Line Contingencies Research Articles

Distributionally robust sequential load restoration of distribution system considering random contingencies

AbstractNatural disasters would destroy power grids and lead to blackouts. To enhance resilience of distribution systems, the sequential load restoration strategy can be adopted to restore outage portions using a sequence of control actions, such as switch on/off, load pickup, distributed energy resource dispatch etc. However, the traditional strategy may be unable to restore the distribution system in extreme weather events due to random sequential contingencies during the restoration process. To address this issue, this paper proposes a distributionally robust sequential load restoration strategy to determine restoration actions. Firstly, a novel multi‐time period and multi‐zone contingency occurrence uncertainty set is constructed to model spatial and temporal nature of sequential line contingencies caused by natural disasters. Then, a distributionally robust load restoration model considering uncertain line contingency probability distribution is formulated to maximize the expected restored load amount with respect to the worst‐case line contingency probability distribution. Case studies were carried out on the modified IEEE 123‐node system. Simulation results show that the proposed distributionally robust sequential load restoration strategy can produce a more resilient load restoration strategy against random sequential contingencies. Moreover, as compared with the conventional robust restoration strategy, the proposed strategy yields a less conservative restoration solution.

Analysis of contingency scenarios towards a suitable transmission pathway in the southern interconnected grid (SIG) of Cameroon

In order to improve on system reliability, utilities conduct contingency scenarios to determine the impact of component outages to the network. It is usually challenging to conduct this analysis due to the complex grid structure. However, the huge consequences of an equipment outage on the grid and the connected loads are compelling enough for researchers to find ways to mitigate these events. Several blackouts have occurred in various parts of the Southern Interconnected Grid (SIG) of Cameroon due to the transient state of the power system. The most recent was the blackout that kept a greater part of the country's political capital, Yaounde, for several days with no electricity. The paper analyses 44 single line (N-1) contingency scenarios in the SIG using ETAP in order to evaluate the impact of line outage events on grid stability. A combined performance index was calculated from the bus voltage security index (PIV/Vsp), real power flow index (PIΔP), reactive power flow index (PIΔQ) and the branch overloading security index (PIS/Ssp). The results showed that the Mangombe - Oyomabang line outage (scenario 22) was the most severe with the load flow failing to converge, followed by the Oyomabang - Ngousso line 1 (scenario 36) with combined performance indices of 992.6191 and 79.74415 respectively. Other scenarios showed overloaded lines, overloaded transformers, high power losses and under/over bus voltages. This study has identified lines with high vulnerability and should be used by utility operators to enhance the reliability of the transmission network and minimize its susceptibility to cascaded power outages.

A proposed PMU-based voltage stability and critical bus detection method using artificial neural network

Voltage stability detection is currently still becoming the main issue in the modern integrated renewable energy power systems. To assess the voltage stability, the classical methods based on continuation power flow (CPF) technique were used to show nose curve. However, the classical methods require complete model of power system and long computation time. Data driven analysis and synchronized real time measurement technologies currently are developing in power systems monitoring, including the stability detection. The detection method is built based on the historical event model and uses the real time measurement as an input. For that reason, the algorithm to detect the voltage instability and critical bus is proposed using the artificial neural network (ANN) technique to represent the historical event model using the PMU measurement data. The ANN model architecture for this application is developed by creating seven hidden layers consisting of one normalization, four rectifier linear unit, one softmax and one sigmoid layer. To warrant the accuracy, the k-fold cross-validation is used. The algorithm is simulated on modified IEEE 14 test system which consider different loading scenario, line contingency, number of PMU and Photovoltaic (PV) integration. To mimic the actual historical data, the synthetic data is generated and labelled. The result shows that the proposed method can represent the complete power system model by giving high accuracy which for voltage stability detection is > 97% and critical buses detection is > 96% for all scenarios. Moreover, the required computation time is between 16 and 18 s per detection which makes the scalability to the real time detection is reasonable.

