PV system check

Re: PV system check

zoneblue wrote: »

Sorry, my diagram is not that flash. The batterys are 2V cells, in series. The PV, for the TS60 would be 6 in parallel, so string fuses, or else for mppt, two strings of 3, no fuses?.

Correct. You look at the "series protection fuse" rating for the panel and the Isc (short circuit output current) too... Typically, Ifuse is less than 2x Isc--So you need a fuse to protect a shorted panel from being fed by two or more other panels in parallel.

I'm aware that over-sized inverters are mentioned here a lot. My uneducated reasoning at the time was that if we cant afford a proper inverter at present then starting out with the better end of the Chinese stuff. Hence i overrated it, so as to not push the thing so hard. All we need is a washing machine, an iron from time to time, you know those girl sort of 'necessary' things. I have a bunch of (lighter) woodworking gear which i'll content to run one at a time. Now the points you mention lead me to rethink this. They come in 2kW,3kW,4kW,5kW. Will 2kW run the gear listed above OK? Some of those motors have high start up draws. The inverter standby load will be less with the smaller model. Although it will be on a remote switch and used maybe only an hour every other day.

It is hard to know... Some inverters have very good surge (2x continuous rated watts/VA)... Others will not even output rated power. "Cheap" MSW inverters are probably cheap enough to take a risk and see what happens.

Expensive TSW inverters--You would be better off getting ideas from people that use that same brand/model and see if would meet your needs (here or elsewhere).

In some cases, you get a small TSW inverter to run your "critical loads" (electronics, fridge, lighting--that can be damaged by MSW inverters). And get a second big old MSW inverter to run your shop tools (and larger than that, a 5+kW generator to fire up when needed).

In any case, short/heavy cables with a capable battery bank is a requirement to ensure there is enough current to run the inverter when supply surge current/heavy AC loads.

So 250 or 300A inverter fuse? The 100A shunt is a typo, it and the DC one are swapped.

Yep--that is what you are looking at if you want 4kW max power (and, in theory 8kW surge support).

Yes, 50mV for the shunt, i wonder if i could use the entire length of the inverter cable as the shunt there. The battery computers use a shunt in the -Bat link. So the same probs there. Ill look into the shunts some more, thanks.

Copper wire is not "temperature stable"--If you are OK with roughly +/- 50% accuracy--then you can do that...

I guess the alternative is some hall effect sensors. I also haven't figured out yet how to RMS correct for the pulsed waveforms that some of these power devices create.

Good enough vs exact is a real problem with measuring power... Inverters have a 120 Hz sine wave squared current wave form... How does any specific monitor read DC and 120 Hz sine squared current--Have not a clue... In the end, if you are within 10%--that is probably good enough for monitoring a system.

The thing would need to be smart enough to recognize the short term volt drop from high loads, ie kicks in after some interval at the threshold value. I think /something/ is needed, for the worse case scenario, bat is low and we are out and some persistent DC load drains the battery. But in general vigilance and alarms will work better than LVD.

One way is to set your voltage point to 11.5 volts / 23.0 volts (12 volt / 24 volt numbers) minimum under load set point voltage (or higher). The 10.5/21.0 volt inverter input, from my point of view supports a 11.5/23.0 volt battery output and 1/2 volt drop for wiring/fuses/shunts.

If the battery is not under heavy load (i.e., somebody left a few lights and fan on)--Now you don't have the 1/2 volt wiring drop and the inverter shuts off at "dead battery" range of 10.5/21 volts.

With the TS60 the PV will need to all be in parallel, ie 6 strings of one, ie 28V. I don't really want to stump a $1000 for the TS-mmptp-60 not knowing how hot it will run. If i did go mppt it would be FX80, or 2 mmpt 45A models. I guess i can always connect 5 of the panels to the TS60, at the hottest time of the year which is soon, keep an eye on the real world current and review. My plan to summer bias the array by 6 degrees will go on hold until the water diversion gets installed and the controller settled.

MPPT charge controller is great for large arrays with longer wire runs (use much less copper in your wiring). And for matching non-standard Vmp-panels to the voltage required to properly recharge your battery bank (a 24 volt batter bank needs around 35<Vmp-array<40 volt for efficient use with a PWM controller... And both MPPT and PWM need Vmp-array>35 volts for full power operation in hot weather.

Ive heard good things about those, but they dont seem to be available here? Even though most of the development seems to be grid tie focused, Im picking that we will see some pretty dramatic changes in all this gear in the next 2 years, and stumping $1500 (FX80+mate+sensors) now, seems just wrong to me. And Mppt seems to have a lot more to go wrong, if it is to be worked hard. As PV comes down and down, arrays get bigger, higher amp controllers will be needed. Hot water diversion will stop being seen as an optional extra.

It is very difficult to "grow" a solar PV system over time--As well as migrate between pure GT system (panels+GT Inverter) vs full off grid power (+battery bank+charge controller+off grid inverter+....).

Yes. Amp counting will work well enough for the first few years, but the danger as i read it, is that as the battery capacity degrades the monitor reads more and more optimistically, allowing deeper discharges leading to faster degradation. Nasty spiral.

Some folks use 50% of battery capacity as their "bank AH set-point". Then just run 100% to 0% SOC without having to explain to guests/spouse/etc. that things are "going south" if the battery is below 50% SOC.

