Severe Contingencies Research Articles

Justification of Effective Reactive Power Reserves With Respect to a Particular Bus Using Linear Sensitivity

A constant reactive power (Q) reserve (CQR) is measured by the difference between the maximum reactive power generation and its current. However, all reactive power sources must not affect the entire power system when severe contingencies occur. Thus, generators must be evaluated as to whether or not they will have an impact on the maximum permissible loading of buses. This definition considers some generators to still have reactive power reserves when the system faces voltage collapse. This remaining reactive power reserve, although available, cannot be used in an arbitrary manner. This paper proposes the concept of an effective reactive power reserve and it is calculated on the basis of a sensitivity analysis. The computed reactive power reserve is analyzed using static and dynamic simulations in bulk power systems.

Using PV and QV curves with the meaning of static contingency screening and planning

This paper investigates the use of the QV and PV curves in the planning scenario, when some system reinforcements and contingency screening are analyzed. The idea is to employ the continuation method to obtain the system load margin and the QV curve to calculate the reactive power reserve associated with each bus. Combining these approaches enables one to identify the system critical area, the actual operating conditions, the most severe contingencies and the most important reinforcements. The methodology proposed may provide important signals to system planners, since the planning is focused under the voltage security point of view. The ideas are tested with the help of some academic systems and two real (a Brazilians and a Paraguayan) systems with all the system limits considered.

Optimal Allocation of Dynamic VAR Support Using Mixed Integer Dynamic Optimization

Dynamic security of power systems is a major concern during contingencies, especially in present environment when there is inadequate transmission line capacity and dynamic reactive power support. This paper addresses dynamic VAR allocation to ensure acceptable transient voltage performance and short-term stability against severe contingency. To filter out severe contingency, severity indices are proposed. An algorithm based upon trajectory sensitivity analysis is used to select candidate VAR locations. The overall problem is formulated (in dynamic framework) as mixed integer dynamic optimization (MIDO). To solve MIDO problem, it is converted into mixed integer nonlinear problem (MINLP) by complete discretization using simultaneous discretization approach. The effectiveness of proposed approach is illustrated using New England 39-bus system.

Generation re-dispatch algorithm against voltage collapse in Jeju island system with a frequency control by high-voltage direct current

A new algorithm for the determination of adequate generation re-dispatch as a protective measure against voltage instability is presented in this study. The study applies the method to the Jeju island system in Korea with a frequency control by the current sourced converter (CSC) high-voltage direct current (HVDC) system, which regulates the local system frequency by the active power injection from the mainland system. In order to transfer the active power using the CSC HVDC system, reactive power consumption is essential, and the frequency control of the HVDC is very fast. Therefore voltage instability may occur if the dynamic reactive support is not enough in the local power system at severe contingencies such as generator's outages. The authors suggest generation against voltage curves ( g - V curves), constructed by generation continuation power flow, and propose the procedure to determine voltage stability limit of active power generation at each generator using g-V curves. An illustrative example of the proposed algorithm is shown as applied to the Jeju island system.

Protective Relay Settings of Tie Line Tripping and Load Shedding for an Integrated Steelmaking Cogeneration System

This paper presents the protective relay settings of tie line tripping and design of load-shedding scheme for an integrated steelmaking industrial power system to ensure system stability after a severe fault contingency on an external utility power network. The transient stability analysis of the industrial power system with the equivalent external Taipower system has been executed for an actual fault contingency in 2005. The mathematical models of 13 cogeneration units in the plant and several large generators of a nearby Taipower thermal plant are included in the computer simulation to solve the system transient response for the fault contingency to verify the accuracy of the models used. To prevent tripping of critical loads due to an external fault contingency, the undervoltage and under/overfrequency relays with proper settings are applied for the tripping of external and internal tie lines. To restore system stability of the industrial power system for the islanding operation after tie line tripping, load shedding is designed to disconnect a proper amount of noncritical loads by considering the governor response and protective constraints of cogeneration units. According to the transient stability analysis of the industrial power system with the proposed protective relay settings and load-shedding scheme, it is found that the voltage sag and oscillation problem caused by fault contingency in the external Taipower system can be mitigated if the tie line tripping is executed in time to prevent the shutdown of critical loads and tripping of cogeneration units.

