Severe Contingencies Research Articles

Flexible Ramping Services in Power Systems: Background, Challenges, and Procurement Methods

In recent years, the share of renewable energy sources (RES) in electricity generation portfolio has been growing, primarily supported by local and international energy policies to reduce the carbon footprint of the electricity sector. However, the large-scale integration of variable and uncertain RES has adverse effects on the power system operations. Therefore, the power system operators should apply advanced scheduling models and enhance the flexibility of the systems to compensate for the variability and uncertainty of RES. To deal with these challenges and covering the ramp scarcities and severe contingencies, not only more operational reserves are required but also the urgent actions is needed to develop the efficient markets, contracts, incentive mechanisms, effective scheduling models, and managerial approaches for encouraging suppliers to participate in flexibility procurement. This paper provides a background of the problem and a comprehensive review of the literature about flexible ramping markets and its procurement mechanisms.

Distributed FACTS for Power System Transient Stability Control

The high penetration of renewable energy sources, combined with a limited possibility to expand the transmission infrastructure, stretches the system stability in the case of faults. For this reason, operators are calling for additional control flexibility in the grid. In this paper, we propose the deployment of switchable reactors and capacitors distributed on the grid as a control resource for securing operations during severe contingencies and avoiding potential blackouts. According to the operating principles, the line reactance varies by switching on or off a certain number of distributed series reactors and capacitors and, therefore, the stabilizing control rule is based on a stepwise time-discrete control action. A control strategy, based on dynamic optimization, is proposed and tested on a realistic-sized transmission system.

Frequency Stability Assessment of Power System Using Frequency Stability Indices and Artificial Neural Newwork

Frequency stability assessment (FSA) is an evaluation of a dynamic behavior in power system under severe contingency. One of the significant approach in FSA is using frequency stability indices (FSI) to evaluate the performance of the power system and identify the stability status of the power system. Many methods for frequency stability assessment is Time Domain Simulation, Frequency Stability Index and Individual Generator’s Frequency referred to Slack Generator’s Frequency.

Screening model of incremental risk events for reliability analysis of transmission system

Screening of severe contingency event is vital in transmission system operation and planning. This study investigates the reliability issue of load loss with static analysis of contingency events of the transmission system. As not all high-risk events provide a more critical impact than its sub-events, the risk index is not always efficient for contingency screening. In this context, this paper proposes a contingency screening model with incremental risk for transmission system. Firstly, the significance of both the risk and the incremental risk of contingency events is analyzed. We show that the use of incremental risk can avoid the “risk masking effect”, which is defined as the situation that high-risk events with weak reference value may mask the events with valuable information about more critical cases. Secondly, the optimization objective of contingency screening is set to maximize the incremental risk of contingency event. The purpose is to directly identify the events that describe a more critical impact than its sub-events and to improve the screening efficiency over that by the risk assessment. Finally, a recursive screening algorithm is designed to identify the set of N − k events with high incremental risk. Case studies on the IEEE-RTS and a provincial grid in China were carried out to show that the events with incremental risk only account for a small proportion of the total high-risk events. Thus, identification of N − k events with more critical impact than sub-events is achieved for system risk analysis in that enumeration of all the high-risk events is not required.

Experiences on Remediation of Special Protection System for Kinmen Power System in Taiwan

The isolated power system in Kinmen Island, Taiwan has already implemented the special protection system (SPS) to prevent various critical contingencies that occurred during the past decade. However, when a severe N-2 contingency occurred on July 26, 2013, the SPS has not properly armed its under-frequency load shedding. After reexamining transient recorded waveforms, it was observed that the system still has sufficient spinning reserve and the coordination between the under-frequency relay and the current SPS was not well considered. In this article, the current SPS is further modified in order to avoid the occurrence of the excessive load shedding under similar N-2 contingencies. Extensive time-domain dynamical simulations are performed first to study the relationship between the amount of the load shedding and frequency deviations under these extreme N-2 contingencies. Based on these simulation results, the remediation scheme of the SPS will be proposed by modifying the existing SPS design logic and the amount of load shedding. More comprehensive input triggered states are proposed for identifying these severe N-2 contingencies. Both simulation verifications under the proposed remedial scheme and actual transient recorded waveforms under a severe contingency after commissioning this remediated SPS are provided to demonstrate the effectiveness of this proposed remediation scheme.

