Low Frequency Electromechanical Oscillations Research Articles

Power system structure and oscillations

Electromechanical oscillations are inherent to interconnected power systems. However, the frequency of the oscillations and the number of generators that oscillate in any electromechanical oscillatory mode depend on the structure of the power system network. Low frequency electromechanical oscillations occur when existing generation/load areas are connected to other similar areas by relatively weak transmission lines. Weak interconnections are obvious in many interconnected systems, for example, when two independent electric grids are interconnected for the first time through one or two tie lines. However, in systems that have been interconnected for some time, such as the US/Canadian interconnected systems, and that are being stressed by increased load, weak links are less obvious. Often, the first signs of trouble are low frequency oscillations becoming unstable. The connection between loading and stability is not always obvious. It is also unclear which contingencies may lead to oscillatory instability. This tutorial examines in detail the relationship between low frequency oscillations and weak interconnections in the transmission system network. The basis of the analysis is the observation that generators in specific areas of a power system behave coherently in low frequency oscillations and that groups of coherent generators are separated from other groups of coherent generators by weak interconnections. This observation is also the starting point for dynamic system reduction.

Performance comparison of three identification methods for the analysis of electromechanical oscillations

This paper describes the results of a study to evaluate the performance of three identification methods for the study of low frequency electromechanical oscillations. The three identification methods considered are: the Steiglitz-McBride algorithm; the eigensystem realization algorithm; and the Prony method. The identification methods are used to identify low order linear systems of power systems modeled in standard transient stability programs. This is accomplished by processing the system response to a simple probing pulse. The frequency domain characteristics of several identified systems are compared using three power systems with lightly damped electromechanical modes.

Oscillatory Stability Considerations in Transmission Expansion Planning

Due to fast economic growth on the island of Taiwan, load demand in Taiwan Power Company (TPC) has been rapidly increasing in the past ten years and is expected to continue throughout the next decade. By 1995, the peak load demand in Taiwan is forecasted to reach 16,000 MW, an increase of 60% from the present level of 10,000 MW. Several new generating units have been under construction to meet the future demand. As a result, additional transmission lines are needed to avoid overloads on present lines. The objective of this paper is to report the results from a recent study which was aimed at reaching an optimal transmission expansion plan by comparing the relative merits of three feasible alternatives: EHV lines (345 KV), UHV lines (765 KV), and HVDC lines (± 250 KV). At present, the main factors considered in transmission expansion planning in TPC are the maximum power transfer limits, fault currents, transient stability, system losses, and cost [1]. In addition to these factors used in current practice, oscillatory stability of the power system under each expansion plan will be extensively studied. In particular, the low frequency electromechanical oscillations which tend to appear in a longitudinal power system such as the one in Taiwan will be investigated. The dampings of these oscillation modes are essential in system operation and planning, especially in the case when the transmission line is heavily loaded.

Development of analytical techniques for system damping evaluation

The paper traces the background, conditions and system study work leading to the development of analytical techniques currently in use for system damping evaluation in England and Wales. The early study work, providing indications of possible low-frequency electromechanical oscillations, is examined together with the early simplified models used for eigenvalue analysis evaluating the damping behaviour of the system during such oscillations. The mechanism producing negative damping of such low frequency electromechanical oscillations is analysed. The development of the detailed eigenvalue analysis program package PRASM currently in use by the CEGB is described in detail. Finally, sample results from the extensive analysis carried out on the CEGB/SSEB oscillation incidents of December 1978-January 1979, utilising the PRASM program package, are given with commentary on likely future developments.

Optimal decentralized damping of low-frequency oscillations under control energy constraints

The problem of damping the low-frequency electromechanical oscillations in electrical power systems is dealt with by a parameter optimization approach. The design procedure is then applied to the synthesis of the stabilizing decentralized controllers for two nationwide power networks.

Optimal Decentralized Damping of Low-Frequency Oscillations in Electrical Power Systems Under Control Energy Constraints

The problem of damping the low-frequency electromechanical oscillations in electrical power systems is here dealt with by a parameter optimization approach. The outlined design procedure is then applied to the synthesis of the stabilizing decentralized controllers for two nation-wide power networks.