Solar Panel mismatched voltage wiring question

[mackerman]

Hi all!

My first post as a forum member so be kind...my question is regarding a solar installation that has mismatched solar panel voltages tied together at a panel combiner. Here's the details:


I'm helping out at a NP facility that has a 20KW solar array using several MPPT charge controllers each with a Vmax of 150 volts.

There are five arrays, each with eight rows of seven 75 watt, 17v panels tied in series. Each row is connected to a combiner with an appropriate circuit breaker.

The rated outputs of the panel array is 152 v at 25 C. This is more than the Vmax of the charge controllers. Worse, when the Voc is calculated at 0 C, the Vmax jumps to 170 V.

Apparently a solar tech had tried to deal with the Vmax issue by taking one panel out of the series connection in each array. That is to say that there were seven rows of panels with a Voc of 152 v, and one row of six panels with a Voc of 130 V. All eight rows of panels are fed to a combiner where the panels are paralleled together prior to being fed to the charge controller.

This wiring method did serve to bring down the overall voltage from the combiner to the charge controller to about 147.8 volts Voc at 15 C, but it puts it above the Vmax for the charge controller when the temperature drops as it has in the last few weeks.

I'm proposing to change the wiring for the panels so that only six panels will be connected in series, then reconnecting the "extra" panels back into the array by adding more circuits to the combiners. My net change will be the voltage from the array will be less, but the amps should actually increase so the overall output from the solar array will not change dramatically.

I've tried to find information about the mismatching voltage connection between the rows of solar panels, but haven't found a definitive answer on what happens when this is done. To put it the best way I know, assume that there are seven 17 V panels wired in series, parallel wired to six 17 V panels wired in series. Seems to me that the six panels would somehow be damaged by the extra voltage generated by the seven panels.

Sorry for the long post, but hopefully I've stated the problem properly.

Thanks in advance for your thoughts, advice and opinions.

[inetdog]

Apparently a solar tech had tried to deal with the Vmax issue by taking one panel out of the series connection in each array. That is to say that there were seven rows of panels with a Voc of 152 v, and one row of six panels with a Voc of 130 V. All eight rows of panels are fed to a combiner where the panels are paralleled together prior to being fed to the charge controller.

The physical arrangement of the panels in rows has very little to do with their functioning. It will affect the length of wire used and therefore the size it needs to be, and it will affect access for maintenance or testing. The most important effect is probably that of shading. If there will be any partial shading of panels which are electrically connected in series, setting it up in the right way will allow you to get the most power from the shaded string. If you build strings with different shading conditions and then put them in parallel to the same MPPT input, you will lose power.

The electrical arrangement of the panels, on the other hand, is critical. For identical panels you must use an identical number in each series string or else you must parallel only strings of the same number of panels and then run them to a different MPPT controller input for each length string you use.

The short string could be damaged by the back voltage under some conditions, but if you put a blocking diode in series with the short string only, you will not harm it. You will just get less (or no) power from it.

Your problem is made worse by the fact that you have 56 panels in each array. Since 7 panels is too much, your options include
1. going to 11 strings of 5 panels each, with one panel left over. The extra panels from the five arrays could be relocated (or just rewired) to create one more string of 5. This may be your best option, since you are not using one of the panels right now anyway!
2. Add four more panels to each array to allow you to use 10 strings of 6 panels.
3. If the voltage and current will be within the specs of the CCs, you could also use all 56 panels by rewiring to 14 strings of 4 panels each. I suspect that this may not be compatible with your DC input limits on the CCs.

If you go to all shorter strings, be sure to recalculate the size of the wires from the combiners to the CCs to make sure that they can handle the current and will not be losing too much power from the voltage drop.

[mackerman]

As I suspected...

Thanks for the reply. I'm in total agreement that the panels must all be of the same voltage when paralleled at the combiner, but that's not the way I found it. I've already configured the array to work with six panels in series, and will be adding the "orphaned" panels back into the combiners as soon as the wire and breakers show up.

I appreciate your thoughts and time in replying.

Mark