Andrey Bernstein

Power Grid Vulnerability to Geographically Correlated Failures - Analysis and Control Implications

We consider line outages in the transmission network of the power grid, and specifically those caused by natural disasters or large-scale physical attacks. In such networks, an outage of a line may lead to overload on other lines, thereby leading to their outage. Such a cascade may have devastating effects not only on the power grid but also on the interconnected communication networks. We study a model of such failures and show that it differs from other models used to analyze cascades (e.g., epidemic/percolation-based models). Inspired by methods developed for network-survivability analysis, we show how to identify the most vulnerable locations in the network. We also perform extensive numerical experiments with real grid data to estimate the effects of geographically correlated outages and briefly discuss mitigation methods. The developed techniques can indicate potential locations for grid monitoring, and hence, will have impact on the deployment of the smart-grid networking infrastructure.

Performance evaluation of resource allocation policies for energy harvesting devices

We focus on resource allocation for energy harvesting devices. We analytically and numerically evaluate the performance of algorithms that determine time fair energy allocation in systems with predictable and stochastic energy inputs. To gain insight into the performance of networks of devices, we obtain results for the simple cases of a single node and a link. Due to the need for low complexity algorithms, we focus on simple policies (some of which proposed in the past as heuristics) and analytically derive performance guarantees. We also evaluate the performance via simulation, using real-world energy traces that we collected for over a year, and in a testbed of energy harvesting devices developed within the EnHANTs project.

Explicit Conditions on Existence and Uniqueness of Load-Flow Solutions in Distribution Networks

We present explicit sufficient conditions that guarantee the existence and uniqueness of the load-flow solution for distribution networks with a generic topology (radial or meshed) modeled with positive sequence equivalents. In the problem, we also account for the presence of shunt elements. The conditions have low computational complexity and thus can be efficiently verified in a real system. Once the conditions are satisfied, the unique load-flow solution can be reached by a given fixed point iteration method of approximately linear complexity. Therefore, the proposed approach is of particular interest for modern active distribution network setup in the context of real-time control. The theory has been confirmed through numerical experiments.

Experimental Validation of an Explicit Power-Flow Primary Control in Microgrids

The existing approaches to control electrical grids combine frequency and voltage controls at different time-scales. When applied in microgrids with stochastic distributed generation, grid quality of service problems may occur, such as under- or overvoltages as well as congestion of lines and transformers. The COMMELEC framework proposes to solve this compelling issue by performing explicit control of power flows with two novel strategies: 1) a common abstract model is used by resources to advertise their state in real time to a grid agent; and 2) subsystems can be aggregated into virtual devices that hide their internal complexity in order to ensure scalability. While the framework has already been published in the literature, in this paper, we present the first experimental validation of a practicable explicit power-flow primary control applied in a real-scale test-bed microgrid. We demonstrate how an explicit power-flow control solves the active and reactive power sharing problem in real time, easily allowing the microgrid to be dispatchable in real time (i.e., it is able to participate in energy markets) and capable of providing frequency support, while always maintaining quality of service.

Existence and Uniqueness of Load-Flow Solutions in Three-Phase Distribution Networks

We present sufficient conditions for the existence and uniqueness of load-flow solutions in three-phase distribution networks. The conditions can be efficiently verified for real distribution systems.

Handling large power steps in real-time microgrid control via explicit power setpoints

We consider a microgrid with real-time control using explicit power-setpoints. Sudden power-steps, such as load disconnections or load in-rushes, directly affect the decisions of the microgrid controller that aims at avoiding voltage or line-ampacity violations. When trying to completely avoid these violations, the grid operation may be too restricted, which may lead to large suboptimality. However, temporary violations of the steady-state bounds are allowed by grid standards and could enable the exploitation of the flexibility of other resources to better control the systems state. In this paper, we propose a method by which such temporary violations are controlled so that they remain within the limits imposed by grid standards and safe operation. The method is experimentally tested and validated on a real microgrid.

Aggregation of power capabilities of heterogeneous resources for real-time control of power grids

Aggregation of electric resources is a fundamental function for the operation of power grids at different time scales. In the context of a recently proposed framework for the real-time control of microgrids with explicit power setpoints, we define and formally specify an aggregation method that explicitly accounts for delays and message asynchronism. The method allows to abstract the details of resources using high-level concepts that are device and grid-independent. We demonstrate the application of the method to a Cigre benchmark with heterogenous and low-inertia resources.

Real-time control of microgrids with explicit power setpoints: Unintentional islanding

We propose a method to perform a safe unintentional islanding maneuver of microgrids. The method is derived in the context of a framework for the real-time control of microgrids, called Commelec, recently proposed by the Authors. The framework uses a hierarchy of software agents that communicate with each other using a common, device independent protocol in order to define explicit power setpoints without the need of droop controllers. We show that the features of the framework allow to design a generic control method for treating unintentional islanding with the following properties. First, the method is able to choose the best candidate slack resource, based on the information obtained from the agents. Second, as the agent responsible for the grid has a global view of the networks status and its resources, it is possible to optimize the performance of the network during and after the islanding transition. Third, after the islanding maneuver it allows for the online switching of the slack resource to that with the best capabilities to face the networks needs. Finally, the method is suitable for inertia-less systems as the control is performed using explicit power setpoints and it does not rely on the frequency signal. We illustrate the benefits of the proposed method via simulation on the LV microgrid benchmark defined by the CIGRÉ Task Force C6.04.02, by comparing its performance to that of the standard droop-based method called load drop anticipator.

A supercapacitor agent for providing real-time power services to the grid

Supercapacitors-based storage systems are expected to play a key role in microgrids in view of their capability to compensate high-power imbalances. We define an agent for the control of supercapacitor arrays within the context of the novel control framework Commelec, proposed by the Authors as a composable method for real-time control of active distribution networks with explicit power setpoints. An important function of such an agent is to advertise the real-time power capabilities and operational preferences of the supercapacitor array based on local information. Given the small energy capacity of such a device, its internal state can largely vary from one setpoint implementation to the next one. For this reason, the use of an accurate model is crucial in the agent definition. We show that it is possible to infer the real-time power capabilities of the device by using simple measurements on the supercapacitor array suitably coupled with an accurate representation of the cells composing the array. Results show that the agent is able to speak for the resource, thus allowing its use from an external controller.