Application of Multi-step Undervoltage Load Shedding Schemes to the KEPCO System

This paper deals with improvements to the special protection schemes (SPS) which have been applied to the low probability and high impact contingencies in the Korea Electric Power Corporation (KEPCO) system since 2004. Among them, the SPS for voltage instability in the Seoul metropolitan area is considered in this paper, and is a form of event-based undervoltage load shedding with a single-step scheme. Simulation results based upon a recent event that occurred on 765kV lines show that the current setting values of the SPS have to be revised and enhanced. In addition, by applying response-based multi-step undervoltage load shedding (UVLS) schemes to severe contingencies in the system, more effective results than those of the existing single-step SPS can be obtained. Centralized and distributed UVLS schemes are considered in the simulation. ULTC-based load recovery models and over excitation limiters (OXL) for the KEPCO system are also included in the long-term voltage instability studies.

Missing-Sensor-Fault-Tolerant Control for SSSC FACTS Device With Real-Time Implementation

Control of power systems relies on the availability and quality of sensor measurements. However, measurements are inevitably subjected to faults caused by sensor failure, broken or bad connections, bad communication, or malfunction of some hardware or software. These faults, in turn, may cause the failure of power system controllers and consequently, severe contingencies in the power system. To avoid such contingencies, this paper presents a sensor evaluation and (missing sensor) restoration scheme (SERS) by using auto-associative neural networks (auto encoders) and particle swarm optimization. Based on the SERS, a missing-sensor-fault-tolerant control is developed for controlling a static synchronous series compensator (SSSC) connected to a power network. This missing-sensor fault-tolerant control (MSFTC) improves the reliability, maintainability, and survivability of the SSSC and the power network. The effectiveness of the MSFTC is demonstrated by a real-time implementation of an SSSC connected to the IEEE 10-machine 39-bus system on a Real Time Digital Simulator and TMS320C6701 digital signal processor platform. The proposed fault-tolerant control can be readily applied to many existing controllers in power systems.

The MVA-Volt Index: A Screening Tool for Predicting Fault-Induced Low Voltage Problems on Bulk Transmission Systems

In this paper, a new method for steady-state screening of buses to identify possible fault-induced delayed-voltage recovery problems is presented. The method is particularly useful for (1) prioritizing substation relaying and breaker maintenance, (2) evaluating impacts of system improvement alternatives, (3) identifying most-effective locations for undervoltage load shedding, and (4) locating voltage (reactive) support equipment. The method is based on a single index value for each bus that is determined by considering the amount of load impacted by a fault at that bus using a weighted summation. The steady-state screening algorithm has been validated using dynamic simulations and has been used to successfully identify as a potentially severe contingency the location of an actual event that occurred on the Southern Electric System in the summer of 1999.

A fast corrective load shedding control scheme to prevent AC/DC systems voltage collapse

AbstractThis paper proposes a corrective load shedding algorithm to restore power flow solvability of AC/DC systems after severe contingencies. Firstly, contingency parameterization power flow is constructed to describe a set of multiple contingencies, including generators, HVDC links, ac lines and transformers. The post‐contingency critical point is quickly determined through optimal multiplier power flow and a simple optimization‐based outage parameter updating procedure. The updating strategy is better improved by the analysis of stability surface. At the nose point, the sensitivity analysis is performed to select the most effective control measure. Then, the corrective load shedding sensitivity‐based linear programming model is constructed to improve the stability margin. The numerical simulations on a practical 1621 bus system testify that the proposed method can prevent system from voltage collapse effectively and efficiently. Copyright © 2008 John Wiley & Sons, Ltd.

Multi-criteria contingency ranking method for voltage stability

The objective of this paper is to propose a fast, accurate, and robust method for ranking contingencies according to their severities as far as voltage stability is concerned. In general terms, the proposed procedure consists of obtaining the operating state and computing performance indices for each contingency of a predefined list. The contingencies are then ranked according to their performance indices. The proposed method can be used as a real time operation tool, since its computational effort is very small. The proposed method is able to pick most of the severe contingencies, resulting in very good capture ratios. Simulation results are shown for small test and large realistic power systems.

