Solar Storage System : System testing

Yesterday I finished all the wiring on the control board, the high power DC wiring, AC distribution wiring and sensor and fan control.

The only thing left to do is add the Raspberry PI and it's I/O boards and finish writing the software. I'll start on that next week, I've ordered a case for the R-Pi, ribbon cable and a few plugs and sockets to connect all the boards - annoyingly only two of the I/O boards have a hole to add a mounting post, the largest I/O board (PiFace) doesn't, and the smallest (2 in, 2 out analogue channels) is too small to have one, so I'll have to mount them in their own case and figure out some way of keeping them stable.

So in the meantime I wanted to test the assembled hardware. 9 SSR relays plus associated heatsinks, temp sensors and cooling fans, the three inverters, DC fuseboard, AC fuseboard, AC distribution block, and all the wiring and crimped connections.

I dug out nine nice old chunky switches to control the relays, fitted them to a bit of wood lying around, connected all the wiring up, used my DC PSU to provide the 12V for the relays and fans and, after a final check for short circuits, connected the battery and charger and turned the input mains power on.

I'm happy to say that everything works fine. At least to start with!

All three inverters running consumed about 45A, which is about half of what they should be drawing.

A quick check revealed that the main battery disconnect SSR was dropping about 1V, the 1KW inverter DC SSR was dropping about 0.7V, and the 500W SSRs dropping about 0.5V.

So the 1KW inverter voltage was 1.7V down, and both 500W inverters were 1.5V down. Not ideal, as the inverters are producing about 1KW between them. Without any voltage drop they produce approx 1.5KW. It'll do for now, my typical evening usage is 900W so it will cover that nicely.

The main battery disconnect relay did get nice and toasty, which tested the temp sensor and cooling fan which worked perfectly. Cut in at about 70 degrees C and turned back off at about 60 degrees C.

After running the system for 20 minutes the two smaller inverters turned their cooling fans on at an internal temp of about 45 deg C, and I know that the 1kw inverter cooling fan works when I tested it in isolation.

So, happy that everything was working properly I turned off both inverters, checked the battery charger and it's relays (both good), and was just turning everything off to finish for the evening.

The battery charger was on and charging at it's minimum of 6.6A. I turned the main battery SSR off, and the charger current sat at 6.6A. Uh? What's it charging? The battery is disconnected. Oh no it's not! The SSR has fried itself and short circuited. I was monitoring it's temperature and it only got upto about 80 degrees C, and I have checked the specs of reputable brand SSRs and they are good for 125 degrees C. Just goes to show that if you buy cheap SSRs, you need to derate their maximum current and their maximum temperature by a lot!

Not a big problem, as it was dropping 1V across it I think I will change it for a good old fashioned 300A physical relay. I didn't want any moving parts anywhere (yes, I know, the fans are moving parts!), but to get the system working at maximum output power it's looking like I'll need to change all the DC SSRs for normal relays.

If any readers know of any eBay sellers selling decent 300A relays (and 100A relays too) please do let me know.

Well, it is a prototype system so finding out parts aren't suitable is part of the game! :)

Piccy of the controller board (excuse the mess at the bottom of the board - this is all the controller wiring connected to the switches, this will all be tidied up when the R-Pi and I/O boards are installed)

Components from left to right, top to bottom:-
Top row
* DC current shunt (partially hiding behind the black box)
* DC fuse box (the black box just under the DC current shunt), 250A main fuse, 75A for 1KW inverter, 60A for each of the 500W inverters, 100A for the charger)
* Main battery SSR (fried)
* Charger DC relay
* DC ground distribution block (hey, it's huge I know, it's what I found on EBay and I couldn't be bothered to chop it down :) )
2nd row down
* DC relays for 1KW inverter
* DC relay for 500w inverter #1
* DC relay for 500w inverter #2
3rd row down
* 1kw inverter
* 500w inverter #1
* 500w inverter #2
4th row down (just below the 500w inverters)
* AC relay for 500w inverter #1

* AC relay for 500w inverter #2
* 4 x main fuses for the AC connection for the three inverters and the charger
* Main AC distribution block
5th row down
* Main 240V AC grid connection (almost off picture to bottom left)
* Charger 240V AC connection (almost off picture to bottom left)
* AC relay for 1kw inverter
* AC relay for 2.5kw charger
* Temporary manual switches used to test the other components

Solar Storage Unit starting to come together

I've got all the bits I need at last, and the external case is being built and should be here in a couple of weeks.

