Severe Contingencies Research Articles

Optimal Placement of Dynamic Var Sources by Using Empirical Controllability Covariance

In this paper, the empirical controllability covariance (ECC), which is calculated around the considered operating condition of a power system, is applied to quantify the degree of controllability of system voltages under specific dynamic var source locations. An optimal dynamic var source placement method addressing fault-induced delayed voltage recovery (FIDVR) issues is further formulated as an optimization problem that maximizes the determinant of ECC. The optimization problem is effectively solved by the NOMAD solver, which implements the Mesh Adaptive Direct Search algorithm. The proposed method is tested on an NPCC 140-bus system and the results show that the proposed method with fault specified ECC can solve the FIDVR issue caused by the most severe contingency with fewer dynamic var sources than the Voltage Sensitivity Index (VSI) based method. The proposed method with fault unspecified ECC does not depend on the settings of the contingency and can address more FIDVR issues than VSI method when placing the same number of SVCs under different fault durations. It is also shown that the proposed method can help mitigate voltage collapse.

Large-scale stochastic power grid islanding operations by line switching and controlled load shedding

Recently, there has been an increasing concern regarding the security and reliability of power systems due to the onerous consequences of cascading failures. Among many emergency control operations, controlled power grid islanding is a last resort yet powerful method to prevent large-scale blackouts. Islanding operations split the power grid into self-sufficient operational subnetworks and avoid cascading failures by isolating the failed elements of the power system into a non-operational island. In this paper, we consider a two-stage stochastic mixed-integer program to seek the optimal islanding operations under severe contingency states. Line switching and controlled load shedding are the main tools for the islanding operations and load shedding is considered as a measurement to gauge system’s inability to respond to disruption. The number of possible extreme contingencies grows exponentially as the size of the grid increases, and this results in a large-scale mixed-integer program, which is a computationally challenging problem to solve. We present an efficient decomposition method to solve this problem for large-scale power systems.

Analysis of Infeasible Cases in Optimal Power Flow Problem

Abstract: In the context of smart grid transformation of existing electricity networks, optimal power flow (OPF) and security-constrained OPF (SCOPF) studies remain to be very important for power system planning, operation and market analysis. OPF study involves finding the (global) optimum solution to a set of nonlinear algebraic equations, subjected to a set of equality and inequality constraints. When the system is heavily stressed, particularly following a severe contingency, the conventional OPF methods may fail due to the problem or solution infeasibility, or inability to select proper initial values. The soft constraint handling approach and repetitive constraint relaxation in finding the causes of infeasibility could be either tedious, or may not be practical for the large-scale problems. This paper presents an alternative approach, based on use of meta-heuristic method, to pinpoint the main reasons for the failure of solution algorithms in nonlinear optimization, in general, and OPF problem, in particular. The presented approach is illustrated on commonly used IEEE 14-bus and 30-bus test networks.

Contingency Analysis and Identification of Dynamic Voltage Control Areas

Contingency analysis has been an integral part of power system planning and operations. Dynamic contingency analysis is often performed with offline simulation studies, due to its intense computational effort. Due to a large number of possible system variations, covering all combinations in planning studies is very challenging. Contingencies must be chosen carefully to cover a wider group of possibilities, while ensuring system security. This paper proposes a method to classify dynamic contingencies into different clusters, according to their behavioral patterns, in particular, with respect to voltage recovery patterns. The most severe contingency from each cluster becomes the representative of other contingencies in the corresponding cluster. Using the information of contingency clusters, a new concept called dynamic voltage control area (DVCA) is derived. The concept of DVCA will address the importance of the location of dynamic reactive reserves. Simulations have been completed on the modified IEEE 162-bus system to test and validate the proposed method.

