During non-severe fault conditions, crowbar protection is not activated and the rotor windings of a doubly-fed induction generator (DFIG) are excited by the AC/DC/AC converter. Meanwhile, under asymmetrical fault conditions, the electrical variables oscillate at twice the grid frequency in synchronous dq frame. In the engineering practice, notch filters are usually used to extract the positive and negative sequence components. In these cases, the dynamic response of a rotor-side converter (RSC) and the notch filters have a large influence on the fault current characteristics of the DFIG. In this paper, the influence of the notch filters on the proportional integral (PI) parameters is discussed and the simplified calculation models of the rotor current are established. Then, the dynamic performance of the stator flux linkage under asymmetrical fault conditions is also analyzed. Based on this, the fault characteristics of the stator current under asymmetrical fault conditions are studied and the corresponding analytical expressions of the stator fault current are obtained. Finally, digital simulation results validate the analytical results. The research results are helpful to meet the requirements of a practical short-circuit calculation and the construction of a relaying protection system for the power grid with penetration of DFIGs.
The protection system has important influences on the reliability of smart distribution networks. Therefore, the comprehensive reliability assessment of smart distribution networks includes primary distribution networks and a protection system. Because the traditional reliability evaluation of a distribution network does not consider the influence of relay protection and the complex configuration mode of the area-centralized distribution protection system (ACDPS) in the smart distribution network, a comprehensive reliability analysis method was proposed in this article. First, the whole reliability model of the area-centralized protection system, including two major failure modes, is proposed based on the fault tree method. Then, an equivalent model of the area-centralized protection system in circuit breakers or switches for the undesired-tripping mode and the fail-to-operate mode is also established. Based on this, an improved failure mode and effect analysis (IFMEA) method is presented to assess the comprehensive reliability of smart distribution networks considering the influence of fault location and protection system. The proposed method can correctly evaluate the comprehensive reliability of the smart distribution networks. The research results are also helpful to the construction of a protection system in the smart distribution networks.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.