All components are being mounted on a spare cupboard door I had lying around which happened to be the ideal size.

Four rubber feet on the bottom of it with the signal and sense wiring to the controller on the underside, and the DC and AC high power connections on top.

All the DC solid state relays are mounted on small heatsinks with their own fan, controlled by a 70 degree C switch mounted on the heatsink.

The past couple of days I've been mounting the heatsinks, fitting the fans and done the positive DC wiring:-

The heatsinks came with one milled flat space for one SSR, but the top right one required two SSRs fitting, so I had to finish fitting a new DC motor to my CNC mill so I could use that to mill the whole of that heatsink flat so it would accept two SSRs. All things that take time!

You can see fitted to each heatsink the little temperature controlled switches, they just look like transistors. Better than the old style thermostat type switches. The fans require 12V 0.25A which these will handle with ease. I will also add a second feed to all the fans, via diodes, so that the controller can turn them all on when the overall temp in the unit case gets too high. The inverters are also getting one of these temp switches each which will feed into the controller to let it know that inverter is too hot. I also have different style temp switches that I'll (try to) fit in between the battery cells. I want to be able to monitor all parts of the system and either not use parts that have failed (or are just too hot) or shut the whole thing down.

Tomorrow I'll finish the negative DC high power wiring and then the controller wiring, and then mount the inverters and finally the AC wiring side.

This board will sit next to the battery in the case, with the charger unit suspended above this board on its own little shelf. I've also got two temperature speed controlled fans which will get mounted in the case, left and right.

I know this project has kind of grown from a few parts that were going to just be assembled on a desk, to something that monitors itself in loads of ways and will shut off if needed, so it can be put in a garage and completely forgotten about by anyone.

I just hope that at the end of it, my solar panels produce enough extra power to charge the battery in a typical day!

Richard

Inverter progress (yes, there is some!)

I have now received a bench power supply to test my grid tie inverters, and a 1Kw GTI that operates of a lower input voltage.

The new 1Kw GTI was given the once over (resolder a few cracked solder joints, replace heatsink compound with decent stuff, check for short circuits and clean out any rubbish inside it), connected to the bench PSU and sprang into life.

It starts working at approximately 24V - good, cuts out at approx 20V - good, and the MPPT algorithm starts off drawing a very low current and increases the current draw upto the maximum - ideal for use with a battery.

Connected it to grid, the LifePO4 battery via 60A fuse and a 100A current shunt, connected two meters to monitor the DC input voltage (to see what voltage drop there is in the system - a quick way to spot bad or loose connections) and the current passing through the shunt, and then turned on the grid, battery and inverter.

It ramps up the current demand over approximately 30 seconds. The maximum power generated is 850W for approx 1100W input power, so it has 77% efficiency.

No noticable heat being generated after a couple of minutes of operation, and the fan did not come on so I assume is temperature controlled.

So the new 1Kw GTI is working nicely.

I also tested one of the 500W inverters on the bench power supply. The supply can output upto 60V (at 5A) so should easily have enough voltage to satisfy the higher input voltage required by the 500W inverters.

In a previous blog I wrote about how I tried to use one of these 500W inverters. These have comprehensive diagnostic LEDs that indicate the grid voltage and frequency, whether the input voltage is too high or low, a 6 segment bargraph that shows the output power, a blue LED that blinks when the MPPT is tracking the input, and a ref fault LED. I tried connecting one to my battery and after testing the input voltage it simply lit the input voltage too low LED and the fault LED. Hence I thought they required a greater input voltage to start to work.

After connecting them to the bench power supply and increasing the voltage to approx 30V the GTI was doing exactly the same thing.

Increase the voltage even more and it then settled with the High Input Voltage + Fault LEDs lit. Which didn't appear to make much sense. Too high, too low with seemingly no middle ground?

I then tried to connect the grid side of it, just to see if that would affect anything.

