[RE-wrenches] High current dc combiner vibration

Sun Jul 2 12:31:09 PDT 2023

Agree start with data so you know where go

On Sat, Jul 1, 2023, 9:20 AM Kirk Herander via RE-wrenches <
re-wrenches at lists.re-wrenches.org> wrote:

> I'm going to first obtain access to an oscilloscope to characterize and
> screenshot this phenomena, both before and after any mitigation I do.
>
> On Fri, Jun 30, 2023 at 12:46 PM bob--- via RE-wrenches <
> re-wrenches at lists.re-wrenches.org> wrote:
>
>>
>>
>> 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 at 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 at 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 at 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?
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> --
>>>>>
>>>>> *Kirk Herander / **kirkh at vermont.solar <kirkh at vermont.solar>*
>>>>>
>>>>> *Owner|Principal, VT Solar, LLC*
>>>>>
>>>>> *Celebrating our 32st Anniversary 1991-2023!!*
>>>>>
>>>>> dba Vermont Solar Engineering
>>>>>
>>>>> 802.863.1202
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>>>
>>> --
>>>
>>> *Kirk Herander / **kirkh at vermont.solar <kirkh at vermont.solar>*
>>>
>>> *Owner|Principal, VT Solar, LLC*
>>>
>>> *Celebrating our 32st Anniversary 1991-2023!!*
>>>
>>> dba Vermont Solar Engineering
>>>
>>> 802.863.1202
>>> _______________________________________________
>>> List sponsored by Redwood Alliance
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>
> --
>
> *Kirk Herander / **kirkh at vermont.solar <kirkh at vermont.solar>*
>
> *Owner|Principal, VT Solar, LLC*
>
> *Celebrating our 32st Anniversary 1991-2023!!*
>
> dba Vermont Solar Engineering
>
> 802.863.1202
> _______________________________________________
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