ADAPTIVE OPTIMAL COORDINATION OF DIRECTIONAL OVERCURRENT RELAYS OF LARGE SCALE POWER SYSTEMS

Abstract

Adaptive protection of electric power systems is a new and a revolutionary idea. It is still in its conceptual development stage. At present, research work is going on to develop it as a protection philosophy and to identify its vital components. A few research papers are available in the protection literature on the subject, which highlight the importance and the implementability of this protection philosophy. All the researchers stress the need for this concept as it has much to offer in terms of reliability, dependability, security, efficiency, innovation and economy. Since the concept of adaptive protection is new, it is appropriate to give it a definition as visualized by some authors. Some of the definitions are : (A) The adaptive protection is an on-line activity that modifies the preferred protective response to a change in system conditions or requirements. It is usually automatic, but can include timely human intervention. (B) The adaptive protection is a protection philosophy which permits and seeks to make adjustments to various protection functions in order to make them attuned to prevailing power system conditions. (C) The adaptive protection refers to the ability of the protection system to automatically alter its operating parameters in response to changing network conditions to maintain optimal performance. The key concept is to change something in a protection system in response to changes in power system caused by operational or structural disturbances. -iThe need for the development of adaptive protection arose because of certain weaknesses and shortcomings in the existing protection philosophy. Researchers have been realizing these shortcomings for some time now and have boldly proposed a new way of thinking to tackle the problems of protection. One way of thinking is, why not to make the protection relays adaptable to the system changes so that the relay would respond to new operating states in aself-adaptive manner, thereby reducing the intervention of the operators to a minimum and increasing the efficacy of the protection functions. One of the most laborious and time-consuming tasks for a protection engineer is to set and coordinate protective relays in an interconnected power system. Ideally these settings are reviewed whenever loads or other system conditions change enough to appreciably alter the fault currents. Settings are also reviewed prior to temporarily taking a line out of service and as a part of post-fault analysis. However, even a review of settings is a troublesome task. Hence settings are not changed as often as conditions warrant and protection system performance is degraded. This problem was tackled by using computer aided approach in an off-line mode. Based on certain coordination criteria, the settings of relays are computed. In this way the system responds to faults and abnormal conditions in a pre-determined manner. It is not only difficult to identify and analyze all the operating conditions in advance, it is also impossible to determine relay settings that would be optimum for all abnormal and normal operating conditions. There could be a better and efficient method to set and coordinate the protective relays. The method is " Adaptive Coordination of Protective Relays". In this method the relays could respond to the changing system -11- conditions and adapt according to the new conditions. The changing conditions could be operational or structural. The requirements for such a scheme to be implemented include the right kind of efficient algorithms, appropriate digital relays and communication channels. The biggest advantage of this scheme would be to liberate the protection engineer from .the most tedious and time-consuming task of setting and coordinating protective relays. This dissertation aims at the development of the concept and the philosophy of the adaptive coordination of relays in the general framework of adaptive protection. The adaptive coordination philosophy is implemented using the concept of overcurrent relaying. The directional overcurrent relays in an interconnected power system are suitable relays to be exploited for the introduction of the adaptive concepts. There are two coordination philosophies available for the coordination of directional overcurrent relays. One is based on the automation of the traditional interactive algorithms and the other is based on the use of parameter optimization techniques. The former technique is the result of traditional relay coordination philosophy implemented with the help of computers. The protection engineer interacts with the computer and tries to find out the solution to the coordination problem. His experience and knowledge influences the coordination results. Whereas, in the later method a performance function is optimized subject to certain coordination criteria. The coordinated relay settings obtained are the optimal relay settings which satisfy a wide variety of conditions which arise due to different system conditions. The performance function chosen reflects the desired operation of the relays. In the work reported in this dissertation, coordination problem is formulated as a parameter optimization problem with a different characterization which is suitable for adaptive coordination. The solution methods employed efficiently fit in the overall coordination philosophy. The traditional coordination algorithms need break point set. This requires elaborate schemes for topological analysis of the network. The break point set is needed to decrease the number of iterations in the coordination process. It is established, in the dissertation, that if the optimization techniques are used for relay coordination, this removes the need of determining break points of the