Design of a DC/DC converter with MPPT control for a photovoltaic system.

[Zankukaz]

Hello, I am studying how to design a boost converter to apply it to a photovoltaic panel. I’m trying to derive the input control transfer function to design a control system that adjusts the input of the converter, which would be the power supplied by the solar panel. The problem is that I have no idea how to obtain this function. I know how to derive the input-output and control-output functions, but I haven’t been able to get the input control function. Therefore, I’m reaching out for guidance as this is becoming quite overwhelming. I’ve read that this function is very complicated to derive, almost impossible, so the simplest approach might be to look for alternatives. One option would be to place a capacitor on the DC bus between the solar panel and the boost converter, which would add a state variable at the system’s input, allowing me to model the input versus control behavior. Another technique I’ve come across involves controlling the inductor current to create an indirect control system, but I don’t fully understand how these methods work. Any help would be greatly appreciated, and if anyone has references that could guide me through this process, it would mean a lot to me.

 

[BradtheRad]

You have a voltage coming from the PV panel, and an Ampere level.You want to maximize the Watts by loading it with the proper Resistance (or impedance). ...
Or since this is a boost converter, switch back and forth between two resistances. It has a net effect similar to one resistor. Changing duty cycle is how you do this. The optimum goal is to push maximum current into a battery. This assumes your end load is a battery having a certain voltage, or increasing a few volts as it charges.

Once you (or your smart circuitry) find the optimum duty cycle, it helps to install a filter that draws the continual smooth current (optimum current) that simultaneously maintains the optimum smooth volt level coming from your PV panel.

 

[KlausST]

Hi,

I´ve just done an internet search for "MPPT algorithm".
It gave thousands of documents. Even application notes from semiconductor manufacturers. (reliable, high quality informations)

I also see there are several different approaches to get MPPT.

Klaus

 

[Zankukaz]

It seems my question wasn't understood well, so I apologize and will clarify it:

I am working on the control of a boost converter connected to a solar panel, where I want to implement an MPPT control to calculate the appropriate duty cycle for the switch. The control is almost finished, but I am missing a transfer function to design the PI controller. The function I need is Vpv/iC (where iC is the current of an input capacitor connected in parallel to the solar panel and the converter) or Vpv/iL. Either of these two functions would help me finalize the control phase.

The information I’ve received so far is something I already know. As for searching online, most resources I’ve found only address MPPT control from an algorithmic approach, which I have already resolved. Additionally, in the theses I’ve reviewed, they only show results, which isn’t helpful in my case.

Recently, I found a solution (Vpv(s)/iC(s) = 1/C*s) that suggests using only the capacitor current equation to derive the transfer function. My concern is that the voltage of the solar panel also depends on the duty cycle (D), the conditions of the panel, and other factors. Therefore, I fear this approach may not capture the complete dynamics of the system, but I am not sure.

I'll share some schematics to visualize it better.

I would appreciate any guidance or confirmation on whether this approach is appropriate.

 

[BradtheRad]

Perturb & Observe is the algorithm mentioned most often in discussions at Edaboard about MPPT. It implies that optimum draw can't be precisely calculated mathematically ahead of time.

 

[KlausST]

It implies that optimum draw can't be precisely calculated mathematically ahead of time.

exactly.
The behaviour varies with intensity of light and temperature .... so, it varies from from minute to minute.
I´d say this is the reason why the "Perturb & Observe algorithm" was invented. It adjusts itself to the varying conditions.

For sure one could use a temperature sensor and a light intensity sensor and develop a two dimensional control loop for the operating point.
But then still one has not included:
* ageing
* dirt on the surface
* partly shaded panels
* panels with temperature difference
* and so on

Klaus