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Author Topic: Combiner and DC Isolator + Lightning/surge protector  (Read 5000 times)
terosity
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« on: April 29, 2017, 02:10:58 PM »

Hi All,

I need help with dc isolation and combiner.

I'm looking at having 12 Trina solar panels 295w see here http://static.trinasolar.com/sites/default/files/Datasheet-DD05A.08(II.pdf

the plan is having 3 strings of 4 panels running parallel, so this will give me around 160v / 10a per string which could rise to about 180v / 11a or 12a per string.

My original plan was to be able to isolate each string in case of a failure, although this is looking expensive to do.

So what would be the best way to isolate that is cost effective. i then intended to combine the strings into one, so they will each go through a fuse and then out via an mcb with surge protector.

This does basically what i require, however with it chinese and cheap im not keen on using this.

http://www.eco-worthy.com/catalog/worthy-solar-combiner-with-circuit-breakers-string-enclosure-breakers-p-600.html

This then goes onto the Lightning protection. the system will be up in the hills in an open space, so i believe there is a more increased risk.

So to summarise i need to know,

DC Isolation, the best method that is cost effective, the idea was to use before the combiner in the event i needed to change a fuse etc.
MCB Rating for the combined.
SPD for surge protection.

If you can link to any relevant uk sites that would be fantastic.

Thanks
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camillitech
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« Reply #1 on: April 29, 2017, 04:42:31 PM »

Hi there Terosity and welcome,

Someone will probably come along and give you a hundred reasons why you shouldn't but I just use a regular consumer unit 'backwards' if you know what I mean.

Use the DP input as the output, replace the mcb's with 10 x 35 fuses and 'Bob's your uncle'. Looks neat and does the job.

Cheers, Paul
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Iain
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« Reply #2 on: April 29, 2017, 04:57:02 PM »

Hi
Following on from Paul, I believe Hager MCB's are rated for DC as well.

http://www.hager.co.uk/downloads-services/technical-services/mcb-s-dc-rated/56346.htm

Iain
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« Reply #3 on: April 29, 2017, 05:24:02 PM »

I'm aware that commercial PV plants use combiners/MCBs & surge protection per string but is there any statutory regulation that calls for this protection on a domestic PV system?

My system has two 2kw strings connected directly to the dual MPPT inverter inputs via two DC isolators?



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camillitech
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« Reply #4 on: April 29, 2017, 07:58:16 PM »

I'm aware that commercial PV plants use combiners/MCBs & surge protection per string but is there any statutory regulation that calls for this protection on a domestic PV system?

My system has two 2kw strings connected directly to the dual MPPT inverter inputs via two DC isolators?





I don't think there is any need as such Westie but larger 'off grid' set ups typically work at lower voltages than Grid tied. So, for an equivilent output more strings are required and whereas GTI's generally have two or more inputs solar charge controllers usually only have one. I guess it's just a neater way of doing it.

Cheers, Paul
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'Off grid' since 1985,  Proven 2.5kW, Proven 6kW direct heating, SI6.OH, 800ah Rolls, 4.75kW PV ,4xTS45, Lister HR2 12kW, , Powerspout pelton, Stream Engine turgo, 60 x Navitron toobs and a 1500lt store. Outback VFX3048 and 950ah forklifts for backup,
eabadger
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« Reply #5 on: April 29, 2017, 08:13:59 PM »

i am using these per string http://www.ebay.co.uk/itm/222272640574?_trksid=p2060353.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

i have some of these to fit as well, soon...
http://www.ebay.co.uk/itm/SPD-DC-800V-20KA-40KA-House-Surge-Protective-Low-voltage-Arrester-Device-/222307485395?hash=item33c28f06d3:g:dcEAAOSwnbZYIDcH

my understanding is regs for dc or ac are the same, anything above 50v is covered under the lvd regardless of ac or dc.


steve
« Last Edit: April 29, 2017, 08:17:00 PM by eabadger » Logged

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« Reply #6 on: April 29, 2017, 11:13:54 PM »

On the datasheet you provided you have to look at the NOCT ratings not the STC ratings. This means the Normal Operating Cell Temperature ratings. On your panels you will get 7.2 Amps at 30 Volts which equals 220 Watts per panel. You will only achive higher power momentarily when the panels are cold and the sun comes out from behind a cloud, or if you move to the sahara desert!!  I have installed thousands if not tens of thousands of panels and never used string fuses on any. They are not required in the regs. You are installing 3 strings so if one string shorts out the other 2 can only produce max 14 odd amps which is not nearly enough to damage the shorted wire or even heat it up. A shorted panel will only produce 1 or 2 volts which equals 14 or 28 watts nothing to worry about in 4mm solar cable. As far as lightning protection is concerned your inverter will blow up way before the lightning protector does its job. A nearby lightning strike will kill all electronics through the ground connection not through the solar panels. Take the NOCT ratings into account when you size your system and forget about the rest.    stir stir stir
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« Reply #7 on: April 30, 2017, 02:04:38 AM »

my understanding is regs for dc or ac are the same, anything above 50v is covered under the lvd regardless of ac or dc.