Optimal sizing of Battery Energy Storage Systems for dynamic frequency control in an islanded microgrid: A case study of Flinders Island, Australia

Challenging frequency control issues, such as the reliability and security of the power system, arise when increasing penetration levels of inverter-interfaced generation are imposed. As a result of the displacement of convention generation in favour of renewable energy sources, the reduction of frequency response capabilities can be seen. A promising method of overcoming the aforementioned challenges is to utilise Battery Energy Storage Systems (BESS), which provides frequency support by injecting instantaneous power to the grid and back up the conventional generation systems. However, large battery systems increase the cost while inadequate battery capacities result in poor performance. This paper, therefore, proposes an approach for finding the optimum BESS size for an islanded microgrid power system. The determination of the optimum BESS size is based on an existing case study, under which the most severe contingencies of generation loss and load loss have been accounted for, as well as different levels of penetration of renewable energy sources. As a result of using meta-heuristic optimization (Grey Wolf Optimization), the constraint optimization problem has been identified as BESS sizing. Through the use of real-time simulation DIgSILENT PowerFactory software, estimated BESS size can be applied to a standalone microgrid to test the frequency of support capabilities. The simulation has made it apparent that through the selection of the optimum BESS size, the system frequency response is not only mitigated, but improved.

On the Loadability and Voltage Stability of Islanded AC–DC Hybrid Microgrids During Contingencies

The future smart grid encompasses ac–dc clusters known as ac–dc microgrids. For reliability and security purposes, each microgrid hosts a mix of synchronous-based and converter-based distributed resources. However, synchronous-based generators, in particular, are characterized by their limited reactive power capabilities because of the limitation of their excitation systems, which could lead to voltage instability problems when the hosting microgrid is islanded. AC–DC microgrids also comprise controllable thermal and electrical loads. Most modern ac–dc loads have power electronics interfaces that control the voltage at their ends, exhibiting constant power characteristics. AC–DC microgrids with high penetrations of constant power ac–dc loads are vulnerable to voltage collapse. As with other power system architectures, microgrids are subject to contingencies, e.g., a line circuit outage, during which their loadability can be jeopardized. For this reason, the work reported in this paper involved the investigation of the steady-state voltage stability of islanded ac–dc hybrid microgrids (HMGs) during contingencies. Voltage stability analysis was carried out on a 12-bus ac–dc HMG. Several case studies were designed to reveal the likelihood of voltage instability/collapse in microgrids under severe contingencies. The voltage stability analysis also shows that interfacing ac and dc microgrids does not always enhance loadability. The analysis was carried with the goal of raising awareness among microgrid planners and operators of the possibility that a voltage collapse phenomenon can develop in islanded ac–dc microgrids under extreme events. Contingency analysis should therefore be incorporated into planning and operations criteria for islanded ac–dc microgrids.

Interdependent electric and water infrastructure modelling, optimisation and control

The phrase water-energy nexus is commonly used to describe the inherent and critical interdependencies between the electric power system (EPS) and the water distribution system (WDS). In this study, the analytical framework capturing the interactions between these two critical infrastructures is examined and a mathematical model to describe the associated dynamics is developed. Based on the time scale of these associated dynamics, the EPS simulation is conducted using time-series power flows following unit commitment and optimal power flow solutions. The WDS control optimisation–simulation model formulated here is solved using a genetic algorithm solution technique interfaced with EPANET. An integrated simulation engine of the interdependent infrastructure systems was created to conduct long-term simulations. The simulation engine was applied using representative WDS and EPS networks. The implemented control optimisation benefits both systems by reducing the effect of severe contingencies. The results of the simulations conducted prove the applicability of the proposed methodology for long-term, water-energy nexus contingency simulations having both power outages and droughts.

