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Author Topic: Combiner and DC Isolator + Lightning/surge protector  (Read 5006 times)
eabadger
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« Reply #30 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
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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
Scruff
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« Reply #31 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.
« Last Edit: May 02, 2017, 09:06:16 AM by Scruff » Logged
eabadger
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« Reply #32 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.
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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
Scruff
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« Reply #33 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. )
« Last Edit: May 02, 2017, 10:58:19 PM by Scruff » Logged
Scruff
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« Reply #34 on: May 03, 2017, 10:39:10 PM »

Curiosity got the better of me.



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
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