Northern Illinois University
Virtual Renewable Energy Laboratory
College of Engineering and Engineering Technology

Experiment 4: Maximum Power Point Tracking (MPPT) for Photovoltaic Systems

From the I-V curve and P-V curve for a PV module in Figure 1, we can identify several important parameters including the open-circuit voltage V_oc, and the short-circuit current I_sc. The product of the voltage and current is the power delivered by the PV module. The output power is zero at the two ends of the I-V curve because either voltage or current is zero at those points. The maximum power point (MPP) is at the knee of the I-V curve where the product of voltage and current reaches the maximum. In Figure 1, it shows the voltage at maximum power point (MPP) is V_R, and the current at MPP is I_R. We’d like to have the PV module operate at MPP to extract the highest amount of power from it.

Figure 1. I-V curve and P-V curve for a PV module

The I-V characteristics of PV modules will vary with different irradiance and temperature. That means the maximum power point will vary in real time. Figure 2 shows the general MPPT control block diagram. The input of the DC-DC converter is connected to a solar panel. Its output is connected to a load.

The MPPT controller takes feedback from PV voltage V_PV, PV current I_PV, output voltage V_O and current I_O from DC-DC converter, then uses different control algorithms to calculate duty cycle for the DC-DC converter. Table 1 shows the feedback signal for the controller. The control algorithm will automatically adjust the duty cycle to move the PV voltage and current to make it operate at the maximum power point, extracting the maximum available power.

Figure 2. MPPT control for a solar panel

The most commonly used MPPT control algorithms are Perturb and Observe (P&O) and Incremental Conductance.

Perturb and Observe (P&O) control algorithm

The perturb and observe method measures the PV module’s output voltage and current, and then perturbs the operating point to determine the change direction. From the P-V curve in Figure 1, we can see when operating on the left of the MPP, incrementing the voltage increases the power, and decrementing the voltage decreases the power. On the other hand, when operating on the right of the MPP, incrementing the voltage decrease the power, and decrementing the voltage increases the power. Therefore, if there is an increase in power, the subsequent perturbation should be in the same direction to reach the MPP. If there is a decrease in power, the perturbation should reverse direction to reach the MPPT. Figure 3 Shows the flowchart for P&O control algorithm. Table 2 shows how to determine the direction of next perturbation.

Figure 3. Perturb and observe control algorithm

Incremental Conductance control algorithm

The incremental conductance algorithm is based on the fact that the slope of the P-V curve is zero at the MPP, positive on the left of the MPP, and negative on the right. Flowchart of the incremental conductance control algorithm is shown in Figure 4.

Figure 4. Incremental Conductance control algorithm

Learning Objective

Materials and Equipment

Procedure

Lab Report Requirement