Going Solar

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iwire

iwire

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Well it looks like I will e installing solar systems and I have lots of questions

1) How do I figure VD for varying voltages and currents?

2) Does each series 'string' have to be of equel design before parlleling at the combiner box?
 
Bob, I saw an ad for some solar installation courses in Boston a few weeks back, dont remember when, want me to try to track it down again?
 
zog said:
Bob, I saw an ad for some solar installation courses in Boston a few weeks back, dont remember when, want me to try to track it down again?
Click to expand...

Not sure, I am getting training on the installtion of the products but I want to know more about the electrical design end as a lot of that is being placed on my shoulders.
 
zog said:
Bob, I saw an ad for some solar installation courses in Boston a few weeks back, dont remember when, want me to try to track it down again?
Click to expand...

The municipal inspector put a course on Apr 1, Boston. It was very informative regarding inspections. I don't recall the issue regarding strings of equal design. I assume Bob means same watts, per string.
 
Bob I have been designing quite a few systems the last couple of years and would be more than happy to help out. I do it for off-grid cell tower sites so my methods are different from resi-systems because we design for 99.9% availability with back-up gensets for cloudy days, and over killed battery banks, but I know how to throttle back.

For the VD you use the minimum or nominal operating voltage. So for example if it is a 12 VDC system you use 12 VDC, not the open circuit voltage of 17 to 19. For the current you use the maximum short circuit amps. The formula is the same as for AC except use 11.1 for K, or I simplify using 22.2 and just the one-way distance.

Yes keep the series. parallel panels equal voltages, current etc… Don't forget about diodes for shaded panels. The systems voltages are 12, 24, 48, and 96. Use as high of an operating voltage as possible for the application to minimize voltage drops on the DC side.
 
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In the 90's I critiqued a spacecraft design using solar cells feeding a switching power supply. The shadow of the planet cycled the cells.

I can't lay my hands on it now, but it seemed to me that there were somehow two stable equilibrium points for this arrangement, and one of them delivered way-below-normal power.

These two points were obvious if you used a graphical solution for this circuit arrangement. If no one has heard of this, I'll try to fetch the report.
 
cpal said:
Bob
You can start here!!

http://www.iaei.org/subscriber/magazine/05_b/wiles.htm
I for one don't have the answer to you questions.
But this email will get you a neat pDF file that probably will
jwiles@nmsu.edu
Charlie
Click to expand...

The link for the 149 page version of
Photovoltaic Power Systems and the 20005 National Electric Code, Suggested Practices
is at:
http://www.nmsu.edu/~tdi/Photovoltaics/Codes-Stds/PVnecSugPract.html
if you download the 66 MB file

It is nice because, esp. in the appendices he suggest
specific products, e.g. to properly ground modules in
one example.

This is more directed at doing good installations wrt Code,
than design issues, though.

It's worth looking at for anyone doing solar installs.
 
iwire said:
2) Does each series 'string' have to be of equel design before parlleling at the combiner box?
Click to expand...

The short answer is No it doesn't have to, but it will
be equal in an optimal design.

According to: "Photovoltaics: Design and
Installation Manual", (SEI, New Society Publishers):
When two dissimilar modules (or arrays) are wired in
parallel, the current will be additive, but the voltage
will "average".

This book s an OK place to start because
it broadly covers design issues and other important
real world factors like shading, insolation variation by
orientation, geography and time of year. It may be
somewhat simplistic for you. There are probably
more in-depth technical resources, but this one is
a quick read to be aware of what the issues are.
 
let me add this comment on this one i am sure there is new articale in the NEC related to the solar system and i not really recall it right but they did mention OCPD in there.

but however if the solar panel will be tied to the grid system they have specail interlock device so if the uility system fail for some reason it should automatically disconnected . [ The POCO should have the specs on their regulation book if not ask them.]

and some case you will need to change the metering set up as well you will have to use second meter for solar power due the diffrent rates.

dereckbc.,, i heard but i really can't confirmed but i did see some solar system high as 240v DC some case more depending on the solar grid layout and size.

i do know there one commeral buliding near my area did have pretty large solar panel i think about 50 KW or larger [ i am not sure but if i do find it i will post it ]

myself i been thinking maybe i should join in the solar bandwagon due they have good potintal useage with it.

Merci,Marc
 
iwire said:
Well it looks like I will be installing solar systems and I have lots of questions

1) How do I figure VD for varying voltages and currents?
Click to expand...

dereckbc is right on this. I might suggest that instead of using a nominal voltage value that you use the panel's rated voltage from the datasheet along with the short circuit current. Rated current is only slightly less than short circuit current, so it really doesn't matter which one you use. In the datasheet I attached the rated operating voltage of 39.8 V is far enough from a nominal value to throw off your calculation, and that 39.8 is the voltage at the maximum power point of the panel, which is where any decent inverter will cause the panel to operate at. When you design your string layout, make sure you put as many panels in series as possible up to the Vmax of the inverter - typically 600V. I probably shouldn't have to point that out.

2) Does each series 'string' have to be of equal design before paralleling at the combiner box?
Click to expand...

