J. Toyoda

Optimal generating unit maintenance scheduling for multi-area system with network constraints

Two proposals are described for the optimal maintenance problem of an isolated system without network constraints and a multiarea system with network constraints. First, a simple and useful method is presented for the generating unit maintenance problem of the isolated system by means of linear programming and branch-and-bound techniques. Second, a decomposition technique based on virtual load is proposed in order to solve the complicated multiarea generating unit maintenance problem. The goal is to level the reserve margin of every area throughout the year. In the multiarea maintenance scheduling the authors take into account the power arrival capability to every area load demand with network constraints, in contrast to the power supply capability of generating units as generally considered in conventional approaches without network constraints. An example of a five area system with network constraints is used to verify the methodology and illustrate the application of the approach to the multiarea maintenance problem. The results show that the approach is profitable and efficient for the multiarea power system. >

Maintenance scheduling based on two level hierarchical structure to equalize incremental risk

The authors propose a new goal of maintenance scheduling, which is to equalize incremental risk, such as loss-of-load-probability throughout the year, rather than to equalize risk or reserve. It is shown that better schedules can be found using this method. In order to simplify the combinatorial problem and optimize maintenance scheduling, the authors propose an approach based on a two-level hierarchical structure model for solving the maintenance scheduling problem. An example of a system consisting of 19 generating units whose maintenance periods are not the same is used to illustrate the application of the new approach to maintenance scheduling. The results show that the proposed approach is profitable and efficient for large-scale power systems.<<ETX>>

A new index extracted from line flow fluctuation to evaluate power system damping

The authors propose an index to evaluate the damping effect of a power system on a dominant mode of electromechanical oscillation. The index is obtained from numbers of line flow fluctuations in which the dominant mode is strongly observed even in the normal state of the system. To extract the index from the fluctuations, a combined technique is developed which is based on FFT (fast Fourier transform) processing and principal component analysis. The technique is applied to the field data which have been acquired at a single tie-line of a power system in Japan. The regression analysis with respect to the index and the total generation of the system shows that the damping effect of the system becomes strong with an increase of the total generation. This property, which is called a droop characteristic, could be used to determine the steady-state stability region and to identify whether the system is in the normal or alert state. >

Slow-coherency based composite mode oscillatory stabilization by means of a hybrid PSS

This paper addresses the problem of the stabilization of composite mode electromechanical oscillations of generators. A Hybrid PSS of the two-level structure including of a local PSS and a global PSS is introduced. Time-scale decomposition is applied to the decoupling of the local and global stabilization problems, where determination of the global (inter area) control law is based on slow-coherency aggregation. Composite mode electromechanical rotor oscillations are observed, in practice, with weakly interconnected power systems. These oscillations are characterized by local and inter-area modes. In certain cases of operating conditions and/or network topology, the damping of these oscillatory modes may be negligible or even negative. The existing approaches to the stabilization of power system are basically in decentralized way, and the objective of the stability measures are largely concentrated on the damping improvement of local modes by means of a power system stabilizer (PSS), using only the local information. However, for those systems with poorly or negatively damped inter-area modes, applications of the locally tuned PSS cannot bring a total stabilization to the system, and the inter-area oscillations remain unaffected or possibly building-up to excessive margins. In this paper we have introduced the idea of the Hybrid PSS which is of a two-level control structure with local and global inputs, as in Fig. 1. The desired supplementary control signal for composite mode oscillations is obtained by a weighted combination of the local and global (inter-area) stabilization signals.

Implementation of a hybrid power system stabilizer

Dynamic stability enhancement of composite mode oscillations of interconnected power system by means of a hybrid power system stabilizer is discussed here. The proposed Hybrid PSS is of a two-level structure composed of a local PSS for local mode and a global PSS for inter-area mode damping as in Fig. 1. The input of the global stabilizer is the center of inertia speed deviation of slow-coherent generators. The present paper is mainly focused on the practical considerations in implementation of the proposed stabilizer. First, the optimum allocation of this stabilizer is discussed. We introduce an index of modeinput-assignability, by which the best sites of the stabilizer can be identified. Next, for practical applications, an estimation method for the approximation of the input of the global PSS is introduced. Here, an approximated value of the center of inertia speed is obtained from a suitable combination of the rotor speed data of a few properly selected key generators. Lightly damped or even negative damping oscillatory modes associated with rotor dynamics of synchronous generators are widely experienced in practice. When the individual power grids are interconnected with AC tie-lines, it forces the generating units to participate in both local and inter-area oscillations, which are referred to composite mode oscillations. Our research is concerned with the dynamic stability enhancement of interconnected power systems through stabilization of composite mode electromechanical oscillations.

Coherency Based Stabilization of Hybrid Mode Oscillations of Interconnected Power System

Abstract This paper addresses the problem of the stabilization of hybrid mode oscillations associated with generator rotor dynamics. A Hybrid PSS with a two-level structure is proposed. Weighted combination of local and global control signals is used as the supplementary signal for the improvement of the both local and inter-area mode components. Time-scale decomposition is used for the decoupling of local and global control law design. Determination of global control law is based on slow-coherency aggregated model. A procedure for the identification of optimum stabilizer site based on an index of mode-input-assignment is introduced.

On-Line Security Monitoring by Detecting of Specified Modes Through Power Flow Observation

Abstract A transmission line selection algorithm is developed for the security monitoring by detecting of a specified mode through the power flow observation. The algorithm is based on the coherent grouping approach by Chows group. The combination of the synchronizing coefficients and grouping matrices, is used for the selection of the appropriate line for the monitoring. The concept of the flow monitoring is reasonable for the security check, which is confirmed through the field data. The proposed approach has been tested by 19-generator, 28-bus and 39-line model. Application of FFT is also discussed.

GPS Application for Synchronous Monitoring System at Multiple Location

Abstract Tohoku Electric Power Company and Tohoku University are cooperatively studying the effective use of a new monitoring .system using GPS (Global Positioning System). This system ensures achieving synchronous measurements at multiple remote electric supply station by using the time stamp facilities of GPS. First, this paper introduces briefly Synchronous Monitoring System at multiple locations applying GPS techniques. Following section summarizes features of power flow fluctuation gotten by the developed monitoring system. The authors suggest some visual methodologies todisplay the features of the acquired data, and conduct a feasibility study on extracting hidden signals to identify insecure conditions in the power system.

Neural network based preventive control support system for power system stability enhancement

The authors propose an application of a newly developed neural network to the preventive control of a power system. The purpose of the proposed control is to improve the damping effect of the system on electromechanical modes by reallocating load to generators. Since the neural network has flexible learning capability the authors apply it to identify the complex and nonlinear relation between the damping effect and the distribution of generating power. The trained neural network acts as the support system which aids an operator in performing the generating reallocation for enhancing the system stability. Furthermore, the authors develop a new type of neural network which can deal with the equal constraints about the output layer in the error-back-propagation type of neural network because it is important for the generating reallocation to satisfy the equal constraint about the energy balance between generation and load.<<ETX>>

Corrective Generation Rescheduling Based on Neural Network for Preventive Control

Abstract 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.