Nerd question - how does the inverter control panel voltage levels?

[captaindomon]

I have an engineering background but it's in computers, not electrical engineering, so I find all this solar stuff fascinating as I read more about it. Here's my question. The inverter is using MPPT tracking to maximize power output by adjusting operating voltage of the panels, correct? How does the inverter do that? I would think the panel voltage is dependent on manufacturing characteristics - i.e. the physics of each cell and then multiples as the cells are hooked together in series. How does the inverter alter the operating voltage of the panels?

Thanks!

[inetdog]

I have an engineering background but it's in computers, not electrical engineering, so I find all this solar stuff fascinating as I read more about it. Here's my question. The inverter is using MPPT tracking to maximize power output by adjusting operating voltage of the panels, correct? How does the inverter do that? I would think the panel voltage is dependent on manufacturing characteristics - i.e. the physics of each cell and then multiples as the cells are hooked together in series. How does the inverter alter the operating voltage of the panels?

Thanks!

An excellent question. And to answer it you need to know the behavior of a panel (or an individual cell) under different load conditions.

The solar panel will have a maximum output voltage when there is no load on it at all except for the volt meter. This is called the open circuit voltage, V_OC, also written just Voc. This voltage will be relatively independent of the amount of incident light.
As you draw more and more current from the panel the voltage drops, and at a direct short circuit (directly through the ammeter) the output voltage is zero and the current is the short circuit current, I_SC or Isc.
In both of those extremes the output power is zero. There will be a place in between, the Maximum Power Point, MPP, where the product of voltage and current is at its largest value. The voltage and current are called V_MP and I_MP respectively.
The Isc and Imp values will be more or less directly proportional to the amount of light hitting the cells of the panel, and Vmp will be about 80% of Voc (for silicon cells).

The problem is that to draw Imp for different light levels on the panel requires a different effective load resistance. A straight connection to a resistor or to a battery cannot do this. So the MPPT (MPP Tracking) Charge Controller, CC, or inverter will take the incoming DC from the panel at Vmp and draw just enough current, Imp, to maximize the panel output power.
In the process an MPPT GTI will be converting the input DC to AC at a fixed voltage and just regulating the amount of AC power produced.
In the same way an MPPT CC will convert the input DC voltage from the panel Vmp voltage to whatever voltage the battery needs, using a circuit called a buck converter.
The CC will regulate the amount of current it draws from the panels to reach Imp and will deliver the corresponding higher amount of current to the battery bank.

[foo1bar]

How does the inverter do that? I would think the panel voltage is dependent on manufacturing characteristics - i.e. the physics of each cell and then multiples as the cells are hooked together in series.

Manufacturing and environment (temp, amount of light) determine the I-V curve for the panel.

https://en.wikipedia.org/wiki/Maximum_power_point_tracking

Has a graph of a panel's I-V curve - each color is for a different amount of light hitting the panel.

The inverter uses mppt to find the knee of that IV curve and make sure the operating point is at that knee so that the max power is produced.

I don't know the method that the inverter uses to alter the "load" for the panel so that it's operating at maximum power on the panel's I-V curve. I'm guessing some sort of buck-boost with a tune-able input.

[captaindomon]

The problem is that to draw Imp for different light levels on the panel requires a different effective load resistance. A straight connection to a resistor or to a battery cannot do this. So the MPPT (MPP Tracking) Charge Controller, CC, or inverter will take the incoming DC from the panel at Vmp and draw just enough current, Imp, to maximize the panel output power.

Thank you! Interesting. So by altering the amount of current the inverter is pulling, it can move around on the current/voltage function so to speak, until power is optimized. That makes more sense now.

[foo1bar]

Thank you! Interesting. So by altering the amount of current the inverter is pulling, it can move around on the current/voltage function so to speak, until power is optimized. That makes more sense now.

You can either look at it as altering the amount of current the inverter "pulls", OR you can look at it as changing the voltage operating point for the input to the inverter.
They're really two sides to the same coin. If you change the current, you change the voltage. If you change the voltage, you change the current.

[sensij]

Thank you! Interesting. So by altering the amount of current the inverter is pulling, it can move around on the current/voltage function so to speak, until power is optimized. That makes more sense now.

If you have a high frequency switch, you can vary the duty cycle to move the operating voltage to anywhere from Voc (0% duty) to short circuit (100% duty). At a high enough frequency, the circuit can be designed so that the solar panels don't see it as switching, but just a steady consumer of the power generated. In a preturb and observe MPPT algorithm (one of a couple different types of MPPT), that duty cycle is continuously varied... if the resulting power goes up, the duty cycle continues to adjust in the same direction, if the power goes down, the duty cycle adjustment will reverse.