A Two-Step Approach to Wasserstein Distributionally Robust Chance- and Security-Constrained Dispatch

This paper considers a security constrained dispatch problem involving generation and line contingencies in the presence of the renewable generation. The uncertainty due to renewables is modeled using joint chance-constraint and the mismatch caused by contingencies and renewables are handled using reserves. We consider a distributionally robust approach to solve the chance-constrained program. We assume that samples of the uncertainty are available. Using them, we construct a set of distributions, termed ambiguity set, containing all distributions that are close to the empirical distribution under the Wasserstein metric. The chance constraint is imposed for all distributions in the ambiguity set to form the distributionally robust optimization problem. This problem is nonconvex and computationally heavy to solve exactly. We adopt a two-step approach to find an approximate solution. In the first step, we construct a polyhedral set in the space of uncertainty that contains enough mass under all distributions in the ambiguity set. This set is constructed by solving several two-dimensional distributionally robust problems. In the second step, we solve a linear robust optimization problem where the uncertain constraint is imposed for all uncertainty values lying in the polyhedral set. We demonstrate the scalability and robustness of our method using numerical experiments.

Enhanced Combined Protection Scheme Using Dual Setting Directional Overcurrent and Distance Relays Considering Severe Line Contingencies

An electrical power system must have an optimal protection system for enhanced reliability. Numerous studies have attempted to address issues with protection schemes, such as coordination constraint violations due to change in network configuration. Furthermore, in a few articles, the N-1 line contingency is considered without considering the line contingency selection criterion, resulting in a large number of constraints to deal with in the optimization problem. This paper proposes a new relay coordination scheme for dual setting directional overcurrent relay (DOCR) and distance relay based on the severity index of line outages, which is calculated by using an active and reactive performance index. The proposed protection strategy is improved with the help of an intelligent selection of the relay characteristics coefficient for dual setting DOCR along with the other relay settings. Another significant contribution is to provide a comparative analysis of standard and user-defined relay characteristics using genetic algorithm and grey wolf optimization algorithms. The modified IEEE-14 bus test system and IEEE-39 bus test system is used to validate the efficacy of the proposed protection scheme.

Applied Power Wheeling Concept Considering Site-Specific and Variability of VRE under Contingency Events

Unlike non-variable renewable energy (VRE) generators, VRE generators have site-specific, various, and uncertain characteristics that require different approaches for power wheeling cost calculations. The conventional power wheeling method cannot be used to accommodate the variability and site specificity of VRE. This study aims to develop a power wheeling calculation method that accommodates the variability and site specificity of VRE by comparing the postage stamp and MW mile methods in a multi-period optimal power flow simulation. The impact of line contingencies on the total power wheeling cost is also assessed. The simulation uses a modified IEEE 39 bus system as the test network, with three study cases centered on assessing the impacts of variability, site specificity, and line contingency. Based on the simulation, it is understood that the pivotal power wheeling cost is the line used by power wheeling actors, as there are cheaper and more expensive lines. Therefore, the total power wheeling cost is less affected by variability and more affected by the site specificity of VRE. On the other hand, because of the meshed network being employed as the test case, line contingency results in a lower cost for power wheeling actors to use fewer and cheaper lines.

A study on the effect of the stability of the electric power system on the internal and external contingency of the combined energy hub

In this study, the combined energy hub on the gas pressure converter station in South Korea was configured in a detailed model form rather than a conventional model form. In addition to power sources such as fuel cells and TEG, energy storage systems (ESS), photovoltaics (PV), combined heat and power systems (CHP), and transformers and buses are configured, and loads are configured to observe various problems that can occur in each change. And the contingencies that can occur inside and outside the combined energy hub are simulated for the Turbo Expander Generator (TEG) to operate effectively in connection with the power system.Four fault locations of connected load and power generation facilities inside the combined energy hub are selected, and we observe the voltage change of each bus as the circuit breaker operates. In addition, as an external change, the increase in renewable energy and the voltage change due to transmission line contingencies were also observed. The analysis intends to establish a system analysis environment to check whether a problem occurs in terms of voltage and reactive power during the operation of future combined energy hubs.

A streamlined and enhanced iterative method for analysing power system available transfer capability and security

Available Transfer Capability (ATC) is a crucial indicator for ensuring the reliable and efficient electric grid as it helps to manage congestion and ensure that the power flows are kept within safe operating limits. This article presents a novel approach named as modified repeated alternating current power flow (MRACPF) with step-size control mechanism and compares it with the existing techniques like direct current power transfer distribution factors (DCPTDF) and repeated alternating current power flow (RACPF) for analyzing the ATC of the power system during bilateral wheeling transactions in MATLAB environment. The study further assesses the ATC in different multilateral wheeling transactions of two standard test systems (IEEE 30-bus and IEEE 118-bus) and evaluates inter-area ATCs for the IEEE 118-bus system under various operational scenarios. The Composite Security Index (CSI) which is based on both line flow limit and bus voltage limit violations is used to identify and rank the most severe (k-1) line contingencies, and is able to differentiate between secure and insecure state of the power system and furthermore, MRACPF with step-size control mechanism is used to assess ATC during the severe (k-1) line outage and generator outage scenarios for various power transactions. The MRACPF method produces comparable results to the RACPF method for obtaining the Available Transfer Capability (ATC), but it takes less time. This time savings can be significant, especially for larger power systems, where the execution time can be reduced by more than 50%.