Adaptive Tuning of Frequency Thresholds Using Voltage Drop Data in Decentralized Load Shedding

Load shedding (LS) is the last firewall and the most expensive control action against power system blackout. In the conventional under frequency LS (UFLS) schemes, the load drop locations are already determined independently of the event location. Furthermore, the frequency thresholds of LS relays are prespecified and constant values which may not be a comprehensive solution for widespread range of possible events. This paper addresses the decentralized LS in which the instantaneous voltage deviation of load buses is used to determine the frequency thresholds of LS relays. The higher frequency thresholds are assigned to the loads with larger voltage decay which are often located in the vicinity of disturbance location. The proposed method simultaneously benefits from individual UFLS and under voltage LS (UVLS) features which operate in the power system without coordination. Numerical simulations in DigSilent PowerFactory software confirm the efficiency of proposed methodology in the stabilization of the power system after various severe contingencies.

A flexible framework of line power flow estimation for high-order contingency analysis

Abstract Traditionally, power system contingency analysis involves massive power flow calculations, which are usually based on linear approximation methods, such as dc load flow model or distribution factors based method, for fast speed but with compromised accuracy. Particularly, the accuracy of power flow results can deteriorate with the increase of k given N – k contingency analysis. Consequently, the obtained results may provide misleading information by under estimating the impact of some severe contingencies. In order to effectively implement online N – k contingency analysis, we propose a flexible framework of power flow estimation, where generalized line outage distribution factors ( GLODF s) and ac power flow model are integrated together to formulate a two-stage scheme. At first stage, the Monte Carlo sampling technique is used to generate tables of computing errors for the hybrid ac -( GLODF s/ ac ) method. The achieved error information can then provide useful references in the second stage to select either ac - GLODF s based method or ac power flow model for N – k contingency analysis. Theoretically, the framework can provide significantly enhanced accuracy as well as satisfied efficiency. Finally, comprehensive case studies with the IEEE -118 bus system are given to validate the proposed framework.

Transient Stability Enhancement of Multimachine Power System Using Robust and Novel Controller Based CSC-STATCOM

A current source converter (CSC) based static synchronous compensator (STATCOM) is a shunt flexible AC transmission system (FACTS) device, which has a vital role as a stability support for small and large transient instability in an interconnected power network. This paper investigates the impact of a novel and robust pole-shifting controller for CSC-STATCOM to improve the transient stability of the multimachine power system. The proposed algorithm utilizes CSC based STATCOM to supply reactive power to the test system to maintain the transient stability in the event of severe contingency. Firstly, modeling and pole-shifting controller design for CSC based STATCOM are stated. After that, we show the impact of the proposed method in the multimachine power system with different disturbances. Here, applicability of the proposed scheme is demonstrated through simulation in MATLAB and the simulation results show an improvement in the transient stability of multimachine power system with CSC-STATCOM. Also clearly shown, the robustness and effectiveness of CSC-STATCOM are better rather than other shunt FACTS devices (SVC and VSC-STATCOM) by comparing the results in this paper.

New genetic algorithms for contingencies selection in the static security analysis of electric power systems

The importance of a reliable supply of electric power in industrial society is unquestionable. In control centers of electrical utilities, an important task is the security analysis, even for those companies that already have the modern smart grids. In this task, a contingency is the operation outage of one or more devices, while contingencies selection is the determination of the most severe contingencies on the system. Despite the current technological advances, an analysis of all possible contingencies is impracticable. In this paper, a method to efficiently perform the selection of multiple contingencies is presented. The issue is modeled as a combinatorial optimization problem and solved by genetic algorithms, developed for this application. A robust method, which considers power flow and voltage, is presented and tested over IEEE-30 test system and over a large real life system, considering double outages of branches. The results showed accuracy close to 100%, when compared with an exact method.

