and voltage zero crosses match.

              In any case I'd start at .1uF and go by factors of 3. So .1uF, .33uF, 1uF, 3.3uF etc. You can parallel caps to get more capacitance. They are about $1 a piece so you can get an assortment of values and try them.

              Needless to say this isn't a good idea unless you are VERY familiar with electrical/electronic work and can take appropriate precautions. At those voltages, even the charged capacitors can be dangerous after powerdown.

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              • #8
                Just a point of interest. X2 capacitors are generally not recommended for current carrying applications although this is often seen. KMET actually has a notice on the data sheet that X2 capacitors should not be used for voltage dropping supplies. The qualities that make them fail safe are the things that make them fail over time. KMET is thinking of offering a line that can carry current because so many want to do this, they won't be cheap. Consider a motor run oil type and fuse it.

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