# Optimization of Distribution Network Current Protection for Inverter-Based Distributed Power Access
## 6 Calculus Analysis  
The 10 kV distribution network simulation model shown in Figure 5 is constructed in PSCAD X4 (v4.6) simulation software.  
Figure 4: Adaptive full line current protection action logic diagram.  
Figure 5: Diagram of a 10 kV distribution network in a certain city.  
The present study is concerned with the impact of the factor under examination on the sensitivity and stability of the protective device. In the process of performing short circuit current calculations, the following specific parameters of the system power supply, lines, and loads were selected:  
(1) System power parameters  
In this study, the standard rated voltage value of 10.5 kV of the 10 kV distribution network was selected as the reference value of the system side voltage.  
In this study, the standard rated voltage of 10.5 kV was selected as the reference value for system voltage in the 10 kV distribution network. To guarantee the precision and comprehensiveness of the analysis, the system impedance values of this distribution network in both maximum and minimum operational modes are taken into account in this study. This method allows for a comprehensive evaluation of the characteristics of the grid under different operating states.  
(2) Line parameters  
In the 10 kV distribution network, the line types are primarily classified into two categories: overhead and underground cables. Given that the nature of faults on underground cable lines is typically persistent, automatic relclosing equipment is typically not installed on these lines. In contrast, overhead lines are commonly equipped with protection measures, including three-stage current protection, as well as pre-accelerated automatic relclosing devices, with the objective of increasing the speed of system recovery. In contrast, the protection configuration of cable lines is more straightforward, relying primarily on a single current protection mechanism. The selection of overhead lines is as follows: \(X=0.347~{}\Omega/\text{km},~{}R=0.27~{}\Omega/\text{km}\). The selection of cable lines is as follows: \(R=0.259~{}\Omega/\text{km}\), \(X=0.093~{}\Omega/\text{km}\).  
In this study, line segments AB and BC are designated as overhead transmission lines, with a length of 2 km each, while segments CD and DE are set as underground cable transmission lines, reaching an extended length of 7 km vs. 14 km, respectively. The distributed power sources are connected at bus C of the grid. A table of specific parameters for each line segment is derived from the unit parameters using a calculation. This table (Table 1) provides the necessary data to support the analysis and design of the distribution network.  
Protection 1 is installed upstream of IIDG. The setting value of the line BC in conjunction with the line CD Current Section II protection is \(I_{set.1}^{II}=2.85\) kA. In the distribution network protection design, the third section current protection of line BC (often referred to as time-limited overcurrent protection) is rectified according to a reliability factor of 1.3, and its setting value is determined as follows: \(I_{set.1}^{III}=1.69\) kA.  
The transient currents generated by inverters typically decline rapidly to a stable level within a few grid IF cycles. Consequently, the set action times of the time-limited current interruption protection and the fixed-time overcurrent protection are considerably longer than the rapid decay of the transient currents [25]. This configuration guarantees that the protective apparatus is able to make determinations and take actions based on a more enduring current state subsequent to the dissipation of the transient current's effects. Accordingly, this study excludes the impact of transient currents on the system [26].  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Bus Route** & **Impedance Value (f)** \\ \hline AB & \(0.53+\text{j}0.695\) \\ BC & \(0.53+\text{j}0.695\) \\ CD & \(1.812+\text{j}0.6\) \\ DE & \(3.627+\text{j}1.303\) \\ \hline \hline \end{tabular}
\end{table}
Table 1: Impedance values of various line sections.