A Resilience-Based Architecture for Joint Distributed Energy Resources Allocation and Hourly Network Reconfiguration

As a result of the recent innovations in the deployment of plug-in electric vehicles (PEVs), this technology can play an important role as a distributed energy resource in supplying the system demand of the power systems of the future. This paper introduces a methodology for optimal coordinated allocation of wind farms (WFs), energy storage systems, and PEV's parking lots considering demand response programs and hourly distribution network reconfiguration in normal and severe contingency conditions. In the proposed methodology, the participation of different types of loads is also examined. The objective function is to minimize the total costs of purchased power from the upstream network and WFs, along with the costs of commercial/industrial loads flexibility and residential loads curtailment. To validate the performance of the proposed methodology, it is implemented on the well-known IEEE 33-bus distribution test system. The simulation results validate the feasibility and effectiveness of the proposed approach.

When to Island for Blackout Prevention

Cascading outages leading to large-area blackouts can be mitigated using intentional controlled islanding (ICI). An ICI scheme must define the most suitable time to split the system after a severe contingency (i.e., when to island), as well as to quickly determine the set of branches to be disconnected to create stable islands (i.e., where to island). Most of the works in the literature, however, focus on the latter and little has been done to address the “ when to island ” problem. To fill this gap, this paper proposes a unified methodology to determine the most suitable moment for islanding. First, the proposed methodology uses real-time information to estimate the rotor angles of generators, which, in turn, serve to define the suitable number of coherent generators and the actual coherent groups. It then adopts the concept of area-based center of inertia-referred rotor angle index to determine the actual time of islanding. The unified methodology for determining the number of islands to be created, the coherent generators, and the time for islanding is tested using the IEEE 39- and 118-bus test systems. Multiple case studies are presented to demonstrate the adaptability and effectiveness of the proposed unified methodology to different system conditions.

Transient stability‐oriented assessment and application of preventive control action for power system

The modern power system is becoming more complex and dynamic because of increasing penetration of renewable energy resources, operating closer to system capacity for economic benefits. In order to maintain the system stability, the system operator is required to initiate appropriate preventive control action under severe contingencies while satisfying the system operating constraints. Real time transient stability assessment (TSA) of power system is proposed in this paper by using wide area measurement system (WAMS) and phasor measurement units (PMUs). An architecture based on least square support vector machine (LS-SVM) has been developed to identify the system stability state in real time. Also, this paper proposes a coherency based selection method to identify the appropriate members for generation rescheduling as preventive operation in insecure operating contingency. Rotor angle trajectories based transient stability index (TSI) is employed to classify the generators as either critical or non-critical generators in a power network. Accordingly preventive control action in the form of generation rescheduling can be initiated to achieve stability. The method has been demonstrated on the IEEE 10-machines, 39-bus system. The proposed methodology is effective and capable of handling complex power system models with multiple contingencies.

Linear daily UC model to improve the transient stability of power system

As coherency of generators decreases, the risk of rotor angle instability increases, especially under severe contingencies. The slow coherency as a network characteristic may be controlled by the locations of committed generators. Unit commitment (UC) problem is conventionally carried out regarding operational and network constraints. In this study, a two-step strategy is developed to promote the slow coherency via the network constrained UC (NCUC) model on a daily horizon. First, conventional NCUC is executed. The most important generators with both economic and coherency merits are then determined as representative generators. In the second step, the Slow Coherency Based Unit Commitment (SCBUC) is re-optimisedaccording to the results obtained from the first step, using a multi-objective function. The first part of the multi-objective function is devoted to the cost of generation, start-up, and shutdown of generators. The goal of the second part of the multi-objective function is to maximise the coherency between the committed generators to reach a transient stability margin. The proposed model is converted to a mixed integer linear programming model. The performance of the proposed method of promoting transient stability is investigated using the dynamic IEEE 118-bus test system.

