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2000 | Buch

Introduction Kapitel lesen Erstes Kapitel lesen

Power System Oscillations

verfasst von: Graham Rogers

Verlag: Springer US

Buchreihe : Power Electronics and Power Systems

Enthalten in: Professional Book Archive

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Über dieses Buch

Power System Oscillations deals with the analysis and control of low frequency oscillations in the 0.2-3 Hz range, which are a characteristic of interconnected power systems. Small variations in system load excite the oscillations, which must be damped effectively to maintain secure and stable system operation. No warning is given for the occurrence of growing oscillations caused by oscillatory instability, since a change in the system's operating condition may cause the transition from stable to unstable. If not limited by nonlinearities, unstable oscillations may lead to rapid system collapse. Thus, it is difficult for operators to intervene manually to restore the system's stability. It follows that it is important to analyze a system's oscillatory behavior in order to understand the system's limits. If the limits imposed by oscillatory instability are too low, they may be increased by the installation of special stabilizing controls.
Since the late 60s when this phenomena was first observed in North American systems, intensive research has resulted in design and installation of stabilizing controls known as power system stabilizers (PSS). The design, location and tuning of PSS require special analytical tools. This book addresses these questions in a modal analysis framework, with transient simulation as a measure of controlled system performance. After discussing the nature of the oscillations, the design of the PSS is discussed extensively using modal analysis and frequency response. In the scenario of the restructured power system, the performance of power system damping controls must be insensitive to parameter uncertainties. Power system stabilizers, when well tuned, are shown to be robust using the techniques of modern control theory. The design of damping controls, which operate through electronic power system devices (FACTS), is also discussed. There are many worked examples throughout the text.
The Power System Toolbox© for use with MATLAB® is used to perform all of the analyses used in this book.
The text is based on the author's experience of over 40 years as an engineer in the power industry and as an educator.

