Preventing wide-area blackouts through adaptive islanding of transmission networks

Lead Research Organisation: Durham University
Department Name: Engineering and Computing Sciences

Abstract

Recent blackouts and disturbances have shown that the twin drivers of: a) commercial pressures for better utilisation of transmission and distribution networks and b) increased penetration of Distributed Generation (DG) are likely to reduce security margins and lead to a higher probability of blackouts. This interdisciplinary project, involving power engineering, graph theory and operational research, will investigate methodologies to limit the occurrence and cost of blackouts through preventive splitting of large networks into islands when a cascade fault is imminent. The formed islands should preserve a good demand/generation balance, without violating any transmission constraint and avoiding electromechanical instability of any generator. The challenges addressed in this project include identification of conditions when preventive islanding can safely be activated, establishing techniques for forming islands and/or isolating a sick part of the network, and demonstrating innovative methods for control of islands with a high penetration of DG. The proposal applies to Complexity Science call, as we strive to understand how small-scale local behaviour of elements of an electrical grid influences the resilience of the grid as a whole, and helps to prevent catastrophic consequences.A major novelty of the proposed approach is that it is measurement, rather than model based. The decision to island a power system is usually taken when the system is in a highly disturbed state and its actual model is significantly different from the normal one. Hence, we propose to base the analysis on actual measurements gathered from Wide Area Measurement Systems (WAMS), which are expected to become widely available in the future, as a part of the next-generation monitoring and control technologies necessary for enabling the vision of low-carbon renewable-based energy flows. Two aspects of this project are particularly adventurous: its unique interdisciplinarity linking mathematics, operational science and power engineering, and its future-oriented application to networks with a high DG penetration, i.e. achieving 2020/2050 carbon reduction targets without sacrificing security and quality of supply.From a mathematical point of view, problem of islanding connects in an exciting way with modern analysis of metric spaces. Taking the view that grid reactances can give rise to a new metric structure on the grid, we will analyse the problem of islanding in the mathematical context of much studied isoperimetric problems. Our main tool will be spectral analysis of the discrete Laplace operator, which will be weighted to accommodate the physical information and used to identify possible candidates for balanced islands. We will employ discrete version of calculus and discrete Morse theory to capture the information about the power flows, to develop alternative techniques for identifying balanced islands, and to assess the effect of disconnecting an island on the rest of the network.In practical terms, the islanding of transmission network is a challenging graph partitioning problem. Unlike the usual graph partitioning problems, in which the partitioning criteria are known a priori (and remain constant), the difficulty in islanding electricity transmission comes from the need to adjust the partitioning to the very recently observed state of the network. As a result, the known graph partitioning techniques, which have proved so successful in the context of sparse matrix reordering or very large scale integration design, are not directly applicable. The solution we propose is to extend the existing techniques to take into account measurement-based (hence dynamically changing) graph properties and design/implement novel graph partitioning heuristics. The new techniques will combine graph dissection heuristics with randomisation and will be guided by the theoretical tool of spectral analysis of the Laplace operator.

Publications

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Bialek J (2016) Benchmarking and Validation of Cascading Failure Analysis Tools in IEEE Transactions on Power Systems

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Sanchez-Garcia R (2014) Hierarchical Spectral Clustering of Power Grids in IEEE Transactions on Power Systems

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Bukhsh W (2013) Local Solutions of the Optimal Power Flow Problem in IEEE Transactions on Power Systems

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Trodden P (2013) MILP formulation for controlled islanding of power networks in International Journal of Electrical Power & Energy Systems

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Trodden P (2014) Optimization-Based Islanding of Power Networks Using Piecewise Linear AC Power Flow in IEEE Transactions on Power Systems

 
Description A. We have developed an efficient algorithm that uses spectral and hierarchical clustering to reveal the internal connectivity structure of a network representing a power grid. Those clusters then can be used as a basis for network islanding as a preventive tool against spreading of cascading blackouts.

B. A methodology was developed to identify "the point of no return" when network islanding has to be activated to prevent a blackout..

