We are talking to Kirk offline about this but the main issue here is that the enclosure for the 2000A combiner is made out of steel which is ferrous.  That means that when the 120 Hz ripple current induces into the chassis it can vibrate just like a transformer.

Low frequency transformer inverters are have higher ripple than the newer HF transformer units
but they have some AC ripple on the battery cables as well.

This combiner is evidently been around for about 8 years and we haven't heard any complaints but maybe others have been silent on this.  I am not surprised at all that this happens.

Aluminum chassis has the problem that it is a good conductor so when the whole enclosure forms around the cables, the aluminum can act as a shorted turn in a transformer.  It is usually just
lossy but should vibrate much less, if at all.

There are other issues in switching to aluminum that Robin mentions, here:

/"One reason is cost, the other is fasteners, We would need to use press nuts instead of snail shells. // //Snail shells and sheet metal screws don’t do well in aluminum. It is too soft. Aluminum would not //
//vibrate from magnetic ripple activity though."

/He has some ideas to make aluminum work so we will try those./
/
Now, plastic could work.   Just has to be real thick.    Darn physics !

Damping can help, like Kirk tried (more can be done here too). Keeping the cables together best as you can inside the chassis may help as well but moving and routing
the cables might also reduce the buzzing some more.

That's the latest buzz from here.

🌜boB🌛
    K7IQ

On 6/30/2023 8:45 AM, david quattro via RE-wrenches wrote:
Ferrite cores can be used for the battery cables to pass through to reduce common mode current

On Fri, Jun 30, 2023 at 3:59 AM Kirk Herander via RE-wrenches <re-wrenches@lists.re-wrenches.org> wrote:

    I have -15- sets of 4/0 cable (1 per 32 kwh stack) into the
    combiner. They are paired and within .25 " of one another in wire
    cable tray for 80% of their respective distance.

    The inverter to combiner sets are 2/0 for each respective
    inverter. They are paired (not twisted) in cable tray, anywhere
    from 10 -20' in length. Maybe I can send some photos, but can't at
    present due to computer issues.

    I used cable tray because the CT-rated cable is in free air, no
    derating of current required. But merely pairing the leads may be
    the issue.

    I could rearrange the inverter cables and twist the sets, but
    doing it from combiner to battery cabinets is impossible.

    And I'm wondering if this is really necessary if the customer is
    willing to live with the issue. What are the ramifications down
    the road? Loosening of connections I suppose would be one.



    On Fri, Jun 30, 2023 at 12:40 AM Jerry Shafer via RE-wrenches
    <re-wrenches@lists.re-wrenches.org> wrote:

        Kirk
        As the batteries come up to charge does the vibration subside.
        Are the battery cables spun together "corkscrew like" l have
        seen this correct DC noise. I have an 8 on 8 just like your 10
        and l have no noise at all, mine come in from each side of the
        battery combiner, how is your install configured, how is the
        cable length same close or very short to very long some length
        is better on FLA's due to resistance under load and charge.
        More information may help with a solution.
        Fun times solar
        NABCEP PV Inspector

        On Thu, Jun 29, 2023, 8:19 AM Kirk Herander via RE-wrenches
        <re-wrenches@lists.re-wrenches.org> wrote:

            All,

            I am in the middle of commissioning a 480kwh BP LFP
            battery bank, connected to -10- OB Radian 8kw
            inverter/chargers.

            For the DC combiner I am using midnite 2000 amp combiner
            boxes, modified to mount  class T fuses on the positive
            bus, for both battery inputs and inverter outputs.

            For starters, I set the master inverter at a max charge
            rate of 20 AC amps, which equates to a DC charge of about
            74 A. All the other inverters are automatically set to
            this charge rate as well.

            Turning the chargers on one by one, I started to get quite
            an audible vibration from the combiner box at a combined
            charge of about 400 to 500 A.

            So I took the cover off of the combiner, which was
            vibrating noticabllly by feel. Most of the noise was from
            the cover not fitting tight. But the The box itself is
            also vibrating by feel.

            I took the combined charge up to 640A dc. This level of
            vibration definitely increases with an increased charge
            rate. And I am surprised because after all, we’re not
            talking AC here we’re talking DC. It almost reminds me of
            a large AC transformer in its behavior. The vibration did
            not seem to be originating at the inverters, then
            resonating to the combiner.

            I can dampen the vibration by adding a few things, but I’m
            surprised that this is happening. All connections in the
            combiner are tightened properly. Ideas?










--
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            *Owner|Principal, VT Solar, LLC*

            */Celebrating our 32^st  Anniversary 1991-2023!!/*

            dba Vermont Solar Engineering

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--
    *Kirk Herander / **kirkh@vermont.solar*

    *Owner|Principal, VT Solar, LLC*

    */Celebrating our 32^st  Anniversary 1991-2023!!/*

    dba Vermont Solar Engineering

    802.863.1202

    _______________________________________________
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