(482b) A Model-Based Framework for Fault-Tolerant Dispatch of Distributed Energy Resources

Distributed energy resources (DERs) are composed of modular energy generation units such as fuel cells, photovoltaic arrays, small-scale wind turbines, battery storage and other distributed generation technologies that are deployed close to the point of consumption. The modular nature of DERs allows them to be integrated with existing grid infrastructure or implemented in a stand-alone manner. DERs have aided in the integration of sustainable energy resources into the power grid, but these new resources pose fundamental challenges including fluctuations in generation from intermittent availability of renewable resources, as well as the resulting increased network communication necessary to coordinate local generation supply with load demand. These and other challenges can be dealt with through use of process control and monitoring techniques (e.g., [1]-[4]).

 Process control is necessary to ensure that the load demand is met and that the economical operation of each DER is maintained. However, the stability and performance of DER smart grids have not been rigorously assessed in the presence of faults at the local and network levels. This is significant given the fact that the distributed power market is driven by the need for reliable high-quality power, and the substantial impact local disruptions in power flow can have when the DERs are integrated to support grid operations. In this context, fault-tolerant control (FTC) is an important tool for reducing the possible deterioration in power quality in the face of faults and uncertainties, resulting in increased reliability of the network.

 In prior work, FTC has been studied in the context of a small-scale network of solid oxide fuel cells (SOFCs), where the focus has been on the detection and recovery from faults at the local level without supervisory oversight [5]. DER health monitoring took place through use of an alarm threshold on a properly designed observer-based output residual. Exploiting inherent actuator redundancy in SOFCs, a number of stabilizing controller configurations were designed, and a methodology for active switching between the configurations in the event of a threshold breach was developed. The main contribution of this work was the characterization of a stable region within which each controller configuration could operate. However, the faults considered were limited to total and abrupt component failures, thus negating the need for fault estimation and accommodation. Furthermore, the detection scheme was stability-based, and thus not suited for detecting performance-degrading faults that do not compromise stability. The results also did not address the supervisory actions needed in the case of irreparable local faults potentially leading to total system failure.

 The objective of this contribution is to extend our previous work through the development of a model-based framework for integrated fault detection, estimation and accommodation in DER networks. The proposed framework extends our previous results in three important directions. In one direction, fault estimation capabilities are explicitly integrated into the fault detection mechanism through the use of a data-based moving-horizon parameter estimation scheme which uses past input and output data to generate an estimate of the local fault magnitude. A key feature of this scheme is that it can still be used for fault identification even after fault recovery, thus allowing for timely fault detection in the event of consecutive system faults. This is an advantage over residual generation approaches where the alarm threshold needs to be redesigned following every fault recovery event. In another direction, the proposed framework considers the use of fault accommodation, rather than reconfiguration, to compensate for local faults. Through a stability analysis, an explicit characterization of closed-loop stability for each component is obtained as a function of the fault size and the controller design parameters, and is used as a metric in determining the appropriate post-fault response. Finally, an optimization-based supervisory control system is designed to reconfigure the power demand distribution in the network by adjusting the set-points of the functioning DERs to compensate for the loss in power supply caused by irreparable local failures. The developed fault-tolerant dispatch framework is illustrated through a simulation case study.

 References:

 [1] S. Barsali, M. Ceraolo, P. Pelacchi, and D. Poli, “Control techniques of dispersed generators to improve the continuity of electricity supply,” Proceedings of IEEE Power Engineering Society Winter Meeting, pp. 789–794, 2002.

 [2] M. Marei, E. El-Saadany, and M. Salama, “A novel control algorithm for the DG interface to mitigate power quality problems,” IEEE Transactions on Power Delivery, 19, 1384-1392, 2004.

 [3] M. Marwali and A. Keyhani, “Control of distributed generation systems - part I: Voltages and currents control,” IEEE Transactions on Power Electronics, 19, 1541–1550, 2004.

 [4] M. Deshmukh and S. Deshmukh, “Modeling of hybrid renewable energy systems”, Renewable and Sustainable Energy Reviews, 12, 235 –249, 2008.

 [5] Y. Sun, S. Ghantasala and N. H. El-Farra, “Monitoring and Fault-Tolerant Control of Distributed Power Generation: Application to Solid Oxide Fuel Cells,” Proceedings of American Control Conference, pp.448-453, 2010.