Extreme Contingencies Research Articles

Exploring the Spillover effects of tail risk fluctuations in the RMB exchange rate—The time-frequency and quantile connectivity perspective

This study constructs spillover indices from a volatility spillover network perspective using the Quantile Vector Autoregression (QVAR) model, capturing the RMB exchange rate's tail risk and time-frequency effects across varying shock sizes. Empirical results show that the QVAR-based spillover index more effectively captures the tail risk spillover effects across different quantiles. In the time domain, spillovers between RMB exchange rates are dynamic and particularly sensitive to extreme contingencies. In the frequency domain, RMB exchange rates demonstrate significant spillovers, primarily at low frequencies. During extreme upward events, dynamic observations show high-frequency spillovers surpassing low-frequency ones as dominant drivers in the tail spillovers of the RMB exchange rate. Additionally, the analysis of tail dependence indicators indicates a strong asymmetry in RMB exchange rate correlations, emphasizing market participants' heightened sensitivity to unfavorable shocks. These findings can serve as a reference for policymakers to strengthen risk management of the RMB exchange rate.

Simultaneous voltage control and frequency oscillation damping in Hydro-Québec’s network by using an updated 735-kV shunt reactors automatic switching system

The Hydro-Québec transmission system is among the most extensive and complex power grid in North America due to its very long lines (over 1000 km) joining the generation facilities in the north of the Québec province to the main centers of consumption in the south. Since the 1990s, Hydro-Québec has adopted a defense plan, consisting of various types of protection solutions including different remedial action schemes (RAS) and automatic action strategies, to increase the stability and reliability of its transmission system against extreme contingencies. One of these protection strategies is the automatic switching of shunt reactors (MAIS) 1MAIS is the acronym of the French name of the project as Manoeuvre Automatique d’Inductance Shunt (MAIS). which has been in operation since 1997. The goal of the MAIS system is to keep the voltage profile of the whole 735-kV grid in an acceptable range to increase the transmission capacity. The MAIS system at each substation is entirely independent from neighbour substations and will close or trip shunt reactors just based on local measurements. As a periodical upgrade in all existing protection solutions in the Hydro-Québec power system, new functionalities have been added into the existing MAIS system to develop its new version to improve both the voltage profile and the frequency response. This paper presents this new version including the algorithm and the concept behind the new additional functionalities. The simulation results show the efficiency and performance of the proposed method. On that basis, a deployment on Hydro-Québec’s grid to replace almost all the existing MAIS system with this new version will be realized by 2030.

Enhanced energy balancing and optimal load curtailment strategy for DC microgrid integration in hybrid AC/DC distribution networks

AbstractUnleashing the potential of distributed renewable energy sources (RESs), intelligent and autonomous microgrids are becoming pivotal in attaining net‐zero carbon emission goals. Hybrid AC/DC microgrids rise as cutting‐edge microgrid topologies, capitalising on the best of both AC and DC systems. However, the integration of intermittent renewables and uncertainties in loading poses stability challenges. Advanced bidirectional converter controls provide efficient power exchange, but in extreme contingencies, a resilient supervisory control framework (load management/load curtailment approach) is inevitable to withstand/avoid unplanned renewable disruptions/blackouts. Moreover, the operational paradigm shift towards achieving net‐zero emissions, isolated operation of RESs, and conventional load shedding methods are anticipated to encounter substantial challenges, necessitating the development of alternative strategies. In order to improve the stability of hybrid microgrid systems in islanding scenarios, this research presents an energy balancing and load curtailment strategy. The proposed method aims at optimising resource utilisation, prioritising essential loads, and executing an optimal load curtailment strategy (if required), thereby augmenting the stability of systems. Unlike a meta‐heuristic or exhaustive search, which depends on 2n − 1 possible combinations and become unworkable as load numbers increase, the suggested methodology is based on a mathematically modelled load restriction method. By including load criticality, this strategy effectively prevents blackouts even with an increasing number of loads, providing a significantly more useful and practical solution. Additionally, the proposed charging algorithm ensures that the energy storage system imports energy from the grid during off‐peak hours and maximises power generation from the DC subgrid. The efficacy of the proposed strategy is validated using a modified IEEE‐33 bus system as a test case for a hybrid AC/DC microgrid. Simulation results demonstrate the effectiveness of the MILP‐based load curtailment approach in maintaining system stability and preventing blackouts during unforeseen events.

