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.