A Hybrid Approach Toward Security-constrained Reactive Power Planning in Electricity Markets

In this article, a hybrid approach for reactive power planning has been proposed that makes use of both numerical and analytical methods. Optimal power flow formulations with four different objectives have been solved for a system-intact and for a few severe contingency cases with the help of mixed-integer non-linear programming. For each objective, ranking of optimal bus locations for reactive power sources have been obtained. The results, so obtained, have been used to arrive at an overall performance sensitivity factor for locating reactive power sources employing an analytic hierarchical process algorithm. The proposed method for reactive power planning has been demonstrated on a IEEE 24-bus reliability test system.

Corrected transient energy function-based strategy for stability probability assessment of power systems

A new model describing the uncertainty of fault clearing time for probabilistic transient stability assessment of power systems is proposed. In this, a corrected transient energy function-based strategy is developed to evaluate the probabilistic instability index of systems. The advantage of this approach is that evaluations of critical clearing time in the system stability probability assessment are only conducted for very severe contingencies, an extremely small fraction of all considered contingencies. Hence, the computation efficiency has been remarkably enhanced. Case studies on a representation of the North China power system are reported to show that the proposed assessment strategy is effective and practical.

Time-delay compensation of a wide-area measurements-based hierarchical voltage and speed regulator

This paper presents a delay-insensitive version of a wide-area measurements-based two-level hierarchical controller. This hierarchical structure consists of a central controller, at the secondary level, dedicated to inter-generator interactions compensation, and conventional controllers (automatic voltage regulator, power system stabilizer, and speed governor), at the primary level, to dampen local oscillations. First, a Smith prediction approach is used to preserve performance in the presence of large remote-measurement time delays and communication time delays between central and local controllers. Second, an optimization algorithm is used to considerably reduce the complexity of the controller and thereby increase its reliability. Finally, sets of realistic tests are performed to assess the robustness of the proposed structure in the presence of uncertainties over time delays and power system parameters. Simulation results reveal that time-delay compensation is effectively required to enhance the hierarchical-structure performance in realistic situations. Furthermore, the performance of local controllers is considerably improved by the secondary-level controller action. The stability margin of the system is improved in the presence of severe contingencies.

Evaluation of small signal stability of a power system

In reality, a power system is never in steady-state, as there are persistent fluctuations in load levels with associated adjustments in generation. The growing complexity of power systems, as well as the system exposition to various contingencies, have exacerbated the system's steady-state condition. Small disturbances that are always present in power system networks include small variations in loads, generation and switching operation of power system equipment. It is important to maintain the load-generation balance such that the system frequency remains nominal. Multiple severe contingencies with complex interaction would cause system instabilities, and thus deteriorate the system condition. This paper explores transient stability of a power system subjected to small disturbances using time domain analysis, eigen-values analysis, participation factors analysis, and transient security assessment using transient security index. Simulations have been conducted on the IEEE 14-bus system and the results presented.

Fast voltage contingency analysis of power systems using model trees

This paper presents a model tree (MT) based approach for fast voltage contingency ranking, of most severe contingencies for online applications in energy management systems. To measure the severity of contingencies, a bus voltage violation based scalar performance index is used. Model trees are developed to estimate the real value of voltage performance index (VPI) and rank the single line outage contingencies, according to their severity. Full AC power flow is performed, to generate the training and testing patterns for the proposed MT under each contingency. The effectiveness of the proposed approach is tested on IEEE test systems. Once trained, the MT method gives fast and accurate ranking of contingencies for unknown load patterns, hence it is suitable for online applications in energy management systems.

Evaluation of available transfer capability using fuzzy multi-objective contingency-constrained optimal power flow

In trying to determine the available transfer capability (ATC), this paper primarily sets out to develop a fuzzy logic approach to parallelizing contingency-constrained optimal power flow (CCOPF). This algorithm may be used by utilities to optimize economy interchange for severe contingencies analyzed without disclosing details of their operating costs to competitors. In fact, the ultimate objective of fuzzy multi-objective CCOPF (FMCCOPF) is to carry out the minimization of both the base case (pre-contingency) operating cost and the post-contingency correction times as conflicting but fuzzy goals. Besides, the Benders decomposition is applied to partition the fuzzy formulation with contingency constraints, which allows for post-contingency corrective rescheduling, motivated by the improvement of computational efficiency using parallel processing. The feasibility of the proposed method is comprehensively realized by a comparison with the conventional optimal power flow (OPF) and the CCOPF with respect to the same array of transactions, base case, and generator/line outages for the IEEE-30 bus system and the IEEE-118 bus system.

