## 5 Grid-Connected Photovoltaic Power Generation System
### Principle of Photovoltaic Systems  
PV power generation is a renewable energy technology that converts solar radiation into electricity through photovoltaic modules [31; 32; 33]. When sunlight hits the semiconductor materials in the PV panels, the photovoltaic effect releases electrons, generating electrical current. The electricity produced is clean, renewable, and low-maintenance. With technological advancements, costs have decreased, making PV an integral part of sustainable energy.  
Maximum Power Point Tracking (MPPT) is a key technology in PV systems. It dynamically adjusts voltage or current to ensure the system operates at its maximum power point, optimizing panel output and enhancing overall efficiency [34; 35]. In environments with fluctuating sunlight, MPPT algorithms continuously track the optimal operating point, ensuring maximum power generation under varying weather conditions. The structure of the PV grid-connected system is shown in Figure 16.  
In this study, the current of the typical harmonics is primarily analyzed on the output current of the photovoltaic system, which is the current fed into the grid through the inverter. This point is where changes in solar radiation directly affect the harmonic content and distortion characteristics of the current. In Figure 16, the structure of the photovoltaic system is shown, and the harmonic analysis is based on the output current at the inverter output terminal. Additionally, the proposed AVA-LADRC is applied to the photovoltaic system to address disturbances caused by variations in solar radiation and optimize system performance. AVA-LADRC dynamically adjusts the control step size and real-time system parameters to ensure MPPT and minimize power loss. The algorithm works in coordination with the inverter control system in the photovoltaic system, effectively responding to environmental changes.  
Figure 15: THD of phase A current at different times under voltage sag conditions: (**a**) 0.6 s; (**b**) 0.9 s.  
ANN methods have significant advantages in MPPT due to their strong non-linear modeling capabilities and high precision in power tracking. The conventional inputs for an ANN-based MPPT system are typically solar irradiance, temperature, photovoltaic voltage (\(V_{PV}\)), and photovoltaic current (\(I_{PV}\)). In this section, alternative inputs based on the importance of the power-voltage relationship slope are proposed. These inputs include the error signal (\(dP/dV\)) and the second input, which is the rate of change of the error signal (\(\Delta E\)). At the maximum power point, the error signal is zero (\(dP/dV=0\)). When the photovoltaic voltage deviates slightly from this point, a response is required to address this deviation. The second input (\(\Delta E\)) represents the trend of the error change, indicating whether the error is increasing or decreasing. Figure 17 illustrates the ANN architecture used for MPPT,  
\[E(n)=\frac{\Delta P}{\Delta V}=\frac{V_{PV}(n)I_{PV}(n)-V_{PV}(n-1)I_{PV}(n-1)} {V_{PV}(n)-V_{PV}(n-1)} \tag{22}\]  
\[\Delta E(n)=E(n)-E(n-1) \tag{23}\]  
where \(E\) represents the error signal and \(\Delta E\) represents the rate of change of the error signal.  
In the UPQC system presented in this paper, the PV power-generation system is integrated on the load side, operating in parallel with the load, and together with the UPQC, they collaboratively optimize power quality. The parameters of the photovoltaic power-generation system are shown in Table 2.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Parameters** & **Value** \\ \hline Temperature & 25 \({}^{\circ}\)C \\ Maximum Power Point Output Voltage & 250 V \\ Maximum Power Point Output Current & 40 A \\ Open-circuit Voltage & 330 V \\ Short-circuit Current & 48 A \\ \hline \hline \end{tabular}
\end{table}
Table 2: Parameters of photovoltaic power-generation system.  
Figure 16: Photovoltaic system working principle block diagram.