[RE-wrenches] Battery Bank to Inverter Wiring

[boB at midnitesolar.com]

On 6/28/2015 9:20 AM, Allan Sindelar wrote:
 >>>>I'm not an EE, but I can't see what difference twisting would make in
the absence of a building/collapsing field as is normal with AC.<<<<

It is little known that with the typical inverters that use the heavy 
power transformers,
have a lot of AC 120 Hz (or 100 Hz for 50 Hz systems) current mixed in 
with that DC battery current.

The AC output current reflects back to the input as ripple current.  
This is why you want to
have at least short as possible Sbattery cable runs AND keep the wires 
as close together as possible.

Twisting the battery cables may help a bit but that is probably overkill.

The problem has to do with inductance in the battery cables.  You can 
use as big of cable
as you can fit in to reduce resistance, but that will not help to lower 
the inductance.
The problems you can sometimes have with high inductance is that L-C 
resonance at the
inverter can raise the peak voltages seen at the inverter input 
terminals and can be hard on
the inverter.

Then again,  the high frequency, lighter weight inverters will typically 
keep most of that ripple
inside,  between the DC input and AC output and battery cable inductance 
will not be as much
of a problem on the battery cables.

boB Gudgel




On 6/28/2015 9:20 AM, Allan Sindelar wrote:
> As a matter of course I have always run the positive and negative 
> conductors of high-current cable pairs together, but have never 
> deliberately twisted them, and have never known of any related problems.
> The most obvious example of this would be 4/0 battery/inverter cables 
> in a 24V system, with a 250A GJ-class breaker or (prior to that) a 
> 300A or 400A Class T fuse. It's pretty tough to thread a twisted pair 
> of 4/0 USE/RHH/RHW cables through a 2" elbow or LB from inverter 
> enclosure to battery enclosure.
> I'm not an EE, but I can't see what difference twisting would make in 
> the absence of a building/collapsing field as is normal with AC.
> I have twisted AC conductors together in the past when clients have 
> expressed concerns about EMF from their equipment and wiring, but only AC.
> Allan
>
> *Allan Sindelar*
> allan at sindelarsolar.com <mailto:allan at sindelarsolar.com>
> NABCEP Certified PV Installation Professional
> NABCEP Certified Technical Sales Professional
> New Mexico EE98J Journeyman Electrician
> Founder (Retired), Positive Energy, Inc.
> *505 780-2738 cell*
>
> **
>
> On 6/27/2015 2:40 AM, John wrote:
>>
>> That is why for years we have been twisting those leads around each 
>> other.  I was told it was to cancel out the opposing fields on the 
>> wires, but for whatever the correct technical reason is,  we have 
>> always twisted those heavy wires.         John V.
>>
>> *From:*RE-wrenches [mailto:re-wrenches-bounces at lists.re-wrenches.org] 
>> *On Behalf Of *Jarmo.Venalainen at schneider-electric.com
>> *Sent:* Saturday, 27 June 2015 5:45 a.m.
>> *To:* RE-wrenches
>> *Subject:* [RE-wrenches] Battery Bank to Inverter Wiring
>>
>> Hi:
>>
>> From time to time over the years I've come across systems where the 
>> routing of DC cables between the batteries and the inverter has been 
>> the cause of  issues.
>>
>> I'm not referring to wire thickness or quality of terminations.  For 
>> the purposes of this discussion, just assume that wire thickness and 
>> terminations are perfect.
>>
>> What I am referring to is the routing of the positive and negative 
>> battery cables.  In particular, the loop area within the + and - 
>> cables as shown in the image below,
>>
>> The problem I've seen in systems with a large loop in the setup is 
>> that the inverter does not provide good surge power and can even go 
>> into low voltage shutdown during large surges.
>>
>> Recently this happened again and I wanted to get a better feel for 
>> it, so I did some math.
>>
>> For a cable length of about 12', the loop is an inductor which has a 
>> value of inductance of about 1 uH for side by side cables and as much 
>> as 6 uH for cables about 1 foot apart.
>>
>> This inductance is greatly multiplied by any ferrous metal in the 
>> loop and can easily be in the range of 10's to 100's of uH.  Examples 
>> being cables which run in steel conduits or along the steel frame of 
>> a motor home.
>>
>> Inductance causes a voltage drop proportional to the rate at which 
>> the current is changing.  To get an idea of how large that rate can 
>> be for typical inverters, I did surge tests with a 5kW inverter and 
>> found that the rate of change of current can be as high as 100A per 
>> milli-second or 100,000 Amps/second.
>>
>> Given that, the voltage drop of the wire inductance is then , Vdrop = 
>> (rate of change of current) x (inductance),
>>
>> Vdrop for 1 uH = (100,000 A/s) x (0.000001 H) = 0.1V
>> Vdrop for 10 uH = (100,000 A/s) x (0.000001 H) = 1.0V
>> Vdrop for 100 uH = (100,000 A/s) x (0.000001 H) = 10.0V  clearly this 
>> is a problem.
>>
>> Have any of the wrenches had systems with this issue?  If so, how often.
>>
>>
>> JARMO
>>
>>

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