The smooth operation of supply networks is crucial for the proper functioning of many systems, ranging from biological organisms such as the human blood transport system or plant leaves to man-made systems such as power grids or gas pipelines. Whereas the failure of single transmission elements has been analysed thoroughly for power grids, the understanding of multiple failures is becoming more and more important to prevent large scale outages with an increasing penetration of renewable energy sources. In this publication, we examine the collective nature of the simultaneous failure of several transmission elements. In particular, we focus on the difference between single transmission element failures and the collective failure of several elements. We demonstrate that already for two concurrent failures, the simultaneous outage can lead to an inversion of the direction of flow as compared to the two individual failures and find situations where additional outages may be beneficial for the overall system. In addition to that, we introduce a quantifier that performs very well in predicting if two outages act strongly collectively or may be treated as individual failures mathematically. Finally, we extend on recent progress made on the understanding of single link failures demonstrating that multiple link failures may be treated as superpositions of multiple electrical dipoles for lattice-like networks with collective effects completely vanishing in the continuum limit. Our results demonstrate that the simultaneous failure of multiple lines may lead to unexpected effects that cannot be easily described using the theoretical framework for single link failures.
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