Optimal Cost of Voltage Security Control Using Voltage Dependent Load Models in Presence of Demand Response

This paper proposes a new framework for corrective voltage control (CVC) of power systems. It ensures a desired loading margin (LM) after encountering severe contingencies while minimizing the corresponding control costs. The framework is divided into primary CVC (PCVC) and secondary CVC (SCVC) stages for restoration of voltage stability and ensuring a desired LM. These stages are based on the sequence and quickness of the control actions required in post-contingency state of the system. The PCVC sub problem deals with the condition faced by a power system subject to voltage instability as the result of severe contingencies. Such control is merely devised to restore system stability. Next, in the SCVC sub problem that follows PCVC, the system operating point is modified such that a desired LM is ensured, and hence voltage security of the system is achieved. The active and reactive power redispatch of generation units and involuntary load curtailment are employed along with the voluntary demand-side participations as control facilities in PCVC and SCVC sub problems, by deploying a proper voltage dependent static model for loads. The proposed framework is examined on the IEEE 118-bus system. The numerical results substantiate the effectiveness of the proposed approach.

Real-Time Voltage Stability Assessment Method for the Korean Power System Based on Estimation of Thévenin Equivalent Impedance

This study aims to develop a real-time assessing methodology for a power systems voltage stability. The proposed algorithm is based on the Thévenin equivalent (TE) impedance estimation method, which applies the phasor measurement unit technology practically. To present an accurate analysis of the real-time situations of a power system, the developed voltage stability index can be used as useful information for system operators to establish appropriate countermeasures. Moreover, by considering the results of voltage stability margin calculation within the Korean power system, the effect of voltage stability on the dynamic behavior of the system is presented. Furthermore, to increase the accuracy, load model parameter estimation is introduced in this algorithm. The load model might be used for calculating the stability margin more accurately. The power-voltage curve is drawn in theory using the TE voltages and impedances. To validate the case study of the proposed method, simulations were executed using the Matlab software. The simulations demonstrated the effectiveness of the proposed method and detected voltage stability/instability under severe contingency scenarios.

Relaxations of AC Maximal Load Delivery for Severe Contingency Analysis

This work considers the task of finding an ac-feasible operating point of a severely damaged transmission network while ensuring that a maximal amount of active power loads can be delivered. This AC maximal load delivery (AC-MLD) task is a nonconvex nonlinear optimization problem that is incredibly challenging to solve on large-scale transmission system data sets. This work demonstrates that convex relaxations of the AC-MLD problem provide a reliable and scalable method for finding high-quality bounds on the amount of active power that can be delivered in the AC-MLD problem. To demonstrate their effectiveness, the solution methods proposed in this work are rigorously evaluated on 1000 $N$ - $k$ scenarios on seven power networks ranging in size from 70 to 6000 buses. The most effective relaxation of the AC-MLD problem converges in less than 20 seconds on commodity computing hardware for all 7000 of the scenarios considered.

Novel Transient Power Control Schemes for BTB VSCs to Improve Angle Stability

This paper proposes two novel power control strategies to improve the angle stability of generators using a Back-to-Back (BTB) system-based voltage source converter (VSC). The proposed power control strategies have two communication systems: a bus angle monitoring system and a special protection system (SPS), respectively. The first power control strategy can emulate the behaviour of the ac transmission to improve the angle stability while supporting the ac voltage at the primary level of the control structure. The second power control scheme uses an SPS signal to contribute stability to the power system under severe contingencies involving the other generators. The results for the proposed control scheme were validated using the PSS/E software package with a sub-module written in the Python language, and the simple assistant power control with two communication systems is shown to improve the angle stability. In conclusion, BTB VSCs can contribute their power control strategies to ac grid in addition to offering several existing advantages, which makes them applicable for use in the commensurate protection of large ac grid.

