Abstract:
The present invention provides a new application for an inductive coupler ( 12 ) used proximate a power transmission line ( 14 ). The magnetic field based power measurement method and system discussed herein in unique and differs from the prior technology by using near-real time intervals and periods that occur systemically in parallel with electrical systems rather than an accumulated period with little or no active capacity or proactive features. The inductive coupler ( 12 ) described herewith is used to collect, measure, and/or extract electromagnetic changes.

Description:
TECHNICAL FIELD 
   The invention relates to management and distribution of electrical power, and in particular systems for monitoring the state of electrical power lines and distribution grids on a real time or near real time basis. 
   BACKGROUND OF THE INVENTION 
   Utility companies, power distribution companies and similar entities in the United States are currently handicapped by an inability to accurately monitor in real time the state of power lines and distribution grids. Thus, problems and inefficiencies often go undetected, resulting in needless expenditures and waste. The present invention is designed to address and rectify this situation. 
   Electrical power in the United States is largely transmitted in the form of alternating current at a predetermined frequency most frequently 60 Hz. As is known, when current passes through a transmission line, it generates an electro magnetic field that varies in frequency and intensity with the current flowing through the line. These fluctuations plotted against time, are in the form of a sinusoidal wave. As is also known, reflected waves traveling in the opposite direction of the current flow include harmonics, transients and variations that reflect conditions down the transmission line. Such harmonics, transients and variations in current and voltage as a function of time can be observed, characterized and analyzed using known mathematical techniques. See Arrillaga,  Power System Harmonic Analysis  (John Wiley &amp; Sons, 1996), chapters 4, 6, 7 and 9. Events occurring to the transmission line such as shorts, lightning strikes, changes in load and similar conditions also generate transient variations that are reflected in the electro magnetic field surrounding the transmission line. The magnitude and frequency of such harmonics, transients and variations can be measured and in accordance with the invention, the data collected and processed to reflect events and occurrences affecting the transmission grid. Thus, in accordance with the invention, monitoring and measuring changes in the electro magnetic field surrounding the transmission line, enables real time or near real time monitoring of the state of the transmission line. The state of the line may be charged with high or low voltage, or be without current or charge. As used herein the terms “real time” and “near real time” refers to time required for a computer to receive and process one or more input data streams and output a signal or value based upon the input data. Thus, in most instances, “real time” will be measured in seconds or fractions of a second. 
   Stewart U.S. Pat. No. 5,982,276, the disclosure of which is hereby incorporated by reference, describes a system for communicating information between subscribers over power transmission lines which normally convey electrical power to a plurality of diverse electrical sites for providing electrical power to electrical devices disposed at these diverse electrical sites. This communication system makes use of an inductive coupling to receive the transmitted information from the magnetic field surrounding the power transmission line. The inductive coupler is preferably a ferroceramic type of inductive coupler having a sensitivity such as 10 −23  volts. The present invention utilizes inductive couplers to detect changes in the electromagnetic field surrounding a transmission line, measures the changes and utilizes the data to identify conditions and events occurring on the transmission line. 
   SUMMARY OF THE INVENTION 
   The present invention provides a new application for an inductive coupler used proximate a power transmission line. The magnetic field based power measurement method and system discussed herein in unique and differs from the prior technology by using near-real time intervals and periods that occur systemically in parallel with electrical systems rather than an accumulated period with little or no active capacity or proactive features. The inductive coupler described herewith is used to collect, measure, and/or extract electromagnetic changes. 
   The present invention provides a system and method for gathering data from one or more inductive couplers. The collected data reflects power surges, demand shifts, and system breakdowns on a real time or near real time basis. Data collection may be timed by reference to an atomic clock signal in order to obtain accurate results from a plurality of locations at once. The system provides the capability of using the collected data in conjunction with stored data, such as historical data, local and nation power grid layout data, topographical information and dynamic information such as weather data to identify, characterize and locate power transmission problems and conditions that need attention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying description: 
       FIG. 1  is a schematic representation of a power management system according to the invention; 
       FIG. 2  is a flow chart illustrating steps utilized in the practice of the invention; 
       FIG. 3  is a graphical representation of an event detected by the system of  FIG. 1 ; 
       FIG. 4  is a schematic representation of an inductive coupler according to the invention. 
