# Fault Analysis and Protection Principle for the Distribution Networks Integrated with PV and BESS
## 1 Introduction  
To achieve the \"dual-carbon\" goals, China is accelerating the development of the power grid with a high percentage of renewable energy. BESS can balance the intermittency of PV generation by storing excess electricity during periods of sufficient sunlight and releasing it during peak demand or insufficient sunlight, thereby ensuring stable power supply and enhancing grid reliability and flexibility. Under these circumstances, an increasing number of PV and BESS are being connected to the distributed networks every year. However, the large scale of PV and BESS has obviously increased the complexity of the power grid [1, 2, 3, 4]. PV and BESS belong to the category of inverter interfaced resources (IIR) [5]. In a distribution network with IIR connected, the directional overcurrent protection may suffer from protection failure issues due to the altered fault current characteristics of the distribution network caused by the IIR connection [6, 7]. This brings a series of new technical challenges to the protection of the power system [8, 9, 10].  
References [11,12] analyzed the fault characteristics of PV employing negative sequence current suppression strategies for FRT, respectively. However, recent grid connection specifications require PV and BESS to absorb negative sequence current during asymmetric faults to suppress the negative sequence voltage at the point of Common Coupling (PCC). In response to the new grid connection requirements, [PERSON] [13,14] proposed an FRT control strategy to comply with the newly emerging grid codes and FRT requirements. In addition, current research on directional elements mainly focuses on distribution networks connected with PV or wind power [15, 16, 17], and there is relatively little research on new directional elements for distribution networks connected with PV and BESS. [PERSON] [18] presented an approach for identifying fault for distribution networks with PV through negative sequence current magnitude analysis during asymmetric faults. However, the applicability of this approach to PV and BESS systems implementing different FRT control strategies remains undetermined. [PERSON] [19] introduced a directional protection scheme utilizing both amplitude and phase attributes of injected negative-sequence impedance for asymmetric fault orientation. For balanced fault scenarios, Reference [20] established a detection mechanism based on positive-sequence impedance variations combined with active power angle measurements, integrated with supplementary directional estimation logic. Notably, the strategies in References [19,20] were specifically designed for negative sequence current suppression control architectures, leaving their validity under negative sequence current absorption control strategies unverified.  
Aiming at the above-mentioned deficiencies, this work discusses the fault characteristics and protection principle for the distribution networks with PV and BESS. First, it studies the FRT control strategy of PV and BESS under negative sequence current absorption. Then, it analyzes the negative sequence impedance characteristics of PV and BESS connected to the distribution network during asymmetric phase-to-phase faults and the distribution of active and reactive currents during symmetric faults. Finally, a fault direction detection principle is proposed, which is based on the negative sequence impedance angle during asymmetric faults and uses the reactive current and the active current during symmetric faults. The proposed principle is verified by simulation results.