Voila! It goes through the self check, then checks the input voltage check, THEN turns on the fault LED (regardless) AND either input voltage too low or too high, along with the grid voltage and frequency green LEDs, and after a couple of seconds the blue MPPT light comes on, the fault LED goes OUT and the power generation bargraph starts to work as it starts outputting power.

So the 500W inverters do work. I've no idea who designed them, but the fault light doesn't mean fault when you first turn it on, but it may do later on, after it's started working.

Input voltage required to start them is approx 24V, low input voltage when they stop working is approx 22V.

So I have two 500W inverters (can generate 400W each) and a 1kW inverter (generates 850W), so 1650W of GTI output power.


Another problem has occured to me now. A GTI inverter doesn't care what the power consumption of my house is, it'll just generate the maximum output power for all the input power it can get, and if I am not using that power, it will just be exported to the grid.

So if I was to turn on all the GTIs they would generate 1650W, I typically use 800~900W in the evening, and so approximately half of that would be exported, which kind of semi-defeats the whole purpose of storing it in the battery in the first place.

I have considered using a grid-interactive inverter (Outback Raidian for example). With one of these I would need to add a second fuse box that is powered purely by the Outback, and connect my low demand loads to it (no cookers or tumble dryers, etc). I can then connect my solar PV GTI to the Outbacks *output*, and in the event of a loss of grid power the Outback disconnect from the grid and uses the battery to supply power to my house loads and to also keep the solar GTI online and feeding solar power to the house loads. (a slight problem with this is when in this mode, if the solar PV is generating more power than I am using, it charges the battery in an uncontrolled way which can easily fry the battery)

For the moment I will carry on with the three GTIs I have. As I have three of them, 500W, 500W and 1000W, I can control them individually and roughly match my power demands. Normal power demand ni the evening without the TV and AV system on is approx 500W, so one 500W GTI will supply that nicely with minimal loss (where 'loss' = the GTI output being more than the load and the excess being exported). With the TV and AV system on load is approx 800~900W, so the 1KW GTI will supply that with 100~200W being exported.

I will have to experiment with the hysterisis points around the 500, 1000 and 1500W points, and the time before turning another inverter on, and time before turning an inverter off, taking into account an inverter takes about 30 seconds before it's generating full output power, so loads like kettles (on for maybe 2 to 3 minutes), microwaves (on for 1 or 2 minutes) do not turn extra inverters on which will be of very little use before the extra load goes away.




Richard

Inverters (again...)

Status update:-

PowerJack : No reply to enquiries in to what CPU they are using in this years models.

OpenEnergyMonitor forum : No replies so far to my post asking for model numbers of Powerjack inverters that are known to have the ATMega CPU.

The Chrome browser does do a nice job of automatically converting German to English - going the other way I'm guessing is laughable but I'll join the German forum and post there anyway, at the very least I'll make a couple of members laugh at the translation!

I've ordered a dual bench power supply (0 - 60V @ 5A or 0 - 30V @ 10A) which will let me play about with various input voltages for the inverters I've got and see where they "wake up" and what the current consumption waveform is like - I have heard people have discovered their MPPT inverter when connected to a battery source, wildly varies the current drawn between almost nothing and double their rated input current - not at all good for longevity.

(I have various power supplies, 5V - 17V @ 35A, 24V @ 45A, 0V - 14V @ 1A, but nothing that goes upto the 55V these inverter can require, and nothing with current limiting, save a selection of fuses!)


I've traced out the DC voltage sensing circuitry in these 500W inverters and now know what is needed to get that part working from 26V, so when the power supply arrives I'll do that and test one. What the rest of the inverter does when working off a low voltage totally depends upon the design and CPU code.


Other alternatives I've been looking at are large online UPS modules - the problem is that any decent UPS uses a DC battery voltage of 72V, some even more, so they're unusable for my project.


Also looking into just getting an 8WK non grid tie inverter, adding a second fuse board and swapping over all lighting and all the sockets (apart from kitchen, utility room, garage, workshop - anywhere that has a high power load) and a switch over relay to switch back to grid power when the battery goes flat - the switch over does concern me, I suspect if the inverters waveform was 180 degrees out from the grid there could be an instantaneous change from (say) +270V to -270V (rough absolute max voltages) which would, well, I'm not exactly sure what it would do. Generate a short burst of high frequency noise for one. And I imagine any inductors will try to maintain the +270V and generate a lot of back EMF.