Corrective Voltage Control Scheme Considering Demand Response and Stochastic Wind Power

This paper proposes a new approach for corrective voltage control (CVC) of power systems in presence of uncertain wind power generation and demand values. The CVC framework deals with the condition that a power system encounters voltage instability as a result of severe contingencies. The uncertainty of wind power generation and demand values is handled using a scenario-based modeling approach. One of the features of the proposed methodology is to consider participation of demand-side resources as an effective control facility that reduces control costs. Active and reactive redispatch of generating units and involuntary load curtailment are employed along with the voluntary demand-side participation (demand response) as control facilities in the proposed CVC approach. The CVC is formulated as a multi-objective optimization problem. The objectives are ensuring a desired loading margin while minimizing the corresponding control cost. This problem is solved using e-constraint method, and fuzzy satisfying approach is employed to select the best solution from the Pareto optimal set. The proposed control framework is implemented on the IEEE 118-bus system to demonstrate its applicability and effectiveness.

Weakest Buses Identification and Ranking in Large Power Transmission Network by Optimal Location of Reactive Power Supports

The detection of voltage collapse is essential to avoid possible voltage collapse for the preventive control actions and voltage security assessment. One effective way to know the locations where voltage collapses could be appear is to identify weakest buses in the systems. The weakest bus is the first point where voltage collapses appear in a severe contingency. This paper proposes a technique to evaluate the weakest bus in large scale power system based on the optimal position of reactive power supports. To solve the optimization problem, Differential Evolutionary (DE) technique is used. The fitness function consists of cost, power losses and Load voltage stability index (L mn ) which satisfying all operational constraints. L mn is used as the indicator for voltage stability margin and weakest bus identification. The method is applied on standard IEEE 30 bus, 57 bus and 118 bus test systems to show their comparative computing effectiveness.

A Wide-Area Measurement Systems-Based Adaptive Strategy for Controlled Islanding in Bulk Power Systems

Controlled islanding is the last countermeasure for a bulk power system when it suffers from severe cascading contingencies. The objective of controlled islanding is to maintain the stability of each island and to keep the total loss of loads of the whole system to a minimum. This paper presents a novel integrated wide-area measurement systems (WAMS)-based adaptive controlled islanding strategy, which depends on the dynamic post-fault trajectories under different failure modes. We first utilize an improved Laplacian eigenmap algorithm (ILEA) to identify the coherent generators and use the slow coherency grouping algorithm to guarantee coherent stability within an island. Using the identification result, we then define the minimum coherent generator virtual nodes to reduce the searching space in a graph and utilize the k-way partitioning (KWP) algorithm to obtain a preliminary partition of the simplified graph. Based on the preliminary partition, we consider the direction of power flow and propose a variable neighborhood heuristic searching algorithm to search the optimal separation surfaces so that the net imbalanced power of islands is minimized. Finally, the bidirectional power flow tracing algorithm and PQ decomposition power flow analysis are utilized to determine the corrective controls within each island. The test results with the New England 39-bus system and the IEEE 118-bus system show that the proposed integrated controlled islanding strategy can automatically adapt to different fault modes through generator coherency identification and effectively group the different coherent generators into different islands.

A new two-stage framework for voltage stability enhancement incorporating preventive and corrective control actions

Summary Nowadays voltage stability is an important concern in power system operation due to many blackouts caused by voltage collapse. This paper presents a new two-stage framework for voltage stability enhancement that in the first stage a preventive control action is applied to increase the voltage stability margin of the system under normal condition. In addition, in the second stage an optimal load shedding scheme as one of the most effective tools to save system stability against severe contingencies is implemented. To do this, a new special protection system design based on definition of a multi-objective function containing the objectives of voltage stability margin, deviation of voltage limits, branches' overload, and load shed amounts is proposed. A theta gravitational search algorithm is also suggested to solve the multi-objective non-linear optimization problem to determine the optimum amounts of load cut as well as the other objective functions. Furthermore, for avoiding either excessive load shedding or voltage collapse, both voltage threshold and critical time of load shedding implementation are determined by dynamic simulations. Azarbayejan area of Iran's power system in 2012 peak load time is applied as a test case of the presented framework. Effectiveness of the proposed approach is illustrated by comparing its simulation results with the results of the current load shedding scheme and two other optimization approaches. Copyright © 2014 John Wiley & Sons, Ltd.