Volatility spillovers and Asymmetric effects of Chinese A-share markets—Enterprise-Level data Based on high-dimensional social network models

ABSTRACT This paper investigates volatility spillovers and their asymmetry in the Chinese A-share market from a firm-level perspective. We calculate the connections of the 5-minute high-frequency realized semivariance volatilities of 81 stocks by using a high-dimensional social network approach. The results show that (i) the volatility spillover effect of corporate-level stocks is significant, with high dynamic total connections, and exhibits sector clustering characteristics. (ii) Both extreme contingencies and economic stimulus can exacerbate the asymmetry of volatility spillovers. During periods with economic stimulus, positive volatility spillovers are much larger than negative ones; at other times, negative spillovers are larger than positive spillovers. (iii) There is a positive linear relationship between the asymmetries in the volatility spillovers received and released by individual stocks. (iv) The financial sector is more sensitive to positive volatility spillovers and dominates the positive volatility spillover system. The telecommunication services and utilities sectors are net emitters of negative volatility spillovers. Our research has substantial ramifications for investors and policy makers with respect to distinguishing different transmission paths of upside and downside risks, conducting better portfolio optimization and establishing risk warning mechanisms.

Enabling resiliency using microgrids with dynamic boundaries

With the increased frequency of adverse weather and manmade events in recent years, the issue of the power distribution system (PDS) resiliency has become drastically important. Microgrids with various types of distributed energy resources (DERs) have capabilities to enhance the PDS’s resiliency against high impact low probability (HILP) events. Resiliency is defined as the ability of the system to keep supplying critical loads even during and after extreme contingencies. This paper presents a systematic method to segmentize parts of a PDS into microgrids with flexible boundaries to enhance resiliency and mitigate negative impacts from anticipated threats. The method is aimed at anticipation and preparation to HILP events in advance: alternative flexible boundaries of the microgrids must be preplanned during normal operation and dynamically changed prior to or during disturbances. The Mixed-Integer Linear Programming (MILP) is formulated and applied to select switching actions to adjust microgrids’ boundaries as well as to supply critical loads, while meeting system constraints. Networked microgrids are merged and reconfigured as necessary to maximize supply to critical loads, driven by a factor-based resiliency metric, which is obtained using the Analytical Hierarchy Process (AHP). The modified IEEE 123 bus system was utilized for validation of the proposed method. Compared with the traditional fixed-boundary microgrids, this approach determines the most resilient network configuration to supply high-priority critical loads. The developed method can be employed in power system planning, operation, and in decision-making to enhance operational resiliency and invest in system upgrades appropriately.

Robust expansion planning and hardening strategy of meshed multi-energy distribution networks for resilience enhancement

As an integrated energy system (IES) incorporated with electricity, gas, and heating networks, the meshed multi-energy distribution network (MMDN) can operate more economically with higher energy utilization efficiency. To achieve high reliability, MMDN is mesh-constructed and radial-operated, complicating the operation scenarios involved in planning. Existing coordinated planning models seldom consider network reconfiguration and hardening strategies collaboratively for extreme contingencies, which is not consistent with actual conditions. In addition, considering the multi-stage discreteness of the expansion planning problem, the existing robust algorithms may feedback invalid cuts. The neglect of such issues could introduce a momentous impact on the optimality of solutions. This paper presents a resiliency-oriented expansion planning and hardening model of MMDN, and the hierarchical algorithm is developed. Specifically, the model is formulated as a min–max-min optimization problem including investment and operation levels. The investment level problem optimizes planning schemes constrained with the robustness verification of the operation level problem. The operation level problem examines the operational cost and feeds back iterative information to the investment level. An additional cuts generation method is developed in the operation level, which effectively deals with invalid feedback information caused by the existing binary variable parameterization. Furthermore, accelerating strategies including infeasible region reduction, multi-cut feedback, and contingency set reduction are developed. To be solved by off-the-shelf solvers, the model is cast as a mixed-integer linear programming (MILP) problem by adopting the convex hull relaxation and linearization techniques for operational constraints with high accuracy. Numerical test results justify the effectiveness of the model.

