# Optimization of Distribution Network Current Protection for Inverter-Based Distributed Power Access
## 2 IIDG Power Models
This section provides the theoretical basis and background for the control strategies in the next section, explaining how the IIDG controls its output in response to changes in grid conditions, and the next section applies the control strategies discussed in the next section to a real grid fault scenario, analyzing how these strategies affect the short circuit current characteristics of the distribution network. Characteristics of Large-Capacity Distributed Power Sources and Their Impact on Access to Urban Distribution Networks  
In the context of the distribution grid, the current of an inverter-based distributed generation system (IIDG) is subject to adjustment in the event of a fault, which is in accordance with the prevailing IIDG current within the distribution grid. This adjustment is made in accordance with the prevailing IIDG current within the distribution grid and is further subject to further adjustment in accordance with the IIDG current. It is of the utmost importance that this dynamic adjustment mechanism is in place to ensure the stability and reliability of the grid in the event of an abnormal occurrence [15]. When the Positive Sequence Voltage at the Shunt Point Is Greater than 0.9 Times the Rated Voltage (\(E_{sp}>0.9E_{N}\))  
In constant power output mode, the inverter-based distributed generation system (IIDG) is capable of maintaining a stable and unaffected output power, despite the occurrence of grid faults. The implementation of a negative sequence suppression control strategy ensures that the output current of the IIDG maintains three-phase symmetry, even in the event of a fault. This control strategy ensures that the output current adheres to a specific mathematical model, thereby guaranteeing the equilibrium and stability of the power system. When the Positive Sequence Voltage at the Shunt Point Is Between 0.2 and 0.9 Times the Rated Voltage (\(0.2E_{N}<E_{gp}<0.9E_{N}\))  
Upon transitioning to a voltage support state, an inverter-based distributed generation system (IIDG) initiates reactive power output, thereby assisting in the stabilization of grid voltage. In this state, the IIDG plays a role in maintaining grid voltage stability by supplying the requisite reactive power, thereby enhancing the grid's resilience to voltage fluctuations. This function is of paramount importance for the stable operation of the grid, particularly in situations in which there is a risk of voltage fluctuations. The d-axis and q-axis positive sequence reference currents, \(I_{d\_ref}\) and \(I_{q\_ref}\), output from the IIDG are shown in Equations (5) and (6). \[I_{q\_ref}=k_{1}(\frac{U_{N}-E_{gp}}{U_{N}})\frac{P_{ref}}{\sqrt{3}U_{N}} \tag{5}\]  
\[I_{d\_ref}=\min[\sqrt{k_{2}^{2}(\frac{P_{\max}}{\sqrt{3}U_{N}})^{2}-I_{q}^{2}},\frac{P_{ref}}{1.5E_{dp}}] \tag{6}\]  
where \(k_{1}\) determines the reactive power support capacity during the IIDG fault; \(k_{2}\) is the IIDG current limiting factor; and the output is generally not greater than 1.2 times of the rated current. When the Positive Sequence Voltage at the Shunt Point Is Less than 0.2 Times the Rated Voltage (\(E_{gp}<0.2E_{N}\))  
Upon transitioning to a voltage support state, an inverter-based distributed generation system (IIDG) initiates reactive power output, thereby assisting in the stabilization of grid voltage. In this state, the IIDG plays a crucial role in maintaining grid voltage stability by supplying the necessary reactive power, thereby enhancing the grid's resilience to voltage fluctuations. This function is of paramount importance for the stable operation of the grid, particularly in situations in which there is a risk of voltage fluctuations. \[I_{q\_ref}=k_{2}\frac{P_{ref}}{\sqrt{3}U_{N}} \tag{7}\]  
\[I_{d\_ref}=0 \tag{8}\]  
In the event of a fault in the grid, an inverter-based distributed generation system (IIDG) injects reactive power into the grid-connected point, thereby effectively boosting the voltage level at that point.