## 3 MPPT Techniques
### Off-Line MPPT Techniques  
The off-line MPPT method measures the short circuit current or the open circuit voltage by disconnecting the PV panel from the system (also referred to as off-line parameters). Since the PV panel is disconnected from the rest of the system during the calculation, it is referred to as off-line MPPT. These MPPT techniques do not continuously track the current or voltage; instead, they measure MPP based on apriori data. These methods have the major benefit of requiring fewer voltage and/or current sensors. The MPPs for changing insolation and temperatures are not precisely tracked by such systems. The following methods belong to this category: curve fitting (CF) [19], look-up-table (LUT) [96], pilot cell (PC) [97], fractional short-circuit current (FSCC) [98], and fractional open-circuit voltage (FOCV) [99]. Figure 6: Block diagram representation of PV MPPT. The CF method uses a mathematical function to approximate the power characteristic of photovoltaic modules. To build a mathematical model and equation to explain the output characteristics, all manufacturing information and data are needed [19]. Equation (5) is used to measure the PV module characteristic, where \(a,b,c\), and \(d\) are the coefficients determined by sampling the \(I_{PV}\), \(V_{PV}\), and \(P_{PV}\)[100]. After calculating these coefficients, the voltage at which the power achieves maximum is determined using Equation (6). \[P_{PV}=aV_{PV}^{3}+bV_{PV}^{2}+cV_{PV}+d \tag{5}\]  
\[V_{MP}=\frac{-b\pm\sqrt{b^{2}-3 ac}}{3a} \tag{6}\]  
However, due to the high complexity of the measurement, this approach is somewhat difficult. Every parameter of the formula must be calculated based on atmospheric conditions, manufacturing technology, and the material used to construct the cell. Estimating optimum operating conditions takes a lot of time and effort. Figure 7: Classifications of MPPT techniques. In the look-up table method, the PV module current (\(I_{PV}\)) and voltage (\(V_{PV}\)) are measured to track the MPP. Technical information about the PV panel, such as the output voltage for varying irradiance and temperature levels, is stored for various climatic situations. The lookup method then tracks the MPP by comparing the measured \(V_{PV}\) and \(I_{PV}\) with the stored data. As a result, a significant amount of data is stored in the lookup memory for reliable MPP monitoring and increased productivity [96]. This approach is complicated and error-prone because it requires a large amount of data collection, storage, and several sensors to monitor the precise MPP. In a pilot cell algorithm, a constant voltage or constant current approach is used, where the \(V_{OC}\) is measured using a PC rather than the PV module/array [97]. As shown in Equation (7), the \(V_{OC}\) of the pilot cell is determined by multiplying a constant (\(K_{1}\)). The constant \(K_{1}\) is pre-determined to track the array's MPP in response to variations in temperature or irradiances. \[V_{ref}=K_{1}\times V_{oc}(\mathit{pilot\ cell}) \tag{7}\]  
where \(K_{1}<1\). In addition, the parameters of the pilot solar cell must match those of the PV module it represents. As a result, every pilot cell or PV module must be optimized, raising the system's energy cost [97]. The FSCC is also known as the constant current (CC) technique [98]. Based on the fact that its short circuit current and IMPP are roughly linearly related, Equation (8) expresses how they relate to one another. The current is regularly estimated when the PV module or array is experiencing a short circuit condition via a power switch. \[I_{MPP}=K_{SC}\times I_{SC} \tag{8}\]  
where \(K_{SC}=0.78\)-0.92. The fractional open circuit voltage (FOCV) method uses the same operational phenomenon as the fractional short circuit current (FSCC) method and is also known as the constant voltage (CV) method. This system operates by approximating the MPP voltage ratio set on the specified \(V_{OC}\) limits.