Storage system in parallel to PV array (before MPPT): help !

Good morning everybody,

I'm writing to ask for technical supoprt: i'm developing a bidirectional battery charge regulator for a storage system to be connected in a parallel to a PV array, therefore before the MPPT of the grid connected inverter. The battery charge/discharge management is based on specifc self-consumption criteria, obsiously considering the BMS informations.

My concern is about the behaviour of the MPPT of the inverter when the device works as an auxiliary power source, i.e. transferring power from the battery to the inverter in order to make up for insufficient power coming from the PV: the MPPT will try to force a voltage that depends on the IV curve of the power source, but in this scenario we have two power sources.

Considering that the PV has its own IV curve that changes randomly based on solar radiation and temperature, how should the auxiliary power source behave to make sure that the MPPT will always drain all the available power from the PV and only the necessary from the battery (without going crazy) ?

Thank you for supporting me

Hi,

If I've understood what the question is. Maybe power path ICs or perhaps solar battery charger ICs offer a packaged solution or can stimulate ideas about how to deal with the issue. Maybe two comparators feeding and AND: one comparator for current, one for voltage; switching the battery path in via a relay of some kind or a transistor when voltage and (or?) current fall below a required minimum; maybe that would need an additional control regarding how to respond if both sources (solar and battery) are below minimum values required to power MPPT, if that could ever occur. That suggestion may require some thought about using hysterisis and switching both in and out softly to avoid moments of no power otherwise.

d123 thank you for your answer. The hardware has already been designed and a protoype is currently under test. My request is about the control strategy to be implemented in the microprocessor.

From a different standpoint you could say the battery is chief source of power, drawing charge as needed/available from the PV panels.
Whenever battery voltage drops, it automatically draws power from the PV.

You might get by with a voltage regulator at the PV, set for 13.5 or 14 V. Thus the battery will not overcharge.

I don't think this will work. I guess that the MPPT/inverter is designed to be connected in parallel with the grid.
This means that it doesn't know if it outputs "enough" power. It will always output the maximum power available from the PV.
This means that the DC/DC converter can't know in which direction the energy should flow, to or from the battery.
Some feedback from the AC power side (grid) is needed to make that decision, and it is probably a difficult control problem to
find the best battery current and avoid oscillations.

std_match said:

This means that the DC/DC converter can't know in which direction the energy should flow, to or from the battery.
Some feedback from the AC power side (grid) is needed to make that decision, and it is probably a difficult control problem to
find the best battery current and avoid oscillations.

Click to expand...

Obviously the DC/DC converter operation is based on the feedback coming from a meter that monitors the power flow from/to the AC main. Oscillations are not an issue because every change is very slow.

In general, we should expect the storage device to disturb MPPT operation. Without knowing the MPPT control algorithm (e.g. modulation frequency), it's hardly possible to design the auxiliary respectively.

Yeah this sounds like a really bad idea. As you draw it, the MPPT is going to see the I/V characteristics of both the PV and the DC/DC, which could easily throw off the the MPPT. The DC/DC and MPPT should be a unified controller, or the DC/DC should come after the MPPT.