Batteries are not perfect... The vendors usually define an end of life battery at 80% of capacity... And if you only use 100% to 50% SOC--You certainly can simply use the battery as it gets below 80% capacity...

The numbers--I try not to get too warped up in them... Basically, if you can measure within 10% of the "real" or "predicted" numbers--You are doing well... And batteries that are within + 20% of rated capacity--they are OK too.

There are trade-offs to be made here... Cheap batteries will give you the best bang for your buck--And if you murder your first set or two (not uncommon)--much less out of pocket risk. But you will get around 5-8 years from them (and may get as little as 3 years or less from cheap/automotive/marine batteries).

Better batteries, you can 10+ years... And fork lift batteries will get 15-20+ years. However you will need a larger array for forklift batteries (more self discharge, especially as they age. And they tend to use more distilled water over time (cost/hassles of distilled water). There are folks that have gotten used fork lift batteries (may have 80% capacity guarantee) and been very happy (a few have even got "scrapped" Forklift batteries and changed/bypassed cells with good results).

-Bill

Re: PV system check- UPDATE

Assuming those are 4x4's... They do not have a lot of strength when subjected to bending forces (something like a 10' pressure treated 4x4 is good for sustaining a 125 lb load--or something like that).

I would highly suggest diagonals or even 3/4" exterior (painted and sealed) plywood with lots of nails (I use screws--but for earthquake codes they actually require nailing) to make diaphragm (if you have parallel legs) or gussets if you do not (a "triangle" to strenthen the "T" section down near the ground--to reduce the "unsupported" length).

I have a friend, back when we were "kids", put a 4x6 on his van for a a bumper--A simple parking lot tap was enough to snap it like a twig...

zoneblue wrote: »

2. Breaker woes. When i picked up the classic lite 150 the midnite dealer gave me a DIN DC only breaker for the controller input, and a panel mount AC/DC breaker for the controller out. Trying to mount this eclectic lot, led me to try to go all DIN rail, but theres some issues there too. The DIN ones only go as high as 60amps. My system will run at around 60A, and that feels too tight to me? If the 60 is ok, then i will get a twin for the neg side,. In that case im confused about which way to mount them. TO my mind when the battery is charging, the controller side is more positive, but in a fault situation the battery side will be more positive. Midnite say to mount the pos side to the battery. But it also says to not reverse polarity!?

You may need to talk with Midnite (or try their forum)... Some breaker are marked Line and Load and others are marked +/- ... It depends on what is being protected... Sometimes it is the direction of the arc control (heat rises) and sometimes it is the actual direction of current flow (they use small rare earth magnets to "blow out" the Arc when the contacts open--wrong polarity and the Arc can "blow" the wrong direction.

For breakers marked "line/load"--I put the Line towards the source of the most current in a single fault condition... With a solar array to controller wiring, that would be (my guess) the solar array (solar PV panels do not supply more than Isc--so breakers on the main solar wiring don't make sense--For combiner boxes, the one protected string is the "load" and the source of current is the rest of the array feeding a shorted string).

For Controller to Battery--The Battery is "Line" where the current is, and the controller is the "Load" even though it is the source of the current in normal operation.

Alternatively if i go all panel mount. But then the midnite guy said that you cant use the AC panel mount breakers for the PV side because "the controller pulses them" and they are called pv breakers for a reason.

Not sure I understand that as an issue--But many AC breakers have very low DC interrupt ratings (may work on 240-600 VAC, but have 48 VDC DC rating, and DC amp interrupt rating can be reduced too).... That is why usually you need "special" DC breakers that are designed for >>48 VDC and high current interrupt capabilities.

3. Lastly the local expert said that regs here require string fuses regardless of the no. of strings. In my case i was following the standard logic around here that you only need string fuses when the the number of strings less one current exceeds the string fuse size, usually 3+ strings. In my case Isc is 8.8amps, but the sticker string fuse is 20 amps. That means i can do 3 strings without string fuses, and ive got combiner bits appropriately.

2x 8.8 = 17.6 amps--So, yes, in this case you could have used 3 parallel connected strings without breakers/fuses. But the inspectors are "god" here--So what they say goes--And usually it is not a problem unless A) it costs a lot of extra money or it is unsafe (see NEC DC GFI rant of mine).

His preference is to run PV cable to each string and mount the string breakers inside. That also doesnt match the cable i have. If i go that route ill need more cable (which in itself is ok as it was cheap) but im back to more DC breakers. Is there really no way around using both DIN and panel mount? I suppose if i do all the above then ill have 6 string breakers, 2 controller in, all DIN, and the pair of panel mounts for the controller out plus a pair for the DC load board. Thats more doable i guess, as i have a section of din rail that will hold the 6, and the other four can go in the side of the case.

Are you saying he did not want a weather proof DC Combiner box (with single pole fuses/breakers out at the array)? But, instead, he wanted the combiner box mounted in the home/battery shed?

I can see wanting the Combiner Box in shade (or even a weather resistant enclosure--maybe). But forcing small wire runs from array seems a bit petty... Perhaps it is their way of making it "painful" to add more solar strings later and possibly overloading the combined current wiring....

-Bill