A distributed robust control strategy for electric vehicles to enhance resilience in urban energy systems

Resilient operation of multi-energy microgrid is a critical concept for decarbonization in modern power system. Its goal is to mitigate the low probability and high damaging impacts of electricity interruptions. Electrical vehicles, as a key flexibility provider, can react to unserved demand and autonomously schedule their operation in order to provide resilience. This paper presents a distributed control strategy for a population of electrical vehicles to enhance resilience of an urban energy system experiencing extreme contingency. Specifically, an iterative algorithm is developed to coordinate the charging/discharging schedules of heterogeneous electrical vehicles aiming at reducing the essential load shedding while considering the local constraints and multi-energy microgrid interconnection capacities. Additionally, the gap between electrical vehicle energy and the required energy level at the departure time is also minimised. The effectiveness of the introduced distributed coordinated approach on energy arbitrage and congestion management is tested and demonstrated by a series of case studies.

Application Contingency Analysis of An IEEE 30 Bus System Using Matpower

Contingency analysis is significant for power system protection. Internal component failures or external causes such as lightning and equipment overloading can cause power system contingencies. Multiple outage scenarios are crucial in today’s deregulated environment for how the transmission network is utilized. The transmission network is highly pressured due to rising load demand and the need to operate economically. Contingency analysis of an IEEE 30 bus system is provided in this article utilizing Matpower software, which includes the source code. Matlab and Matpower are suggested as platforms for developing a screening process that examines the severity of each non-islanding branch failure, generator outage, MVA, and voltage violation. The generator terminal voltages will be permitted to fluctuate between 0.95 per unit and 1.05 per unit, as specified in the case30 input data files, to provide a broader practical zone. The performance index (PI) value is generated by adding the apparent performance index and the voltage performance index to rank the severity of the line outages. For the extreme contingency situation, a load flow study was performed. The four most severe scenarios, generator 22, generator 13, generator outage 23, and branch outage 28, should be constantly monitored to keep the electrical system safe.

Enhancement of power system stability by real-time prediction of instability and early activation of steam turbine fast valving

Turbine FV is a stability enhancing method used to protect power systems against the loss of synchronism after extreme contingency events. To avoid unnecessary FV action, smart control schemes can be used based on real-time instability prediction. In the last decade, several WAMS-based instability prediction methods have been developed. However, relatively long cumulative latencies in WAMS structures significantly reduce the efficiency of preventive control and can lead to system instability. To speed up FV action initiated by smart control scheme, this paper proposes a fast stability prediction method which is completely different from other methods described in literature. It uses only local measurements and requires performing simple mathematical operations. The basis of this method is the prediction of the magnitude of the power-angle characteristic. Just after the fault clearance, the method allows to predict transient instability and initiate MFV action. Based on the magnitude of the power-angle characteristic, the steady-state stability margin can be also computed. When this margin is too small, SFV action can be initiated as well. The validity of the proposed method has been verified by simulations performed for a large-scale real power system and detailed models of power system elements.

Impact of COVID-19 Pandemic on the Workload of Diagnostic Radiology: A 2-Year Observational Study in a Tertiary Referral Hospital

Rationale and ObjectivesTo evaluate the impact of COVID-19 pandemic on diagnostic imaging workload in a tertiary referral hospital.Materials and MethodsRadiological examinations performed in pre-pandemic period (2015-2019) and in pandemic period (2020-2021) were retrospectively included. Based on epidemiological data and restriction measures, four pandemic waves were identified. For each of them, the relative change (RC) in workload was calculated and compared to the 5-year averaged workload in the corresponding pre-COVID-19 periods. Workload variations were also assessed according to technique (radiographs, CT, MRI, ultrasounds), body district (chest, abdomen, breast, musculoskeletal, head/neck, brain/spine, cardiovascular) and care setting (inpatient, outpatient, emergency imaging, pre-admission imaging).ResultsA total of 1384380 examinations were included. In 2020 imaging workload decreased (RC = -11%) compared to the average of the previous 5 years, while in 2021 only a minimal variation (RC = +1%) was observed. During first wave, workload was reduced for all modalities, body regions and types of care setting (RC from -86% to -10%), except for CT (RC = +3%). In subsequent waves, workload increased only for CT (mean RC = +18%) and, regarding body districts, for breast (mean RC = +23%) and cardiovascular imaging (mean RC = +23%). For all other categories, a workload comparable to pre-pandemic period was almost only restored in the fourth wave. In all pandemics periods workload decrease was mainly due to reduced outpatient activity (p < 0.001), while inpatient and emergency imaging was increased (p < 0.001).ConclusionEvaluating imaging workload changes throughout COVID-19 pandemic helps to understand the response dynamics of radiological services and to improve institutional preparedness to face extreme contingency.

