Series Parallel PV in Battery systems [RE-wrenches]

Hugh Piggott hugh at scoraigwind.co.uk
Tue Nov 21 15:27:27 PST 2006


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I find the descriptions in this thread very confusing.  But I did 
find this previous exchange which clarifies things a bit for me.  I 
normally group strings in parallel maximum of 3 strings at a time to 
one fuse, and I have been told this is OK with the BP modules I use.

At 2:43 pm -0800 31/10/04, Bill Brooks wrote:
>
>Gary and Darryl,
>
>Without going into a long and confusing explanation, any crystalline-silicon
>module on the market today requires series fusing when 3 or more strings are
>connected in parallel. Use the max series fuse rating on the back of the
>module for simplicity and fuse longevity.
>
>The assumption about 1 or 2 strings is that the inverter is listed to
>provide no backfeed current. I believe UL 1741 is being adjusted to test for
>this, but I have not seen much in the way of bulletproof documentation from
>any manufacturer on this subject of no backfeed current. Maybe some of the
>manufacturer reps can chime in here. When I say "bulletproof", I mean that
>it is undeniably clear that UL has evaluated and tested this function of the
>inverter so that the pickiest inspector is satisfied with the documentation.
>
>Blocking diodes are not required by the code unless the manufacturer
>requires their use with their modules. Blocking diodes can not be used in
>lieu of fusing in PV arrays. They should not be used unless you know what
>you are doing.
>
>Bill.
>
>
>-----Original Message-----
>From: Darryl Thayer [mailto:daryl_solar at yahoo.com]
>Sent: Sunday, October 31, 2004 5:38 AM
>To: RE-wrenches at topica.com
>Subject: Re: Series fuses/combiner boxes [RE-wrenches]
>
>HI Gary
>I went through th e very same question last summer.
>For series strings, you can place in series as many
>panels your charge control/inverter or module voltage
>will allow.  This calculation is the Voc/module x
>number of modules x 1.25 (code safety factor) x Temp
>correction factor (the temp diff from 25C to the 99%
>probability not this cold local temp=DT i.e 1+.05*DT)
>This is the voltage calculation and it must be less
>than 600 volts or the limit of the
>controller/inverter.
>
>For Parallel strings John Wiles has a white paper
>which says the max current through the module from all
>sources must not exceed the max series fuse limiting
>that current.  That is the max of the Isc of all
>parallel collectors or other equipment must not exceed
>the max series fuse as labeled on the module.
>
>As an example if you have two modules in parallel each
>of 3 amps Isc, and the fuse protection is 2 X 3 x
>1.25= 7.5 amps  And if one modules fails and the
>second module supplies Isc the first  module could see
>3 amps plus 7.5 amps reverse current through the fuse
>or the failed module would have 10.5 amps.  To comply
>with the code the module must have 10.5 max fuse size.
>
>
>This is very limiting of course.  And I have wondered
>the use here of reverse diodes.  Either at the breaker
>or on each module.  The second solution is to fuse
>each module at the junction box.
>
>Now it is my turn.  What is the ruling on Blocking
>Diodes?   What is the accepted fuse holder that can be
>installed in module junction boxes?
>
>Thanks Daryl
>
>--- Gary Higbee <gary at windstreamsolar.com> wrote:
>>  Wrenches,
>>
>>  I'd appreciate some help in determining when we need
>>  fuses or breakers for
>>  parallel strings. For some reason I thought that two
>>  strings in parallel are
>>  OK, but that for three and above we need to use the
>>  appropriate combiner box
>>  with breakers or fuses. This feels too
>>  vague--especially as I am now being
>>  asked to review and approve systems (for the Energy
>>  Trust of Oregon) with
>>  more than two parallel circuits.
>>
>>  Please shed some photons on this--when do we not
>>  need and when do we need
>>  parallel string fuses or breakers, and where does
>>  this correspond to in the
>>  code--or make common sense?
>>
>>  Thanks so much!
>>
>  > Gary

-- 
Hugh

Scoraig Wind Electric
http://www.scoraigwind.co.uk/


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