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