Additional findings:

1. Using the standard DC approximation for load flow in constructing islands in electrical networks often leads to infeasible solutions due to reactive power and voltage limits being violated.

2. Finding optimal islanding solutions using the full AC load flow equations is computationally intractable for large networks. However a new piecewise linear approximation of the AC load flow is tractable and reliably yields feasible results that are close to optimal.

3. It is possible to use measures of generator coherency within the optimisation approach to ensure the dynamic stability of the islanded system.

4. Local optima of power flow problems are rare. We showed that they are commonly associated with cases where it is beneficial for the network to loose real or reactive power, and created a library of instances.
Exploitation Route The outcomes may be used to develop a detailed network splitting scheme.
Sectors Energy

 
Description The cost to society of wide scale electricity blackouts is huge. It is recognised by industry that there is no solution in place for this problem so it will recur and with increasing frequency due to the growth or intermittent renewables. However implementing any solution will be difficult and costly in itself. At the current state of knowledge there is no single agreed solution approach, so the importance of work in in this area is to create a suite of tools which can be combined to give a practical robust system. Our work has demonstrated that an optimizaton approach can yield practical solutions in tractable times for important aspects of this problem. The general problem of reconfiguring electricity networks is of increasing importance beyond the blackout prevention area. Switching components in and out of a network for operational cost and robustness reasons is of increasing interest due to the renewable growth and smart grids. The piecewise linear model developed in this project makes many of these problems more computationally tractable. We are currently studying a practical problem from Hydro-Quebec and expect to apply methods and insights from the project on that. A library of optimal power flow (OPF) problems with local optima produced during this project is being used widely in analysing and testing new approaches to solving OPF problems.
First Year Of Impact 2012
Sector Energy
Impact Types Economic

 
Description IEEE
Geographic Reach Multiple continents/international 
Policy Influence Type Influenced training of practitioners or researchers
Impact Chris Dent has the following roles within IEEE Power and Energy Society: - Vice Chair, Reliability, Risk and Probability Applications Subcommittee - Chair, LOLEWG (which shares experience between practical adequacy studies) - Chair, Task Force on Capacity Value of Solar Power - Chair, Working Group on Review of IEEE Standard 859 (transmission reliability data collection terminology) These contribute to development of industrial and academic knowledge and practice worldwide in the field of power system reliability analysis
 
Description Invited presentations to UK industrial mathematics community 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Professional Practitioners
Results and Impact Dr Chris Dent has given invited presentations at:
- Turing Gateway to Mathematics event on "Maths and Public Policy - Cities & Infrastructure", see http://www.turing-gateway.cam.ac.uk/mpci_mar2015-programme
- KTN Industrial Mathematics community event, to stimulate interest in links between the mathematical sciences community and energy systems applications
- KTN "Mathematics in Energy Systems" workshop, see https://www.eventbrite.co.uk/e/energy-catalyst-brokerage-event-london-tickets-19192257559?aff=erelexporg

This outreach activity drew together experience from a number of different EPSRC projects.
Year(s) Of Engagement Activity 2015,2016
 
Description Tutorial day on Energy System Planning Under Uncertainty 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Other audiences
Results and Impact Tutorial day for academics and practitioners. Around 40 delegates including speakers. Aim was to translate methods from mathematical science to application disciplines. Slides and videos of talks available on website, see URL.
Sponsored by Hubnet and CESI. Other grants listed indicate source of methodology presented in one or more talks, or payment of travel for a speaker.
Year(s) Of Engagement Activity 2016
URL http://icms.org.uk/workshops/energytutorialday
 
Description Visit to industry colleagues in California to broaden impact links 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Industry/Business
Results and Impact Visit to colleagues in California, including CA Independent System Operator, Public Utilities Commission, Stanford Research International, Electric Power Research Institute. Aim is to broaden impact of EPSRC work on uncertainty quantification in complex computer models, including discussion of future collaboration and industry projects.
Year(s) Of Engagement Activity 2017