Wednesday, July 26, 2017

Tutorial on PV panel simulation

I released a tutorial on how to simulate a PV panel.

This is the first step towards simulating the interaction of renewable energy systems with the grid and also on how to build smart grids and microgrids with renewable energy. In the past I have simulated converters for interfacing PV to the grid and I have usually assumed the PV panel as a dc source. As I was focused on the control of the power converter, assuming the PV panel as a dc source, was not a problem. But if I need to simulate how PV can be used in any system, I need a more accurate model of a PV panel. In the least, I need to be able to replicate the V-I characteristics of the PV panel. It would be a good addition if I could also model how the output of the PV panel is affected by radiation, temperature etc.

The problem is that a lot of the papers that deal with PV are pretty heavy into solid state physics. The widely popular current-source with anti-parallel diode equivalent circuit for a PV panel is usually used. However, where most of the papers defer is how to generate the constants that the current generated and drawn by the diode are expressed with respect to. The only data available from a PV panel specification are of limited use when it comes to solving these equations. In order to be able to completely express the equations, constants have to be determined by solving non-linear equations.

My objective was to generate a PV model that was reasonably accurate and also understandable to a power electronics engineer without an in-depth understanding of device physics. And I found that several approximations were possible to determine many of the constants that appeared in the equations.

The most important assumptions were around the two extreme operating points - open-circuit and short-circuit. At short-circuit, the entire photo current generated flows into the short. The short-circuit current at a reference temperature is provided in the datasheet. Therefore, the photo current can be equated to the short-circuit current. Conversely, at open-circuit, the entire photo current flows into the anti-parallel diode. Therefore, the diode current is now equal to the photo-current which at the reference temperature is equal to the short-circuit current. The concept is that the value of the photo current generated depends only on the temperature and solar radiation. At short-circuit, this current flows into the short while at open-circuit, it flows into the diode.

The remaining calculations are merely procedure. Finally, the PV current that has been calculated is modeled as a voltage behind a resistance. A capacitance is connected across the PV terminals to stabilize the voltage.

Check out the report in the tutorial link for more information.