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I have a solar power system (SMA Sunny Boy with 2 strings of panels). When the battery is nearing full charge or the inverter maximum output is reached and excess solar power is available the system throttles the amount of power coming from the solar panels. Observing the panel string voltages and currents it appears that it does this by reducing the string voltages while allowing the currents to be whatever the panel strings can supply. Presumably the voltage measurement is taken after some component that regulates the voltage. What type of circuit would be used to do this?

This graph shows what happens when the power available from the panels exceeds the inverter's maximum capacity of 5kW. Graph showing the throttling effect

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As described in this answer the output current of solar panels is directly related to the output voltage. If you short circuit the panels you will get a very high current - but basically no power, because the voltage of the panales drops towards 0 V. The same is true for a open circuit: In this case the voltage is pretty high, but there is no current flowing. Again, with P = I * U the power output of the panels is very low.

A solar inverter is using a switching DC/DC regulator at the input to controll voltage and current. In the typical use case this circuit would be controlled to find the point of maximum power (the MPP). But when there is a derating, because the inverter is not able to supply as much power as would be possible, it can increase its current draw from the panels and by that automatically decresing the voltage up to the point, where the desired power output is achieved.

In this diagram you can also see the power over panel voltage and current: enter image description here Image from Analog.com

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  • \$\begingroup\$ Thanks for your answer, but it doesn't explain the observed behaviour. On a day where there is full sun on the panels all day the reported current appears to mirror the intensity of the sunlight, producing a roughly cosine shaped curve, and the voltage is varied to limit the power. Another observation is that it often limits one string and not the other, some data from today: string A V=114, I=5.9, string B V=0, I=5.9. \$\endgroup\$ – glennr Jul 30 at 8:55
  • \$\begingroup\$ I can not really follow what exactly your question is. The Sunny Boy has an own MPPT (maximum power point tracker) for every string. When one string is shadowed, it still can get full power from the other one. And when the inverter has to derate the output it can do so by reducing the power from only one string. Only when there is still to much power at the input the other string is also derated (by either increasing or decreasing current). \$\endgroup\$ – jusaca Jul 30 at 10:30
  • \$\begingroup\$ It appears to derate the input power by adjusting the voltage of each string independently while the current for both strings is always the same. There's no shading so both strings always have the same amount of light. If it was adjusting the current to derate the input power I would observe a similar voltage but different currents in each string, which is opposite to what it actually does. FWIW the parameters I'm looking at are 'A.Ms.Vol', 'A.Ms.Amp', 'B.Ms.Vol', 'B.Ms.Amp'. \$\endgroup\$ – glennr Jul 30 at 11:06
  • \$\begingroup\$ As you can see in the graph above there is not much tolerance from the MPP. Increasing the current only slightly requires significant voltage drop. Although the difference between 114V at one string and 0V at the other string with the same current does seem a little odd and I have no explanation for that. But I'm pretty certain that the Sunny Boy is measuring the voltage and current right at the input, without any additional circuitry before (except for the ESS). \$\endgroup\$ – jusaca Jul 30 at 11:06
  • \$\begingroup\$ On further consideration I think this is the right answer. The thing that makes it confusing is that the low voltage part of the IV curve is probably very flat e.g.[1] and the current fluctuations are probably less than the measurement resolution that is available to me. [1] voltaicsystems.com/blog/create-an-iv-curve-for-a-solar-panel \$\endgroup\$ – glennr Jul 30 at 20:49
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It's not complex BUT it may seem confusing at first :-).

PV panels are close to current sources with Iout loaded proportional to insolation.

I do not know exactly how the (extremely competent) Sunnyboy makers achieve their result BUT it could be done by using either raw on/off PWM or (more likely) a buck converter** = PWM with an inductive energy store to provide output energy during the off cycle.

Your description of the system providing full current and variable voltage makes sense if measured at the panels and not the output.
An intelligently controlled buck converter** can 'compel' the panels to assume any V or I which are less than the currently maximum available as long as the converter can dispose of the energy*. A panel loaded to more than its Vmp / Imp values at a given moment will provide LESS power, a current somewhere between Imp and Isc (ie slightly above Imp) and a voltage of whatever the system causes it to have.

eg a panel with Vmp=30V, Imp = 10A, Isc = 11A, Voc = 36V,
so Wmp = 300 W, under current insolation could be loaded down to 10V. It would then provide slightly more than 10A (as closer to S/C) at 10V = 100W.
If the panel was hard shorted by the controller it would provide about 11A at zero voltage so no power.

If raw PWM was used, at say 25% duty cycle into an optimum load, the panel would provide 30V, 10A when loaded and Voc, 0A when unloaded. Wout = 300W x 25% = 75W. Vpanel with a well filtered meter would be
(30 x 25% + 36 x 75% ) = 34.5V APPARENT. I apparent = 10A x 25% = 2.5A. Power apparent = 34.5 x 2.5A (too high) unless an RMS power meter was used that measured V & I simultaneously.

*As long as the VI product is less than the wattage available at the selected voltage. That's a quibble added for completeness. eg if Vmp = 30V and Imp = 10A so Wmp = 300A, you could get say 33V but not at 10A.


** Buck or boost or buck-boost depending on system design.

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