## 2 Adaptability of FDP to PV Station Access  
FDP is the main form of protection for high-voltage transmission lines above 220 kV in China. However, with the integration of new energy sources such as PV stations, the characteristics of fault currents have changed, making traditional FDP less applicable [11]. This section analyzes the adaptability of FDP to PV station access and the impact of CT saturation on protection action.  
The model of FDP in transmission lines connecting PV stations is shown in Figure 1. K1 is an internal fault, K2 is an external fault on the system side, and K3 is an external fault on the PV side. The fault current provided by PV stations is limited in amplitude, showing a weak feed effect, and the power grid side is divided into a strong power grid and a weak power grid according to the short-circuit capacity and voltage support capacity of the power grid. The transmission line is modeled in PSCAD with a length of 100 km and rated frequency of 50 Hz. Line impedance parameters (R, X, and B) are given in per-unit values, and distributed parameter modeling is adopted to preserve electromagnetic transient accuracy. A midpoint connection at 40% of the total length is used to emulate a typical PV connection scenario.  
FDP generally adopts the ratio braking action criterion [10], which compares the differential current to the braking current to determine the appropriate protection action.  
\[\left\{\begin{array}{l}\left|\dot{I}_{\text{p}}+\dot{I}_{\text{g}}\right|>K \left|\dot{I}_{\text{p}}-\dot{I}_{\text{g}}\right|\\ \left|\dot{I}_{\text{p}}+\dot{I}_{\text{g}}\right|>I_{\text{on}}\end{array}\right. \tag{1}\]  
In Formula (1), K is the braking coefficient, and the steady-state fault usually takes the value of 0.8 in engineering practice [10]. I\({}_{\text{on}}\) is the starting value of the protection action, which is the minimum action threshold. \(\left|\dot{I}_{\text{p}}+\dot{I}_{\text{g}}\right|\) is the differential current and \(\left|\dot{I}_{\text{p}}-\dot{I}_{\text{g}}\right|\) is the braking current. The fault current phase difference for internal faults is \(-90^{\circ}\) to \(90^{\circ}\), and the fault current phase difference for external faults is \(180^{\circ}\) to \(270^{\circ}\)[25]. Formula (2) defines the control strategy of photovoltaic inverters under abnormal grid voltage conditions, which limits the amplitude and phase of fault currents to meet LVRT requirements and ensure safe inverter operation.  
\[\left\{\begin{array}{l}i_{d}^{*}=\min(\frac{P^{*}_{\text{p}}}{\text{I}_{ \text{p}}},\sqrt{1.2^{2}-(i_{q}^{*})^{2}})\\ 0,\quad\quad\quad U_{\text{p}}>0.9 pu\\ i_{q}^{*}=\begin{array}{cc}1.2(0.9-U_{\text{p}}),&0.2 pu<U_{\text{p}}<0.9 pu\\ 1.05,&U_{\text{p}}<0.2 pu\end{array}\right.\end{array}\right. \tag{2}\]  
In Formula (2), \(\dot{i_{d}}^{*}\) and \(\dot{i_{q}}^{*}\) are the reference active current and reactive current of the PV side, respectively. It is evident that the PV side operates at a unit power factor under the steady-state conditions of the power grid. When the inverter fails, the maximum current is limited to 1.2 times the nominal current \(I_{N}\), which can be seen in the fact that the amplitude of the PV side current is limited and the phase is controlled. Figure 2 shows the current characteristics on the PV side during an AB-phase fault, with the fault current amplitude being limited.  
Figure 2 shows the PV-side current waveforms during an AB-phase fault. Before 0.30 s, the system operates normally with balanced sinusoidal currents. After the fault occurs, a transient state appears due to the controller's delayed response. Eventually, a new steady state is established with a limited fault current amplitude, demonstrating the effectiveness of the fault current limiting strategy.  
Figure 1: FDP model of transmission line connecting PV station.