Reliability enhancement of radial distribution system by placing the reactive power compensators and distribution systems

Distribution systems (DSs) are highly stressed with addition of newer loads like electric vehicle charging stations and lower scope for expansion due to urbanization. Any line outage could cause interruption to major loads. Reliability studies have gained importance for lowering the frequency and lowering the duration of interruption for supply systems. In this paper a bi-stage method for optimum placement of reactive power compensation devices and distributed generations (DGs) for enhancing voltage stability and system reliability. A new method named delta analysis method is used to optimally locate the reactive power compensation devices and DGs. IEEE-33 radial DS, which is taken as experimental system. Based on the study, the fixing of reactive power compensation devices and DGs are to increase voltage outline of buses and decrease power fatalities. After the placement of DGs, the enhancement in reliability indices following line contingency is studied using MATLAB simulation.

Leveraging Power Grid Topology in Machine Learning Assisted Optimal Power Flow

Machine learning assisted optimal power flow (OPF) aims to reduce the computational complexity of these non-linear and non-convex constrained optimization problems by consigning expensive (online) optimization to offline training. The majority of work in this area typically employs fully connected neural networks (FCNN). However, recently convolutional (CNN) and graph (GNN) neural networks have also been investigated, in effort to exploit topological information within the power grid. Although promising results have been obtained, there lacks a systematic comparison between these architectures throughout literature. Accordingly, we introduce a concise framework for generalizing methods for machine learning assisted OPF and assess the performance of a variety of FCNN, CNN and GNN models for two fundamental approaches in this domain: regression (predicting optimal generator set-points) and classification (predicting the active set of constraints). For several synthetic power grids with interconnected utilities, we show that locality properties between feature and target variables are scarce and subsequently demonstrate marginal utility of applying CNN and GNN architectures compared to FCNN for a fixed grid topology. However, with variable topology (for instance, modeling transmission line contingency), GNN models are able to straightforwardly take the change of topological information into account and outperform both FCNN and CNN models.

Optimal Integration of Hybrid Energy Systems: A Security-Constrained Network Topology Reconfiguration

The integration of distributed energy resources, such as wind farms (WFs) and energy storage systems (ESSs), into distribution networks can lower the economic cost of power generation. However, it is essential to consider operational constraints, including loading margin, which ensures the security line contingency. This study aims to develop a comprehensive hourly distribution network reconfiguration (HDNR) model to minimize the economic cost for the power generation company. The model considers the optimal allocation of WFs and ESSs in terms of capacity and location, as well as the hourly status of the distribution network switches, based on security constraints. The proposed model is applied to an IEEE 33-bus distribution test system, and the capacities and locations of WFs and ESSs are determined. The impacts of security constraints on the optimal capacities and locations of WFs and ESSs, and the hourly configuration of the distribution network, are analyzed based on two case studies. In Case Study I, the model is solved with HDNR conditions, while Case Study II is solved without these restrictions, for comparison purposes. The results show that the optimal allocation of WFs and ESSs is affected by security constraints when HDNR is considered and highlight the crucial role of security constraints under contingency conditions, such as line outages. In the test system, three WFs and two ESSs are optimally allocated, with changes in capacity and location as the loading margin varies.

Analysis of a new voltage stability pointer for line contingency ranking in a power network

Improper management of reactive power in a power network could lead to voltage instability. This paper presents a well-detailed study on voltage instability due to violation of power equilibrium in a power network and introduces a new voltage stability pointer (NVSP). The proposed NVSP is developed from a reduced 2-bus interconnected network to predict the sensistivity of voltage stability to reactive power variation. The simulation results from MATLAB were evaluated on IEEE 14-bus test system. The contingency ranking was achieved by varying the reactive power on the load buses to its maximum loading limit. The maximum reactive power point was taken at each load bus and the critical lines were ranked according to their vulnerability to voltage collapse. The results were compared with other notable voltage stability indices. The results prove that the NVSP is an essential tool in predicting voltage collapse.

