Generation Rescheduling Research Articles

Online Assessment and Control of Transient Oscillations Damping

A new approach to online assessment and control of transient oscillations is proposed. It relies on coupling Prony analysis and the SIME transient stability method, with the twofold objective: on one hand, to improve Prony's applicability and reliability, and on the other hand, to design generation rescheduling patterns able to enhance the damping of poorly damped oscillations and/or stabilize them. Simulations performed on two different systems illustrate the technique and show main features: accuracy, ability to uncover and assess the influence of system machines' generation on damping, and compatibility with real-time requirements. Comparison of the results obtained by the proposed approach with conventional eigenanalysis techniques is provided. Additional method's interesting byproducts are also discussed.

Generation Rescheduling Methods to Improve Power Transfer Capability Constrained by Small-Signal Stability

Small-signal stability, often in the form of low frequency oscillations, has been found to be the limiting factor when determining power transfer capabilities in a number of power systems. This paper proposes a new strategy to improve the power transfer capability constrained by small-signal stability by appropriately rescheduling generation based on sensitivity analysis. In the proposed strategy, a small-signal stability index, which can be computed quickly and reliably, is used to determine the small-signal security status of a system and also for the purpose of contingency screening. Two sensitivity-based dispatch methods are proposed to reschedule the generation in order to maximize power transfer subject to the small-signal stability constraint under a set of selected contingencies. The effectiveness of the proposed methods is demonstrated by comparing with other generation dispatch methods on a real power system model.

Determination of Effectiveness of Transient Stability Controls Using Reduced Number of Trajectory Sensitivity Computations

This paper presents a new approach for determining the effectiveness of preventive generation rescheduling and shunt/series compensation in improving transient stability using trajectory sensitivity (TS) method. It is shown that the effectiveness of changes in parameter values can be judged by evaluating the TS of certain variables with respect to clearing time. Therefore, the need to evaluate a host of sensitivities with respect to many parameters is obviated. This heuristic is motivated by the fact that for simple models and small changes, controllability of trajectories using certain parameters can be inferred from observability in certain output signals. An example of such a parameter and signal pair is shunt susceptance at a bus and the square of voltage at that bus, respectively. Case studies are carried out on a 10-machine 39-bus system with detailed models and 17-machine system to validate this method.

A Practical Preventive Control Method for Transient Stability Based on Time Domain Simulation

A time domain simulation based preventive control method is proposed in this paper. To achieve excellent practicality of the proposed method, the author focused on improving economy of the resulted generation rescheduling, unlimited model application and calculation efficiency of the stability analysis. A new technique is proposed to improve accuracy of Transient Energy Margin estimation from time domain simulation results. A sensitivity analysis technique is employed for stability limit determination of generating power from TEM. Calculation efficiency is also improved by proposed contingency selection. The effectiveness of the proposed method is verified through a calculation example with IEEJ 10 machine power system model.

A comprehensive approach to transient stability control: part II-open loop emergency control

For pt.I see ibid., vol.18, no.4, p.1446-53 (2003). The transient stability-constrained dispatch of an electric power system is a problematic task for the system operator who, under economic pressure, may be reluctant to take expensive preventive actions against very harmful contingencies. The open-loop emergency control (OLEC) technique proposed in this paper aims to relieve such preventive actions by complementing them with emergency ones. This is achieved by combining generation rescheduling, assessed and taken preventively, with generation tripping, assessed preventively but triggered only if the anticipated harmful contingency actually occurs. The relative size of preventive versus emergency control may be modulated so as to furnish a panoply of solutions. Besides, the technique may stabilize simultaneously many contingencies. Simulations conducted on the EPRI 88-machine system illustrate various possibilities of the OLEC technique and compare it with the purely preventive one. Tradeoffs between preventive control and OLEC are also discussed, especially in the context of liberalized electricity markets. It is shown that OLEC is indeed able to provide a good compromise between economics and security, and to realize important savings.

A new efficient nonlinear programming based method for branch overload elimination

A simple and efficient method for eliminating branch overloads in power systems is presented in this paper. The overloads are eliminated by corrective control actions, which are defined through the use of an efficient and accurate nonlinear programming method. Generation rescheduling (GR) and load shedding (LS) are the main controls used. The idea of adaptative local optimization (ALO) is introduced. The computation of appropriate GR using ALO becomes a very efficient process. LS is used as a last resort, when further GR is no longer possible. Heuristics are added in order to speed up the computation process and to take into account some practical aspects of power systems operation into it. A special procedure is carried out in case of critical situations, where emergency control actions are defined. The method's general idea is to keep the new secure operating point as close as possible to the original one, while minimizing the amount of LS. The method can be a helpful tool for operation planning studies, security analysis and reliability evaluation of power systems. Simulations have been carried out for small test to large real life systems in order to show the efectiveness of the proposed method.

