# Optimization of Distribution Network Current Protection for Inverter-Based Distributed Power Access
## 6 Calculus Analysis
### Simulation Analysis of the Effect of IIDG Access on BC and CD Segments of Stage Current Protection
#### 6.1.3 Analysis of the Effect of Different Output Power of IIDG on Fixed-Time Overcurrent Protection  
In the simulation experiment, the end of line L2 was set to experience a three-phase short circuit fault. In this fault scenario, the RMS value \(I_{protect1\_rms}\) of the fault steady state fundamental current at Protection 1 was measured. These data are essential for evaluating the performance of the protection device under real fault conditions, as shown in Figure 8.  
The data presented in Figure 8 unambiguously demonstrate the inverse relationship between the output level of the inverter-based distributed generation system (IIDG) and the fault current at Protection Device 1. In particular, an increase in the output of the  
Figure 8: Minimum two-phase fault currents at the termination of line CD across different IIDG power outputs.  
Figure 7: Sensitivity variations of time-limited overcurrent protection with different IIDG outputs.  
IIDG is accompanied by a corresponding increase in the reduction of the fault current [21]. Furthermore, the precise quantification of the sensitivity of Protective Device 1 when it is employed as a backup time-limited overcurrent protection for a CD line, absent the IIDG being connected to the grid, remains undetermined. Nevertheless, further investigation is required to ascertain the precise influence of IIDG access on the sensitivity of the protection device, with a view to ensuring that the protection system maintains its optimal performance in all circumstances.  
\[K_{sen.1}^{II}=\frac{I}{I_{set.1}^{III}}=1.30 \tag{27}\]  
After connecting to the IIDG, the fixed-time overcurrent protection sensitivity curves for different PV generation outputs can be plotted when a fault occurs at the end of the line CD, as demonstrated in Figure 9.  
Figure 9 analyzes that the grid connection of the inverter-based distributed generation system (IIDG) reduces the fixed-time overcurrent protection sensitivity of the upstream Protection Device 1. When the output power of IIDG reaches the maximum, the fixed-time overcurrent protection sensitivity of Protection Device 1 drops to below 0.97, which is far from the required sensitivity threshold \(K_{sen.1}^{III}>1.2\), highlighting the necessity of optimizing and adjusting the protection system.  
The simulation analysis supports the rationality of the theoretical analysis and reveals that there is a positive correlation between the output power of the IIDG and the reduction of the upstream line current during the fault period after the IIDG is integrated into the distribution network. In order to improve the protection sensitivity, it is recommended to dynamically adjust the current action setting value of the protection device using the real-time output parameters of the PV system to optimize the current protection performance of the BC and CD sections [26; 28; 29]. The accuracy of the theoretical analysis is verified by PSCAD simulation, and the simulation of the optimization method is carried out below to compare the optimized results with the non-optimized results.