Broken solar panel replaced and now Inverter shows low DC at start up

[flierman1945]

A few weeks ago, I had my solar panels remove from my roof and then replaced. Sadly, on removal, one panel was warped and on letting the fixing screws go, the panel glass cracked.
My set up is / was 16 Kyocera KD210 panels wired in 2 strings of 8 in series, then parralled, feedning into a Fronius IG30. The system has an up time of 6 years.
Having been an electrician all my life, I thought it would be easy to source a replacement, but sadly that is almost imposible. All panels have higher working voltages, but I was able to find a Perlite 250 watt panel with a

Max Power Voltage (VMPP)	30.5 V
Max Power Current (IMPP)	8.2 A
Open Circuit Voltage (VOC)	38 V

The Kyocera has
[TABLE="width: 100%"]
[TR="bgcolor: #FFFFFF"]
[TD]

Maximum Power Voltage (Vmpp)​

[/TD]
[TD]26.6V[/TD]
[/TR]
[TR="bgcolor: #FFFFFF"]
[TD]

Maximum Power Current (Impp)
​

[/TD]
[TD]7.90A[/TD]
[/TR]
[TR="bgcolor: #FFFFFF"]
[TD]

Open Circuit Voltage (Voc)​

[/TD]
[TD]33.2V[/TD]
[/TR]
[/TABLE]
I realise there is a mismatch, but convinced myself that when connected to one sided of the array, the existing panels would pull down the perlite output and that it would work ok.
So I now have 2 strings of 8, one string of 8 kyoceras and the other with 7 kyocera and one perlite, paralleled. This then feeds to the Fronius and is well within limit of the inverter. Having the panels back on the roof, I have taken more interest in the start up, but am finding the fronius showing low dc, it then starts a 3 minute resync, this continues for quite some time. Luckilly I have a data logger connected to the inverter and am able to at least see the dc levels being generated.
Having spoken to Fronius, they advise the strart sequence starts at ( for the IG30) at 150 vdc. grid Connection, they say, will not start until dc levels reach around 170v dc, then after the 3 minute sync period, connection to grid should start. But after the sync test and with my dc way above the threshold level 181 to 183vdc, connection fails and Fronius reports low dc and the process of sync starts again. At 185vdc the unit connects to grid, but this is not how it worked prior to the new replacement being fitted.
So, this brings me to my main question, is the very slightly higher generation from one string backfeeding and so as the Fronius completes its final check before connectio, the combined voltage of the 2 strings drops just enough to prevent the fronius starting. I have monitored the dc and can see no drop on a dvm meter, but I wonder if the dc just drops momentarily below the connection level of 170vdc and is my new panel causing me all this grief.
Any views of whether there is any loading effect of one string to the other will be appreciated.

 

[TJ Anderson]

The Fronius has single maximum power power point tracking so is not ideal really for two string operation anyway but is acceptable assuming both are together on same array. I was not a fan of Fronius due to low max input voltages which limited you to parallel strings when you could have run single (whole array in series) on a Power One or SMA and achieved start up voltages earlier. As for putting a different make panel in, I have fortunately not had to do this yet but am sure will have to at some stage due to all the PV panel manufacturers that went to the wall after the boom. You have selected the right way round so to speak in regard to ISC and VOC of the new panel although it will work at different efficiencies in terms of output for given sunlight. There really is not much you can do with what you have there, it wont do any damage.