A New Efficient Nonlinear Programming-Based Method for Branch Overload Elimination

A simple and efficient method for eliminating branch overloads in power systems is presented in this paper. The overloads are eliminated by corrective control actions, which are computed by an efficient and accurate nonlinear programming method. Generation rescheduling and load shedding are the main controls used. The idea of adaptative local optimization is introduced, making the computation of appropriate generation rescheduling a very efficient process. Load shedding is used as a last resort, when further generation rescheduling is no longer possible. Heuristics are added in order to speed up the computation process and to take into account some practical aspects of power systems operation. A special procedure is carried out in case of critical situations, where emergency control actions are defined. The method's general idea is to keep the new secure operating point as close as possible to the original one while minimizing the amount of load shedding. The method can be a helpful tool for operation planning studies, security analysis, and reliability evaluation of power systems. Simulations have been carried out for small test to large real life systems in order to show the effectiveness of the proposed method.

Preventive control of voltage security margins: a multicontingency sensitivity-based approach

Summary form only given, as follows. This paper addresses the problem of changing the operating point of a power system in order to keep voltage security margins with respect to contingencies above some minimal value. Margins take on the form of maximum pre-contingency power transfers either between a generation and a load area or between two generation areas. They are determined by means of a fast time-domain method. The paper first discusses the use of a general optimal power flow, in which linear voltage security constraints are added. The simultaneous control of several (possibly conflicting) contingencies is considered. Then, the paper focuses on the minimal control change objective. Among the possible controls, emphasis is put on generation rescheduling and load curtailment. Examples are presented on an 80-bus test system as well as on a real system.

Preventive Control of Voltage Security Margins: A Multi-Contingency Sensitivity-Based Approach

This paper addresses the problem of changing the operating point of a power system in order to keep voltage security margins with respect to contingencies above some minimal value. Margins take on the form of maximum precontingency power transfers either between a generation and a load area or between two generation areas. They are determined by means of a fast time-domain method. The paper first discusses the use of a general optimal power flow, in which linear voltage security constraints are added. The simultaneous control of several (possibly conflicting) contingencies is considered. Then, the paper focuses on the minimal control change objective. Among the possible controls, emphasis is put on generation rescheduling and load curtailment. Examples are presented on an 80-bus test system as well as on a real system.

Enhanced ac power‐flow solutions for reliability analyses

AbstractThe paper presents an advanced Newton‐Raphson power‐flow algorithm for reliability evaluation with special regard to the prevention of non‐convergent situations. Remedial actions in order to relieve violations of operational constraints and some generation rescheduling functions are considered. Included is a reliability analysis for a sample power system in order to compare the resulting reliability indices using different power‐flow algorithms.

A fuzzy control for network overload alleviation

This paper presents a prototype of a fuzzy system for alleviation of network overloads in the day-to-day operation of power systems. The control used for overload alleviation is real power generation rescheduling. Generation Shift Sensitivity Factors (GSSF) are computed accurately, using a more realistic operational load flow model. Overloading of lines and sensitivity of controlling variables are translated into fuzzy set notations to formulate the relation between overloading of line and controlling ability of generation scheduling. A fuzzy rule based system is formed to select the controllers, their movement direction and step size. Overall sensitivity of line loading to each of the generation is also considered in selecting the controller. Results obtained for network overload alleviation of two modified Indian power networks of 24 bus and 82 bus with line outage contingencies are presented for illustration purposes.

Transient stability-constrained maximum allowable transfer

A method is proposed for allocating generation so as to maximize power between areas of interconnected power systems under stability constraints. This transient stability-constrained maximum allowable transfer (MAT) method consists of screening a large number of contingencies, scrutinizing the dangerous ones and suggesting generation rescheduling patterns to stabilize them. The MAT method is based on SIME, a hybrid stability method. Like SIME, MAT is accurate and free from simplifying assumptions about modelling, stability scenarios and instability modes. In addition, by controlling all dangerous contingencies simultaneously, the method succeeds in being fully compatible with requirements for real-time preventive monitoring and control. The method is illustrated on the South-Southeast Brazilian power system, operating under highly stressed conditions imposed by large power between areas.

98/01308 Effect of generation rescheduling or voltage stability margin: Srivastava, K. N. and Srivastava, S. C. Electrical Power and Energy Systems, 1997, 19, (1), 11–17

98/01308 Effect of generation rescheduling or voltage stability margin: Srivastava, K. N. and Srivastava, S. C. Electrical Power and Energy Systems, 1997, 19, (1), 11–17

A coherency based rescheduling method for dynamic security

For power system online dynamic security analysis (DSA), preventive control or remedial action should be an integral part of the function if instability for a contingency is detected. Research done so far in online remedial action has been in rescheduling generation and most of the suggested methods for determining such preventive control use the sensitivities of the stability energy margin to the generator power injections. For any approach using the energy margin, computation time is a concern and a somewhat different approach was suggested that determines control action by maximizing coherency of the generators without the use of sensitivities. In this paper, a new coherency based sensitivity method is proposed for generation rescheduling. Different coherency indices have been defined and then compared by ranking the contingencies according to these indices as well as the energy margin index. Since the coherency indices are always functions of the rotor angles, the sensitivity trajectories of a coherency index, such as the most critical rotor angle, with respect to changes of generation can be calculated at every time step of the integration process. This paper suggests that these sensitivities calculated shortly after fault clearing be used for rescheduling the generation. The calculation of these sensitivities are obviously faster than the calculation of the energy margin sensitivities. This paper also shows, with test results using several different systems, that the rescheduling achieved by this method provides the necessary remedial action. It is also noted that this method is intuitively more direct as it uses the sensitivities of the worst affected generator angles for rescheduling.