Cost optimized dynamic design of offshore windfarm transformers with reliability and contingency considerations

Application of Dynamic Thermal Rating (DTR) for optimal utilization of transmission system components has been addressed in several studies. For Offshore Wind Power Plants (OWPPs), as a consequence of intermittent wind generation combined with favorable ambient conditions and strict design regulations, power transformers tend to be over-dimensioned. Experience suggests that transformer lifetime exceeds OWPP life by tens of years, therefore DTR-based design of transformers can facilitate the optimization of OWPP export system.The recent breakthroughs in Thermo-Electric Equivalent (TEE) and aging models of transformers suggest that temporal development of Top Oil (TOT) and Hot Spot (HST) temperatures can be calculated efficiently, which when combined with monitoring of moisture and oxygen development can accurately assess the transformer health in real-time using Degree of Polymerization (DP). However, the economic optimization of offshore windfarms substations using DTR and lifetime utilization-based transformer design during the OWPP planning phase has not been addressed in the literature due to the risks involved in balancing system reliability with windfarm profitability.In this paper, a novel strategy is proposed to utilize transformer DTR for cost optimization of offshore windfarm export system during the OWPP design phase. Net Present Value (NPV) is used to assess capital cost impacts on transformer and Offshore Substation (OSS) design in year 0, and yearly revenue change over OWPP lifetime due to increased losses and possible energy curtailment as a result of smaller transformers. The unique methodology accounts for varying wind turbine availability due to maintenance and concurs with OWPP design requirements for OSS transformer contingency by generating statistical scenarios based on distribution functions and Markov Chain (MC) models respectively. Moreover, the MC models are adapted for two fundamentally different design concepts for OWPP export system: n-1 contingent and non-contingent. The implementation of probabilistic Markov models to resolve stressful periods with increased transformer loading is distinctive to this paper and offers a unique perspective for DTR-based transformer size optimization. Long-term site assessment data for OWPP including historical wind generation is used for transformer thermal assessment (TOT and HST) based on its load and ambient conditions. Furthermore, the reliability of design for varying construction and commissioning conditions is ensured by tracking DP-based transformer lifetime utilization over the operation period for three scenarios of oxygen and moisture development: conservative, high moisture and high oxygen.The case study of a 1200 MW offshore windfarm with two 220 kV parallel export circuits and four 33/220 kV transformers in one OSS located 50 km off the east coast of UK has been used in this paper. 100 scenarios of wind turbine availability and transformer contingency are simulated and the optimal design is determined for the worst-case scenario. Results indicate that optimal transformer rating can result in high thermal stress and minor energy curtailment during extreme contingency conditions for both the design concepts. However, the reduction in transformer size to align with OWPP operation life can reliably improve the windfarm business case significantly, particularly for the n-1 contingent design concept with conservative transformer moisture and oxygen conditions.

Contingency Analysis for Improved Power System Stability of the Nigerian 330 Kv, 48-Bus System Using Series Facts Controllers

Stressed electric network, either due to increased loading because of system expansion, or due to extreme contingencies, often result in voltage limit violations and overload of lines leading to extreme degradation in the system's damping torque, metamorphosing into fluctuations and dynamic instability. This paper employs two series, Flexible Alternating Current Transmission System (FACTS) controllers; Static Synchronous Series Compensator (SSSC) and Thyristor Controlled Series Compensator (TCSC) to investigate the system responses during line outages for dynamic stability improvement. The commercial version of Power System Analysis Toolbox (PSAT) was used to model the Nigerian 48-bus power system network with simulation of line outages using breakers on lines 6-8 (Oshogbo to Ikeja West transmission stations) and 35-39 (Ugwuaji to Ikot Ekpene transmission stations). SSSC and TCSC were optimally positioned on line 21-28 close to bus 21 (Jos transmission station) after continuation power flow simulation and computation of voltage sensitivity factor of all the load buses. Results show that the two FACTS controllers effectively improved the system stability by reducing the rotor angle and speed and by damping the post outage oscillations of the bus voltage and real power.

