Security-constrained expansion planning of low carbon power systems

© 2021 Sebastian Tomas Puschel LovengreenPower systems worldwide are experiencing a radical transition from a traditional configuration based on conventional thermal prime movers with synchronous generators to systems increasingly dominated by variable renewable energy sources that are asynchronously connected to the system via power electronic interfaces. Consequently, the level of system synchronous inertia is rapidly decreasing, with potentially severe effects on frequency stability. In Australia, the recent decommission of large coal-fired power plants, recurrent brown-outs over hot summer days, in addition to a major blackout in the South Australia region in 2016, have thus raised significant concerns as to the capability of the current system and market arrangements to adapt to the varying technical, economic and environmental requirements. Also, Australia's weak grid faces a high risk of separation during extreme weather conditions or in the event of cascading failures. These contingencies may become particularly severe in low-inertia grids, where the sub-regions resulting after system split need to procure enough resources to respond to low (importing areas) and high (exporting areas) frequency conditions. In this work, a multi-service frequency response model is developed to represent the frequency behaviour of a system (or subsystems) considering different contingencies, frequency response resources, inertia availability and demand sensitivity to frequency changes. This model is used to build frequency response constraints that guarantee the allocation of the resources necessary to contain frequency deviations after load, generation and transmission contingencies. These frequency response constraints are used to assemble a multi-area security-constrained unit commitment, capable of minimising the total cost of operation considering the technical characteristics of synchronous units, renewable energy resources availability, DC power flows, transmission constraints, and the allocation of reserves (including fast frequency response resources) and inertia to comply with statutory limits for the rate of change of frequency, frequency nadir and zenith, and quasi-steady-state frequency. This unit commitment model, cast as a mixed-integer linear program, is investigated on test systems representing the Australian power system and validated via dynamic simulations. The results show how the system's frequency resilience to extreme events can be effectively enhanced for both high- and low-frequency conditions that arise in different areas after a generation, load or interconnector contingency. As decreasing inertia levels put pressure on the system frequency security adequacy, investing in technologies to support it becomes necessary. Using the unit commitment model as the framework to study low-inertia power system operation, this work presents an expansion model based on Dantzig-Wolfe decomposition and delayed column generation algorithm that inherently embeds the associated frequency security constraints in the expansion process. As a result, the expansion model guarantees minimum investment and operation cost decisions and frequency security adequacy. A case study application representing the actual characteristics of the Australian power system is presented, using the parameters provided by the Australian Energy Market Operator in the context of the Integrated System Plan. This work studies the optimal development paths for the system under uncertainty, considering the investment in transmission lines, battery energy storage systems, pumped-hydro storage systems, and synchronous condensers. The expansion model is validated and tested using different scenario trees that help identify the investment flexibility provided by various technologies in the presence of uncertainty. The novelties of this work include a multi-area security-constrained unit commitment model capable of allocating frequency security resources, including fast frequency response to withstand generation and load contingencies, and transmission contingencies leading to system split. Furthermore, the model can co-optimise the largest contingency size and guarantee the feasibility of transmission of contingency reserves through the interconnectors after contingency. It also presents a state-of-the-art security-constrained expansion planning model capable of making investment decisions that consider frequency response constraints and fast frequency response. To the best of the author's knowledge, this is the first time an expansion planning model with these characteristics has been presented and applied on a real system to make investment decisions not only on transmission and storage assets but also on synchronous condensers. Some of the novel elements presented in this work have had a substantial impact on real-world applications. The frequency response adequacy model has been used to analyse the frequency security of future energy scenarios in Australia in the context of the Independent Review into the Future Security of the National Electricity Market conducted by Australia's Chief Scientist. A graphical tool derived from the frequency adequacy model has been used by the Australian Energy Market Operator to analyse the frequency behaviour of the South West Interconnected System in Western Australia. Also, recently, UK's National Grid Electricity System Operator has started applying a new framework to evaluate investment decisions in new transmission assets based on the least worst weighted regret approach presented in this work