Maximum power point traking

Question

I'm having trouble understanding maximum power point tracking. How can I track the Maximum Power point? How do I consider the effects of load on Voltage and Current while power point tracking?

Thanks for your help.

Answer 1

Others have told you about how the voltage is tracked to get maximum power out of the solar panel. Here I will tell about how the basics of how the required voltage across solar panel actually changes. The actual voltage change is done by the MPPT DC-DC converter, which changes the ratio between the input DC voltage (from the solar panel) and the output DC voltage (the load). By changing this ratio, it keeps the output voltage constant while presenting a different voltage at its input, which is the solar cell.

Another way to tell it, is if you consider the load as a resistance. Then the when you see the load from the solar panel (input), the resistance gets multiplied or divided by the square of the voltage transformation ratio. The MPPT algorithm (inside the MPPT controller) is such that this 'referred' resistance presented to the solar panel by the converter matches the 'correct' V/I ratio so that the solar cell is operating at maximum power.

Answer 2

The first thing you will need to do is observe the power by monitoring the voltage and current coming from the cell with analog electronics or IC's. You then multiply the current and power (best done digitally) and then try and find the maximum point.

To find the maximum point, you need is some kind of control scheme the best one is called Preturb and Observe that monitors the point on the power curve and 'climbs up the hill'. There are variations and other ways to find the maximum power point, but this one is the most widely used and is not to difficult to implement or debug.

Here is the link for the paper which the image was pulled.

Answer 3

A converter with MPPT tracking maximises its power output into a load.

The operating algorithm makes a small change to its conversion control, which is controlling either the current it attempts to draw from the panel, or the voltage gain of the converter. If that results in increased output power, it makes another small change in the same direction. If the output decreased, it makes a small change in the other direction.

That way, the operating point dithers around the maximum power output point. It is constantly moving because it needs to keep tracking variables like the light input and the temperature. The power falls off quite slowly either side of the optimum point, so there is no need to be very precise when tracking the maximum.

Note that the controller doesn't need to do anything complicated like measuring the panel output voltage and current, multiplying them, and figuring out which way to change things. It just measures its output power, usually into the battery bank it's charging, higher output current is more power, and maximises that.

Answer 4

A dc-dc converter is generally used in a PV system as an interface between a PV string and a load, as shown below:

The control parameter d refers to the duty cycle of the converter. Assuming that M(d) is the voltage conversion ratio, the relationship between the input and output voltages (currents) for the dc-dc converter can be written as: $$V_{in} = \frac{V_{out}}{M(d)}, I_{in} = \frac{M(d)}{I_{out}}$$ Divide Vin into Iin, we have: $$ R_{in} = \frac{V_{in}}{I_{in}} = \frac{V_{out}/M(d)}{M(d) \cdot I_{out}} = \frac{1}{M(d)} \cdot \frac{V_{out}}{I_{out}} = \frac{R_{out}}{M(d)}$$ In the PV system, we have $$R_{pv} = \frac{R_{load}}{M(d)}$$ where Rpv and Rload refer to the equivalent resistance for PV and load.

Then, the operating condition resulted by a PV string connected with a dc-dc converter is shown as below:

As shown in this figure, when you change d, the operating point will be changed even if Rload is constant. The aforementioned part is how the MPPT works with duty cycle. With a proper MPPT method by changing d, the PV will work towards the optimal voltage.