Friday, August 28, 2015

Verify inverter MPPT algorithms with Keysight’s new PV array simulators

While I normally avoid simply promoting new products in my blog posts, when Keysight Technologies announces a new power product, I feel obligated to mention it here. After all, this is Keysight’s power blog!

So, yesterday, Keysight Technologies announced two new photovoltaic array simulators. Click here for the press release.

The two new models are the N8937APV (208 Vac 3-phase input) and N8957APV (400 Vac 3-phase input). Both are autorangers and provide up to 15 kW, 1500 V, and 30 A on their outputs. Autoranging power supplies cover more output voltage and current combinations than power supplies with rectangular output characteristics. Click here for a previous post on autorangers and here for a post on the power supplies on which these two new models are based. These models can be put in parallel to provide a single output of up to 90 kW! They complement the family of Solar Array Simulators (SAS) that have been available from Keysight for decades.

Pictured below is the front panel of the two new photovoltaic array simulator models (15,000 W in a 3 U package):
So what is a photovoltaic array simulator? It is a specialized power supply that has an output characteristic that mimics the output characteristic of a solar panel (or a collection of solar panels known as a solar array). Photovoltaic (PV) simply refers to something that generates electricity when exposed to light so solar panels are PV devices. Solar panels have an output characteristic called an I-V curve that looks something like the solid line shown below. Isc is the short circuit current, Voc is the open circuit voltage, and Imp and Vmp are the current and voltage at the maximum power point.
Solar arrays are made by taking many solar panels and connecting them in series and parallel combinations for more power. When put in series, the total voltage increases. When put in parallel, the total current increases. Solar inverters take the DC output power from a solar array and convert it from DC into AC that can be used to power AC-mains devices (like those that plug in the wall in your home). So manufacturers of solar inverters are interested in testing their inverters and using a PV array simulator helps them. Instead of connecting their inverters to a real solar panel array that operates only when there is sunlight shining on it, they “simulate” the power output of the array with a PV array simulator. This enables them to test the inverter in many different conditions that affect the I-V curve of a solar panel, such as the temperature surrounding the panel, the angle of the sun on the panel, and cloud cover. Inverters must work when solar panels are subjected to all variations of these parameters, and waiting for them to occur with an actual solar array and the sun is not practical.

The inverter manufacturers are very interested in harvesting as much power as possible from the array, so they design their inverter circuitry to include Maximum Power Point Tracking (MPPT) algorithms that ensure their inverters operate at Pmax (the maximum power point) shown on the I-V curve above. The new Keysight photovoltaic array simulators allow engineers to test their MPPT algorithms.

So the next time you see a rooftop of solar panels, or a parking lot covered with them, or a field filled with panels collecting sunlight and converting it into electrical energy, think about the inverter connected to those panels converting the DC into AC for your use….hopefully, the inverter was fully tested with a Keysight SAS or one of these new photovoltaic array simulators!