Navitron Renewable Energy and Sustainability Forum

Energy/Electricity Storage and Use/Grid Connection => Off-Grid, Batteries & Inverters => Topic started by: terosity on April 29, 2017, 02:10:58 PM



Title: Combiner and DC Isolator + Lightning/surge protector
Post by: terosity 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


Title: Re: Combiner and DC Isolator + Lightning/surge protector
Post by: camillitech 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


Title: Re: Combiner and DC Isolator + Lightning/surge protector
Post by: Iain 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


Title: Re: Combiner and DC Isolator + Lightning/surge protector
Post by: Westie 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?





Title: Re: Combiner and DC Isolator + Lightning/surge protector
Post by: camillitech 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


Title: Re: Combiner and DC Isolator + Lightning/surge protector
Post by: eabadger 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


Title: Re: Combiner and DC Isolator + Lightning/surge protector
Post by: rogeriko 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:


Title: Re: Combiner and DC Isolator + Lightning/surge protector
Post by: Scruff 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.


Title: Re: Combiner and DC Isolator + Lightning/surge protector
Post by: camillitech 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


Title: Re: Combiner and DC Isolator + Lightning/surge protector
Post by: eabadger 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


Title: Re: Combiner and DC Isolator + Lightning/surge protector
Post by: rogeriko 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.


Title: Re: Combiner and DC Isolator + Lightning/surge protector
Post by: eabadger 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


Title: Re: Combiner and DC Isolator + Lightning/surge protector
Post by: rogeriko 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.


Title: Re: Combiner and DC Isolator + Lightning/surge protector
Post by: eabadger 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


Title: Re: Combiner and DC Isolator + Lightning/surge protector
Post by: biff 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


Title: Re: Combiner and DC Isolator + Lightning/surge protector
Post by: Westie on April 30, 2017, 12:35:04 PM
I completely agree with the surge protection but I don't see why over-current MCB's are of any use?

MCB's  normally to protect against excess current, however string the current can never exceed the Imax of any one panel in a single string anyway?  (typically 10A)

Also, say you had a short circuit somewhere on an array, how would an MCB protect it?  The S/C current will only flow around the panels in the loop contained within the shorted section?

The only way you could hope to protect against a string short would be to have an over-current device between each panel, even then what rating would you use , Imax?  But you want Imax !

When you think about it, the safety of micro-inverter arrays are way superior.









Title: Re: Combiner and DC Isolator + Lightning/surge protector
Post by: Tiff on April 30, 2017, 02:23:34 PM
What Westie said. If anything, more chance of causing a fire than preventing one.

http://bpva.org.uk/media/38266/new-guide-to-installlation-of-pv-systems-mcs_20130530161524.pdf

PV modules are current-limiting devices, which require a non-standard approach when designing fault protection systems, as fuses are not likely to operate under short-circuit conditions.


Title: Re: Combiner and DC Isolator + Lightning/surge protector
Post by: eabadger on April 30, 2017, 03:17:05 PM
the mcb on arrays are a way of isolating not protecting.
the 150a mcb on inverters are both isolating and protection on batteries in case of inverter s/c

steve


Title: Re: Combiner and DC Isolator + Lightning/surge protector
Post by: Westie on April 30, 2017, 03:30:09 PM
the mcb on arrays are a way of isolating not protecting.
the 150a mcb on inverters are both isolating and protection on batteries in case of inverter s/c

steve

Actually if you have a system with DC coupled batteries then you definitely need MCB's to protect against current backfeeding from the batts into a short on the string.


Title: Re: Combiner and DC Isolator + Lightning/surge protector
Post by: jonesy on April 30, 2017, 03:40:15 PM
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
Do you mean static?  ie high voltages generated by friction.
SPD stop very little and need regular replacement.  They are one-shot devices. The ones in my phone socket went sometime this year and we've had no lightning, so maybe just a surge somewhere. I only noticed as I saw a massive increase in the broadband error rate and a reduction in speed down to 20M  ;D
I had a satellite box go a bit fut (blown transistor) following a lightning storm in the neighbourhood, but I can't be sure it was the storm.  The dish is about 20m from the house, so I'd guessed it was a difference in ground potential during the strike.  The transistor only affected channels on vertical polarisation and the channels I mostly watch are on horizontal, so it could just have been one of those things.
Panels are not lightning magnets.  Are they any more likely to be hit than any other part of the house eg TV aerial?  
The Hager breakers taking DC are good, but IIRC it's something like 45s for a B curve to trip at rated current. The MCB protects the cable. I've a Merlin Gerin dual conversion UPS which has type B MG breakers on DC, but the spec sheets don't show it last time I checked.