Extra Low Voltage:

< 50VAC &
< 120VDC

Except boats where ELV is less than 50VDC but then building regs don't apply because they aren't buildings.
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camillitech
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« Reply #8 on: April 30, 2017, 07:39:38 AM »

forget about the rest.    stir stir stir
javascript:void(0);
I find that the only thing that works for lightning protection is lucky underpants  hysteria
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'Off grid' since 1985,  Proven 2.5kW, Proven 6kW direct heating, SI6.OH, 800ah Rolls, 4.75kW PV ,4xTS45, Lister HR2 12kW, , Powerspout pelton, Stream Engine turgo, 60 x Navitron toobs and a 1500lt store. Outback VFX3048 and 950ah forklifts for backup,
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« Reply #9 on: April 30, 2017, 09:38:43 AM »

lightening you are spot on, nothing you can do, but my experience says that static or near strike discharge is far more likely and causes as much despair and failure of sensitive stuff.
have you ever seen the spark arrestors working in telephone sockets?
bloke here was hospitalized some years back, hit in the chest by his fax machine!!

steve
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1600w PV main array at 24v, excide 2v 1000a forklift cells now x 2, 320w PV secondary array at 12v. Enfield 1944 ex RAF 5.6kw diesel genset (now in pieces, big ends gone), Petter AC1 28v diesel charging set at 2.8kw.
1kw wind turbine.
26kw wood stove back boiler to underfloor heating and dhw
rogeriko
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« Reply #10 on: April 30, 2017, 10:14:15 AM »

When I was living in the Greek islands we used to get lots of lightning storms and indeed it was the telephone cables that caused the most damage. In one case the lightning hit the telephone pole at least 200 meters from the house, then travelled along the cable slung between 3 poles, completely melting it in the process. Then it got to the incoming box to the house where the mains supply was also located. There it jumped to the mains cable completely melted the 10mm cable to the earth rod and travelled into the house destroying TV's fridges and everything with a transistor or chip inside. There is indeed nothing you can do and for the price of some of these useless protection devices just buy some extra panels. I do not recommend using any breaker type of disconnect because they will just arc inside and catch fire. If you put a solar panel in the sun and touch the wires together (short circuit, of course you have to cut off the plugs first or use a short piece of wire with bared ends) when you separate the wires you will get an arc just like an electric welder. That is what will happen inside your MCB disconnecter. I install solar farms, we are talking fields of panels and I demonstrate this to new electricians/employees and after they have seen a continuous sustained 100mm spark from 20 panels at 700v they are extremely cautious and carefull with their work.
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eabadger
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« Reply #11 on: April 30, 2017, 10:28:39 AM »

so you use fuses to comply with LVD? i think your example is flawed as this is the s/c current at very low volts not 700v as you state, when you disconect the panels by separating the plugs do you get an arc? no because they are at 700v.

static discharge via telecom cables is more likely scenario in most cases, yours sound horrific, i have seen the spark arrestor flashing many times in our telephone socket as it discharges.
 

steve
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1600w PV main array at 24v, excide 2v 1000a forklift cells now x 2, 320w PV secondary array at 12v. Enfield 1944 ex RAF 5.6kw diesel genset (now in pieces, big ends gone), Petter AC1 28v diesel charging set at 2.8kw.
1kw wind turbine.
26kw wood stove back boiler to underfloor heating and dhw
rogeriko
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« Reply #12 on: April 30, 2017, 10:44:59 AM »

When you disconnect the plugs on any working pv system where the current is flowing you get a huge arc, it melts the plugs. You can only safely disconnect the DC plugs when there is no current flowing, ie when the inverter is off. AC off first.
As far as string fuses are concerned the commercial ABB 3 phase inverters used on solar farms have glass tubular fuses fitted inside on each string.
« Last Edit: April 30, 2017, 10:48:47 AM by rogeriko » Logged

eabadger
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« Reply #13 on: April 30, 2017, 11:04:52 AM »

i get the impression that the OP is battery/off grid? i can see on a gti it would be under load. but not on a solar charge controller.
isolating strings i have never done before, but am doing on my upgrade, in past i wished i had, am also fitting mcbs to inverters somthing again i wished i had done in the past.
fingers crossed after fitting SPD to telephone and data cable not had anymore static issues or lost equipment, so maybe just luck or they may work?

steve
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1600w PV main array at 24v, excide 2v 1000a forklift cells now x 2, 320w PV secondary array at 12v. Enfield 1944 ex RAF 5.6kw diesel genset (now in pieces, big ends gone), Petter AC1 28v diesel charging set at 2.8kw.
1kw wind turbine.
26kw wood stove back boiler to underfloor heating and dhw
biff
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« Reply #14 on: April 30, 2017, 11:24:18 AM »

We had a very bad lightning storm years ago,
                                  Heavy rain, then big hail stones that battered our roof for a good 5 minutes, No sleep possible, so we got up and had a look out to see how the W/T was getting on and watched for a while as the flashes lit up the whole area right out to sea,,then a bolt hit the lawn about 10ft or so from the tower,,up the tower it went and right out to the end of the tail with little blue triangular flickerings . Both our inverter and controller survived but the one brain on the Symmetra popped, the big blue ac plug on our stanby generator got a melted pole and the sound went on our tv for about an hour or so. The turbine got fried to a black crisp, The Generator was not connected nor was the Symmetra rm. We were running the house with the chinese 2kw inverter and controller which tripped but were successfully restarted,,yet the sound got knocked out on the flatscreen. The whole thing did not make sense but that is what happened. The Generator and the symmetra were both disconnected. The generator would have been that bit closer to the strike on the lawn which did not leave much of a mark, maybe a light burn about 18" which I did not notice for weeks.
  It seems to me, that lightening don,t obey our laws of physics.
                                                                     Biff
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