Abstract:
Remote indication devices are each a combination of electrical parameter measurement circuitry, a CPU, and a communication device. Each remote indication device is electrically or electromagnetically coupled to a power distribution circuit conductor to monitor the current flowing in the conductor. The monitored conductor may be an energized power line (“phase”), or a current return line (“neutral”). In either case, the direction of power transfer at each measurement point can also be monitored by each remote indication device. Each remote indication device processes the monitored electrical current information to convert it into a form suitable for data transmission. The information is then presented to a monitoring station. The monitoring station collects and analyzes the electrical current information from the plurality of remote indication devices and is programmed to identify one or more particular power line segments that appear to have a fault.

Description:
CLAIM TO PRIORITY 
     The present application claims priority to U.S. Provisional Application No. 60/598,928, filed Aug. 5, 2004, and entitled “REMOTE FAULT MONITORING SYSTEM.” The identified provisional patent application is hereby incorporated by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to power distribution in utility power grids, and more particularly, to detecting and localizing faults on an electric utility power distribution circuit. 
     BACKGROUND OF THE INVENTION 
     Power distribution circuits typically used by electric utilities can experience various faults that disrupt service to consumers of electricity. Causes of such faults include electrical insulation breakdown or mechanical failures. Most commonly, faults are manifested as short circuits from line to ground, but line to line short circuits and open circuit faults can also occur. When these events occur, they are readily detectable and safety devices, such as circuit breakers, can be automatically actuated to shut down the distribution circuit. It is important for the utility and its customers to have the problem located and repaired as quickly as possible so that electrical service can be restored with minimal down time. In practice, since power transmission or distribution circuits extend over large distances, repair crews must patrol the entire line section. Locating faults is thus presently time consuming and expensive due to the lost revenue and the cost of lengthy troubleshooting. 
     Systems known in the art for localizing power line faults include stand-alone (independently-functioning) fault detectors deployed on distribution feeders that emit light or audible alarm signal when they detect an abnormally high current magnitude. This approach does not eliminate the need for repair crews to examine long stretches of power line as part of troubleshooting a line fault. 
     There are other devices known in the art that can be equipped with contact outputs for use with a SCADA system, but it appears that no integrated system approach exists to date for localizing faults potentially occurring over a long stretch of power line. 
     Another problem experienced by utilities involves a reverse EMF generated by certain loads in a power outage. Motorized equipment and appliances (air conditioners, for example) that are loads on a broken distribution circuit can, following a fault, generate an EMF in the broken circuit section that appears as a flow of power from the load in a direction opposite the ordinary flow of power during normal system operation. For example, in the case of a line fault affecting one of three phases of a power distribution circuit, a three-phase motor powered by the remaining two phases will continue running, and may well operate as a power generator producing an EMF back onto the wire of the faulty phase. Measuring line current magnitude, alone, near the fault point, is therefore sometimes insufficient to distinguish between an actual fault and a momentary imbalance of the distribution circuit. 
     Yet another drawback associated with conventional current measuring devices are the hazards associated with installing the devices on the high voltage power lines. 
     SUMMARY OF THE INVENTION 
     One aspect of the present invention utilizes conventional communication technologies, along with application software, and remote indication devices, to enable rapid recognition and notification of faulted line segments to facilitate rapid isolation (manual or automatic) and repair thereof. 
     In one example embodiment, the remote indication devices are each a combination of, but not limited to, electrical parameter measurement circuitry, a CPU, and a communication device. Each remote indication device is electrically or electromagnetically coupled to a power distribution circuit conductor to monitor the current flowing in the conductor. The monitored conductor may be an energized power line (“phase”), or a current return line (“neutral”). In either case, the direction of power transfer at each measurement point can also be monitored by each remote indication device. 
     According to another example embodiment, each remote indication device processes the monitored electrical current information to convert it into a form suitable for data transmission. The information is then presented to a monitoring station via a suitable mode of communication such as a WAN implemented via land-based telecommunications, two-way radio, microburst cellular, satellite-based telecommunications, or the like. The monitoring station collects and analyzes the electrical current information from the plurality of remote indication devices and, having a database associating each remote indication device with its corresponding location along the power distribution network, is programmed to identify one or more particular power line segments that appear to have a fault. 
     A monitoring station can determine an existence of a fault condition in various ways, such as by recognizing a change of current magnitude and/or direction from one of the remote indicator devices, or by recognizing a condition where neighboring remote indicator devices report disparate electrical current measurements. The fault information may be presented graphically, or made available via other report format suitable for enabling dispatch of repair personnel to the fault locations(s). In addition, the determined fault location information may be utilized to initiate transmission of control messages to automatic switching equipment located on the distribution system to further isolate faulty segments and allow energizing of non-faulty segments in accordance with a rules-based procedure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating one example system-level arrangement according to one embodiment of the present invention. 