You can directly connect panels to batteries. The only reason I stopped was too high a voltage once the batteries were fully charged. Batteries should be protected just above the maximum demand, or cable size, whichever is smallest.


Title: Re: Combiner and DC Isolator + Lightning/surge protector
Post by: terosity on April 30, 2017, 06:03:46 PM
Hi all thanks for the replies.

Im looking for a way to safely turn off any string if required and all if needed hence why the mcb's.

So you are saying SPD are a waste of money ?

What is the safe way to disconnect a string if needed.

Thanks


Title: Re: Combiner and DC Isolator + Lightning/surge protector
Post by: Westie on April 30, 2017, 06:57:19 PM
Hi all thanks for the replies.

Im looking for a way to safely turn off any string if required and all if needed hence why the mcb's.

So you are saying SPD are a waste of money ?

What is the safe way to disconnect a string if needed.

Thanks

Every solar inverter I've looked at has built in surge protection on the string inputs. But personally I would fit external devices as well. It's easier to replace those rather than the ones built into the inverter in the event you do get unlucky.

BTW if your panels are up on a roof and can't be touched from anywhere (eg. by leaning out of a window)then it's better not to earth the supporting frames, always remember to isolate the arrays if your working on the roof off a ladder / scaffold.  Also be aware that a static charge can build up on the array frames. Personally I always flash them down to earth before I touch them. The last thing you want is a static jolt when perched on the top rung.   sh*tfan:

For isolation I would use a straight forward DC isolator they're around £20 each, bring each string back to it's own isolator, do your combiner wiring on the inverter side of the DC isolators. 





Title: Re: Combiner and DC Isolator + Lightning/surge protector
Post by: camillitech on April 30, 2017, 08:42:33 PM


Every solar inverter I've looked at has built in surge protection on the string inputs.

But he does not have a solar inverter Westie, this is in the 'off grid' section and he's using a solar controller. However, as pointed out by Roger and Steve, methinks it's a waste of time anyway.


Title: Re: Combiner and DC Isolator + Lightning/surge protector
Post by: Westie on April 30, 2017, 10:06:59 PM


Every solar inverter I've looked at has built in surge protection on the string inputs.

But he does not have a solar inverter Westie, this is in the 'off grid' section and he's using a solar controller. However, as pointed out by Roger and Steve, methinks it's a waste of time anyway.

OMG... Sorry guys. I've accidentally strayed into an area I know nothing about, all my comments, as Camillitech pointed out, relate to Grid Tied Solar inverter - Sorry terosity I'm sure the off grid guys will sort you out!
 surrender:





Title: Re: Combiner and DC Isolator + Lightning/surge protector
Post by: camillitech on April 30, 2017, 10:44:33 PM


Every solar inverter I've looked at has built in surge protection on the string inputs.

But he does not have a solar inverter Westie, this is in the 'off grid' section and he's using a solar controller. However, as pointed out by Roger and Steve, methinks it's a waste of time anyway.

OMG... Sorry guys. I've accidentally strayed into an area I know nothing about, all my comments, as Camillitech pointed out, relate to Grid Tied Solar inverter - Sorry terosity I'm sure the off grid guys will sort you out!
 surrender:





Methinks you still need 'lucky underpants' even with a solar inverter Westie  :crossed Far better than surge protectors  ralph:

Cheers, Paul


Title: Re: Combiner and DC Isolator + Lightning/surge protector
Post by: Scruff on May 01, 2017, 03:13:19 AM
(https://mindbodyplate.files.wordpress.com/2014/05/underpants.jpg)


Title: Re: Combiner and DC Isolator + Lightning/surge protector
Post by: bautsche on May 01, 2017, 09:34:04 AM
I've only skim-read the post, so if I'm missing the point, feel free to flame me... ;-)

If you're direct attached (i.e. DC and high amperage), go for something like this:
https://inverterdrive.com/OrderCode/20988/

It's a fuse box and if you pull the handle it disconnects.