I don't know if Article 690 speaks to this, but you will take an efficiency hit if they're not equal. The reason is that the inverter will adjust the input impedance that it presents to the panels to adjust the voltage and current for maximum power output. If you have non-identical strings paralleled each string should see a different impedance to track maximum power, but they'll both see the same impedance, which means both string's efficiencies suffer. Basically what rexowner said about the voltage averaging.

And make sure you have a Grounding Electrode Conductor run continuously or Burndy crimped from each inverter to your grounding electrode system:smile:. We were trying to borrow a crimper at the last minute but eventually pulled continuous #6 solid through sealtite into a loadcenter then out to the ground rod. A couple people asked why we had 7 wires and 7 acorn clamps.

Click here to see where my experience came from.
 
frenchelectrican said:
but however if the solar panel will be tied to the grid system they have specail interlock device so if the uility system fail for some reason it should automatically disconnected . [ The POCO should have the specs on their regulation book if not ask them.]
Click to expand...

Any inverters listed to meet UL 1749 (I think that's the right number) will meet this conceptual requirement. When such inverters are first powered up on the DC side they "listen" to the AC line for 5 minutes or so to ensure the line is stable and so they can perform voltage and frequency matching. Of course each POCO will have to be convinced if they haven't dealt with a grid tied system before, but that's where John Wiles might be able to help out - there must be a standard procedure for convincing POCOs.

Also, if you're going to need any technical support, don't buy Xantrex inverters. Those of us in the solar decathlon were complaining about their responsiveness and we got a lot of nods and smiles from the folks at the National Renewable Energy Laboratory - apparently Xantrex has this reputation.
 
Wow, I can't thank you all enough. :smile:

The pdf file looks like a gold mine of info. :cool:

I think I will need some time to read the 150 pages then if I have more questions I can ask. :smile:


The system I am working on will have 280 panels. (10 strings of 28).

It will have battery back up and be tied to the utility.
 
Bob I have a whole bunch of info on this subject..if you would like some just PM me and we can make arrangements for transfer..The only thing I know for sure is we are suppose to be going green and the only green going is to the solar panel manufacturers..very costly and pay back is long term..
 
I don't know about back East - but out here they are installed by YAHOO's for the most part, who dominate the market - little or no code compliance whatsoever. JMSO Anyway, for the most part the AC side of the inverter is about all I care about often, as I point in the direction of the nearest hippy whenever an inspector is looking to have words with someone on their installations.

(Sorry - the more I hear about anything 'green' - I want to climb into a tower.... :rolleyes: )

I am sure you will at least be using a chapter 3 method for much of the premise portion of the wiring. IMO the best option is conduit from roof top at a large nema 3 can(s) to wherever the inverter will be. Depending on the batteries you may want to wall them off in a separate room with ventilation, and for utility tied you'll have to check with your POCO for that, as they may want a disco readily accessible. (Locally they want it smack dab on the front of the building! Or remote shut off under thier sole control.)

As for your questions....
  1. Not sure how to get the load for the inverter, (Assume it is its capacity) but a DC voltage drop calc is nearly the same as the AC one if I remember right.
  2. They should be, as you're looking to get the same voltage from each string. Many of these get series'ed up to get to the operating voltage, then paralleled to the inverter or charger. (Like batteries) If they were different, one set would act as a load to the others....
I wish you luck, and I hope you tell us all about it along the way. I would like to know more - but have a hard time stomaching protein deficient hack Eco Fascist's telling me about electrical work with a 'hollier than thou' additude. I can bear it from you though.... :D
 
e57 said:
I don't know about back East - but out here they are installed by YAHOO's for the most part...
Click to expand...

E:

I am not sure I would have put it the same way, but we
probably share many of the same sentiments. I have
been thinking about this reading the posts on solar,
but I really haven't been sure how to bring it up.

It's good to see more professionals, like those on this forum,
get more involved and specifically knowledgable, on both
the installation and especially inspection side. As PV
moves from relatively more "off-grid / remote" to more
mainstream utility interactive installs, and genuine E.C.'s
get involved, I am hoping this gets better.

I have also had my frustrations, such as explaining to
PV *INSTRUCTORS* the difference between a cable and
a raceway (thinking, "WTF are you an instructor and
what are you teaching the other ignoramuses?")
Or, a look in many PV combiner boxes
is like a look into a pot of spaghetti. I'm sure there
are many stories.

There are some good PV contractors out there, though,
and as has been discussed elsewhere, there is no shortage
of crappy AC electrical work being installed.

Article 690 has been significantly revised and expanded,
which will hopefully help. Other hopeful stuff is
happening, e.g. John Wiles' efforts to educate
the unwashed PV installers is also a big step in the
right direction.

Anyway, I am cautiously optimistic it will get better.
 
I have seen some good installs - but most have been just shoddy - with no inderstanding of electrical workmanship at all.

Under-rated UF cable, or my favorite is non-sun light rated single conductors just hap-hazardly stapled to a buildings exterior with NM staples - yet people pay big money to these guys dominating the market who have a few 'installers' they 'trained' off the street for low ball money - to chuck some over priced equipment on the roof so someone can feel as green as the Jones'. It just ticks me off. All of it.
 
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