Saying that, this is how offline UPSs work, a simple relay without any inverter waveform synchronising.

There is also the problem that may arise if some unsuspecting person plugs a high load into the inverter powered circuit. I can get an 8WK inverter with a 30 sec peak of 16KW which is way more than one device (connected via a 13A slow blow fuse) should use, but instantaneous switch on currents can be very high, 10 times rated power isn't uncommon, so 30KW for a few mS.


Hence I will continue to pursue the GTI solution for now. This new PSU can supply enough voltage to "turn on" any GTI, so I can check my modifications and also connect my scope and see what the voltage and current draw is like. Also the new 1KW GTI should be hear next week, and that should be fine for working from my battery.


I've also read a lot of (valid) concerns from people about these inverters that just plug into a wall socket, and taking their concerns into account in the final system I'll have to modify the GTIs to remove any output sockets and make sure they're hard wired with proper earths and protection in place, enclose all components in a decent ventilated box / case, and also locate the system next to the fuse board and put it on its own spur. All common sense really.

GTI Inverter headaches

Just a quick post to update readers on the avenues I'm exploring about this problem.

One possible solution I have found is that some Chinese inverters use an ATMega controller CPU. There are a group of great guys in Germany who have discovered these, interfaced to the CPU (this type of CPU is easy to reprogram) and written their own version of the firmware that allows the GTI to be reliably used with a battery.

I found this post here that refers to the German forum and details the code a little
OpenEnergyMonitor.org
http://openenergymonitor.org/emon/node/1658#comment-21742

The German forum is dasWindrad.de, here is the linked post
http://www.daswindrad.de/forum/viewtopic.php?f=19&t=860&sid=b206245d4f6cf8c43dd314799e57875a&start=80

which could quite possibly be the answer to my problems if I spoke German, which I don't.

The GTIs which they say use these ATMega CPUs are made by PowerJack.

A blog from what appears to be a PowerJack employee (in the design dept?) is here
http://powerjack888.blogspot.co.uk/2014/05/lf-psw-power-inverter-from-power-jack_26.html

And I've send a message to the fellow, Chang Jack, asking if he can point out which 2014 products use the ATMega CPU.

Another possibility is REUK:-
http://www.reuk.co.uk/Grid-Tie-Inverters.htm

This chap built his own GTI and has made all the designs and source code available for anyone to build their own GTI, and obviously that will be easy to disable MPPT and optimise it to work off a 24V battery, but it's not exactly scalable if I want 10 inverters.

I will keep this blog updated when / if I hear back from my enquiries.

Oh, one last thing I intend to try with the little 500W inverters I have (that don't recognise the 26V from the battery as enough voltage to start working) is to adjust their voltage divider (there's a convenient potentiometer) to see if I can persuade them to at least start, and then see how their MPPT behaves with a low resistance (the battery) supply.

Inverter problems...

I purchased two 500W grid-tie inverters to go into the system, with a plan to expand upto 4, 6 and 8 depending on how quickly the battery discharged in the evening. (not draining the battery completely (to my pre-defined level) means I'm not utilising all the power the system captures during the day)

These came from China via EBay, so I gave them the usual strip down, inspect, rebuild treatment.

Quite a few solder joints were lacking in solder, albeit not actually broken or loose, but they were re-soldered anyway.

They had attempted to apply some vibration resistant measures (basically squirting special sealant around and under anything that has enough weight to move due to vibration), but in quite a few places this was woefully inadequate so their 'stuff' was removed and enough new sealant applied to do the job.

One worrying thing was in one location the sealant they had used was covered with small particles of metal swarf, they must have applied it and then somehow the board got metal swarf blown over it, and it stuck to the still sticky sealant - rather worrying!

One thing to pay special attention to in these things is any extruded heatsinks. They come in very long lengths and the manufacturer cuts them up into the required length. During cutting up this can leave sizeable bits of swarf that are only just attached to the heatsink that are hiding between the fins. There were a few inside these items. They won't cause problems if they stay where they are, but the slightest vibration can dislodge them and you then have a 0.5 ~ 1cm long sliver of metal floating around inside, just what you need to short out something and let the magic smoke escape.

