FAULT DETECTION AND LOCATION ALGORITHMS FOR THREE-TERMINAL TRANSMISSION LINES

Abstract

Transmission lines are invariably essential elements for the reliable operation of the power system network. The two-terminal transmission line is the most traditional configuration of the line. However, the increase in power demand has led to the configuration of three-terminal transmission lines (TTL) that are preferred within the power grid due to the economic and rightof- way constraints. As the transmission lines usually have the chance of susceptible to numerous faults, fault detection and accurate fault location are critical necessities to repair the faulty area and decrease the outage time. However, the conventional method used for the protection of the two-terminal transmission line cannot be directly imposed on the TTL due to the presence of infeed current and fault resistance. The conventional method based on the distance relay suffers from under-reach due to infeed current from third terminal. Further, the high value of fault resistance increases the limitation. The first stage of present research work deals with the development of fault location method for a single-circuit TTL and the limitation of the conventional distance protection scheme. The proposed technique utilizes time-synchronized voltage and current signals from all three terminals. Initially, fault detection based on the estimation of the superimposed voltage of tap point with reference to all three terminals has been carried out. Subsequently, utilizing the above three estimated superimposed voltages, the faulty section identification criterion is formed. Finally, the fault distance and resistance has been obtained. The performance of this scheme remains unaffected during the wide variation in fault and system parameters. Various fault and system operating conditions of the TTL reported in this thesis are simulated in the PSCAD/EMTDC software package. The extension of single-circuit to double-circuit TTL results in further challenges for protection schemes owing to the mutual coupling between zero-sequence components of two circuits. Very few researchers have made attempts to provide the solution for the double-circuit TTL to identify the faulty circuit, faulty line section, and fault location. This thesis deals with the development of fault detection and location scheme for double-circuit TTL. The fault detection and faulty circuit/ line-section identification have been carried out by estimating the superimposed components of current at remote end with reference to all the three terminals. Subsequently, the estimation of fault distance is performed by utilizing the phase angle of voltage and current at the fault point. The performance of the proposed technique is evaluated for a wide variation in system and fault parameters. The proposed technique provides stability against closein fault, more sensitivity towards high resistance fault, and better selectivity for discriminating the internal and external fault. The proposed technique is also tested for cross-country faults, and its accuracy remains consistent even with considerable errors in the measurement of current transformers (CTs) and capacitive voltage transformers (CVTs). An iterative approach is also developed and tested to determine the fault location in single-circuit TTL. It is based on the derivation of three indices by solving non-linear equations with iterative method utilizing synchronized measurements from three ends of the line. It also provides a solution in case of absence of synchronization between the remote end measurements. The presented technique applies to all types of fault on three-terminal transposed/untransposed as well as the non-homogeneous transmission line. This method is tested for comprehensive variation in the type of fault, line length, fault inception angle, complex fault impedance, source impedance, and CT saturation condition. Its accuracy remains almost constant, even considering errors in line parameters as well as synchronization and noise in the measured signals. It was observed that the initial guess in the iterative process has a minimal impact on the accuracy and convergence of the presented technique. To overcome the limitation of the phasor-based methods and to further improve the accuracy of fault distance estimation, a novel fault location method based on the traveling wave has been proposed. The polarity of Wavelet Transform Modulus Maxima (WTMM) is obtained using the first level detail coefficient of the current traveling wave at each end, is utilized to identify the faulty line section. Then, the fault is discriminated as either before or after the middle of the faulty line-section by observing the change in polarity among the first two consecutive traveling waves. Finally, the fault location is estimated at a particular terminal on which the faulty line-section is connected. The obtained results proved the effectiveness of the proposed method for locating all types of internal ground faults. Further, the comparative analysis with several existing methods shows the supremacy of the proposed method in terms of the fault location error for high resistance faults at different fault locations. The reported work is likely to contribute significantly to the area of the transmission line protection. The different techniques developed will be particularly useful for single-circuit as well as double-circuit (homogeneous and non-homogeneous) TTL.