Will excessive voltage drop confuse an SMA inverter

wirenut

We've reached a point where solar panels are cheaper than copper wire. If I have a certain power production figure in mind it makes more financial sense to oversize the array and use smaller wire with a lot of voltage drop. The overall power production should be the same but will the voltage drop confuse the MPPT in the inverter?
Specifically I'm looking at an installation where the array must be 1,700 feet from the house. A 10 kW array would produce enough power but the copper wire to get it there is really expensive. I'd have to run 4/0 to keep the voltage drop under 2%.
Financially it makes more sense to install a 12 kW array with #6 wire. I'll lose about 14% in the wiring but the overall result should be the same amount of production.
Would 14% voltage drop cause any problems with the inverter? Would it still be able to find the MPP of the array?
Has anyone ever done an installation like this before?

Thanks,

inetdog

Re: Will excessive voltage drop confuse an SMA inverter

wirenut wrote: »

We've reached a point where solar panels are cheaper than copper wire. If I have a certain power production figure in mind it makes more financial sense to oversize the array and use smaller wire with a lot of voltage drop. The overall power production should be the same but will the voltage drop confuse the MPPT in the inverter?
Specifically I'm looking at an installation where the array must be 1,700 feet from the house. A 10 kW array would produce enough power but the copper wire to get it there is really expensive. I'd have to run 4/0 to keep the voltage drop under 2%.
Financially it makes more sense to install a 12 kW array with #6 wire. I'll lose about 14% in the wiring but the overall result should be the same amount of production.
Would 14% voltage drop cause any problems with the inverter? Would it still be able to find the MPP of the array?
Has anyone ever done an installation like this before?

Thanks,

The high voltage drop in the wiring will change the shape of the current versus voltage curve seen by the inverter, and will probably change the simple rule of thumb that Vmp is roughly 80% of Voc, but the algorithms in the SMA inverter should still zero in on the MPP of the system, which will be different from the MPP of the panels alone.
If you are in a situation where the copper cost for low VD would be more then the cost of the additional panels, then your analysis is absolutely correct.
But be sure to do your wiring cost calculation using a larger size of aluminum wire, which costs about 1/3 as much for the same VD as copper.

The other options to look at are:
1. Making the string voltage as high as possible while keeping winter Voc below the inverter limit.
2. Consider putting the inverter at the panels and running high voltage AC (480 volts, for example) from the inverter to the house. You will need to figure in the cost of transformers for this approach.

Cariboocoot

Re: Will excessive voltage drop confuse an SMA inverter

As long as the Voltage on the inverter's input while under power is within its operating range it will work. The MPPT function is capable of dealing with variations in both Voltage and current, and expects them in the normal course of operation.

wirenut

Re: Will excessive voltage drop confuse an SMA inverter

Thanks for the replies. I have looked at aluminum wire but it is till rather expensive and will require much larger conduit and more labor to handle. My concern was that with added current would come added voltage drop and the mppt tracking would get confused. I'm sure it doesn't normally expect and increase in PV output (more current) to mean a significant drop in voltage. I may call SMA and see what they have to say about it also.

David and Laura

Re: Will excessive voltage drop confuse an SMA inverter

Re #4 - Are we correct in inferring that you planning to install the wire from the array to the Sunny Boy in protective conduit, regardless of the wire gauge? 1,700 feet of conduit isn't cheap... Depending on what your AHJ says, and where the cable is being run (or buried), you may be better off with armoured cable.

Can you let us know what SMA says?

inetdog

Re: Will excessive voltage drop confuse an SMA inverter

wirenut wrote: »

Thanks for the replies. I have looked at aluminum wire but it is till rather expensive and will require much larger conduit and more labor to handle. My concern was that with added current would come added voltage drop and the mppt tracking would get confused. I'm sure it doesn't normally expect and increase in PV output (more current) to mean a significant drop in voltage. I may call SMA and see what they have to say about it also.

Since the voltage from the panels normally drops rather sharply as the current drawn increases, what the MPPT controller sees will not differ all that greatly from what it sees when connected with a low wire resistance.
The only thing that may happen is that the operating point where the controller gets maximum power may not be the same panel operating point which would produce maximum power with a low resistance connection. But it will still be the maximum power that can be delivered through those wires to the controller, which is exactly what you want!

Don't confuse the changes in output current which come from changes in light level with the changes in output current that happen at a constant light level as the MPPT controller searches for the MPP.

As I mentioned elsewhere, the result of the high voltage drop will be that the Vmp will be a different percentage of the Voc at different light levels rather than being close to constant for all light levels with a low voltage drop connection.
A very cheap "phony" MPPT device which just measures Voc and then sets its operating voltage to a fixed fraction of that will not work correctly, but SMA and other reliable manufacturers do a lot better job than that!