## 1 Introduction  
The increasing global demand for energy transition has spurred significant advancements in renewable power generation technologies, such as wind and PV systems, in recent years. Alongside these technological developments, novel challenges have emerged more prominently, including issues of power and energy balance as well as transient stability [1, 2, 3]. Furthermore, grid-forming technology, which enhances system inertia and short-circuit capacity, increases the multi-station short-circuit ratio for renewable energy sources, and improves grid impedance characteristics, is poised to be a key enabling technology for building future power systems. Currently, the protection of renewable energy transmission lines still predominantly relies on traditional power-frequency-based protections, which were originally designed for synchronous-generator-based power systems. However, due to the influence of inverter grid-forming controls, the fault characteristics exhibited during transmission line faults differ significantly from those of traditional synchronous generators [4]. This discrepancy leads to degraded performance of conventional protections, potentially resulting in maloperation or failure to operate [5; 6; 7; 8]. Therefore, in the context of accelerating deployment of grid-forming renewable energy plants, there is an urgent need to develop highly reliable protections specifically tailored for grid-forming renewable energy transmission lines. Current differential protections have been widely adopted as the primary protection due to their absolute selectivity and rapid response. With the increasing integration of PV systems into the power grid, improved pilot protections and novel pilot protections based on waveform similarity have emerged [9; 10]. The pilot protection based on differential impedance is presented in [11]. However, it relies on precisely synchronous voltage and current data at both line terminals. The pilot protection based on pattern recognition in [12] is dependent on differential calculations, exhibiting limited robustness against noise interference. In [13], the proposed protection only requires current samples, but it depends on simulation tests and field operation records to determine the protection threshold, whose performance is easily affected by fault resistance and time window length. The pilot protection based on differential current energy ratio is presented in [14], which is easily affected by fault resistance and noise interference. In addition, the fault current response of grid-forming inverters exhibits notable differences compared to that of synchronous generators. To characterize these discrepancies, quantitative measures, such as cosine similarity, Euclidean distance, etc., are utilized to construct protection criteria [15; 16]. However, their effectiveness has not yet been verified in grid-forming PV systems. High-frequency fault components are utilized in the directional pilot protection in [17]. Alternative protections based on active control strategies have also been proposed in recent studies. In [18], wavelet transform is employed to extract transient time-frequency features of current waveforms. However, the use of wavelet transform introduces significant computational complexity. Furthermore, the influence of line capacitance current on healthy phases during auto-reclosing operations are omitted. In [19], a time-domain-based protection is proposed based on the first derivatives of the differential voltage and current. Nevertheless, the reliance on first-order derivatives renders the method sensitive to high-frequency harmonics, thereby compromising its overall protection performance. In [20; 21], the performance of the distance protection is analyzed, and improved protections are proposed. However, the virtual impedance control of the PV inverter is not considered. Moreover, for high-voltage transmission lines, distance protection is hardly configured as the primary protection.