Determination of available transfer capability using multi-objective contingency constrained optimal power flow with post-contingency corrective rescheduling

In a new competitive electricity market, accurate information should be shared to provide nondiscriminatory access to all participants. Key information to determine how much power can be shipped through the network is dubbed available transfer capability (ATC). This paper presents a methodology for the calculation of ATC, which is performed through a fuzzy logic approach to parallelizing contingency constrained optimal power flow (CCOPF). This algorithm may be used by utilities to optimize economy interchange for severe contingencies analyzed without disclosing details of their operating cost to competitors. In fact, the main objective of fuzzy multi-objective CCOPF is to determine the minimization of both the base-case (pre-contingency) operating cost and the post-contingency correction times as conflicting but fuzzy goals. Also, Benders decomposition is adopted to partition the fuzzy formulation with contingency constraints, which allows for post-contingency corrective rescheduling, motivated by the improvement of the computational efficiency using parallel processing. The IEEE-30 bus system is employed to test the proposed algorithm and the results are comprehensively demonstrated by a distinct comparison between the conventional optimal power flow and the CCOPF with respect to the same array of transactions, base-case, and generation/line outages.

Decentralized adaptive backstepping control of electric power systems

In this paper, a decentralized adaptive backstepping excitation controller, tuned using a Particle Swarm Optimization technique (PSO), is designed for stability enhancement of multi-machine power systems. To achieve decentralization, each machine is modeled as an independent uncertain dynamic subsystem, where the uncertainty is a disturbance that represents the effects of the rest of the system on that particular machine. This disturbance is expressed as a polynomial function of electric power deviation, and its parameters are adapted using PSO. The proposed technique is illustrated with a two-area benchmark power system. This system exhibits inter-area oscillations which are effectively damped with the proposed decentralized controllers under severe contingencies, for which traditional power system stabilizers fail.

Voltage Stability Study of the PJM System Following Extreme Disturbances

This paper presents results from an analysis of voltage stability following severe contingencies in the PJM system. The analyses involved the use of steady-state (OPF) and short- and long-term simulation tools. The results highlight the importance of adequate load modeling in assessing the performance of and solutions to voltage instability. Undervoltage load shedding (UVLS) is shown to be beneficial in arresting instability. Optimal power flow (OPF) analyses (supplemented with dynamic simulation results) provide valuable insight into the best locations and required minimum levels of UVLS. However, UVLS may not be able to address the serious scenarios where fast voltage dynamics develop immediately after an outage

Decentralized multivariable voltage and speed regulator for large-scale power systems with guarantee of stability and transient performance

This paper presents a novel procedure for the design of decentralized regulators for large power systems with a formal proof of ‘global’ stability. The distinctive feature of the solution is that both voltage and rotor speed dynamics are regulated simultaneously contrary to most of the solutions proposed so far in the literature. First, the traditional multimachine power system algebraic-differential equations are reformatted into suitable state equations, more appropriate for modern control tools. Secondly, a voltage and speed controller based on this model is proposed. The design consists of first cancelling some of the dynamical model non-linearities using non-linear excitation and valve input. The resulting subsystems are stabilized by auxiliary controls with linear and non-linear components. The non-linear component, which uses local signals to dominate those with interconnections, is derived from a stability criterion involving the Lyapunov function of the entire power system. The gains of the linear component are computed from the solution of an algebraic Riccati equation similar to the one involved in the full information H ∞ problem. These gains guarantee that effects of interconnection signals on voltage and speed dynamics are considerably reduced. The benefit of the proposed scheme is that the voltage regulation characteristic ensures a good post-fault voltage profile which helps improve rotor oscillations damping. Simulation results on a realistic power system confirm that the system stability is considerably improved in presence of severe contingencies.