         FIG. 2A  is a simplified schematic diagram illustrating an arrangement for measuring line current magnitude and phase according to one embodiment of the present invention. 
         FIG. 2B  is a vector diagram illustrating a reference phase angle VE, a second phase angle VI relative to VE, and a theoretical phase reversal of VI. 
         FIG. 3  illustrates a theoretical set of current measurements along an electrical distribution line that can be analyzed according to one embodiment of the present invention to identify and localize a potential fault condition. 
         FIGS. 4A and 4B  illustrate two theoretical sets of current measurements made at two different moments in time along the same electrical distribution line that can be analyzed according to one embodiment of the present invention to identify and localize a potential fault condition. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In this detailed description of the various embodiments, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural or logical changes may be made without departing from the scope of the invention. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the invention is to be defined by the claims. 
       FIG. 1  illustrates an example system configuration according to one embodiment of the present invention. High voltage transmission lines  100  carry generated electricity to distribution substation  102 . Substation  102  includes step-down transformers  104  and  106  that reduce the line voltage to an amplitude suitable for distribution. The electricity is distributed into a set of distribution lines (also referred to herein as “feeders”) indicated generally at  108 , the set includes feeder  108   a . Although feeder  108   a  is shown schematically as a single line in  FIG. 1 , it should be understood that feeder  108   a  can represent a three-phase distribution circuit (delta or wye), or a single phase circuit. Each distribution line can be disconnected from the substation  102  by a corresponding circuit breaker indicated generally at  110 . Feeder  108   a  has a corresponding circuit breaker  110   a.    
     Along feeder  108   a , remote indication devices  112   a ,  112   b , and  112   c  (generally referred to hereinafter as  112 ) are installed. Each remote indicator device  112  includes a current meter. In one embodiment, current metering is accomplished by a current transformer  114   a ,  114   b ,  114   c  (generally referred to hereinafter as current transformer  114 ) having a secondary winding electromagnetically coupled with the feeder  108  wire, which is the transformer&#39;s primary winding. Signal conditioning circuitry  116   a ,  116   b ,  116   c  (generally referred to hereinafter as  116 ) receives signals from the secondary winding of the associated current transformer  114  and outputs a voltage, current, frequency, or other suitable signal that represents the magnitude of the current conducted through the primary side of the current transformer (the feeder  108  wire). The output of the signal conditioner  116  is fed into an analog-to-digital converter (A/D)  118   a ,  118   b ,  118   c  (generally referred to hereinafter as  118 ), which periodically samples the signal conditioner&#39;s output and generates information readable by a CPU  120   a ,  120   b ,  120   c  (generally referred to hereinafter as  120 ) such as a microprocessor-based digital system, the information representing sampled electrical current measurements. The CPU  120  buffers the measured electrical current information. Each CPU  120  is interfaced with a corresponding communications subsystem  122   a ,  122   b ,  122   c  (generally referred to hereinafter as communications system  122 ). Each communications system  122  can communicate over a communications channel implemented using conventional land-based telecommunications, two-way radio, microburst cellular, satellite-based telecommunications, or the like. Each CPU  120  is programmed to pass the buffered measured electrical current information to the corresponding communications system  122  for transmission to a monitoring station. Communications may be achieved by direct communication as illustrated by dotted line  130   a , or by relaying data frames via intermediate repeater, as illustrated by dotted lines  130   b  and  130   c.    
     The monitoring station may be distributed, as illustrated in  FIG. 1 . In the illustrated example embodiment, a substation advisor gateway  124  receives the information transmitted by each remote indicator device  112 , and retransmits the information (optionally in an aggregated format and according to a different communications protocol) via wide area network (WAN) to a control system including a computer running specialized distribution management software  128 . A description of an example substation advisor gateway is included in the priority document of the present application, and has been previously incorporated by reference. In another embodiment, a capacitor bank controller may be used in place of a substation advisor gateway. A description of an capacitor bank controller is also included in the priority document of the present application. In one embodiment, access is provided to the information via web browser access  129 . 
     In some applications, as discussed in greater detail below, it is desirable for the electrical current information to include not only current magnitude, but also the phase angle of the measured current relative to a reference phase.  FIG. 2A  schematically illustrates an example arrangement for measuring current magnitude, as well as relative phase angle to an AC voltage signal. For simplicity, a distribution circuit  208  is shown as a single line. However, it is to be understood that circuit  208  may be a three phase circuit. A remote indicator device  212  is coupled to circuit  208 . Current transformers CT are coupled to each of the phases of the power distribution circuit  208  for measuring the current carried in each line. Voltage tap VT is coupled to one of the phases of circuit  208 , and includes a step-down transformer for converting the measured signal to an amplitude suitable for reading by electronic instrument. Each of the current transformers CT and voltage tap VT is connected to a respective analyzer  202 ,  204  that provides a vector angle corresponding to the phase angle of the measured parameter. Thus, the phase angles of the currents sensed by each of current transformers CT are represented by vectors VI (only one is shown in  FIG. 2B ), and the reference phase angle of the AC voltage, as sensed by VT, is represented by vector VE. Vectors VI and VE are fed to an analyzer (not shown) that is configured to measure the relative phase angles of each vector VI and vector VE. 