NEW IMPLEMENTATION OF THE LP ALGORTHM FOR OPTIMAL REAL POWER RESCHEDULING

ABSTRACT A new implementation of an LP algorithm for security-constrained preventive rescheduling of real power is described. Rescheduling of generation and load (if required) is used to maintain a secure condition by avoiding line overloads. A dual linear programming formulation of the optimal rescheduling is developed. The weighted minimum deviations in real power generations and loads are chosen as the objective function in the LP formulation. Inclusion of inequality constraints on active line flow limits and equality constraint on real power generation-load balance assures a solution representing a secure system. Transmission losses are also taken into account in the constraint function. Change in customer demands can be brought into the objective function. The line losses are also brought into the optimization in the form of an equality constraint. The algorithm has options to choose selected generators or loads to take part in the optimization. The proposed method is tested on four representative power systems. These tests show very encouraging results.

Corrective Generation Rescheduling Based on Neural Network for Preventive Control

This paper proposes a corrective generation rescheduling method based on a neural network for the purpose of reducing the risk of post-outage overloading. The neural network evaluates the severity of line contingency and its weighting factors are utilized for the generation rescheduling. The simulation study on the IEEE 30-bus system shows that the proposed method can become an useful decision support tool for system operators in the alert state of power systems.

Methodology and computer package for generation rescheduling

Generation rescheduling is a cost-effective way to enhance steady-state and dynamic security. The paper presents an improved formulation for generation rescheduling and develops an iterative solution procedure. The proposed method can be used to assist operators to eliminate potential steady-state insecurity, dynamic insecurity, or their combination. Steady-state security is established by the well known (n – 1) criterion which deals with thermal limits of transmission lines. Dynamic security is defined in such a fashion that transient stability of the system is maintained under any of the contingencies which may occur at each end of every transmission line of a power system. An attractive feature of the proposed method is that it is reliable, realistic and easy to understand since dynamic security is checked in the method by an integration procedure. Simulation results of a 36-machine 200-node practical power system are demonstrated.

Effect of generation rescheduling on voltage stability margin

One of the recent concerns that power utilities are facing is to maintain the required system voltage stability margin. This paper investigates the effect of generation rescheduling in enhancing the overall system voltage stability margin. A new formulation of generation rescheduling in view of minimization of the slack reactive power injection has been suggested. In addition, conventional optimal power flow (OPF) formulations, with their objectives being the minimization of total generation cost and minimization of total system transmission loss, and another formulation of generation rescheduling in view of the maximization of the minimum singular value of the power flow Jacobian have been studied. All the four formulations have a more realistic voltage dependent model of the generators and the synchronous condensers. The study has been carried out on three sample systems.

COMPOSITE RELIABILITY EVALUATION OF INTERCONNECTED POWER SYSTEMS

ABSTRACT A power network can be divided into several subsystems such as generation, transmission and distribution subsystems. The most popular approaches for reliability evaluation, namely contingency enumeration and Monte-Carlo simulations, have been combined in this paper to form a hybrid algorithm for reliability assessment of composite generation and transmission subsystems. The main objective of this hybrid approach is to enumerate transmission outages and then sample generation states, conditioned to each enumerated transmission state, to evaluate the annualized reliability indices of the entire system and different load centers Within the proposed hybrid algorithm for generation rescheduling, correction of voltage levels or alleviating of component overloading in order to minimize the amount of load curtailment, a linear programming model is implemented. This algorithm has been utilized for assessing the reliability of the Unified Egyptian Power System. Moreover a weak point analysis is carried out...

A generation rescheduling method to increase the dynamic security of power systems

In this paper, a new method is proposed for dynamic security dispatch. The idea of coherent behavior of generators is used to find a new generation configuration with better transient stability behavior. Rescheduling the generation in the power system improves system security by increasing the critical clearing time or the transient energy margin without changing total generation. The calculation in the proposed method is simple and direct; the generation rescheduling depends only on the original generation and inertia constants of the machines, and their rotor speeds at the fault clearing time. In this work, the transient energy function (TEF) method is used to check the dynamic security of the system by analyzing all credible three phase faults with single line outages. If the worst energy margin is not acceptable, the proposed rescheduling is done, and the security is checked again. The method has been tested on the Ontario Hydro 55-bus 11-generator system, the IEEE 145-bus 50-generator system and the IEEE 162-bus 17-generator system. Results on several loading levels of the 162-bus 17-generator system are also shown.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>