network. Thus eliminating the use of elaborate topological analysis programs. A simple but efficient method is proposed to determine the backup/primary relay pairs by using the linked-list type of data structure known as LINKNET. The backup/primary relay pairs are automatically generated for the close-in as well as the far-bus fault currents. Two integrated software packages OPCORD and OPCON have been developed for coordinating the directional overcurrent relays of an interconnected power system. Different software packages like load flow analysis, fault analysis, determination of backup/primary relay pairs, main coordination program, etc. have been integrated. The programs could be run as interactive packages or as off-line 'batch' processing packages, depending on the choice of the user. The information needed for the coordination is stored and used by the software. No data base management packages are needed to manage huge data generated by the program. An efficient method has been developed to generate, store and retrieve the data as per the requirements of the coordination. Data generation, updating, retrieval and use are all combined in the same program. The problem of sympathy trips of relays in an interconnected power system has been studied in detail. A sympathy trip of a relay is the tendency to operate for a fault which is otherwise of no concern to this relay. The various reasons which give rise to the sympathy trip tendency of a relay have been classified into different categories. It is concluded that the use of adaptive coordination would remove the sympathy trip tendency of certain relays which are caused due to a change in the system conditions. A full system adaptive optimal coordination algorithm has been developed and tested successfully for different test power transmission systems. The program continuously monitors the system for any disturbance, structural or operational. If a disturbance is detected, the program is invoked immediately. Nothing is assumed apriori. The algorithm calculates everything according to the present conditions of the network. No assumed contingencies are considered. The determination of the pickup current settings and the time multiplier settings are the essence of any overcurrent coordination process. The pickup current value is normally fixed before the coordination is attempted. This procedure does not ensure correct setting of the relays. Therefore, it is proposed to dynamically vary it as the load current varies. This step would make the relays more sensitive to the system changes. Therefore, an expression for pickup current setting has been developed which dynamically incorporates any change in load current into the new value of pickup current. Due to large number of interconnections and ever growing demand, the size and complexity of the present day power systems, have increased tremendously. Therefore, it is becoming difficult and time consuming to solve the coordination problem of large and complex power networks. Hence there is a need for an efficient decomposition technique for solving the coordination problem of large power networks. Thus an efficient decomposition technique for the optimal coordination of directional overcurrent relays of large power networks has been developed and successfully implemented. The large power network is decomposed into number of sub-networks called blocks by identifying boundary buses, branches and relays. The blocks are analyzed sequentially and the boundary data table is updated during the solution process. All the components of the software are fully integrated. It is observed that the larger the number of blocks, lesser is the overall execution time for the solution of the coordination problem. An adaptive optimal coordination algorithm has been developed which is based upon the decomposition approach. The relay settings are determined in an adaptive manner, block-wise. Considerable execution time and the memory requirements are saved if the decomposition approach is used for the coordination process. A new concept of local optimal coordination has been developed and reported in this dissertation. A local disturbed region is identified along with all the backup/primary relay pairs in response to a structural change or an operational change in the power transmission system. The coordination problem is solved only for this local region using local optimization. The methods developed do not employ any data base management packages or any elaborate topological analysis programs. -viThe concept of local optimal coordination has been used to develop an adaptive coordination philosophy. This method is an ideal method to solve the coordination problem of the affected part of the network only, in an on-line manner. Only a small portion of the power network is identified and its coordination problem is solved efficiently. The time taken for execution is negligible in comparison with the full system coordination method. The memory requirements are also drastically reduced as a small portion of the network is dealt with. During the progress of the present work, it was realizedlthat the concepts of control theory can be easily applied to the problem of relay coordination. In future coordinated control and protection schemes are sure to be developed and implemented. Therefore, we have formulated the classical coordination problem as an adaptive control systems engineering problem. It is made clear that this is a closed-loop, non-linear, multidimensional control system with variable parameters and intermittent control. The concept of adaptive relay coordination control system is presented and discussed. The structure of the main component of this control system, i.e., the adaptive relay coordination controller has been given. Other components of the control system are also presented and discussed.