DEVELOPMENTS IN DIGITAL PROTECTION OF TRANSMISSION LINES

Abstract

Restrictions on right-of-way and limitations on building new transmission lines necessitate optimization of transmission line networks. This imposes challenges on their protection. Transmission lines, thus needs to be protected by comprehensive and quite complicated protective schemes so that the power interruption is reduced to minimum with regard to the time of interruption and the area affected by power interruption. Distance relay is an economic option that can be widely used for the protection of transmission lines. The distance calculation performed by a distance relay is incorrect due to the ground fault resistance, pre-fault system conditions, mutual coupling effect and shunt capacitance influences when protecting two terminal parallel transmission lines. The first stage of present research work deals with the development of an adaptive digital distance relaying scheme, motivated by the desire to automatically respond to the effects of transmission line parameter uncertainties, mutual coupling between parallel lines, network conditions and uncontrolled parameters such as fault resistance and fault location. In the proposed scheme, initially, the detailed analysis of the apparent impedance as seen from the relaying point, taking into account the effects of transmission line and system parameter uncertainties is carried out. This would enable to observe the effects of different parameters on the relay characteristics. In order to improve the performance of the conventional distance relay, the proposed scheme alters compensation factor adaptively based on the mode of operation of parallel lines. Finally, using radial basis function neural network (RBFNN), the proposed adaptive scheme modifies the relay characteristics, considering the changing network conditions and the uncontrolled parameters such as fault resistance and fault location. Distance protection of traction overhead equipment (OHE) is a special case of protection of EHV lines. The differences arise from the complexity of the traction supply system, feeding methods and the nature of load. Different efforts have been made by the different researchers to overcome the problem of protection of traction OHE. However, none of these schemes could completely provide solutions to the complex problem of the protection of traction OHE, as they might fail either in case of high resistance fault at the farthest end or in case when the load current drawn by locomotives is rich in odd harmonic components. This thesis deals with study of thyristorized electric locomotives and its effect on conventional distance relays used for the protection of traction OHE. In order to xv evaluate the impact of thyristorized locomotives on the performance of protective relays, the traction system is modeled using dynamic simulation software (PSCAD/EMTDC). Based on extensive simulations, it has been observed that thyristorized locomotives inject significant amount of current harmonics in the primary side. Moreover, change in drive frequency and steady state/dynamic conditions of locomotive significantly affect the performance of the conventional distance relays. To resolve this issue, a new high speed protection scheme is proposed which uses powerful analyzing and decomposing features of Wavelet Packet Transform (WPT). Numerous computer simulations have been carried out in respect of realistic data of 25-kV single-phase 50 Hz Indian railways AC traction system. Simulation results establish that the relay is capable to discriminate amongst internal faults, external faults, high resistance faults and heavy loads, and detects internal faults within half a cycle. The compact transmission configuration such as the double-circuit line sharing the same and the different right of way has emerged as a result of the search for compact and efficient systems with minimal environmental hazards. Protective relaying techniques are therefore, required to provide adequate protection to such systems. Although, a large number of parallel line protection techniques have been developed in the last three decades, still no method has reached a stage of perfection to deal with entire spectrum of different fault conditions, thereby leaving a scope of research for improvements in this area. In the present work, two different approaches, based on differential protection and digital protection, for double-circuit lines have been developed. In the first approach, a new WT based percentage differential protection scheme, for double-circuit line sharing the same right of way, is proposed. Data for verifying the proposed scheme were generated by modeling the sampled power system by using PSCAD/EMTDC program. This scheme works satisfactorily for different fault conditions which includes high resistance fault, inter-circuit fault and unusual condition such as single ended supply. Though, the proposed method eliminates many of the problems associated with double-circuit lines sharing the same right of way, the accuracy & effectiveness of this scheme can not be guaranteed for different configurations of parallel lines. Moreover, the proposed scheme fails in case of simultaneous same phase fault and also in case when one of the parallel lines is disconnected due to maintenance or fault. In the said situations, it relies on distance algorithm being used as a back-up protection. Hence, in the second approach, an attempt has been made to modify the digital algorithm XVI to suit the different configurations of double-circuit lines so as to make it potentially a stand alone scheme. To validate the proposed scheme, numerous computer simulations have been carried out on the realistic data of a part of the Indian 400 kV power transmission system. A series of tests have been carried out using test signals generated by modeling a whole 400 kV system in PSCAD/EMTDC, and the results have proved the accuracy and effectiveness of the proposed scheme. The proposed scheme provides stability against close-in fault, more sensitivity towards high resistance fault and reliability for discriminating in-zone and out-zone fault during complete loss of generation at buses. Furthermore, the suggested scheme avoids the requirement of distance algorithm as a backup