On 6/29/2015 3:15 PM, Roy Rakobitsch wrote:
In my experience, adding large electrolytic filter caps at the
inverter seems to help quite a bit with ac ripple issues and increases
inverter efficiency a bit as well.
Yep, that usually helps. Turns out that if you get just the right
amount of battery cable inductance AND just
the right amount of inductance, then you can have an LC resonant circuit
that makes the voltage go
sky high and let out the smoke.
But typically, raising the capacitance lowers the resonant frequency
enough below 120 Hz that it
helps the situation.
As far as batteries being part of the loop, yes they are. Just picture
their layout as
a piece of wire and you can get a feel for that contribution to the loop.
You could also place a capacitor or two right where the battery cables come
together again.
But you gotta stop somewhere. This is more educational or a way to
visualize what is going on so that you use the best wiring practices
when at all possible.
There are other related best practices sort of relating to this as well,
as far as wiring chargers and inverters electrically close to the batteries.
If you have the charge controllers wired too close to the inverter
battery terminals and also have long battery cables, then the
inverter will tend to draw more current from the charge controllers
rather than the batteries and cause problems with the CC.
By wiring the CC positive and negatives closer to the battery
terminals, then the inverter MUST draw its power from the battery
terminal... The battery acts as kind of a filter for the CC and the
CC doesn't get jerked around so much by the inverter's input
ripple current. This can help reduce wear and tear on the
CC somewhat.
But we've been used to wiring the batteries to bus bars inside of
a main connector box (E-panel, flexware, whatever), and aren't
really given the opportunity to make nice best practice connections,
except maybe to make the battery cables as short as possible
and twist them as best we can.
boB
Roy Rakobitsch
NABCEP Certified Small Wind Installer®
NABCEP Certified Solar PV Installer®
Certified Advanced Tower Climbing, Safety & Rescue
Wind/PV Design Engineer
Windsine LLC
631-514-4166
www.windsine.org <http://www.windsine.org>
On Jun 29, 2015 5:05 PM, "Ray Walters" <r...@solarray.com
<mailto:r...@solarray.com>> wrote:
This is fascinating because while I knew the DC cables had an AC
component, I never realized how much inductance played in the volt
drop to the inverter. Thanks for bringing this to our attention
Jarmo.
Which brings up another question based on Benn's question: isn't
the battery itself part of the inductive loop? and would the lead
inside and steel cases act to increase the inductance? Should we
consider changing battery layouts not just to shorten cable
lengths, but to counter inductance in the batteries themselves?
If needed, we could actually wire the batteries to "twist" by
putting every other 2 v cell in the opposite row.
My guess is that it wouldn't be worth it, but I've been wrong many
times.......
R.Ray Walters
CTO, Solarray, Inc
Nabcep Certified PV Installer,
Licensed Master Electrician
Solar Design Engineer
303 505-8760 <tel:303%20505-8760>
On 6/29/2015 3:55 PM, Benn Kilburn wrote:
What about when you have the (+) and (-) terminals at opposite
ends of a larger battery bank? Sure you can run the conductors
side-by-side once they meet up on their way to the inverter, but
would you consider the distance between the end terminals a
"large loop"?
Ideally the battery string is laid out so the end terminals are
close together and close to the inverter, but this isn't always
the case.
Benn Kilburn
SkyFire Energy Inc.
780-906-7807 <tel:780-906-7807>
On Jun 29, 2015, at 10:28 AM,
jarmo.venalai...@schneider-electric.com
<mailto:jarmo.venalai...@schneider-electric.com> wrote:
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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From: "b...@midnitesolar.com <mailto:b...@midnitesolar.com>"
<b...@midnitesolar.com <mailto:b...@midnitesolar.com>>
To: RE-wrenches <re-wrenches@lists.re-wrenches.org
<mailto:re-wrenches@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-boun...@lists.re-wrenches.org
<mailto:re-wrenches-boun...@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@sindelarsolar.com_ <mailto:al...@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 <tel:505%20780-2738> 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-boun...@lists.re-wrenches.org_] *On Behalf
Of *_Jarmo.Venalainen@schneider-electric.com_
<mailto:jarmo.venalai...@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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