     In another example embodiment, the reference phase angle of the voltage of one of the lines is not tapped near the point of the current transformer CT, but instead is measured at the distribution station. Certain waveform characteristics of the line currents are transmitted to the monitoring station (not shown), where the relative phase angles are computed. For example, a communications channel such as communication  130   a  ( FIG. 1 ) can continuously or periodically carry phase synchronization information such as zero crossing points or wave crests of the current waveforms to a receiver and signal analyzer at the monitoring station. 
     In other embodiments, the reference phase is taken from current waveform monitoring, and vector VE would represent the current phase angle of the measured parameter. Such monitoring can be made of the current flowing in the neutral conductor at the remote indicator site or at another site. Alternatively, current waveform monitoring can be performed at a site different from the phase current monitoring location, such as line current monitoring at the distribution station. 
     Referring again to the arrangement of  FIG. 2A , under normal (non-fault) conditions, the phase angles of the current vectors VI will be about 120 degrees apart from one another. One of the phase angles of the VI vectors will be relatively near the phase angle of vector VE, the difference representing the extent of power factor degradation of the VE phase. 
     Assuming a particular VI and VE were derived from measurements made from different phases of circuit  208 , any dramatic change in their relative phase angles would be indicative of a fault condition occurring on either the VE or the VI phases. If, for example, the amplitude of vector VI drops suddenly, it can be inferred that the associated line has been broken. Referring to  FIG. 2B , if the phase angle of VI changes by 180 degrees relative to the VE reference phase angle, it can be inferred that a ground fault has occurred and reverse EMF generated by the load, rather than the utility, is now supplying the current in the line. 
     Referring now to  FIG. 3 , a method of analyzing current measurements for identifying and/or localizing a line fault according to one example embodiment is described.  FIG. 3  illustrates the measured current magnitude and direction of one particular line. In this example, direction refers to the relative phase angle of the current vector. FD 1 -FD 7  are each a current measurement made at a different point along the line. Measurements FD 1 -FD 4  are all in the same direction and have approximately equal magnitudes. Measurements FD 5 -FD 7  are dramatically different from FD 1 -FD 4 . Measurements FD 5 -FD 7  have an opposite direction and a different magnitude. Given this information, a control system can infer that a ground fault has occurred along a segment of line located between the measuring point of FD 4  and of FD 5 . A repair crew can be dispatched directly to the suspect segment of the distribution line. 
     According to another example embodiment, a method of identifying and/or localizing, faults involves measuring current flowing in the neutral wire of a power transmission or distribution circuit. It is understood that in cases of a line to ground fault (short circuit to ground), the power generated or distributed down the faulty line returns to its source (the generator or distribution point) through a ground path. In single phase or Wye-type three phase distribution circuits, the ground path includes the grounded neutral conductor. Therefore, one method of fault detection/localization is based on the principle that the neutral line will carry at least a detectable portion of the ground fault current. 
       FIGS. 4A and 4B  illustrate one example embodiment utilizing measurement of neutral current. Remote indicating devices having current measuring capabilities are placed along a power distribution line at points FD 1 -FD 7 , each measuring the current flowing in the neutral wire. At time 10:04:30, no fault condition is present and the neutral wire is carrying approximately 18 Amps. At time 10:05:00, the current measured at points FD 1 -FD 4  is substantially greater, while the current at FD 5 -FD 7  is somewhat less than what was measured previously. Given this information, a control system can infer that a ground fault has occurred along a segment of line located between the measuring point of FD 4  and of FD 5 . A repair crew can be dispatched directly to the suspect segment of the distribution line. The method of applying current measuring devices to the neutral wire, rather than the high voltage wires, provides significant reduction in measurement system equipment and installation cost. Other benefits, such as reduced system complexity and improved reliability, can also be realized. 
     Although specific embodiments have been illustrated and described herein for purposes of description of an example embodiment, it will be appreciated by those of ordinary skill in the art that a wide variety of alternate and/or equivalent implementations calculated to achieve the same purposes may be substituted for the specific embodiments shown and described without departing from the scope of the present invention. Those with skill in the mechanical, electromechanical, electrical, and computer arts will readily appreciate that the invention may be implemented in a very wide variety of embodiments. This application is intended to cover any adaptations or variations of the various embodiments discussed herein, including the disclosure information in the attached appendices. Therefore, it is manifestly intended that this invention be limited only by the claims and the equivalents thereof.