Happy birthday to me! As the weather gets worse I get to cast a jaded eye upon why my (Fronius) GTI spends so much time offline, it doesn't help itself by taking 3-4 minutes in between tries BUT why oh why does it cycle around trying so much and getting nowhere!
It consists of an input filter & reservoir connected to the panels followed by a high voltage isolated SMPS followed by an LC filter to a DCbus that is connected to the grid via an Hbridge and LCL filter all standard stuff. BUT it has lots of software shortcomings, one of them is its startup sequence.
As soon as the input voltage exceeds its UVLO threshold it starts its boring 4 minute connect timer, when that expires it starts up the SMPS!! And that's the problem, it just starts it, no attempt at any kind of soft start ramp! Its trying to charge 1200uF to ~400V from only ~1000uF charged to <200V, unsurprisingly unless the source impedance (governed by insolation) is low enough the ubiquitous DCLOW or POWER LOW message flashes up and the whole boring 4 minute restart timer starts again!
This explains why these things can follow the sun down and happily run at <20W but completely fail to restart until the sun is bright, very frustrating, just give me the code so I can put a soft start in there please
Enclosed is a pretty picture of a simulation at about 1.25mS the SMPS starts, the green trace being the DC input voltage and the red the PV power, by 1.75mS the DC input trips the 150V UVLO.
Of course simulation is an extrapolation of reality according to one hypothesis but it beats sitting out in the cold trying to capture it on the scope.
I hope this helps others understand some of there weird GTI behavior when the sun is low and maybe encourage some manufactures to improve there software and testing
EDIT :- I added a scope shot, not of an actual failure thats very hard but a typical start at 50% insolaton and at ~2.8 seconds there is the sudden drop in voltage caused by there hard SMPS start, when insolaton is lower <20% this causes a start failure.