## 1 Introduction  
After a large number of distributed generations (DGs) are connected to the distribution network, the topology of the traditional distribution network has been dramatically transformed from the original single-power supply radial network structure to a complex network structure with multiple-power supplies. The operation mode, power flow, and short-circuit current fault characteristics of the distribution network are complex and variable [1], making its protection face significant challenges [2]. Depending on the grid-connected interface, DGs can be divided into synchronous-based DGs (SBDGs) directly connected to the grid and IBDGs connected to the grid by inverters [3]. Among them, the fault characteristics of IBDGs differ from traditional synchronous generators due to the low inertia, high controllability, and poor overcurrent capability of inverters [4]. The output currents of IBDGs are affected by their inverter control strategy, low voltage ride-through (LVRT), and fault conditions. The uncertainty of IBDGs' output currents makes the setting and delay coordination of traditional passive-detection-type protection face significant challenges in the distribution network with IBDGs [5], especially in the case of high penetration of IBDGs. Most available studies have improved traditional passive-detection protection methods to make them applicable to distribution networks with DCs. Traditional overcurrent protection is improved in [6; 7; 8], modifying the protection setting values according to the DG output's fault characteristics. This method is simple to implement and has high sensitivity. However, it is difficult to set and coordinate the protection in the active distribution network due to its variable operation modes, complex branches, and numerous DG access points. To solve the above problems, an overcurrent protection scheme based on wide-area communication is proposed in [9; 10; 11; 12]. After receiving current information, the centralized controller adaptively calculates the set value of each protection, which requires high communication requirements. In [13], an efficient protection method is proposed by comprehensively using the local fault voltage and current information without communication, but is only applicable to the distribution network with SBDGs. Compared with single-end protection based on local information, pilot protection using electrical information at both ends of the feeder has superior operational performance in multi-power networks. Adjacent impedance information is used to construct protection criteria in [14; 15; 16] for fault identification and isolation with high protection reliability. All of the above methods use voltage information. However, the practical medium/low voltage distribution network feeders are generally not equipped with voltage transformers, which limits the application of these schemes. In [17; 18], current differential protection applied in transmission lines is improved and introduced into the distribution network, which has good selectivity, but high requirements for data time synchronization. Traditional passive protection methods seldom consider the influence of the control strategy of IBDG. However, the control strategy of IBDG directly determines its fault characteristics, which is critical for protection. In [19], the idea of control-protection synergy is first proposed: fault detection is achieved using the control strategy of the IBDG. In [20], a protection scheme based on controlling the negative-sequence current of the IBDG output is proposed, but fault types must be judged first to determine the given value of the negative-sequence current. Reference [21] reviews active detection-based fault identification methods, and the feasibility and physical limitations of the injection detection signal operation are described. In [22], a differential protection scheme based on non-rated frequency injection is proposed to detect and isolate faults.