Real-Time Federated Cyber-Transmission-Distribution Testbed Architecture for the Resiliency Analysis

With the ongoing automation driven by the push toward the smart electric grid and the advancement in associated cyber infrastructure, the interaction between physical [electric transmission and distribution (T&D) systems], cyber (communication, automation, and control), and human (grid operators and decision-makers) is increasingly becoming more complex. This creates the requirement of analyzing the effect of the transmission system on the distribution system and vice versa with consideration of the additional complexity of the cyber infrastructure. Such an integrated testbed will also help with resiliency analysis, where resiliency refers to the ability of the system to continue serving energy to the critical loads even with limited extreme contingencies. Interaction of the physical power grid with the cyber layer can be effectively modeled using real-time (RT) simulator for developing and validating various operational and control algorithms. Testbeds using RT simulators with multiple capabilities have been developed at different institutions. Still, no single existing testbed can offer full scalability while simultaneously meeting high fidelity requirements for resiliency experimentation. Co-simulating federated testbed assets can provide a scalable experimentation platform that can be leveraged for verification and validation. In this article, an architecture is developed for federated cyber-physical testbed. A local federation with two real-time simulators is developed: real-time digital simulator (RTDS) and OPAL-RT have been interfaced using VILLAS framework for end-to-end testing. Also, a real-time linear predictor is developed and integrated here to address the communication latency impact on geographically allocated federated RT simulation. Finally, resiliency analysis tools are formulated and utilized for T&D systems. As an illustrative use case, the resiliency of a T&D test system is simulated, and the results are analyzed. A 179-bus Western Electricity Coordinating Council (WECC) transmission system is developed using OPAL-RT/ HYPERSIM, and a modified IEEE 13 node feeder system is modeled in RTDS/RSCAD and interfaced for resiliency analysis.

Improvement of Power System Transient Stability in the Event of Multi-Phase Faults and Circuit Breaker Failures

This paper proposes a new event-based logic of the breaker failure protection to improve power system transient stability by shortening the total clearing time of extreme contingencies involving circuit breaker failures. Stability improvement is achieved by using dual-timer protection with shorter safety margin for multi-phase faults close to the power plant and longer safety margin for remote multi-phase faults and single-phase faults. The use of the output signal obtained for a short safety margin depends on how the substation is configured. In the breaker-and-a-half configuration, this output signal is used to speed up the opening of all adjacent circuit breakers. In substations with the dual (or triple) busbar configuration the output signal obtained with a short safety margin is used to speed up the busbar splitting. A case study was performed for a real large-scale power system. Simulation results confirmed the advantages of the proposed logic. Breaker failure protection with the proposed logic has been implemented in a substation of a large power plant.

Measuring and Enhancing Microgrid Resiliency Against Cyber Threats

Recent cyber attacks on the power grid have been of increasing complexity and sophistication. In order to understand the impact of cyber-attacks on the power system resiliency, it is important to consider an holistic cyber-physical system specially with increasing industrial automation. In this study, device-level resilience properties of the various controllers and their impact on the microgrid resiliency is studied. In addition, a cyber-physical resiliency metric considering vulnerabilities, system model, and device-level properties is proposed. Resiliency is defined as the system ability to provide energy to critical loads even in extreme contingencies and depends on system ability to withstand, predict, and recover. A use case is presented inspired by the recent Ukraine cyber-attack. A use case has been presented to demonstrate application of the developed cyber-physical resiliency metric to enhance situational awareness of the operator, and enable better proactive or remedial control actions to improve resiliency.

