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