In the previous lecture, we discussed how a PV panel can be loaded. We concluded that in order to extract the maximum power from a PV panel during changing our environment conditions it's necessary to use an active load that is capable of finding, and tracking the maximum power point. In this lecture, we'll discuss how the location of the MPP can be found in practice when we don't know what the irradiance, and temperature conditions are. Let's start from the power voltage curve of a PV panel that you already know. Normal operating conditions. This curve has a single peak, the PMP located at the VMP, and IMP. You may remember from your math analysis classes that a maximum or minimum of a continuous function can be determined by solving the roots of the first order derivative of the function. In the case of the power voltage characteristic of the panel, we can find its peak by solving for the voltage where the power or voltage derivative equals 0. In practice, we can use this mathematical property of the PV curve to actively search for the PV voltage that will result in the numerical derivative of the power that equals 0. This is achieved by changing the operating point on the PV characteristic in small increments. After each increment, we check if we have reached the peak by verifying if the power voltage derivative equals 0. If not, we determine on which side of the peak we are currently located. If the power voltage derivative is positive, then we are located on the left side of the peak. Whereas if it is negative, we are located on the right side of the peak. By making the right adjustments, we can move closer to peak. Based on these observations, we can define a simple maximum power point tracking algorithm as follows; at the beginning of each iteration, we measure the present voltage, current and power output of the period generator, and calculate the difference in power relative to the previous iteration, denoted here as k minus 1. If the power difference is zero or very small, it means that we are located at the peak of the power voltage curve. However, if the power difference is greater or lesser than 0, we calculate the difference in the p voltage relative to the previous iteration. Using the power and voltage differences, and assuming that the steps are small enough, we can use a principle of numerical differentiations to estimate the power voltage derivative, and determine on which side of the peak we are located. For example, if we are located on the right side of the peak, we need to move the current operating point left on the PV curve. This can be done by controlling, and decreasing the PV voltage. The video on the right exemplifies how the simple maximum power point tracking algorithm works in practice. Active load will start searching for the MPP from the open-circuit voltage when the panel is first connected to the load. Then it will iterate through the MPPT algorithm relatively fast with a frequency between 10, and 20 hertz. After each iteration, the operating point is moved in small increments towards the peak. Once the peak is reached, the operating point will oscillate around it. As you know, in practice, a power voltage curve will not remain constant, and it's continuously changing with gradients in temperature therefore, the MPPT will continually search, and track the location of the maximum power point. This algorithm is also known as the perturb, and observe MPPT. It has the advantages of simplicity, and low computational demand, as well as it will work for most PV panel types, and PV systems. On the other hand, increasing the MPPT tracking speed will generally decrease its accuracy. Moreover, it can track in the wrong direction during fast change conditions such as fast-moving clouds. Nonetheless, the perturbation observe is the most used MPPT algorithm today. Moreover, it is one of the two most commonly used hill-climbing methods for maximum power point tracking together with incremental conductance algorithm, which uses a similar principle of operation as the perturb, and observe. Both methods are local MPPT algorithms, which we'll discuss shortly. Other MPPT methods are IV sweep method, where the operating point on the current-voltage characteristic is change very fast from the open-circuit to near the short-circuit current. During this week, the current-voltage curve is recorded, and the maximum power point determined. This method has the advantage of being very accurate in determining the global maximum power point. However, some energy is lost during the sweep, such that IV sweep cannot be done very often. Another common MPPT method is based on the observation that the ratio of the MPP voltage over open-circuit voltage is relatively constant for crystalline silicon PV modules, and also that the MPP voltage does not change significant complete with the region. In this method, the open-circuit voltage is measured periodically, and the MPP voltage is estimated based on this fixed ratio assumption, and it's being kept constant by the active load. However, most simple MPPT methods have limited occurs seen partial shading conditions. As you remember from the previous lectures, PV modules are commonly equal with multiple bypass diodes to protect the cells from reverse biasing, and damage during partial shading. However, partial shading can also lead to the formation of multiple power peaks in such modules as depicted in the figures shown below. In such situations, local maximum power point tracking algorithms such as the perturbation observe, and incremental conductance can track the wrong peak. As you can see in this example, partial shading has caused the formation of three power peaks. As a perturbation observe MPPT starts operating from the open-circuit voltage. It searches until it finds the first speak, and remains there as the power voltage derivative is zero at this peak. In this case, this peak is not a global maximum, and more power could be extracted from the panel if the panel would be operated at the second peak. A global MPPT method, such as IV sweep is needed, and often used to find the global power peak for PV panels, which are expected to be shaded. Now that we have learned how the MPP of MPPT power can be found without the knowledge of irradiance, and temperature. How can the MPPT be implemented in practice by an active load? Generally, any load can be transformed into an active load by means of a power converter depicted in figure, which we'll discuss in more detail in our next lecture. Using a power converter, the MPP of a generator can be tracked for a wide range of irradiance, and temperature conditions. Assuming that the load can handle the maximum power output of the panel the converter will monitor the input voltage, current, and power of the PV generator, and is able to control either the PV voltage or current, effectively controlling the operating point on the IV characteristic of the PV generator. Given these conditions, the converter is able to actively search for the maximum power continuously using one of the MPPT algorithms presented. Moreover, the converter adapt to the PV voltage, and current to the load ratings such that it can be operated safely, and for a wide range of environmental conditions. To summarize, MPPT algorithms are used to actively search for the maximum power that can be extracted from a PV generator at any given time without knowledge of the actual irradiance, and temperature conditions. Local MPPT algorithms have limited tracking accuracy in partial shading conditions. In this situation, the global MPP can be found using the IV sweep method. However, it cannot be used with the high frequency due to energy losses during the sweep. Lastly, the MPPT algorithms are implemented into the control software of the power converter, which will be discussed in the next lecture.