Inhaltsverzeichnis

Frontmatter
Chapter 1. Introduction
Abstract
Electric power systems are among the largest structural achievements of man. Some transcend international boundaries, but others supply the local needs of a ship or an aeroplane. The generators within an interconnected power system usually produce alternating current, and are synchronized to operate at the same frequency. In a synchronized system, the power is naturally shared between generators in the ratio of the rating of the generators, but this can be modified by the operator. Systems, which operate at different frequencies, can also be interconnected, either through a frequency converter or through a direct current tie. A direct current tie is also used between systems that, while operating at the same nominal frequency, have difficulty in remaining in synchronism if interconnected.
Graham Rogers
Chapter 2. The Nature of Power System Oscillations
Abstract
Power system oscillations are complex, and they are not straightforward to analyze. Therefore, before going into any detail, I will use an example to show the basic types of oscillations that can occur. The example two-area system is artificial; its model was created for a research report commissioned from Ontario Hydro by the Canadian Electrical Association [1,2] to exhibit the different types of oscillations that occur in both large and small interconnected power systems. A single line diagram of the system is shown in Figure 1. There are two generation and load areas interconnected by transmission lines. Each area has two generators. The generators and their controls are identical. The system is quite heavily stressed; it has 400 MW flowing on the tie lines from area 1 to area 2. In all cases, the active load is modelled as 50% constant current and 50% constant impedance; the reactive load is modelled as constant impedance. Using the two-area system as the basis, I will discuss the different types of oscillations that can occur in this system and, by implication, other interconnected systems.
Graham Rogers
Chapter 3. Modal Analysis of Power Systems
Abstract
In Chapter 2, I discussed the oscillations that may occur in interconnected power systems. By looking at different models, and with different disturbances, I showed examples of the different types of oscillation that can occur. To do this, I performed a considerable number of 10-second nonlinear simulations. It is apparent that in larger systems the use of transient simulation for the analysis of system oscillations could be very time consuming. To study inter-area oscillations, it is often necessary to run simulations for longer than 10 s; 30 s is quite common in practice. Not only is the use of non-linear simulation time consuming, but also it is often difficult to interpret the results. Larger systems may have a number of interarea modes at very similar frequencies, and it can be quite difficult to separate them from a response in which more than one is excited.
Graham Rogers
Chapter 4. Modal Analysis for Control
Abstract
In Chapter 3, I defined and applied modal analysis to understanding the nature of power system oscillations. However, it is necessary to do more than understand; controls, which modify the natural behaviour of the interconnected synchronous generators, must be designed. While power systems are essentially nonlinear, we have seen that their oscillations about an operating point can be predicted accurately from a linearized system model. For oscillation damping control design, we can use this to justify the application of linear control theory.
Graham Rogers
Chapter 5. Power System Structure and Oscillations
Abstract
Electromechanical oscillations are inherent to interconnected power systems. However, the frequency of the oscillations and the number of generators which oscillate in any electromechanical oscillatory mode depend on the structure of the power system network.
Graham Rogers
Chapter 6. Generator Controls
Abstract
In this chapter, I will discuss some aspects of the normal generator controls, i.e., the speed governor and the automatic voltage control. It is normal for these controls to be set up more or less independently of the requirements of the generator in an interconnected system. The governor’s purpose is to control speed, and an exciter with an automatic voltage regulator is used to control generator terminal voltage. Or at least this is the stated purpose. Once a generator is synchronously interconnected with other generators, the system plays an important part in both speed and voltage control. When generators are synchronized, their electrical speeds are identical, whether or not the generators have speed governors. In such a situation the speed governors largely control the distribution of the power between the generators: the operators adjust generator power by changing the reference input to the governor. Although not quite the same, the action of the automatic voltage regulators is similar. The voltages in an interconnected power system are close to their nominal values, the automatic voltage regulator essentially controls the reactive power supplied by the generator, at least until this reaches one of its limits.
Graham Rogers
Chapter 7. Power System Stabilizers
Abstract
Power system stabilizers have been used for many years to add damping to electromechanical oscillations. Essentially, they act through the generators excitation system in such a way that a component of electrical torque proportional to speed change is generated (an addition to the damping torque) [1,2] Of course, it is easy to say that this is done, and the mechanism varies depending on whether the mode is a local mode or an inter-area mode[3]. Never the less, an effective stabilizer does produce a damping torque over a wide range of input frequencies [4]. Less effective stabilizers may only produce a damping torque over a very small frequency range, which leads to problems when system changes cause the system’s oscillatory modes to change.
Graham Rogers
Chapter 8. Power System Stabilizers — Problems and Solutions
Abstract
In Chapter 7, I showed that power system stabilizers act efficiently to damp the electromechanical oscillations in interconnected power systems. When the stabilizers are correctly tuned, the resulting damping control is robust. Power system stabilizers are cost effective when compared to the alternative, purely electronic controls, e.g., static VAr compensators - I will consider these in chapter 10. However, over the years of power system stabilizer use, a number of problems have arisen. These are often cited as reasons not to install power system stabilizers, but this is not valid. Methods to rectify the problems have been found and, in this chapter, I will discuss the problems and their solutions in some detail.
Graham Rogers
Chapter 9. Robust Control
Abstract
Control systems are often thought of in terms of a driving signal and an output response, as a regulator in which a quantity must be kept as constant as possible despite disturbances applied to the system, or as a servomechanism. There are many regulators in power systems, for example, the automatic voltage regulator and the governor, they are necessary to ensure that the quality of the power supply is maintained.
Graham Rogers
Chapter 10. Damping by Electronic Power System Devices
Abstract
So far, I have considered the damping of power system oscillations by power system stabilizers that act through the excitation systems of the generators. The generators act to amplify the power of these controllers so that they can add damping to the electromechanical oscillations. There are other system elements which can act as power amplifiers, for example, high voltage direct current links, static VAr compensators and FACTS devices. Generally, these devices are placed in a power system for some reason other than to add damping to power system oscillations. However, once installed, their control design may be able to increase the damping of certain electromechanical modes as well as satisfying the primary requirements of the device. Because electronic devices are not directly involved with electromechanical oscillations, damping using these controls is not as straightforward as damping using power system stabilizers on the generators. It is particularly difficult to attain robustness to large changes in operating conditions. I will show that this is due to the large changes in the positions of the poles and zeros of the transfer functions of the devices when the operating conditions change.
Graham Rogers
Backmatter
Metadaten
Titel
Power System Oscillations
verfasst von
Graham Rogers
Copyright-Jahr
2000
Verlag
Springer US
Electronic ISBN
978-1-4615-4561-3
Print ISBN
978-1-4613-7059-8
DOI
https://doi.org/10.1007/978-1-4615-4561-3