Page |1 PANIMALAR INSTITUTE OF TECHNOLGY, CHENNAI-123 PART-B 1. Explain the importance of stability analysis in power system planning and operation. Power system stability: Power system stability may be broadly defined as that property of a power system that enables it to remain in a state of operating equilibrium, under normal operating conditions and to regain an acceptable state of equilibrium after being subjected to a disturbance. Design and operating criteria for stability For reliable service a bulk electricity system must remain intact and be capable of withstanding a wide variety of disturbances. Therefore, it is essential that the system be designed and operated so that the more probable contingencies can be sustained with no loss of load (except that connected to the faulted element) and so that the most adverse possible contingencies do not result in uncontrolled, widespread and cascading power interruptions. Design and operating criteria play an essential role in preventing major system disturbances following severe contingencies. The use of criteria ensures that, for all frequently occurring contingencies, the system will, at worst, transit from the normal state to the alert state, rather than to a more severe state such as the emergency state or the in extremis state. When the alert state is entered following a contingency, operators can take actions to return the system to the normal state. Normal design contingencies The criteria require that the stability of the bulk power system be maintained during and after the most severe of the contingencies specified below, with due regard to reclosing facilities. These contingencies are selected on the basis that they have a significant probability of occurrence given the large number of elements comprising the power system. The normal design contingencies include the following: (a) A permanent three-phase fault on any generator, transmission circuit, transformer or bus section, with normal fault clearing with due regard to reclosing facilities. (b) Simultaneous permanent phase-to-ground faults on different phases of each of two adjacent transmission circuits on a multiple-circuit tower, cleared in normal time. (c) A permanent phase-to-ground fault on any transmission circuit, transformer, or bus section with delayed clearing because of malfunction of circuit breakers, relay, or signal channel. (d) Loss of any element without a fault. (e) A permanent phase-to-ground fault on a circuit breaker, cleared in normal time. (f) Simultaneous permanent loss of both poles of a dc bipolar facility.
Page |2 PANIMALAR INSTITUTE OF TECHNOLGY, CHENNAI-123 The criteria require that, following any of the above contingencies, the stability of the system be maintained, and voltages and line and equipment loading are within applicable limits. These requirements apply to the following two basic conditions: (i) (ii) All facilities in service. A critical generator, transmission circuit, or transformer out of service, assuming that the area generation and power flows are adjusted between outages by use of ten minute reserve. Extreme contingency assessment The extreme contingency assessment recognizes that the interconnected bulk power system can be subjected to events that exceed in severity the normal design contingencies. The objective is to determine the effects of extreme contingencies on system performance in order to obtain an indication of system strength and to determine the extent of a widespread system disturbance even though extreme contingencies do have very low probabilities of occurrence. After an analysis and assessment of extreme contingencies, measures are to be utilized, where appropriate, to reduce the frequency of occurrence of such contingencies or to mitigate the consequences that are indicated as a result of simulating for such contingencies. The extreme contingencies include the following: (a) (b) (c) (d) Loss of the entire capability of a generating station. Loss of all lines emanating from a generating station, switch station or substation. Loss of all transmission circuits on a common right-of-way. A permanent three-phase fault on any generator, transmission circuit, transformer, or bus section, with delayed fault clearing and with due regard to reclosing facilities. (e) The sudden dropping of a large-load or major-load centre. (f) The effect of severe power swings arising from disturbances outside the NPCC interconnected systems. (g) Failure or misoperation of a special protection system, such as a generation rejection, or load rejection, or transmission cross-tripping scheme. System design for stability The design of a large interconnected system to ensure stable operation at minimum cost is a very complex problem. The economic gain to be realized through the solution to this problem is enormous. From a control theory point of view, the power system is a very high-order multivariable process, operating in a constantly changing environment. Because of the high dimensionality and complexity of the system, it is essential to make simplifying assumptions and to analyze specific problems using the right degree of detail of
Page |3 PANIMALAR INSTITUTE OF TECHNOLGY, CHENNAI-123 system representation. This requires a good grasp of the characteristics of the overall system as well as of those of its individual elements. The power system is a highly nonlinear system whose dynamic performance is influenced by a wide array of devices with different response rates and characteristics. System stability must be viewed not as a single problem, but rather in terms of its different aspects. Instability in a power system may be manifested in many different ways depending on the system configuration and operating mode. Traditionally, the stability problem has been one of maintaining synchronous operation. Since power systems rely on synchronous machines for generation of electrical power, a necessary condition for satisfactory system operation is that all synchronous machines remain in synchronism or, colloquially, “in step” this aspect of stability is influenced by the dynamics of generator rotor angles and power angles relationships. Instability may also be encountered without loss of synchronism. For example a system consisting of a synchronous generator feeding an inductor motor load through a transmission line can become unstable because of the collapse of load voltage. Maintenance of synchronism is not an issue in this instance; instead, the concern is stability and control of voltage. This form of instability can also occur in loads covering an extensive area supplied by a large system. In the evaluation of stability of concern is the behavior of the power system when subjected to a transient disturbance. The disturbance may be small or large. Small disturbances in the form of load changes take place continually, and the system adjusts itself to the changing conditions. The system must be able to operate satisfactorily under these conditions and successfully supply the maximum amount of load. It must also be capable of surviving numerous disturbances of a severe nature, such as a short-circuit on a transmission line, loss of a large generator or load, or loss of the equipment. For example, a short circuit on a critical element followed by its isolation by protective relays will cause variations will actuate generation controls; the changes in voltage and frequency will affect loads on the system in varying degrees depending on their individual characteristics. In addition, devices used to protect individual equipment may respond to variations in system variables and thus affect the system performance. In any given situation, however, the responses of only a limited amount of equipment may be significant. Therefore, many assumptions are usually made to simplify the problem and to focus on factors influencing the specific type of stability problem.
Page |4 PANIMALAR INSTITUTE OF TECHNOLGY, CHENNAI-123 2. Explain the classification of power system stability. Power system stability is a single problem; however, it is impractical to study it as such. Instability of a power system can take different forms and can be influenced by a wide range of factors. Analysis of stability problems, identifications of essential factors that contribute to instability, and formulation of methods of improving stable operation are greatly facilitated by classification of stability into appropriate categories. These are based on the following considerations: The physical nature of the resulting instability; The size of the disturbance considered; The devices, processes, and time span that must be taken into consideration in order to determine stability; and The most appropriate method of calculation and prediction of stability. Fig.1 gives an overall picture of the power system stability problem, identifying its classes and subclasses in terms of the categories described in the previous section. As a practical necessity, the classification has been based on a number of diverse considerations, making it difficult to select clearly distinct categories and to provide definitions that are rigorous and yet convenient for practical use. For example, there is some overlap between mid-term/long-term stability and voltage stability. With appropriate models for loads, on-load transformer tap changers and generator reactive power limits, mid-term/long-term stability simulations are between transient, mid-term and long-term stability. Similarly, there is overlap between transient, mid-term and long-term stability: all three use similar analytical techniques for simulation of the non-linear time domain response of the system to large disturbances. Although the three categories are concerned with different aspects of the stability problem, in terms of analysis and simulation they are really extensions of one another without clearly defined boundaries. While a classification of power system stability is an effective and convenient means to deal with the complexities of the problem, the overall stability of the system should always be kept in mind. Solutions to stability problems of one category should not be at the expense of another. It is essential to look at all aspects of the stability phenomena and at each aspect from more than one viewpoint. This requires the development and wise use of different kinds of analytical tools. In this regard, some degree of overlap in the phenomena being analyzed is in fact desirable.