Transformerless inverters, the differences

[ggunn]

Both, yes, but not (one and not the other), parentheses added for clarity. That's what SMA told me, anyway. The SMA guy could have been mistaken.

Just to be clear about what I have been saying all along, in a case where there are multiple connection points to a conventional SMA inverter (yes, I know that on a SBxxxx with integrated DC disconnect they are inputs to a combiner, not multiple inputs to the inverter itself), a common practice is to home run the DC+'s to the inverter (yes, to the integrated combiner located at the inverter) and to combine the DC-'s at the array.

In the case of a Sunny Boy transformerless inverter with multiple inputs (as in http://www.solar-net.nl/zonnepanelen/files/images/Manual-SMA-SB2100TL.pdf, see page 22 for the two DC inputs), if you combine the DC- connectors at the array you must also combine the DC+'s, because the current in a DC+/DC- pair must be the same on both conductors, because of the way that that inverter detects ground faults, because in a transformerless design the DC- is ungrounded and measuring current between the DC- and ground will not detect a fault. In a transformerless SMA inverter a fault is detected when a difference in current between DC+ and DC- is sensed, and the inputs are monitored separately.That's how a SMA field rep explained it to me, and I was passing it along as an answer to the question about what is different about transformerless inverters.

He may have been mistaken, and in which case I am as well. So sue me. ;^)