Control Strategy of Improving the Transient Voltage Stability of Grid-Connected Large-Scale Wind Farm

Because mostly used variable speed wind turbines based on doubly fed induction generator (DFIG) do not provide with transient voltage support capability under fault conditions, the security and stability of grid is influenced by wind farms integration in the case of severe short circuit fault contingency occurred on the grid side. The modified power converter controller model with transient voltage support capability and pitch control model used for post-fault stability control are implemented in DIgSILENT/PowerFactory. The models validation and the contribution to transient voltage stability enhancement are verified by power system simulation containing large wind farms. From simulation results, it can be illustrated that modified power converter controller with transient voltage support capability can provide dynamic reactive power to support grid voltage recovery and the modified pitch control model can reduce mechanical power , preventing wind turbines from over-speeding and also providing more reactive power generation stability for the power converter controller in the duration of power system large disturbance. The conclusion is presented that the control strategy can effectively improve the transient voltage stability of the gird.

고속 스위칭에 의한 송전선로 과부하 해소 연구

Because of computational burden and complex topology of substation, a transmission overload relief using circuit-breaker switching was very complex and difficult. However, a on-line algorithm for reducing the overloads in transmission lines has made progress due to the advance of IT technology. This paper describes the methodology for alleviating the overloads in transmission lines by circuit-breaker switching. First, the severe contingency lists and substations were selected from the results of contingency analysis. Then the switch combinations are determined using circuit-breakers of the selected substation. The topology changes are limited to equipment outage, bus split, island split, bus merge and island merge. Finally, the fast screening and full analysis methods are used to analyze the overload in transmission lines. To verify the performance of the proposed methodology, we performed a comprehensive test for both test system and large-scale power systems. The results of these tests showed that the proposed methodology can accurately alleviate the overloads in transmission lines from online data and can be applied to on-line applications.

A Decentralized Multivariable Robust Adaptive Voltage and Speed Regulator for Large-Scale Power Systems

Abstract This papter introduces a decentralized multivariable robust adaptive voltage and frequency regulator to ensure the stability of large-scale interconnnected generators. Interconnection parameters (i.e. load, line and transormer parameters) are assumed to be unknown. The proposed design approach requires the reformulation of conventiaonal power system models into a multivariable model with generator terminal voltages as state variables, and excitation and turbine valve inputs as control signals. This model, while suitable for the application of modern control methods, introduces problems with regards to current design techniques for large-scale systems. Interconnection terms, which are treated as perturbations, do not meet the common matching condition assumption. A new adaptive method for a certain class of large-scale systems is therefore introduces that does not require the matching condition. The proposed controller consists of nonlinear inputs that cancel some nonlinearities of the model. Auxiliary controls with linear and nonlinear components are used to stabilize the system. They compensate unknown parametes of the model by updating both the nonlinear component gains and excitation parameters. The adaptation algorithms involve the sigma-modification approach for auxiliary control gains, and the projection approach for excitation parameters to prevent estimation drift. The computation of the matrix-gain of the controller linear component requires the resolution of an algebraic Riccati equation and helps to solve the perturbation-mismatching problem. A realistic power system is used to assess the proposed controller performance. The results show that both stability and transient performance are considerably improved following a severe contingency.

Voltage Stability Constrained Optimal Power Flow Using NSGA-II

Voltage stability has become an important issue in planning and operation of many power systems. This work includes multi-objective evolutionary algorithm techniques such as Genetic Algorithm (GA) and Non-dominated Sorting Genetic Algorithm II (NSGA II) approach for solving Voltage Stability Constrained-Optimal Power Flow (VSC-OPF). Base case generator power output, voltage magnitude of generator buses are taken as the control variables and maximum L-index of load buses is used to specify the voltage stability level of the system. Multi-Objective OPF, formulated as a multi-objective mixed integer nonlinear optimization problem, minimizes fuel cost and minimizes emission of gases, as well as improvement of voltage profile in the system. NSGA-II based OPF-case 1-Two objective-Min Fuel cost and Voltage stability index; case 2-Three objective-Min Fuel cost, Min Emission cost and Voltage stability index. The above method is tested on standard IEEE 30-bus test system and simulation results are done for base case and the two severe contingency cases and also on loaded conditions.