If you've got an inverter, something like this will do:
http://uk.rs-online.com/web/p/non-fused-switch-disconnectors/3813578/

HTH
Eric



Title: Re: Combiner and DC Isolator + Lightning/surge protector
Post by: eabadger on May 01, 2017, 11:11:35 AM
do the spd's work? not on lightning strike that i am sure, but static build up in air? i dont know, i know i lost items before i fitted them, but i also disconnect the telephone line now, which is also now isolated from the solar technology by wifi, so kind of too many changes to confirm.
But, in the scheme of things they are peanuts so i would fit no matter what.
biggest issue when my mppt died was i had a standard pwm controller, but i had to reconfigure all the panels to get it working.

after all for most of us it is all a big experiment.

steve


Title: Re: Combiner and DC Isolator + Lightning/surge protector
Post by: Westie on May 01, 2017, 04:08:37 PM
do the spd's work? not on lightning strike that i am sure, but static build up in air? i dont know, i know i lost items before i fitted them, but i also disconnect the telephone line now, which is also now isolated from the solar technology by wifi, so kind of too many changes to confirm.
But, in the scheme of things they are peanuts so i would fit no matter what.
biggest issue when my mppt died was i had a standard pwm controller, but i had to reconfigure all the panels to get it working.

after all for most of us it is all a big experiment.

steve

Yep, they definitely work.  The cheapo ones just use voltage dependent resistors (VDRs), there's usually one or more connected per conductor to earth, they're okay for attenuating low energy surges, the best 'arrestors' have VDR's and avalanche diodes. Serious energy triggers the avalanche diode with virtually shorts the conductor to earth.  When you get a surge of high enough energy to trigger the diode it's normally destroys it, I've had them literally blow apart, but the fact it shunted all that energy to earth usually saves the device that's being protected.

It the surge protector doesn't have a fat connection to earth don't bother buying it, the best one's have VDR's, Avalanche diodes, and a 4 or 6mm separate earth cable to dump the energy down.  I use APC and Furse arrestors to protect comms lines (PSTN, ISDN, Ethernet etc.)  I'm afraid Pauls lucky underpants are chocolate fireguards where I live, I'm about a half mile away from a 750ft (disused) TV mast and it's an absolute lightning magnet, which is good as it's all the lightning ever strikes. Sadly, tho all our overhead  telephone lines run along a road that runs along side the mast, within about 100Mtrs. When the mast is struck megaamp currents are conducted down it to earth, a huge electromagnetic pulse induces high voltages into the phone lines which then looks for ways out, before I started fitting arrestors I regularly replaced master sockets/ phones/faxes and routers, now I just replace arrestors.

These arrestors ain't cheap ... They're around £100 a pop for a PSTN line. Mine have blown twice in the last 3 yrs, I now buy the VDR's and avalanche diodes to repair them myself, that's if the PCB hasn't vapourised sh*tfan:.

http://new.abb.com/low-voltage/products/earthing-lightning-protection/furse

http://www.apc.com/shop/au/en/products/APC-ProtectNet-standalone-surge-protector-for-10-100-1000-Base-T-Ethernet-lines/P-PNET1GB








Title: Re: Combiner and DC Isolator + Lightning/surge protector
Post by: Scruff on May 02, 2017, 12:00:59 AM
but i had to reconfigure all the panels to get it working.

How did the real world outputs compare? Same manufacturer or different?


Title: Re: Combiner and DC Isolator + Lightning/surge protector
Post by: eabadger on May 02, 2017, 08:44:22 AM
no not the same make of controller, the pwm was an allegedly 80a Chinese special, input power was well down, when i got the new mppt returned i left string on low voltage just in case, and the figures were well down, when i got round to upping the string length the power went back up.
i am convinced the mppt is worth it, i just bought another 2!! so i hope i am right, i am considering buying another for the wind turbine, the software has a "beta" mppt turbine setup.
i was seeing 2kw spikes from my 1600w strings last week, the mppt's coped well.

back to the spd, the size of earth tag i can see as a major thing, the technology is basic but would work on spikes from static discharge or induced in the tv mast case scenario, it is regs here for them to be fitted on the telephone line, but france telecom man questioned me as to what it was, never seen one before, he just shrugged when i said it was the rules.


steve


Title: Re: Combiner and DC Isolator + Lightning/surge protector
Post by: Scruff on May 02, 2017, 09:03:45 AM
I'm in PWM camp but I have yet to collect my own data on the matter.  surrender:

This is from the designer of one of the better Ah counters.

