# Optimization of Distribution Network Current Protection for Inverter-Based Distributed Power Access
## 2 IIDG Power Models  
The field of distributed generation (DG) encompasses a range of technologies, the majority of which can be classified into two primary categories based on their connection to the grid. The first category encompasses direct-drive motor-type technologies, while the second category includes inverter-type technologies, which are converted by an inverter. Inverter-based DG represents a crucial link between renewable energy sources and the grid, making it an ideal solution for clean energy forms, such as photovoltaic and wind power. Consequently, the study of inverter-based distributed generation (IIDG) is of paramount importance. The equivalent circuit diagram of the inverter grid-connected system can be found in Figure 1. Currently, the majority of inverter grid-connected systems are connected to the grid using the pulse width modulation (PWM) technique. This study is concerned with the inverse control method with low-voltage ride-through capability, which is based on grid voltage-directed vector control. A brief overview of the control process is provided below. The inverter detects the phase of the grid voltage through phase-locked loop technology and adjusts its output current using a vector control strategy orientated to the grid voltage to achieve a synchronous grid connection with the grid. \(E_{dp}\) and \(E_{qp}\) are obtained by means of the dq-transform. In the majority of cases, the d-axis is set to be phase-aligned with the phase of \(E_{gp}\), thereby ensuring that the \(E_{gp}\) component on the q-axis, \(E_{qp}\), is equal to zero. In accordance with the aforementioned information, the positive sequence reference currents, \(I_{dp-ref}\) and \(I_{qp-ref}\), on the d-axis and q-axis can be derived as illustrated in Equations (1) and (2). \[I_{dp-ref}=\frac{P_{ref}}{1.5E_{dp}} \tag{1}\]  
\[I_{qp-ref}=\frac{Q_{ref}}{1.5E_{dp}} \tag{2}\]  
Figure 1: Equivalent circuit of IIDG access to grid. The inverter will no longer generate negative sequence currents after implementing a control method for suppressing them. The d-axis and q-axis negative sequence reference currents \(I_{dn-ref}=0\) and \(I_{qn-ref}=0\) of the inverter will satisfy specific conditions. \[I_{dn-ref}=0,I_{qn-ref}=0 \tag{3}\]  
In typical operational circumstances, the active power reference of the IIDG is typically set to the maximum output power determined by the maximum power point tracking (MPPT) module. Conversely, the reactive power reference \(Q_{ref}\) is generally maintained at zero. In the event of a grid fault, the output power of the IIDG will be regulated by the low-voltage ride-through (LVRT) module. In the event of a fault, the reference currents, \(I_{dn-ref}\) and \(I_{qn-ref}\), in the d- and q-axes are determined by the voltage outer loop control or LVRT module. Subsequently, the output reference voltages \(U_{cd-ref}\) and \(U_{cq-ref}\) of the inverter's d-axis and q-axis can be calculated according to the control algorithm. The control strategy guarantees the stable operation of the IIDG in diverse grid conditions while preserving grid stability in the event of grid faults [14]. \[\left\{ \begin{array}{l}U_{cd-ref}=E_{gd}+swLI_{d}+(K_{p}+(K_{S})^{-1})(I_{d} -I_{d-ref})-swLI_{q}\\ U_{cq-ref}=E_{gd}+swLI_{d}+(K_{p}+(K_{S})^{-1})(I_{q}-I_{q-ref})-swLI_{d}\end{array} \right. \tag{4}\]  
where \(K_{p}\) and \(K_{S}\) are the proportional gain and differential gain coefficients, respectively; \(L\) is the rectifier inductance; and \(I_{d}\) and \(I_{q}\) are the actual \(dq\) current. Next,\(U_{ca-ref}\), \(U_{cb-ref}\), and \(U_{cc-ref}\) are obtained through \(dq0-abc\) transformation, which completes the PQ decoupling control.