## 1 Introduction  
The traditional distribution network relay protection mainly relies on the high short-circuit current, which is nearly 10-20 times the normal current value to detect the fault and disconnect the fault feeder. The high short-circuit current is generated by the system-side rotating synchronous generator [1]. And then, the distribution automation remote terminal units (DRTUs) are used to realize the fault section localization and fault isolation according to the inherent local logic action [2]. These abovementioned strategies can guarantee the safe and reliable operation of the distribution system in case of faults of the power distribution line. With the increasing proportion of renewable sources' access to the distribution network, traditional short-circuit current calculation results cannot reveal the real-time short-circuit variation characteristics and cannot provide theoretical knowledge to obtain the reasonable setting calculation of relay protection, as well as the parameter selection of power equipment, such as circuit breakers. It has been shown that the output short-circuit current of a renewable source is completely determined by its control strategy and control parameters, and its short-circuit current is restricted, distorted, and unable to provide significantly increased current characteristics [3, 4, 5]. However, due to the secrecy of the manufacturers and the variable operating environment in the field, it is impossible to accurately know the control strategy and parameters of all grid-connected converter devices. In the meantime, the number of distributed generators (DGs) in the distribution network is large and widely dispersed, and it is impossible to accurately take into account the control characteristics of the renewable sources to calculate the short-circuit current [6]. Therefore, most of the existing short-circuit current calculations used in the field equate the equivalent output short-circuit current of DGs to zero or the maximum rated value [7; 8; 9], which leads to a large error in the calculation of the short-circuit current of the whole network, and there is a risk of erroneous rejection of the distribution network protection. A lot of research work has been carried out by scholars on the calculation of short-circuit current considering partial-power inverter-type and full-power-type DGs under different voltage drop conditions caused by grid faults. Among them, the output current of partial-power inverter-type DGs is mainly calculated via the voltage drop at the terminal, and the crowbar action would also affect the output current if the voltage dip reaches a predefined threshold [10; 11; 12]. Different from partial-power inverter-type DGs, the output current of full-power inverter-type DGs relies entirely on the control of grid-connected inverters [13; 14; 15]. The results show that the output fault current of a single distributed renewable energy resource is characterized by a segmented function of the terminal voltage during a grid fault. In refs. [16; 17; 18], the approximate equivalent expressions of the short-circuit current are derived from the mechanism of the control strategies of the partial-power inverter-type DGs before and after the grid fault, and the results have been verified in the recorded field data. Similar work has also been carried out for full-power inverter-type DGs in the literature [19; 20; 21]. Although the analysis of full-time short-circuit currents of partial-power and full-power inverter-type DGs during grid faults is not yet able to meet the actual and complicated recorded transient waves in the field, the modeling and analysis of their output steady-state fault currents have been relatively mature, which can satisfy the calculation of short-circuit currents of distribution networks under some constant scene.