Comprehensive control framework for ensuring loading margin of power systems considering demand-side participation

This study presents a comprehensive control framework to ensure desired loading margin (LM) while minimising the corresponding control cost. The proposed framework is divided into corrective control (CC) and preventive control (PC) subproblems. The CC subproblem deals with the condition that a power system encounters voltage instability as a result of severe contingencies. This control is merely devised to restore system stability. The PC subproblem is brought up when the system operates in a stable region but its LM is insufficient. In this manner, the system-operating point is changed such that the desired LM is achieved. One of the features of the proposed methodology is to consider participation of demand-side resources as an effective control facility that reduces control costs considerably. Active and reactive redispatch of generating units and involuntary load curtailment (ILC) are employed along with voluntary demand-side participation as control facilities in CC and PC subproblems. The proposed control framework is examined through case studies conducted on the IEEE 14-bus and the IEEE 118-bus test systems. The results presented demonstrate the effectiveness and efficiency of the proposed framework.

Modulation Controller Design for the 1400 MW New Zealand Inter Island HVDC Link

Abstract The existing HVDC scheme connecting the North Island and the South Island of New Zealand is currently being upgraded. During this upgrade also known as the New Zealand Inter Island HVDC Pole 3 Project, the existing HVDC Pole 1 based on mercury arc valves is replaced by a new HVDC Pole 3 based on light triggered thyristor vales. Both poles, the existing Pole 2 and the new Pole 3 are then operated as a non balanced bipolar HVDC scheme. In this study, a power system stability analysis of the HVDC Pole 2 and Pole 3 connecting the North Island and the South Island is performed. Focus is hereby on the design of special power modulation controller to enhance the dynamic performance of the entire interconnected power system, especially during and shortly after faults and severe contingencies. The proposed modulation control functions are designed for frequency and transient AC voltage support.

Enhancing Power System Security by Proper Placement of Thyristor Controlled Series Compensator (TCSC)

In stressed electric power systems, due to increased loading or due to severe contingencies often lead to situation where system no longer remains in the secure operating region. FACTS devices can play very important role in power system security enhancement. Due to high capital investment, it is necessary to locate these devices optimally in the power system. The scope of this paper is to decide optimal location of Thyristor controlled series compensator (TCSC) by a real power flow performance index sensitivity based approach & line outage distribution factor. The effectiveness of the proposed controller has been tested on modified IEEE 30 bus system using Power world simulator software version 12.0. Index Terms—Line outage distribution factor, power system security, power world simulator software, real power flow performance index, sensitivity analysis, TCSC.

Real-time prediction of event-driven load shedding for frequency stability enhancement of power systems

Maintaining frequency stability is one of the three dynamic security requirements in power system operations. As an emergency control, event-driven load shedding (ELS), which is determined preventively and triggered immediately after fault occurrence, can effectively prevent frequency instability. This study proposes a methodology for real-time predicting required ELS against severe contingency events. The general idea is to train an extreme learning machine-based prediction model with a strategically prepared ELS database, and apply it on-line for real-time ELS prediction. The methodology can overcome the shortcomings of conventional deterministic approaches by its high generalisation capacity and accuracy. It can either be an individual tool or a complement to deterministic approaches for enhancing the overall reliability of the ELS strategy. Its feasibility and accuracy are verified on the New England 10-machine 39-bus system, and the simulation results show that the prediction is acceptably accurate and very fast, which is promising for practical use.