27-08-2013, 04:56 PM
PROTECTOIN OF TRANSMISSION LINES USING Global Positioning System
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Abstract
This is a new technique for the protection of transmission systems by using the Global Positioning System (GPS) and fault generated transients. Global Positioning System (GPS) receivers are equipped with Phasor Measurement Units (PMU), which is called GPS-PMU, to increase the accuracy of fault detection/location by tagging all of the measured data with time stamps. This works based on the data measured by Phasor Measurement Units (PMU). Phasor measurements that occur at the same time are called "synchrophasors", as are the PMU devices that allow their measurement. In typical applications phasor measurement units are sampled from widely dispersed locations in the power system network and synchronized from the common time source of a global positioning system (GPS) radio clock and can output accurately time-stamped voltage and current phasors. Because these phasors are truly synchronized, synchronized comparison of two quantities is possible, in real time. The synchronized voltage and current phasors measured by PMU are transmitted to a Control center(Data Concentrator) for analysis. Data Concentrator will compare the present values with previous values and determine whether fault present in transmission line or not. Once a fault was occurred in the transmission network, the time of those measured data transmitted to relay through communication unit. At each substation relay determine the location of the fault by comparing the GPS time measured locally with those received from the adjacent substations.
INTRODUCTION
Accurate location of faults on power transmission systems can save time and resources for the electric utility industry. Line searches for faults are costly and can be inconclusive. Accurate information needs to be acquired quickly in a form most useful to the power system operator communicating to field personnel.
To achieve this accuracy, a complete system of fault location technology, hardware, communications, and software systems can be designed. Technology is available which can help determine fault location to within a transmission span of 300 meters. Reliable self monitoring hardware can be configured for installation sites with varying geographic and environmental conditions. Communications systems can retrieve fault location information from substations and quickly provide that information to system operators.
TRANSMISSION SYSTEM
Electric power transmission, a process in the delivery of electricity to consumers, is the bulk transfer of electrical power. Typically, power transmission is between the power plant and a substation near a populated area. Electricity distribution is the delivery from the substation to the consumers. Electric power transmission allows distant energy sources to be connected to consumers in population centers, and may allow exploitation of low-grade fuel resources that would otherwise be too costly to transport to generating facilities.
Due to the large amount of power involved, transmission normally takes place at high voltage . Electricity is usually transmitted over long distance through overhead power transmission lines. Underground power transmission is used only in densely populated areas due to its high cost of installation and maintenance, and because the high reactive power produces large charging currents and difficulties in voltage management. A power transmission system is sometimes referred to colloquially as a "grid"; however, for reasons of economy, the network is not a mathematical grid. Redundant paths and lines are provided so that power can be routed from any power plant to any load center, through a variety of routes, based on the economics of the transmission path and the cost of power. Much analysis is done by transmission companies to determine the maximum reliable capacity of each line, which, due to system stability considerations, may be less than the physical or thermal limit of the line.
FAULT DETECTION SYSTEM
Distance protection has played a major role in power line protection since it was first introduced in the early part of the century. It has many advantages over other power line protection techniques and can be adapted for fault location and backup protection. However, like any other power frequency
based protection techniques, it suffers from limitations due to the power system frequency waveform, fault path resistance, line loading and source parameter variations. In particular, the response speed of the relay can not meet the requirements when very high speed fault clearance is required.
Modern developments of the power system network, the demands for fast fault clearance to improve system stability, and the need for alternative protection principles have resulted in the search for methods to increase the speed of relay response. In the late 1970s, this led to the development of “ultra high speed protection” based on the use of Travelling wave and superimposed components. These relays offered the advantages of fast response, directionality, and were not affected by power swing and CT saturation. However, many distinct advantages of the conventional protection techniques were not retained, for example, inherent backup protection. In addition, in themselves, these technique had difficulty in detecting close in and voltage
zero faults.
TRAVELING WAVE FAULT LOCATION
Faults on the power transmission system cause transients that propagate along the transmission line as waves. Each wave is a composite of frequencies, ranging from a few kilohertz to several megahertz, having a fast rising front and a slower decaying tail. Composite waves have a propagation velocity and characteristic impedance and travel near the speed of light away from the fault location toward line ends. They continue to travel throughout the power system until they diminish due to impedance and reflection waves and a new power system equilibrium is reached.
The location of faults is accomplished by precisely time-tagging wave fronts as they cross a known point typically in substations at line ends. With waves time tagged to sub microsecond resolution of 30 m, fault location accuracy of 300 m can be obtained. Fault location can then be obtained by multiplying the wave velocity by the time difference in line ends. This collection and calculation of time data is usually done at a master station. Master station information polling time should be fast enough for system operator needs.
BENEFITS OF TRAVELING WAVE FAULT LOCATION
Early fault locators used pulsed radar. This technique uses reflected radar energy to determine the fault location. Radar equipment is typically mobile or located at substations and requires manual operation. This technique is popular for location of permanent faults on cable sections when the cable is de-energized. Impedance-based fault locators are a popular means of transmission line fault locating. They provide algorithm advances that correct for fault resistance and load current inaccuracies. Line length accuracies of ±5% are typical for single-ended locators and 1-2% for two-ended locator systems. Traveling wave fault locators are becoming popular where higher accuracy is important. Long lines, difficult accessibility lines, high voltage direct current (HVDC), and series-compensated lines are popular applications. Hewlett-Packard has developed a GPS-based sub microsecond timing system that has proven reliable in several utility traveling wave projects. This low-cost system can also be used as the substation master clock.
TRAVELING WAVE FAULT LOCATION THEORY
Traveling wave fault locators make use of the transient signals generated by the fault. When a line fault occurs, such as an insulator flashover or fallen conductor, the abrupt change in voltage at the point of the fault generates a high frequency electromagnetic impulse called the traveling wave which propagates along the line in both directions away from the fault point at speeds close to that of light.The fault location is determined by accurately time-tagging the arrival of the traveling wave at each end of the line and comparing the time difference to the total propagation time of the line.
DATA CONCENTRATOR
The supply of high quality power is an important industry requirement. A phasor measurement unit (PMU) extracts system parameters, such as frequency and synchronized
phasors, to provide reliable data for power quality studies or real-time power system control.
These data are obtained from various stations substations and then sent to a Data concentrator(control unit), where the data are analyzed and control signals are generated. A common time reference is supplied by a global positioning system (GPS) for all acquired data to satisfy the needs of real-time control. PMU data are time tagged with precision better than 1 microsecond and magnitude accuracy that is better than 0.1%.