Browsing by Author "Chen, Qunjie"

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    Critical transitions analysis in self-organized power systems by OPA
    (IEEE, 2016) Ren, Hui; Chen, Qunjie; Wang, Fei; Watts Casimis, David Eduardo; Sun, Chenjung
    Understanding the mechanism of large scale outages is the precondition for designing of an effective prevention scheme. Complex system study shows that sudden transitions of the system from one regime to another may happen due to the nonlinear interaction among consisting components. Power systems are good examples of complex systems, and previous research shows that their critical transitions can be observed and predicted by statistical measures. These critical transitions include voltage collapse and system's entering a higher stress level from a long-term evolutional perspective. This paper continues the analysis of critical transitions in self-organized power system by Oak-PSERC-Alaska (OPA) model, a well-accepted model for the study of cascading blackouts from long-term perspective. Based on the self-organization mechanism designed in OPA, the mechanism of critical transition of power system entering a critical state is explained. The effects of local reliability on the transition is analyzed by comparing different cases. Time series of fractional load-shedding and fractional line loading are constructed and the performance of early warning of these two time series are discussed. The proposed research provide more insights on the mechanism of large blackouts, and easily available early warning signals, which are possible to implement as early warning of large catastrophic power system outage. Moreover, the fact that our research is conducted from a long-term point of view enables it to serve as a planning-assistant tool for a future power system with higher resilience.
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    Network motif as an indicator for cascading outages due to the decrease of connectivity
    (IEEE, 2017) Chen, Qunjie; Ren, Hui; Sun, Chenjung; Mi, Zengqiang; Watts Casimis, David Eduardo
    Complex network study pointed out that, between the macro level and the micro level, there still exist abundant various dimensions addressed as subgraph, including motifs, cliques, cores... The structural property of these subgraphs plays an import role for the robustness in the function of the system. In this paper, the network motif existing in different IEEE test systems and real power grids are identified, and the consistency between the structural feature of motif and power system design for reliable power supply is revealed. By incorporating the transmission line margin into the procedure of finding significant motifs, the risk of cascading overloads due to insufficient transmission capacity and the decrease of connectivity of the power network is reflected by the occurrence of certain motifs in the power network. Through the simulation of cascading failures along the continuous increase of load demand and random line failures, different phases in the evolution of cascading failures are defined, and the patterns in the occurrence of motifs corresponded to different phases are identified. The performance of network motifs as a warning signal for higher risk of large outages is also tested under continuous line/node removal scenario. Test results show that the occurrence of motifs can be a good supplement to the EENS to describe the risk of outages.