protection in case of disconnection of any of the parallel lines due to maintenance or fault. Moreover, the proposed scheme analyzes and solves the problem of simultaneous same phase fault on parallel lines. In order to maintain stability and the efficiency of the system, faults on transmission lines, not only need to be detected rapidly, but they are required to be classified correctly, located precisely and cleared as fast as possible. Considering the above facts, two different approaches, based on RBFNN and Support Vector Machine (SVM), have been proposed to meet out the said requirements in respect of the transmission lines. With the help of Wavelet Transform Modulus Maxima (WTMM), which is obtained using the first level detail coefficient of the three line currents at each end, a new fault detection scheme is proposed. Once fault is detected in the forward direction, the fault classification and location task has been carried out using RBFNN. The proposed fault classifier uses three line currents, whereas, the proposed fault locator uses three line currents and voltages. The trained RBFNN fault classifiers and locators use a non monotonic transfer function based on Gaussian density function and, classifies the faults reliably (90% overall accuracy) and locate the faults precisely (97% of the total test cases), by overcoming the difficulties faced by the conventional BPNNs. Although the neuralnetwork- based approaches have been quite successful, they suffer from limitations such as susceptibility to the variations in source impedances, require long and sometimes uncertain training time, and need a considerable amount of training effort for good performance. In order to overcome the limitations of the neural network based schemes and to further improve the fault classification accuracy, a novel fault classification scheme based on SVM has been proposed. In order to avoid manual fault data generation, a new XVll automatic fault data generation model is developed using PSCAD which can generate 10" (n is the number of variable) training data sets. Wavelet Transform is used for the decomposition of measured signals and for extraction of the most significant features (feature extraction); which facilitates training of SVM, particularly in terms of getting better classification performance. These features are then used as inputs to the three SVM classifiers of the respective phases, which, when added to the output of the ground index can be used for obtaining a decision of fault (with or without ground), either with no fault on any phase or a fault on multiple phases of the transmission lines. The performance of classifier has been evaluated using two-kernel functions, namely, polynomial kernel and radial basis function (RBF) kernel. The performance of the proposed SVM classifier is tested by using fault classification accuracy (Ac). RBF kernel performs better than polynomial kernel. When compared with the other schemes, the proposed SVM based schemeoffers the highest overall fault classification accuracy (95.45%). Multi-terminal and tapped transmission lines, although usually economical in their breaker requirements, need complex relaying for adequate protection and operation. Various attempts have been made for the protection of tapped transmission lines using schemes based on different approaches. However, none of these schemes completely solved the said problem, as they might fail either in discriminating in-zone fault and outzone fault in case the current flow out of bus, or in detecting a fault that occurs near the junction having a source of small short circuit capacity. Hence, two different schemes, namely differential protection and digital protection, for tapped transmission lines have been proposed. Initially, in order to improve the existing differential protection algorithms, a new current differential relaying scheme using Wavelet Transform has been presented. The proposed scheme has been tested at length by using data that were generated by modeling a sample power system using PSCAD/EMTDC software package. The proposed scheme provides high sensitivity for internal faults, and stability for external faults. Some of the novel features of the proposed scheme are its operation under different conditions that includes the current flow out of one terminal in internal faults, insufficient current for tripping and its stability against inadvertent tripping in case of external faults. A comparative evaluation of conventional current differential protection scheme with the proposed scheme for multi-terminal line has also been carried out. Dependability and security have been studied separately. This scheme may fail in case of grounded neutral YXVlll A power transformers at more than two terminals. Hence, there is a need for stand alone primary protective relays for application on tapped transmission lines. In order to overcome the limitations of the differential protection scheme, a new WPT based digital protection scheme has been presented. Testing of the developed algorithm for different fault and system conditions have been carried out with the simulated data of an existing system using PSCAD/EMTDC. It has been observed from the results that the proposed technique distinguishes clearly between internal and external faults on EHV/teed transmission lines. Due to finer characterization of current signal the proposed WPT-based digital relay successfully distinguished low current faults. The relay has shown satisfactory performance under various fault conditions especially for high resistance faults and also for reverse faults in case of weak sources. A comparative study has been carried out between the existing scheme and the proposed scheme by analyzing different fault and system conditions. The results show that the proposed WPT-based scheme is superior to the existing differential protection schemes. The present work is likely to contribute significantly to the area of transmission line protection. The different techniques developed will be particularly useful for doublecircuit lines, tapped transmission lines, traction OHE and transmission line systems. Some suggestions, based on observations and simulations in this area, are proposed at the end of the thesis for the benefit of potential researchers.