So this is a little fault finding mission that I engaged in today, and after chasing a few red herrings and coming up with a hypothesis or two, we managed to pinpoint the problem while using only the logged data.

The scenario: The homeowner recently added a second array of PV panels. The new array is sized the same as the existing one, is installed right next to the first array, and has its own MPPT which is also exactly the same make and model. For some obscure reason, the two MPPTs were fighting each other, taking turns to ramp up the charge current just to give up a few seconds later.

After chasing a few red herrings a colleague said that the most likely culprit is that the two arrays are not isolated: they are somehow interconnected. That's when it occurred to me to check the PV voltage. A cursory check revealed something suspicious: The voltage of the two arrays seemed to be almost exactly the same. Look at /Pv/V in the attached picture.

So I pulled down the logged data for the day, and this is when it became absolutely clear. A PV cell makes around 0.6V open circuit, and around 0.5V at maximum power. That's around a 20% difference. So we would expect a voltage swing of around 20% as the MPPTs try to engage the PV array. Also, since we know that the two MPPTs are fighting, we would expect out-of-phase 20% swings. The data told us something different. This is a screenshot from the data with some columns hidden, showing the PV voltage and current of the two arrays side by side.

Note how one array is working at 5A while the other one is doing nothing, yet somehow there is no difference in voltage. Both arrays seem to have the exact same input voltage at all times.

Notice also how the second one tries to ramp up, completely dropping out the voltage causing both chargers to give up.

Guessed the problem yet?

The two strings are interconnected.

I wonder, is the part where I do not mention: You get free support, help and even some troubleshooting from a certain supplier, if you bought their Blue equipment?

Sorrieee - but that could not just not be left there!

1 hour ago, The Terrible Triplett said:

free support

Well you got to follow the process. First you talk to your installer.  Then the distributor. If that still fails, you are normally referred to third-level support, and if it gets particularly hairy you end up with someone from R & D itself. It's not free... you paid for it :-)

Of course there is something else I ought to have said about this: None of what I did above is inaccessible to the sufficiently talented amateur. The VRM data is available for download to anyone with access to your site data.

Cool man. So in what way were they interconnected? At the DB or at the Panels?

2 minutes ago, PaulF007 said:

in what way were they interconnected?

The PV panels have their positives connected to each other, perhaps the negatives too.

The BlueSolar MPPTs have a common negative (except for older 15A units) anyway, so only the positive side matters theoretically, although it is much better not to have weird loops: They two PV arrays should be completely isolated from each other.

7 hours ago, plonkster said:

The PV panels have their positives connected to each other, perhaps the negatives too.

The BlueSolar MPPTs have a common negative (except for older 15A units) anyway, so only the positive side matters theoretically, although it is much better not to have weird loops: They two PV arrays should be completely isolated from each other.

This makes perfect sense. Is it solved now? What have you done to create the required isolation?

30 minutes ago, SOLARWIND said:

This makes perfect sense. Is it solved now? What have you done to create the required isolation?

The installer made a mistake and paralleled the connections up at some point. It's been rectified now and works perfectly.

(Also, it's on the other side of the country. I just did the troubleshooting in an extremely virtual manner, which is the point of this post :-) ).

On 2018/03/14 at 5:27 PM, plonkster said:

The installer made a mistake and paralleled the connections up at some point. It's been rectified now and works perfectly.

(Also, it's on the other side of the country. I just did the troubleshooting in an extremely virtual manner, which is the point of this post :-) ).

You'd be surprised how many such simple mistakes are made, often.