 

Quote from: Ralph, Bogart Engineering


A very good question! They BOTH have good and bad. Plenty of hype has been written already. Here's my (Ralph's) view:

The "good" for PWM: It is simpler and lower cost technology. Under some common circumstances–it can actually deliver more amps to the battery. That could be when:

(1)days are moderate or warm, with few clouds.

(2) batteries are charging at over 13 volts, (in a 12 battery system) which they almost always are when actually CHARGING.

(3) Panel voltage is properly matched to the battery voltage, for example "12V" panels are being used with a 12V system.

PWM is actually more "power efficient" than MPPT–which means less total power loss in the controller itself. So heat sinks in the design can be smaller (and less expensive). Missing in most analysis of MPPT is that there is always a conversion loss with MPPT, which tends to be higher the greater the voltage difference between battery and panels. That's why PWM can actually beat MPPT under circumstances described above.

Some places that analyze MPPT assume that panels with 30V open circuit voltage are being used in a 12V system. Any good MPPT system will easily provide better performance in that case. They also may assume batteries are charging at 12 or even 11 volts, which is unrealistic. Lead acid batteries are typically below 13 volts only when discharging, or perhaps charging with very little charging current–meaning the actual potential gain in amps is not great.

The benefit for MPPT becomes apparent if you use panels not voltage matched for the battery. If they are not, MPPT will utilize more of the potential energy of the panels. For example, if you use 24 volt panels to charge a 12 volt battery system you must use MPPT, otherwise you would be using your panels very inefficiently. If you are trying to use PWM in that case, you are misusing the PWM technology.

Another potential benefit with MPPT is that if distance between panels and batteries is far, smaller wire can be utilized by running panels at higher voltage to the batteries. Running at twice the voltage reduces wire size to 1/4, which for a long run can be a significant saving in copper wire.

If temperatures are low enough, the slightly less power efficiency of MPPT will be compensated by the higher panel voltages, which will result in a little more battery current. But in actual measurements we made using a commonly sold MPPT solar controller, this would occur at temperatures less than 55 F degrees (in full sun, when charging at more than 13 volts), where there is a slight advantage to MPPT in my location (Boulder Creek, near the California coast). As temperature drops below that (in full sun) MPPT will get some advantage, such as could occur at high elevations in Colorado in the winter. Potentially this would be maximum about a 2.5% improvement in amps output for every 10 degrees F lower in temperature (or 4.6% per 10 degrees C colder. I'm using data from Kyocera KD-140 panels.)

There can be theoretically optimal situations (that I don't personally experience where I live) where MPPT could give some advantage: that is when solar current is present, but the batteries are quite low in charge–but because loads are high and even greater than the solar current the batteries are still discharging despite the solar current. Under these conditions the voltage COULD be at 12.5 volts, or even lower. Again, using data from Kyocera panels, ("Normal Operating Conditions") there is a theoretical maximum gain over PWM of 20% current assuming NO MPPT conversion loss and no voltage drop in the wires to the panels, at 20C (68F). With PWM, the voltage drop in the wires in this case would not affect the charging current. Now if in addition you lower the temperature to below freezing at 28 degrees F (while sun is shining) you might actually get up to a THEORETICAL nearly 30% gain while the batteries are discharging.

The only REALLY BAD part of MPPT, is all the hype surrounding it–for example one manufacturer advertises "UP TO 30% OR MORE" power harvested from you panels. If you are using solar panels properly matched to the batteries, 30% ain't gonna happen unless it's EXTREMELY cold. And your batteries have to be abnormally low in charging voltage–which tends not to happen when it's cold (unless you assume the battery is still discharging while solar is happening). Virtually all the analyses I've seen touting MPPT on the Internet ignore the conversion loss, assume really cold temperatures, assume unreasonably low charging voltages, assume no voltage drop in the wires from panels to batteries, use STC conditions for the panels (that the marketing types prefer) rather than more realistic NOCT conditions, and in some cases assume panels not voltage matched to the batteries.