Risk-Based Contingency Screening Method Considering Cyber-Attacks on Substations

This letter proposes a probabilistic bi-level optimization model for system operators to identify important line contingencies that are initiated from substations. In contrast to classical contingency screening methods, the proposed model investigates the impacts of cyber intrusions against substations, which are rather different from the scenario where lines are on outages caused by natural disasters or physical attacks. Specifically, we consider the cyber-attacks that, to maximize the expected load loss, start from the substation cyberspace and then affect transmission lines to attain the desired outcomes. In addition, the correlation between substations and associated lines that are subjected to cyber-attack is also modeled to simulate the behavior of an intelligent attacker. The numerical results on THE standard IEEE RTS 24-bus and IEEE 118-bus systems validate the effectiveness of the proposed model.

Battery energy storage train routing and security constrained unit commitment under solar uncertainty

The location of solar parks far from load areas may lead to transmission congestion and thus solar curtailment for secure system operation. Battery energy storage (BES) Train as mobile storage can transmit solar energy from site to load centers using a transport network while relieving lines from congestion. Therefore, stochastic security-constrained unit commitment (SCUC) with BES Train is modeled in this paper under solar uncertainty and N-1 critical line contingency. Time-space network models transportation constraints of BES Train routing problem. To reduce the computational burden, this paper proposes modeling of line outage distribution factor (LODF) for DC power flow that reduces the nonzero coefficients of post-contingency network constraints. The benders decomposition technique simplifies stochastic optimization for the mixed-integer linear programming (MILP) problem. BES Train integrated modified IEEE reliability test and 118-bus system is investigated as a case study. Simulations evaluate the effect of BES Train, solar uncertainty and critical line contingency state with LODF on operation costs, BES Train scheduling, solar curtailment, congestion and computational time. The proposed model shows that BES Train is an economical option for large solar integration to reduce transmission congestion and curtail solar power effectively.

Performance and security enhancement using generalized optimal unified power flow controller under contingency conditions and renewable energy penetrations

In this paper, a novel flexible AC transmission system (FACTS) device named generalized optimal unified power flow controller (GOUPFC) is introduced to control the power flows in multi transmission lines and to regulate the voltages and angles at the load buses. The detailed power injection modeling of GOUPFC is presented in this paper. The optimal location of GOUPFC is determined based on line collapse proximity indicator (LCPI). A multi-objective function is framed in terms of average voltage deviation (AVDI), real power loss (Ploss) and average line collapse proximity indicator (LCPIavg) to test the effectiveness of the proposed device. The simulation studies are performed on standard IEEE 57-bus test system under single line contingency and considering various renewable energy source (RES) penetrations. The control parameters of GOUPFC are optimized by using whale optimization (WO-BAT) algorithm, by hybridizing WOA and BAT algorithms, and the superiority of WO-BAT is observed in minimizing the proposed objective function and enhancing the voltage profile.

Sudan National Grid Contingency Ranking Through Fuzzy Logic Approach

The effect of the line outage when rest of system is stable is called contingency study. The outage on the system may be for single line (N-1) contingency or for multiple lines (N-m) contingency where N the total number of lines and m the number of lines out the service. The study of contingency is essential process in planning, operating and control of power systems. The main thrust of contingency studies carried out in power system control centers is to determine the steady state effects of outages. Large power systems require the analysis of all the credible contingence within a very short time so as to exercise the control in the short time available for corrective action. Generally, the system continues to operate in the contingency condition for a considerable duration of time, on occurrence of a line outage. The altered voltage stability margins of all the load buses for the various contingency conditions are to be known prior to monitor and initiate emergency control action to avoid voltage collapse. This study suggests an intelligent technique using fuzzy logic control system to assist in ranking the single contingency (N-1) which occur in the system. The suggested fuzzy logic approach was taken into consideration and applied to the national Sudanese grid.

Load Shed Recovery With Transmission Switching and Intentional Islanding Methods After (N-2) Line Contingencies

Changing power system configuration may result in load shed recovery (LSR) because topology change can provide power flow control in meshed network. Some topologies may favor generation redispatch as compared to others and can eliminate line congestion which leads to LSR. One of the known methods for topology change is called transmission switching (TS) and research conducted in the past showed that TS is an effective means of mitigating load shedding. However, another method of topology control also exits and it is referred as intentional islanding (IIS). In this manuscript, we explore IIS as a potential solution for LSR. IIS based on generator coherency has been presented in literature for mitigating cascading failures. However, IIS has not been explored solely as a LSR mechanism. In this paper, we compare the LSR based on IIS with well known LSR algorithm based on TS. The comparison is performed for IEEE 39-bus system and IEEE 118-bus system. The results show that IIS has a potential to perform better than TS in terms of computational efficiency and LSR.

