When designing a device with max power point tracking (MPPT) capability such as a solar inverter or power optimizer you are going to need to simulate the output of a PV panel or panels to test your MPPT design. This is necessary to verify your design and provide an accurate efficiency spec under different of lighting and temperature conditions that create a wide variety of IV curves. Engineers new to this type of testing often decide to take a standard programmable power supply, connect it remotely to a computer (using GPIB, LAN, USB, etc), and create software with adjustable I-V curve look-up tables with the idea of turning their power supply into a PV panel simulator. This whole concept is shown in the figure below.
From personal experience, I can tell you this method will not work or at least it won’t work how you think it will work. With this type PV simulator system the I-V curve 3 dB bandwidth will most likely be < 1 Hz. With such a slow IV curve update rate it is not possible to verify your design’s MPPT capabilities and efficiency. Here is a quick breakdown why the bandwidth to this solution is so poor:
- The IO latency between the computer and the power supply.
- The supply programming time which consists of the time it takes the supply to process a command and the time for the internal analog circuitry to move the output of the supply to where it should be on the curve.
- The most limiting factor is that this is a closed loop system, which leads to oscillations in the output. Now to get rid of oscillations digital filtering will need to be added to the software which leads to multiple iterations of back and forth adjustments between the computer and supply to zero in on that point on the I-V curve where the supply output should be.
Because of the overhead just discussed you typically cannot achieve output bandwidths better than 1 Hz using this solution to simulate the output of a PV panel. For this reason it is not an acceptable test method to verify your MPPT design or spec the efficiency of your MPPT design. Now does this PV simulation method have a place in the MPPT hardware test cycle? Because of its relative low cost it could be used in long term reliability testing where you are just interested in continually feeding power through your design over a long period of time to make sure it doesn’t break down. Or in the case of manufacturing test where sometimes only static IV curve tests are needed above solution could be used.
So what is out there to simulate the output of a PV panel? There are two main ways to do it: take a power supply and put some custom analog circuitry around it or purchase a solar array simulator (SAS). I have heard of many different approaches using custom analog circuitry, below is a link to a paper that presents one way to do it:
Of course building a solution yourself comes with a high overhead of simulation, layout, testing, and support. If you want a finished solution you could purchase an SAS. An SAS is not a standard power supply. It is more comparable to a high powered current source with a low output capacitance (< 100 nF) to give it a high output bandwidth. Of course with these more advanced capabilities it comes with a higher price tag than a standard power supply. There are not too many companies out there that make SASs. One example is Agilent which makes the E4360A SAS. This is a modular platform that offers the best performance on the market as far as IV curve update rate is concerned. But the E4360A has a limited output voltage range so it can typically only be used for testing microinverters and power optimizes.
In this post we looked at why a standard power supply with some IV curve producing software will not provide an adequate solution for verifying and testing a MPPT design. If you have any personal insight to add to this post please you the “Comment” section below.
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