Centralized coordination of emergency control and protection system using online outage sensitivity index

This paper determines the best locations for centralized load shedding considering the protection systems that can be monitored from transmission system to avoid cascading failures. The vulnerability of power system integrity is determined and ranked using online outage sensitivity index (OSI) considering ampacity of transmission lines, over excitation of generators and transformers, under frequency and under voltage protection relays and low voltage ride through capability of grid connected renewable energy sources. The power system elements are ranked based on the results of outage sensitivity index (OSI) to concentrate the load shedding as control action on areas containing most critical elements prone to outage. The efficiency of proposed multi objective approach is validated through numerical simulations of severe contingencies carried out in DigSilent Power Factory software on 39 bus IEEE standard test system.

Multi-Phase Under Voltage Load Shedding Scheme for Preventing Delayed Voltage Recovery by Induction Motor Power Consumption Characteristics

This paper aims to develop a multi-phase under voltage load shedding (MUVLS) strategy that effectively sheds the load to mitigate delayed voltage recovery (DVR). The major cause of the DVR phenomenon is related to the dynamic characteristics of induction motor (IM) loads. The deaccelerating and stalling of the IM load during disturbances is the main driving force of short-term voltage instability, resulting in an amount of reactive power consumption and excessive current draw. With the economic efficiency of energy use, the proportion of IM loads is gradually increasing, and this trend might deteriorate system stability. This paper focuses on the impact of IM loads in the Korean power system and analyzes the parameter sensitivity of IM loads and the proportion of appropriate IM loads. The proposed procedure for under voltage load shedding (UVLS) applies voltage stability criteria to decide the most efficient load shedding scheme. The determined MUVLS scheme can offer new and more effective remedial actions to maintain voltage stability, considering the characteristics of IM loads. Case studies on the Korean power system have validated the performance of the proposed MUVLS scheme under severe contingency scenarios, showing that the proposed strategy effectively mitigates DVR.

Transient Stability Emergency Control Using Asynchronous Parallel Mixed-Integer Pattern Search

Special protection schemes (SPSs) are designed to monitor system conditions and apply control to avoid system collapse when subjected to a disturbance. Transient stability emergency control (TSEC) is a practical and effective SPS mechanism that utilizes generator tripping and load shedding as control actions to maintain system synchronism during severe contingencies, but faces significant challenges when it comes to a real-world grid of significant scale and complexity. In this paper, considering the characteristic of different control measures, TSEC problem is modeled as a mixed-integer nonlinear optimization problem with differential algebraic equation constraints. The proposed mixed-integer pattern search approach is able to obtain an optimal strategy without explicit derivative information of various complex component models. Furthermore, asynchronous parallelism is exploited and implemented to improve work-load balance and accelerate search procedure. Case studies on three different scale systems, including a real-world electric grid in China demonstrate that the proposed approach is promising to obtain TSEC strategy for large-scale power systems.

A New Formulation to Reduce the Number of Variables and Constraints to Expedite SCUC in Bulky Power Systems

This paper proposes a new formulation to speed up the security constraint unit commitment (SCUC) solution in large scale power systems. The number of variables and prevailing constraints grow drastically as the number of system buses and the credible contingencies increase. So, as the first stage, inessential variables and constraints are eliminated and necessary variables for a tighter formulation are defined. Some constraints and variables are removed and some constraints are modified based on the results of a small sub-problem at each iteration of Benders decomposition algorithm. By utilizing the proposed framework, especially in the case of large number of severe contingencies, a large number of variables and constraints will be eliminated. Up- and down-going reserves are considered at both generation- and demand-sides. The IEEE RTS1 and RTS2 systems are studied as illustrative examples. The SCUC results drawn using the proposed algorithm are compared with those obtained by a conventional MIP-based algorithm to show the effectiveness of the proposed algorithm.