PV input module for DC bus systems

W

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
Does anyone here know if a 'solar input module' exists as a stand alone purchasable component? I'm specifically looking for a DC-DC converter intended to connect a PV input with MPPT to the DC bus of a VFD.

This would seem to be a standard piece of a complete inverter, but I don't know if this can be purchased 'stand-alone'.

Thanks
Jonathan
 
No battery? Just a PV module to a VFD? Or a PV string?

I think a PV charge controller *might* work for your application, depending on specs and how much precision you need. MPPT charge controllers are a thing. But what kind of voltage are you going for?

My guess is the setup will work better with a battery, to provide a buffer and stabilize voltage, even if you're not interested in overnight storage.
 
Right now this is just curiosity. I want to know if the PV interface component found in a 'PV hybrid minisplit' is available commercially for other uses.

Voltage would be in the 350V range. Often no battery is used but rectified grid power creates a voltage floor that keeps the equipment operating.

For an example of a hybrid minisplit, see:


It seems that some common 3 phase VFDs are available with custom firmware that simply connect the PV string to the normal DC bus, and then do MPPT tracking by just adjusting the output. Generally this is for water pumping:


Thanks
Jonathan
 
If a DC/DC converter is used without a battery, the converter would need to provide a constant output voltage over the range of load currents drawn by the VFD, which then corresponds to a range of delivered power up to a maximum value. And so the voltage at the input to the DC/DC converter will have to move to a point on the panels I/V curve such that the power from the PV panel is supplying the instantaneous power demand from the VFD (plus the losses in the converter). And so it will be operating away from the MPPT, unless the VFD is drawing the maximum power available from the panels. The I/V curve to the right of the MPPT has the challenge that it is very nonlinear, but at least the required voltage range to vary the power output is relatively limited. On a PV constant current curve to the left of the MPPT, the power is linear.vs. voltage. But the duty cycle of a buck or boost switching supply would have to move over a wide range to accommodate the variations in instantaneous power demand from the VFD.

I think having a battery is a much better option for a number of reasons, and beyond what I'm mentioning here. You could use an available charge controller for the battery. And the requirements for a DC/DC converter to provide a constant output voltage from a battery's ~constant voltage should be a lot easier to satisfy, and standard designs can be used.
 
@synchro I really appreciate your input. I have a pretty good understanding of the maximum power point of a PV array, MPPT tracking, etc. But for batting ideas around it really helps to have someone else picking them apart.

I've been specifically thinking about the ways to take advantage of solar in a minimalist fashion, specifically using solar when the sun is shining without the use of batteries or using the grid as a 'free' battery.

I've been aware of the solar assisted minisplits for quite a while. But I only just discovered that PV -> standard VFD with custom firmware -> pump is a thing. The latter makes pretty good sense; you do the MPPT by modulating the pump power/speed, with only the energy storage of the DC bus capacitors to give the system time to respond.

I suspect that the 'solar only' minisplits require custom firmware, so that the system properly drops compressor power when the array output drops. But the 'solar assist' minisplits could work with standard firmware since the grid is always there to support a minimum bus voltage no matter what the sun does.

For a 'solar assist' VFD application, I think one would want a DC:DC converter that does the following: when array power is < VFD load, the converter would operate the array at the maximum power point and produce output that just dumps this power onto the bus, reducing the rectified grid current. When the available power from the array > VFD load, the DC:DC converter would operate at a fixed output voltage > rectified grid voltage, as necessary operating the array off the maximum power point to curtail array output.

Would a 'charge controller' with 350V (ish) output rating typically be programable to function in this fashion?

Thanks
Jonathan
 
If you write DC-DC converter (as in the OP) instead of
Code: 
DC:DC converter
you can avoid the automatic forum emoticon to emoji translation. :D

Cheers, Wayne
 
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