The only other thing I found was a few screws clamping the diodes and transistors to the heatsinks not done up particularily tightly, an easy thing to sort.

So after checking them over and reassembling both inverters, I connected one up to the battery, it lit up, performed MPPT and promptly said the input voltage was too low.

Damn!

MPPT can be implemented in numerous ways, constant voltage, open circuit voltage, short circuit current, perturb and observe, incremental conductance and others.

See this PDF from TI for the technical details www.ti.com/lit/an/slva446/slva446.pdf

In essence a PV panel is a constant current device and has to be operated at the highest voltage possible, just where the current starts to dip, to get the maximum power out of it.

To do this the MPPT circuit varies the resistance it presents to the PV panel, the higher the resistance, the higher the voltage, lower resistance, lower voltage.

In one way of operation it will start at it's lowest resistance, lowest voltage, lowest power transfer, and increase the resistance, up and up, until it sees the power just start to dip. When it sees the dip it knows it is working as efficiently as that design can.

If you connect a battery to such a MPPT device, it will move through the range of resistance values it has, trying to find where the maximum voltage is, but of course we're using a battery which is not a constant current device, so the voltage will not change.

If we have a 24V battery, do we choose an inverter that works from a voltage range of 22V to 36V so our battery voltage is at the bottom of the range? Or 14V to 28V so our battery voltage is at the top of the range?

I thought about what would happen at the "other" end of the range and made my decision on that.

For the 22V - 36V MPPT circuit, if it raised its resistance up to the maximum (so the PV panel voltage would be ~36V), with a 24V battery the current would be at maximum at the 22V end, and a lot lower at the 36V end.

For the 14V - 28V MPPT circuit, it would be OK at the high end, presenting a high resistance so the PV panel produces 28V, our 24V battery would be feeding in just below the maximum rated current.

BUT at the bottom end, the 14V PV voltage end, the MPPT circuit would have a low resistance, and a 24V battery would be feeding in far more current than the inverter was designed to handle, and would either blow the fuses or die.

That was my theory anyway, so I chose inverters that have this spec:-
* Vmp 35 - 39V
* Voc 42 - 46V
* MPPT range 18 - 48V
* Input DC 15 - 60V

Trying to get the supplier / seller to provide me with meaningful specs was fruitless, so it was something of a guessing game.

Now I've found out that these do NOT work with a 26V battery I am more confident in saying that the above specs mean the following:-

* Vmp : Voltage where the unit is designed to transfer maximum power
* Voc : The maximum open circuit PV panel voltage.
* MPPT range : The voltage range the MPPT circuit can "track".
* Input DC : Less than 15V DC and the unit won't even turn on. And 60V is the absolute maximum voltage the inverter can handle (but do not run it at this voltage for any length of time)

I suspect that the MPPT voltage goes as low as 18V is because it takes more voltage for the unit to start up and "lock on", but once working it can continue working until the voltage drops below 18V. Once stopped I suspect it needs something close to 35V to start working again.

So I have two very nice 600W (or 500W depending which label you read) grid-tie inverters I can't use.

Back to EBay to buy one with Vmp that covers the operating voltage range of the battery.

Oh, every single manufacturer and seller I spoke to on AliBaba and EBay said grid-tie inverters just cannot be used with a battery. I can see why, you need to think carefully how the MPPT circuit works, take a gamble that it uses the method you think it does, and pray it does NOT use the short circuit method!



Richard

Solar storage project 26-5-14

Inspected the variable 100A charger today.

In spite of my worries about it's construction, a close inspection revealed a couple of screws that needed an 1/8th of a turn to get them nice and tight.

Looking round for dry or solder joints didn't reveal anything.

A quick test with the meter for blown semiconductors showed everything was fine.

It also showed that the metal case of the unit was only connected to the -VE DC output via a small capacitor. Now that isn't to the UK spec (and I suspect most of the rest of the world) where anything with a metal case must have the case connected to earth just in case the internal circuitry shorts out and connects mains directly to the output terminals. This is easily sorted with a new 3 core mains cable with the earth connected to the right places.

Happy that all was good as far as I could tell, plugged in to the wall and turned on.

All good.

