In addition to the having superb electrical specifications, it’s important that a solar array simulator has software that is easy to use and capable of providing the desired test data. These features include:
The success and credibility of solar power depends in large part on the i performed, both at the development and production stages. To get the best results, you need the best test equipment that you can find. That means using a solar array simulator that accurately tracks the I-V curve even under the demanding conditions imposed by modern inverters. You can’t really settle for anything less.
Lead image: Test results via Shutterstock
How Do Photovoltaic Inverters Work?
Photovoltaic (PV) inverters work like any other type of inverter. That is to say they convert direct current to alternating current. PV inverters differ from standard inverters in that the DC input for a standard inverter is normally a steady DC supply, such as a battery, while the DC input for a PV inverter is a solar panel or array. The DC output of a solar panel varies widely as the temperature and amount of sunlight hitting the panel changes.
Figure xx shows the I-V characteristics of a typical solar panel for a given illumination level. Also shown, in the blue trace, is the amount of power that the solar panel will deliver to a load at points along the curve. When the output is shorted, the voltage is zero, so the output power is zero. When the output is open, the current is zero, so again the output power is zero.
At some point along the curve, the power output (which is equal to the panel’s output voltage times the panel’s output current) of the panel is at maximum. This point is called the maximum power point. At this combination of voltage and current, the panel will be delivering the most power it can given the operating conditions. As you can see, the maximum power point is on the knee of the curve
The PV inverter must determine what the MPP is at any point in time and change its input impedance so that the solar panel is always operating at that point. When it can do that, the PV inverter will be drawing the most power from the solar panel.
Inverters use different strategies to find the maximum power point and to continue operating at that point under varying external conditions. Perhaps the most common strategy is called “perturb and observe.”
Inverters that use the “perturb and observe” strategy change their input impedance by a small amount so that the output voltage of the solar panel increases or decreases by a small amount. They then measure the output current and calculate the output power. If the power increases, the inverter makes further small adjustment in that direction until the output power no longer increases. When conditions change, the maximum power point changes, and the inverter once again goes through this process to find this point.
Because this is a very dynamic process, testing inverters is quite complicated. It’s impractical to use solar panels and PV arrays for this testing because their outputs are so dependent on illumination levels. Also, characteristics vary from panel to panel, making comparisons difficult.
The only way to get reliable results is to use a simulator that will faithfully replicate the output of a solar panel or array. To do this, the simulator must not only be able to supply a significant amount of output power, but agile enough to respond quickly to changes in the load impedance. Simulators that can do this will stay on the curve and provide the best test results.
Figure zz. The TerraSAS real-time display updates 20 times per second, allowing you to see how well the simulator tracks the simulated solar panel’s I-V curve.