How can I reduce solar panel voltage of 49-51V to below 48V?

Question

My MPPT's maximum input voltage is 49.5V.
The solar panel's specs are:

• Open circuit voltage (Voc) = 49.40V (plus or minus 3%)
• Maximum power voltage (Vmp) = 40.42V
• Maximum power current (Imp) = 10.02A
• Short circuit current (Isc) = 10.69A (plus or minus 4%)

Since the solar panel's maximum Voc (50.882) could be slightly higher, how can I reduce it to be below 48V?

Would any of below solutions work and practical, or are there better alternatives?

1. Use a set of 10A10 rectifier diodes in series. That uses the rectifier diode's forward voltage of 0.6-1V x 5 to drop the voltage. Paralleling is to make the pair capable of handling 20A (10Ax2.)

^{simulate this circuit – Schematic created using CircuitLab}

2. Use a 48V Zener diode in parallel

^{simulate this circuit} Since the panel's output voltage drops below 48V quickly (I am assuming,) I guess I can use a low watt Zener diode and a low watt resistor (e.g. 0.25W.)

I live in a tropical country where there is good sun light. So I guess Panel could be operating close its max capacity.

Answer 1

You could not use a low watt resistor or Zener. You need to drop around 2.9 V at 10 A, so the resistor would dissipate 29 W, and the Zener could dissipate up to 480 W.

The way a Zener regulates is you size the resistor to drop the difference between the input and output voltage at the maximum load current. So for 2.9 V and 10 A it would be: $$ \frac{2.9 V}{10 A} = 290 m\Omega$$

So at the maximum current draw the dropping resistor and load form a voltage divider that reduces the 50.9 V to 48 V.

When the load is not drawing the maximum the Zener has to make up the difference to keep the voltage drop the same, so let's say the MPPT has a minimum draw of 2 A. This would only drop 0.58 V across the resistor, so the Zener needs to draw 8 A to keep the voltage at 48 V. That's a dissipation of 384 W. Hardly low power. If the MPPT was disconnected or shut off the Zener would draw all 10 A for a \$P_{diss}\$ of 480 W.

This all assumes a steady 50.9 V input, but your input is going to vary over a pretty wide range up to that value as maximum, and that complicates things even further. Whatever the input voltage is there will be a 2.9 V drop when drawing 10 A. So a Zener is not going to be what you want.

The diodes would be a better option, although they'd need to dissipate up to 50 W between \$(10 A \times 1 V \times 5\$) and their drop would vary from 3 V to 5 V with current.

All that said, I tend to agree with the comments, I don't think it's going to be a problem in the real world. You'd probably never see those sorts of voltages from the panel when connected to a load.

Also, you might take a look at this related question.

Answer 2

I tend to agree with the point made in comments, however: it's hard to believe it's that sensitive. This is the V_OC, which is rather a nominal figure, basied on nominal sunlight conditions in order to make useful comparisons between products. Every solar designer knows it's not likely you have this voltage, and you might have considerably more or considerably less depending on many factors. Under what circumstances will your MPPT system ever get V_OC? I see from your comments you live somewhere sunny: in which case your system may well deliver more than V_OC-STC on peak days. In the first place, is the system even in a location which receives 1000 W/m²?

Your MPPT system says it has protection against input high voltage. So perhaps you don't have to use anything.

From linked datasheet

However

• Consider using a non-optimal tilt for your panels. This will reduce their peak voltage without circuitry.
• Consider active monitoring of the voltage, ie, microcontroller + voltage measurement + relay + resistor/diode. Which is pretty much adding your own input over-voltage protection, without constant loss of resistors or diodes.