Analysis of the stability and dynamic responses of converter‐based generation in case of system splits

Here, a methodical research on the stability and response of a converter-based generation model is conducted, focusing on the accompanying effects of a system split scenario in the (European) electricity transmission network. The purpose of this research is to analyse the behaviour of a generic but detailed model in case of extreme contingencies. In particular the focus is set on reduced short-circuit power, high rates of change of frequency, and sudden voltage phase jumps at the point of common coupling (PCC). These effects are analysed by means of electromagnetic transient (EMT) simulations. In addition, the stability of the converter control is evaluated qualitatively. For the investigations carried out here, a single reduced wind turbine model with full-size converter is used focusing on the interactions with the accompanying phenomena of a system split. The results show that reduced short-circuit power as well as a large and sudden voltage phase jumps can be potential reasons for the converter system to disconnect from the grid.

A Subtransmission Metropolitan Power Grid: Using High-Voltage dc for Enhancement and Modernization

Power demand in large or mega cities is increasing as the world urbanizes. Metropolitan transmission networks are continuously upgrading to meet the growing demand for power and to replace old-style generation with local renewable generation and power supplies from cleaner external sources. The cost of land and its scarcity make it difficult to secure new rights-of-way for traditional-style transmission lines. As power transmission levels increase, the risk of exceeding the short circuit capability of existing switchgear equipment as well as other network components becomes a major constraint to the expansion of power networks. Strategies to enhance urban transmission networks must address all of these issues and prioritize solutions that may be easily located within urban boundaries and have short lead times from decision to commercial operation. Advanced transmission technologies can effectively increase the capacity, efficiency, and utilization of existing infrastructure and improve grid resiliency under extreme contingencies or catastrophic events. These technologies have been considered and implemented by electric utilities for urban grid enhancement and modernization.

Fuel cost minimisation and line loadability enhancement using the PMBCA evolutionary technique

In the present situation of the power system, congestion due to contingency is one of the basic issues, which should be settled for better economic/monetary and network proficiency. Hassled power system, due to extreme line contingencies, regularly prompts circumstance where the system never again stays in the protected working region. Under these circumstances, it is an essential goal of the system operator to watch control activity to bring the electricity system into the invulnerable region. The objective function of this paper is fuel cost minimisation by an optimal power flow technique using honey bee colony–particle swarm optimisation-based hybrid evolutionary optimisation algorithm (Particle Movement Bee Colony Algorithm (PMBCA)) and to enhance the line loadability during network contingencies. The proposed approach to relieve transmission line from congestion and for fuel cost minimisation is tried on the IEEE 30-bus standard test system using MATLAB.

A Novel Metric to Quantify and Enable Resilient Distribution System Using Graph Theory and Choquet Integral

Network operators and utilities are challenged with increasing extreme weather conditions, resulting in interrupted power supply to critical loads. Resiliency metrics, which can capture the level of preparedness to resist adverse impact of extreme conditions on a distribution system, can be leveraged in multiple ways to provide better operation of the network and design of the future systems. In this paper, a methodology to quantify resiliency and maintain power supply to critical loads (CLs) during extreme contingencies has been proposed. Resiliency evaluation of power distribution system has been defined as a multi-criteria decision making problem and quantified using graph theoretic approach and Choquet integral. The algorithm proposed in this paper to calculate the resiliency for all feasible network configurations supplying CLs in a network is useful in planning as well as operation of the distribution network. The application of the proposed algorithm is demonstrated through several case studies using two proximal CERTS microgrids and IEEE 123 node distribution system. Simulation studies are also provided for planning of resilient network, by placing additional switches in the considered distribution systems with microgrid.

Design and Deployment of Special Protection System for Kinmen Power System in Taiwan

If an extreme contingency strikes the Kinmen Island power system in Taiwan, it is very likely to cause blackouts. Although an underfrequency protection scheme has been applied, some drawbacks of the protection scheme hinder the Kinmen power system from avoiding blackouts. In the past decade, several blackouts have occurred in the Kinmen power system. To avoid further possible blackouts, a special protection system (SPS) is summoned. This paper describes the design guidelines and deployment procedures of an SPS for the Kinmen power system. The design depends on the Kinmen power system characteristics, and the deployment hinges upon the supervisory control and data acquisition system. Exquisite software was also developed to testify the Kinmen system SPS. Operational records have demonstrated that the proposed SPS can indeed prevent blackouts from occurring in the Kinmen power system. The design, implementation, and operation of the SPS dealt with in this paper might foster further deployments of other SPSs.