The Warning Of Cascading Failures In Power Grids By Network Motifs

As an important part and functional core of critical infrastructures,the importance of the power grids is self-evident.Since 2003,happening of several large blackouts worldwide has caused a huge impact.The integration of large amount of intermittent renewable energy at the transmission level and the distribution level intensifies the variation in power flows.Moreover,under power markets,modern power systems have the tendency to operate under heavy loading conditions and will inevitablely face the contradiction between raising the stability margin and increasing the operational economy.Under these circumstances,providing operators with decision-making assistant information for properly assessing the operational risk will reduce the catastrophic costs of large blackouts.Therefore,there are great needs to perform the warning of the risk of large blackouts.The Research of complex networks point out that mesoscales play an equally significant role with macro and micro structure in network functions.Sometimes local structure feature characterizes network functions better than others.Network motifs are the small building blocks of large networks and can describe the organization characteristics and connectivity of networks,which could reveal the dynamic structure of networks.By combining the topological structure and power flow snapshots of power grids,we study the basic building blocks of power grids and analyze the IEEE test systems and a regional grid according to the Zscores and Significance Profile.Results show that the occurrence of three nodes motif ID38 and four nodes motif ID904 in power grid has statistically significance and the two above motifs are the main components of power grids.The consistency between structure feature of motifs(ID38 and ID904)and design of reliable power supply is also revealed.Based on that,we define the dynamic functional motifs by combining the cascading overloads and the transmission line margin.And the functional connectivity of the power grids is reflected by Zscores of motif ID38 and ID904.Through simulating the increasing of the operational “stress” by continuous load increasing,we verify the warning effectiveness of Zscores of dynamic functional motifs responding to the large blackouts with IEEE-30/57/118 test systems.Also,we observe the variation of Zscores and blackouts scales along continuous line/node removal scenario.Results show that the variation of Zscores can be regarded as warning signals of high operational pressure in the first kind of disturbance.In the second kind of disturbance,Zscores and traditional reliability index EENS have the same varying tendency.Therefore,Zscores of motif ID38 and ID904 can be regarded as effective supplement of warning of large cascading blackouts and provide operators with decision-making assistant information for properly assessing the operational risk.