The other thing that is misleading about MPPT, is that some manufacturers make meters that show both the solar current and the battery current. In almost all cases for a well designed MPPT type the battery current will be greater. The engineers making these know better, but it is implied (by marketing types?) that if you were NOT using MPPT you would be charging your batteries with only the SOLAR current that you read on their meters. That's not true, because the PWM BATTERY current should always be higher than the MPPT SOLAR current. It is the nature of the MPPT that maximum power occurs when the current is lower than the maximum, so they must operate there to get the maximum power. So to properly compare the two you need to compare MPPT with an actual PWM controller in the same circumstances.

Finally, the reason we went to PWM is that I was anticipating that panel prices were going to drop (which they certainly have over the last 5-10 years!) and that the small advantage of MPPT (under conditions where the correct panels are used for the batteries) would not justify their additional cost and complexity. So my thinking, for more total benefit per $, put your money in an extra panel rather than a more expensive and complex technology.



I can't see how manipulating the voltage of a current source is gonna hold much water against a well matched panel to battery Voc minus installation losses myself. Having said that I'm running MPPT for the now because it was more suitable for grid tied changeover.
As an aside I much prefer the PWM buzz to the MPPT whine.  wackoold

I guess a rule of thumb is scale dependant; which is the cheaper +10%; more PV or MPPT.


Title: Re: Combiner and DC Isolator + Lightning/surge protector
Post by: eabadger on May 02, 2017, 07:38:35 PM
i feel, that i get value for money with mppt, i have run pwm and mppt so get an idea, i also really like the web interface on the mppt60a, gets addictive though.
for sure early and late on i get more power with mppt.

out of interest what cross section cable you running the dc bus on? i will have 3 60a mppt 1 x 45a ts and two victron 3000 all at 24v, online calculator says 35mm with 1.5m run and 1% loss or 16mm with 3% loss, real world?
i do have some 50mm stuff like tow rope!! going to be a Heinz to dress it nice.
or i can get some french swa 25mm french swa is same as english but without the swa.


Title: Re: Combiner and DC Isolator + Lightning/surge protector
Post by: Scruff on May 02, 2017, 09:40:57 PM
Yeah you probably do, few years back when I last looked at it >600Wp was worth MPPT, < was more PV.


I tend to call 35mm² an upper limit after that I increase the voltage unless for some reason legacy prevents it.
Thin wall or Welding cable is always an option for < 60V.

2 x 45A controllers on 25mm² (<3m round trip) I'm going to load balance by the time they're both full I daresay I won't care about the cable losses. Battery to bus is 35mm² (<3m round trip).

My avatar has 10mm² controller to battery for Isc 10A, there's 2m x 35mm² from alternator to starter motor, 1m x 50mm² starter to engine battery, 2m x 90mm² engine battery to engine bus & 2m x 35mm² from bus to contactor to domestic battery, 2.5m x 35mm² battery to shunt to chassis & 50mm² 1m chassis to gearbox. (1.4% drop @ 20A)

The house is overkill the motor has room for improvement (shorter runs/ independent alternator).

There's more resistance on the terminations usually than the cable. 200μΩ per junction for a good connection, so cable to lug, lug to lug, lug to cable = 0.6mΩ it adds up after a while not a big deal with solar controllers but it can make huge difference to split charge systems. Which on principle are not a very good idea because the same time and resources invested in 2  x independent charge systems would bare 8 times the fruit but it's not the done thing....  

I think aggregating cable is best; wherever possible run 1 x 50mm² instead of 2 x 25mm² for instance common controller grounds. At least this way when one's working harder than the other there's less impedance for that controller while the other plays second fiddle. If it's a shorter route you can maybe go pv neg to battery to controller. Waste of cable sometimes going pv neg to controller common neg, then battery. (when I do it this way I make sure the controller neg csa = the PV + controller pos. CSA....it's a question for the manufacturer if this is necessary, on the face of it I'd say you might get away with a small ground to controller electronics and the main pv neg bus direct to battery.....if that's shorter. )


Title: Re: Combiner and DC Isolator + Lightning/surge protector
Post by: Scruff on May 03, 2017, 10:39:10 PM
Curiosity got the better of me.

(http://i.imgur.com/PFcJos2.jpg)

According to MS Tech Support this is ok (but not my idea of using a tiny ground just for the TS control electronics). As long as the battery to controller cables are the same gauge. Which I'll take to mean if I'm running 4 parallel controllers with 6mm² feeds from the panels I can run a single negative from the isolators to the battery bus on 25mm²