Coherent Groups Of Generators Research Articles

Representation of coherency-based equivalents in transient stability studies

An equivalent generator model designed to replace a group of coherent generators is discussed. The equivalent generator incorporates the equation of one of the generators in the corresponding coherent group. Any degree of detail is possible in the model representation of the group. Application of the coherency-based equivalent to reduce the size of a given power network is demonstrated in the paper, and results are presented which indicate the performance of the reduced-order model.

Application of coherency and reduced order models to transient stability studies

Study of the transient stability of a large and interconnected power system requires a great deal of computational time. To reduce that time, power system equivalents are employed. A simplified transient stability method, based on the values of the ‘stability measures’, is presented in which generators with small swing are replaced by a new equivalent model, and appropriate simplified models are used for the rest of the generators. Results of this study are used to identify coherent groups of generators. Replacing coherent sets by their equivalent generators will further reduce the size of the system. The validity and accuracy of the method is demonstrated by stimulated tests on a sample power system.

Coherency identification for power system dynamic equivalents

Coherency-based approach to the problem of constructing power system dynamic equivalents has been very successful in applications. The key step in this approach is to identify groups of coherent generators. An analytic study of coherency is conducted. An algebraic characterization of coherency is given. An algorithm, based on the algebraic characterization, to identify coherent groups directly from system data is developed. A physical interpretation of the algebraic characterization of coherency is presented, where the condition for coherency is described in terms of generator inertia constants and their equivalent admittances to the bus of the disturbance.

Coherency identification and equivalents for transient stability studies

This paper presents techniques for identifying coherent generators without conducting a transient stability study and for developing stability equivalents of a group of coherent generators. Identification is based on the property of equal acceleration for coherent machines and it requires only examination of the elements of the Jacobian matrix of the linearized power system model. After the coherent groups have been identified, the stability equivalents are developed on the basis of the power invariance condition at the tie buses and at the internal buses of the coherent generators. These stability equivalents are used in a transient stability study. The techniques are illustrated with a 12-machine system. These techniques greatly improve the efficiency of calculation of coherency based equivalents. They have potential applications in planning studies of large power systems.

Coherency identification for large electric power systems

An important problem associated with contingency and reliability studies for large-scale electric power systems is the use of dynamic equivalent models to represent the behavior of aggregable parts of the system. Fundamental to the aggregation process associated with any coherency-based approach is an ability to identify groups of coherent generators or generators that swing together in response to a contingency. Coherency identification is thus a key step in the reduction process. In this paper, an analytical process is examined which permits identification of coherent groups based upon the closed-form solution of a linearized power system model and the use of the Cayley-Hamilton theorem. The result is an algebraic characterization of coherency. Computationally practical coherency indices are developed which do not require significant computer storage. A physical interpretation of this characterization is discussed which suggests that coherency depends not only of the electrical distances between generators but also on generator inertia constants. The method of coherency identification developed in this paper is evaluated on a detailed model of a 39-bus test system.

Coherency-based multimachine stability study

A new equivalent generator model to replace a group of coherent generators is discussed. The model is simple and can easily be implemented in transient stability programs. Derivation of the parameters of the proposed equivalent generator does not follow the common approach of calculating a new set of parameters for a new or hypothetical generator. The model is derived in such a way that its dynamic equations correspond to those of one of the generators it represents. The approach to the derivation of the equivalent generator equations is general. In the paper the model is derived for application in two different stability programs. In the first program a conventional method of stability analysis is used which incorporates classical generator models. The second program involves a noniterative algorithm in which generators could be modelled with any desired degree of accuracy. As the equivalent generator incorporates the equations of one of the machines in the system, it is possible to model all generators and controllers, including the equivalent generator, with any degree of detail desired. While the main objective of the paper is to introduce the equivalent model and its mathematical derivation, application of the new technique to reduce the order of a power system is also demonstrated.