[RE-wrenches] Battery Bank to Inverter Wiring
Jarmo.Venalainen at schneider-electric.com
Jarmo.Venalainen at schneider-electric.com
Mon Jun 29 09:28:46 PDT 2015
Hi:
>From the comments so far, it appears that within the wrenches group, good
practices are the rule and DC cables are kept side by side, so the problem
I mentioned hasn't come up much.
I have seen "large loop" battery cables a couple of times in solar
installs myself out on islands way down south. Also on boats/RV's/motor
homes.
Please keep the cable loop in mind when you do bench tests of inverters,
as it may screw up the results and make it seem like there is something
wrong with the inverter.
JARMO
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Jarmo Venalainen | Schneider Electric | Xantrex Brand | CANADA |
Sales Application Engineer
Phone: +604-422-2528 | Tech Support: 800-670-0707 | Mobile:
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Email: jarmo.venalainen at schneider-electric.com | Site: www.Xantrex.com
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From:
"boB at midnitesolar.com" <boB at midnitesolar.com>
To:
RE-wrenches <re-wrenches at lists.re-wrenches.org>,
Date:
06/28/2015 10:30 PM
Subject:
Re: [RE-wrenches] Battery Bank to Inverter Wiring
Sent by:
"RE-wrenches" <re-wrenches-bounces at lists.re-wrenches.org>
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
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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