In this brief, by considering realistic network operational settings, we propose a novel model to investigate the cascading failures in interdependent power grids and communication networks. We perform a critical node analysis on single networks to identify the vital nodes from the perspective of network robustness. Moreover, we assess the robustness of an interdependent system composed of an Internet AS-level network and the IEEE 118 Bus. Our simulation results show that assortative coupling of node destructiveness is more robust for densely coupled networks, whereas disassortative coupling of node robustness and node destructiveness performs better for sparsely coupled cases.

In this paper, we use a circuit-based power flow model to study the cascading failure propagation process, and combine it with a stochastic model to describe the uncertain failure time instants, producing a model that gives a complete dynamic profile of the cascading failure propagation beginning from a dysfunctioned component and developing eventually to a large-scale blackout. The sequence of failures is determined by voltage and current stresses of individual elements, which are governed by deterministic circuit equations, while the time durations between failures are described by stochastic processes. The use of stochastic processes here addresses the uncertainties in individual components' physical failure mechanisms, which may depend on manufacturing quality and environmental factors. The element failure rate is related to the extent of overloading. A network-based stochastic model is developed to study the failure propagation dynamics of the entire power network. Simulation results show that our model generates dynamic profiles of cascading failures that contain all salient features displayed in historical blackout data. The proposed model thus offers predictive information about occurrences of large-scale blackouts. We further plot cumulative distribution of the blackout size to assess the overall system's robustness. We show that heavier loads increase the likelihood of large blackouts and that small-world network structure would make cascading failure propagate more widely and rapidly than a regular network structure.

In this paper, we incorporate the dynamical process of frequency control in modeling cascading failure and assessing the robustness of power systems with a high level of penetration of renewable energy sources. Triggered by an initial failure, a power system may be decomposed into multiple subnetworks, where the power imbalance between generation and load can be induced. Frequency control functions including primary, secondary, and emergency frequency control are employed to reduce the frequency deviation to balance the generated power and consumed power. In our simulation model, the power flow process maintains power balance in the steady-state, and a power redistribution process is triggered each time the network changes with the next failure event. The process reiterates until no overloaded component is generated. In the dynamical frequency control process, we differentiate the traditional synchronous machine-based generators from the renewable energy sources that are integrated with power electronic devices and exhibit low inertia. As power electronics integrated renewable sources do not contribute to frequency response, they can be disconnected from the grid due to unexpected frequency deviation. Applying to the IEEE 39-bus power test system, our model captures the salient frequency response process in a failure propagation chain similar to historical scenarios. The frequency deviation is found to increase with the percentage of integrated renewable energy sources, intensifying the cascading failure events. Furthermore, using the IEEE 300-bus test system, we show an increased risk of power outage as the level of penetration of renewable sources increases in the absence of an adequate protective measure.

This paper studies the dynamic behaviors of weak-grid-tied VSCs with simplified transfer functions, which provides an accurate stability analysis and useful indications for tuning system parameters. A reduced-order multi-input multi-output (MIMO) transfer function that contains four single-input single-output (SISO) transfer functions for the weak-grid-tied VSC is first presented. It is found that the four SISO transfer functions share the same equivalent open-loop transfer function, i.e., the same stability conclusion. The Bode plots of the equivalent open-loop transfer function show that the inner current loop behaves as a band-pass filter whose maximum gain is approximately at the frequency of the PLL's bandwidth. By stability criterion, the harmonic amplification and instability occur when its maximum gain exceeds 0dB caused by high PLL's bandwidth, large grid impedance or high active power. It is also found that the target system is less stable when it works as an inverter than as a rectifier, due to the risk of the local positive feedback in the inverter mode. An effective criterion is further proposed to guide the selection of a proper PLL's bandwidth to ensure the stability of the VSC system. Simulation results validate the correctness of the analysis and the efficacy of the criterion.

In this paper, we propose a model to investigate the cascading failures in the coupled system (smart grid) that comprises a power grid and a coupling cyber network. In this model, we take into consideration the effects of power overloading, contagion, and interdependence between power grids and cyber networks on failure propagations in the coupled system, and then use a stochastic method to generate the time intervals between failures, thus producing the dynamic profile of the cascading failures caused by the attack of cyber malwares. We study several coupled systems generated by coupling the UIUC 150 Bus System with cyber networks of different structures and coupling patterns. Simulation results show that the dynamic profile of the cascading failures in a coupled system displays a “staircaselike” pattern which can be interpreted as a combined feature of the typical step propagation profile triggered repeatedly by cyber attacks due to network coupling. Results also show that cyber coupling can intensify both the extent and rapidity of power blackouts. Moreover, the cyber network structure and the coupling patterns affect the propagation of the cascading failures in smart grids. Scale-free cyber networks promote failure spreading, and the higher average cyber node degree also intensifies the spreading. Coupling power nodes with high-degree cyber nodes accelerates the failure propagation compared with random or low-degree couplings.

In this brief, we propose a stochastic model based on state transition theory to investigate the dynamics of cascading failures in communication networks. We describe the failure events of the nodes in the network as node state transitions. Taking a probabilistic perspective, we focus on two uncertain conditions in the failure propagation process: which node in the network will fail next and how long it will last before the next node state transition takes place. The stochastic model gives each overloaded element a probability of failing, and the failure rate is relevant to the degree of overloading. Moreover, the time dimension is considered in the stochastic process, by removing a node after a time delay when its traffic load exceeds its capacity. We employ this proposed model to study the dynamics of cascading failure evolution in a Barabási-Albert scale-free network and an Internet AS-level network. Simulation results reveal the effects of the initial failure pattern, community structure and network design parameters on the dynamic propagation of cascading failures.