## 1 Introduction  
With the deterioration of environmental problems, the shortage of fossil fuels, and the rapid development of power electronics technology, distributed generations (DGs), which use clean energy to generate electricity, have been integrated into the distribution network in large numbers. Therefore, the active distribution network has gradually become the future direction of the distribution system. However, access to many DCs has transformed the distribution network from the traditional single-source radiation-type power supply network to a complex network with multiple power sources and terminals. As a result, the operation mode, power flow direction, and fault characteristics of the active distribution network are complex and variable [1], leading to the traditional three-stage current protection no longer being applicable for active distribution networks [2, 3]. Depending on the grid connection interface, DGs can be divided into synchronous-based DGs (SBDGs) directly connected to the grid and inverter-based DGs (IBDGs) connected to the grid by inverters [4]. The characteristics of fault currents provided by different types of DGs are different. The maximum fault current provided by the SBDG can reach 6-10 times the rated current, significantly influencing the distribution of fault currents in the active distribution network [5]. The fault characteristics of IBDGs differ from those of traditional synchronous generators due to the low inertia, high controllability, and poor overcurrent capability of grid-connected inverters [6]. The fault currents provided by the IBDG are affected by the inverter control strategy, fault conditions, low voltage ride-through, and other factors. The uncertainty of the fault currents brings significant challenges to the traditional protection settings in active distribution networks. With the increasing penetration of DGs in the future distribution network, a novel protection method is urgently needed for the active distribution network with different DG types. Scholars have proposed many protection schemes to solve the problems of active distribution networks. In recent years, the proposed protection schemes for the active distribution network have been divided into the following three categories:  
1. The traditional overcurrent protection based on local information is improved to apply to active distribution networks with complex fault characteristics. In [7; 8; 9; 10], the protective setting value of the overcurrent relay is modified adaptively according to the behavior of DGs during faults, which improves the sensitivity of traditional overcurrent protection and has a good economy. However, the topology of the active distribution network is complex, and there are still problems with protection delay and coordination difficulty between feeder sections after high permeability DGs access. In addition, directional overcurrent relays can adapt to the power flow changes caused by DG access to the distribution network [11; 12]. However, there are still defects in the case of high DG penetration;
2. To solve the problem of protection based on local information, the pilot differential protection using adjacent information is introduced into the active distribution network. Pilot differential protection can be divided into impedance and current differential protection. Some scholars use high-frequency impedance [13], positive-sequence impedance [14], and integrated impedance [15] to construct differential protection that has good selectivity. In [16], the fault is identified according to the phase difference between the bus voltage's positive-sequence fault component and the feeder current's positive-sequence fault component. However, this method is only applicable to the grid-connected microgrid. All of the above-mentioned methods require voltage measurement. However, voltage transformers are rarely installed in low- and medium-voltage distribution networks.