Power system analysis and integration of the proposed Nigerian 750-kV power line to the grid reliability

The present situation of power generation in Nigeria obviously represents a challenge to our ability for rethinking the delivery of energy at maximum efficiency. Previous research on the existing Nigerian 330-kV network grid, recommended that the network be transformed from radial to ring because of high losses inherent in it and the voltage insecurity. In this study, the existing 330-kV network was reconfigured based on the identified regions mapped out for upgrade to form a ringed 750-kV super grid. The bus voltages of some of the buses in the existing 330-kV were upgraded to 750-kV and new transmission lines added to create an integrated super grid with a ring structure as compared to the radial nature of the existing 330-kV grid. These proposed buses have been selected for upgrade based on the fact that they are positioned in critical areas within the topology of the grid that transforms the existing radial structure to a ring one. The method is also cheaper than making the entire network a 750-kV system. Load-flow analysis was carried out on the existing 330-kV Nigerian Grid and the proposed Nigerian 750-kV integrated into the existing grid using Newton–Raphson algorithm. The results analysis of the new network revealed a significant reduction of 30.2% power loss. This was validated using the code-based MATLAB and Power World Simulation model-based software. Contingency analysis was also carried out on both grids using the Power World Simulator. The study revealed that the 750-kV super grid was able to mitigate the losses experienced on the existing grid significantly with better voltage profiles in all the buses. It also revealed that the new network (330-kV and 750-kV integrated) performed better to the single line contingency analysis with less violations occurring and no unsolvable cases.

Individual Incremental loading factor based maximum loadability limit prediction using modern optimization tools

Infrastructure innovation in the power system industry encourages more partakers to participate in the electricity market which improvises the load utilization level. So, the maintenance of power system’s agility with respect to any dynamic update in terms of load level is necessary. Precise prediction of maximum allowable loading point helps to enhance the power system agility and also improvises the total allowable power transfer capability which in turn helps to supply continuous eminent power supply at the minimal cost to the customers by means of encouraging more contracts. Considering the above potential benefits, in this papear by using individual incremental loading factor (IILF) the precise prediction of total loadability limit (TLL) of the system is manipulated with the help of newly evolved meta-heuristic optimization algorithm such as Grey Wolf (GRW) optimizer and Flower Pollination Algorithm (FPA). The allowable single line contingency scenario is considering along with base case scenario to extract the more realistic TLL which helps to maintain the power system balance with respect to the dynamic nature of the load. The proposed maximum loading point extraction manipulation solution problem is tested with the help of three standard IEEE systems such as 30 Bus, 57 Bus and 118 bus systems. The extracted test results show that the predicted maximum allowable loading point enhances the load utilization level without affecting the system securities. The statistical performance measures of GRW and FPA confirmed the better balance of exploration and exploitation in extracting the optimal results.

Upshot of unified power flow controller on the minimization of the severity of overloading on electric power grid

With the objective of discovering the vital line in an electrical power grid (EPG) where the unified power flow controller (UPFC) could be inserted to minimize the severity of overloading (SOL) during contingencies, an investigation into the stiffness of single line contingencies (SLC) on the EPG is presented in this study. An overloading index (OVI) is developed and used in this contribution to arrange the transmission branches of the EPG based on their cruciality and criticality throughout SLC. A harmony search algorithm (HSA), an optimization method, has been adopted to optimally set the parameters of the UPFC to achieve the best minimum SOL of the system at the optimal location. The IEEE 30-bus network was used as the test bed. Results based on the test bed show that placing a UPFC based on the ranking of a well-known contingency severity index (CSI) minimized the SOL to 2.40×1010 as against the 8.05×105 obtained when the placement is done on the ranking based on the proposed OVI. The minimization of SOL is achieved with a reactance and reactive power of 0.036pu and 0.096pu, respectively, of the UPFC. The result also reveals that the proposed OVI identifies with a higher level of precision the vital line in the test bed for placing the UPFC for the purpose of minimizing the SOL during contingencies.