# Online Estimation of Short-Circuit Fault Level in Active Distribution Network
## 1 Introduction  
The accurate estimation of the potential short-circuit fault level is of great importance to quantify the strength of power system [1], set the protective relaying, and select the qualified circuit breakers to trip the electrical components [2]. The higher potential fault level is equivalent to the larger short-circuit current in case of fault, the smaller system impedance, and hence the greater system strength. Traditionally such an estimation in distribution networks is only necessitated in an offline manner at the stage of grid planning. However, the networks tend to be active nowadays and referred to as active distribution network (ADN) with the increasing penetration of the distribution generators (DGs), including wind turbine generators and photovoltaic (PV) systems, etc. Such a trend largely diversifies and complicates this estimation of fault level [3]. The contribution of DGs on the short-circuit current is non-negligible anymore, while on the other hand, the intermittent renewable energy and the associated frequent operation also make the potential fault level change ceaselessly. To validate the breaking capacities of the mounted circuit breakers or monitor their risks of the possible scarce margin, it might not be proper or efficient to evaluate the fault level offline by a fixed grid configuration, as the penetration of DGs makes it difficult to select the grid configuration typical enough to represent theabsolute-maximum short-circuit currents. In addition, some advanced functions, such as the adaptive protection setting [4] and even the adaptive control of DG for stability improvement, also rely on the real-time knowledge of the time-variant short-circuit current or the system strength. Consequently, the online method to evaluate the fault level in active distribution network deserves investigation. Generally, all DGs increase the short-circuit fault level more or less [5], but their contribution depends on their forms and locations. DGs can be roughly classified into two major forms, which are inverter-based DGs (IBDGs) and rotating DGs, respectively. The typical IBDGs, e.g., PV systems or full conversion wind turbine generators, can be regarded as the controllable current sources, whose contribution to the short-circuit currents is limited by their control strategies and usually less than 1.5 times of their rated currents. However, the rotating DGs, such as doubly-fed induction generators (DFIGs), greatly enlarge the short-circuit current, as they can be modeled as voltage sources similar to those conventional synchronous generators [6]. On the other hand, it is believed that, in the low-voltage (LV) distribution networks, the short-circuit current will only be slightly increased with the penetration of DGs [7], whereas in the medium-voltage (MV) and high-voltage (HV) distribution networks, the fault level apparently differs from the traditional level if a large amount of DGs are integrated. Previously, some analytical works have been presented regarding the short-circuit current in ADN. It is confirmed by both simulation and experiments in Reference [8; 9] that the output currents of IBDGs increase in case that the grid voltages drop during fault. In Reference [1], the equivalent current sources are established for the positive and the negative sequence components, considering the inherent characteristics of IBDGs during low-voltage ride-through (LVRT). A simple equivalent circuit is proposed in Reference [10], to evaluate the short-circuit current with IBDGs. The models mentioned above are nonlinear and require the node impedance matrices as well. More importantly, they might not be practically feasible, as the structure and the operation states of ADN change frequently. Some real-time estimation methods that enable online application have drawn enough attention [11; 12]. In Reference [13], the naturally occurring disturbances, such as load fluctuations, are employed to assess the supply impedance and consequently calculate the short-circuit fault current.