DC Solar Home System Design with Battery

mrow

Joined Aug 13, 2020
2
I'm currently working through re-creating a simulation I found on YouTube for an integrated PV system with a battery. I found the tutorial here, for reference:

Here's a layout of the Simulink model:

I'm having trouble understanding how the dc-dc bidirectional converter works. I understand on the left-side of the circuit includes a boost converter, and the SimuLink model includes a basic MPPT algorithm script to adjust the duty cycle on the gate of the MOSFET. However, the bidirectional converter is confusing to me. Here is how the pink blocks are determined:

So, the left-side of the circuit (the boost converter), from my understanding, is trying to optimize the voltage and current to extract the maximum power from the PV panel. However ,by setting the voltage reference value at 48 on the battery control, the voltage at the battery is adjusted until it is 48V. The part that confuses me further is the current PI control, and the resulting duty signal. I was under the impression that the purpose of a bidirectional converter as part of a battery controller is to allow current to flow in each direction, but looking at the circuit I'm having trouble figuring out what exactly this section is doing.

I have searched around for bidirectional converter information but most of the materials I find are more advanced. Could someone help me understand how this circuit works, so I can then adjust the loads based on my system and set the correct parameters (e.g., select an appropriate reference voltage)?

Additionally, how does one select the inductance and capacitance values in such a circuit? I am currently using the same ones used in the video, but it's not clear how these should be adjusted if you change the PV array specs or load specs.

KeepItSimpleStupid

Joined Mar 4, 2014
4,153
I'm going to make a couple of comments.
PI may mean proportional-Integral control. e.g. PI controller.
You have to be able to charge the battery.
You have to be able to take power from the battery,
You also have Maximum Power Point Tracking. That's some V and I where the array is producing the most efficient power.
You also have a system delivery voltage and load to meet.

I trouble-shot one grid-tie PV system that no one was able to figure out why it wasn't working. What happened is that you had limits where the inverter would start and then shut down. They re-configured the arrays (series/parallel combinations) and all was fine.

mrow

Joined Aug 13, 2020
2
Right.

I suppose speaking at a high level, how does the boost converter work in relation to the bidirectional converter? The bidirectional converter is constantly trying to get the bus voltage to 15V it seems, which would oppose the MPPT boost converter's goal of optimizing for max power transfer from the array.

KeepItSimpleStupid

Joined Mar 4, 2014
4,153
I THINK you have two things going on:

The MPPT is controlling the input to create a store (variable voltage, depends on capacitance).

You should be using that same sink to be charging the battery.

Then you have a wide range input DC-DC creating the desired output voltage.

One the array is unable to power the load, the battery does.

It doesnt handle the case where the array and battery supply the load together and it might be more difficult to achieve.

The battery could contribute to the "store". Diode "or". Array and battery.