Max voltage is 29.2V, minimum charging current is as low as 6.3A which is better than I was lead to believe, having been told that the minimum was around 20A. That's good news as I can still charge the batteries (albeit very slowly) when the solar panels are producing as little as (6.3A @ 24V) 150W.

Connected to the battery and out with the digital thermometer to see how hot parts inside the charger get.

Set it to 20A and everything is room temp (20 deg C) apart from the main HF transformer (30 deg C) and a balancing resistor between the high V main push - pull transistors (50 deg C).

Set to 50A charge current and HF transformer is upto 33 deg C, balancing resistor is 65 deg C.

That resistor is a ceramic enclosed wire wound type with has a max operating temp of 250 degrees C, so it is well within it's rated power, although it is down between the main HF transformer and a huge choke on the other side, so the air flow provided by the twin fans won't get that much air past it. I'll test it a bit more later on and may move it up further into the air flow if it gets over 150 deg C.

The absolutely massive heatsink increased in temp by 5 deg C after 10 mins @ 50A.

Overall it runs a LOT cooler than I expected it too. I've only run it upto 75A for a short period, as I suspect a brand new LifePO4 battery will not appreciate being charged at its absolute maximum charge rate for the very first charge!

I'll do a few cycles of full charge & discharge with the inverters and then do a 100A charge and check the charger temperatures.

Overall I'm very happy with it.


Yesterday I realised I'd forgotten to buy any fuses for anything - they're on the parts list but got overlooked. So an order for the large DC fuses and smaller AC fuses plus holders and a nice little DC fuse panel which will be perfect was put in earlier today. Should be here towards the end of the week when I'll be able to lay out everything and sort out the cabling.

The start of my Solar Storage project

This blog is where I'll document my DIY Solar Storage project.

Inspired by Christophe Huberts system which you can read all about here http://www.diyesskit.com/p/what-is-it.html

Brief specs of the first stage of the system:-

* 5KW LifePO4 battery pack
* 10A ~ 100A LifePO4 battery charger (adjustable constant current)
* 2 x 600W output Grid Tie inverters
* 100A solid state relays for the battery charger and battery* 40A solid state relays for the AC input to the charger, AC output of the inverters and DC input of the inverters
* 100A current shunt
* 2 x AC induction current sensors
* Raspberry PI micro computer controller
* 4 x 16 character display for the PI
* 32 digital I/O board for the PI
* 8 analogue input, 8 analogue output board for the PI

Some pictures:-

The 5KW LifePO4 battery pack
(it has 16 x 3.2V 200Ah flat pack cells in it in a 2P8S configuration for 5120W capacity, each cell is 2.5Kg, making the total weight 40Kg - not exactly nice and light)

The two 600W Grid Tie inverters

All the SS relays

The 24V, 100A battery charger

The charger is in bits as I want to check it over very thoroughly before using it. Two reasons for doing this, it was bought from China, and from past experience their QC isn't as comprehensive as we are used too, and secondly the journey from China to the UK may well have shaken things loose or caused some solder joints to crack - as you can see the components aren't exactly small and if they are jarred, the solder joints can easily crack.

Checking it over carefully now can make the difference between it going bang when I plug it in and it becoming a $600 paperweight, and it lasting for years.

After a quick check of the components in it (things like capacitors rated to 105 degrees C rather than 115, or 125, the additional thick wire soldered to the high current tracks on the PCB), leads me to believe that even though it may be able to charge at 100A, it will extend it's life a lot if I use it to, say, 75A maximum.

A variable current LiPo charger was chosen as I can drive the charge current control directly from the PI controller, which will allow me to exactly match the power taken by the charger to the excess power generated by the solar panels. The alternative was to have 'x' smaller (say 10A) chargers and turn more and more of them on or off as the excess power from the solar panels allowed. The latter would work fine but cannot perfectly match the solar excess power to the power going into the battery. Deciding to go with this type of charger is a bit of a gamble though, they aren't cheap and I hope the efficiency of the charger and the extra charge I can get into the battery on a dull day will be worth it. Time will tell.


After I've checked over the charger I have a couple more bits to get (better crimp connectors, 200A fuses, more cable), finish the initial version of the controller code, finish making the buss bars and decide how to lay everything out and find a heavy duty box to put everything in.



Richard