Uneven DC string sizes in combiner box.

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Ctay005

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We have a system that has 3 strings (13, 13, and 12 modules) that will be going into a SMA SB1000TL inverter. My question is, since the SB10000TL only has one DC input, can we combine 3 strings together in a DC combiner if they're not all the same size. We had an electrician a while back mention something about how all strings need to be equal size in order to combine them.
 
They do need to be the same size or else you risk taking a huge efficiency loss. Without going into too much detail, one MPP tracker can only keep a string at a certain voltage to get the most out of the modules. If you have uneven strings then that changes the voltage at which the strings should be at. That's why SolarEdge is so great by employing optimizers at each module and keeps the voltage at a constant 350V regardless.

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Ctay005 said:
We have a system that has 3 strings (13, 13, and 12 modules) that will be going into a SMA SB1000TL inverter. My question is, since the SB10000TL only has one DC input, can we combine 3 strings together in a DC combiner if they're not all the same size. We had an electrician a while back mention something about how all strings need to be equal size in order to combine them.
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It is PV Systems 101, to know that all strings combined, without separate power processing, need to have the same number in series. Some inverter manufacturers might require it specifically, others might just strongly recommend it.

While it is not a good idea to mix dissimilar strings in parallel, it is not technically a violation. So it is difficult to call others out on it, if they make this mistake. It will have a disporportionate performance hit, due to the asymmetry. Worst case scenario, it will either cause all strings to behave as if they only have 12 modules, or it might cause the 12 module string to not even produce.

If you build a system with module-level power optimizers, you can combine different quantities of modules per string on the same inverter, because the power processing is decentralized to the module level, which changes the mix of current and voltage from what the module produces, in order for all strings to have the same total voltage.
 
Ctay005 said:
We have a system that has 3 strings (13, 13, and 12 modules) that will be going into a SMA SB1000TL inverter. My question is, since the SB10000TL only has one DC input, can we combine 3 strings together in a DC combiner if they're not all the same size?
Click to expand...
Can you? Yes. Should you? No.

You might as well just connect three strings of 12; it will run pretty much the same. Selling your customer those two extra modules is, IMO, irresponsible. This is very basic PV design.
 
For situations where new panels with different specifications are added, there is a rule of thumb that if the resulting string voltages are within 5% of each other the power loss going into a common MPPT will be tolerable.
One full panel out of 12 violates this rule, and so will not make efficient use of all of the panels. If the voltage difference is above 20% you might not get any power at all from the short string.

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