       FIG. 5  is a schematic representation of a power transmission grid employing a plurality of magnetic couplers to provide real time information reflecting the condition of the grid. 
   

   DETAILED DESCRIPTION 
   According to one embodiment of the invention as shown in  FIGS. 1 and 2 , a power management system  10  according to the invention includes one or more sensors (inductive couplers)  12  disposed in proximity to one or more power transmission lines  14 . In practice, coupler  12  is clamped onto the power line at a convenient location in order to detect fluctuations in the voltage and current flowing through transmission line  14 . As set forth in detail below, inductive coupler  12  includes an antenna and coils each of which are connected via a microwave cable  11  to a microwave converter  16  such as a HP model 8902 B analyzer, which filters the signals. Microwave cable  11  is configured similarly to a standard coaxial cable with a center conductor and an annular conductive shield, except that the shield of microwave cable  11  is typically a foil or layer of conductive material as opposed to the mesh used in typical coaxial cable. In the system illustrated in  FIGS. 1 and 2 , the center conductor of microwave cable is utilized as a first channel conductor and the shield is used as a second channel conductor. 
   As illustrated, a first two channel clock signal generator  18  and a second four channel clock signal generator  20  provide a clock signal on four channels, two of which are used for system calibration with the second two dedicated to sampling frequency. Currently available technology is limited to a clock frequency of approximately 300 Ghz, however, as faster clocks become available, it is anticipated that such faster signal generators may be utilized in the practice of the invention. The clock signals are input to a digital generator or oscilloscope  24  such as ESG signal generator model E4422B which in turn generates a reference waveform signal to converter  16  and spectrum analyzer  26 . In one variation, spectrum analyzer  26  is a Hewlett Packard Series 89400 vector signal analyzer. Spectrum analyzer  26  utilizes the clock signal to convert the analog signals from microwave converter  16  into digital form. The digital data or Bitstream from analyzer  26 , representing changes in the magnetic and electric fields surrounding power line  14 , is input to a processing system  30  including one or more computers  32  and communications interfaces  36   a ,  36   b ,  36   c  ( FIG. 2 ). Computer  32  may be local or remote relative to the other components. If computer  32  is at a distant location, then suitable means such as a network are provided for transmitting the output of analyzer  26  to computer  32 . 
   In order to prevent spurious inference from signals in the power supply from interfering with the operation of the system components, converter  16 , clock signal generators,  18  and  20 , oscilloscope  24 , analyzer  26  and computer  32 , along with any associated auxiliary components, are enclosed within a Faraday cage  22 . Additionally, the power source for these components is supplied from a source such as the primary side of the step down transformer supplying transmission line  14 , the source being filtered to further isolate the components from transmission line  14 . 
   As schematically represented in  FIG. 2 , system  30  includes a historical database  40  reflecting prior states of the transmission line  14  including a “normal state,” data associated with prior events such as, lightning strikes, line failures or shorts and other abnormal conditions, along with the classification of such events. Database  40  is constructed using frequency and power measurements from inductive coupler  12  and associating the occurrence of known events such as lightning strikes, grounds, equipment failures and similar conditions with transients measured at the time of the event. For example, if a transformer shorts out, inductive coupler  12  will detect a transient resulting from the short and the data associated with the transient will be recorded in data base  40 . When the cause of the transient is identified, it is logged into the database by an operator. Subsequently, when computer  32  observes a transient having the same or similar characteristics, it will identify the source of the transient as a transformer failure based upon the data recorded in historical database  40 . 
   An hypothetical example of an event is graphically represented in  FIG. 3  wherein solid line represents historical data corresponding to the amplitude of a selected parameter, for example reflected voltage at a given frequency over a period p. A change in the amplitude of the amplitude of the parameter at a point during the period, represented by the dotted line at A, indicates an event or changed condition affecting the transmission of power over line  14 . When computer  32  registers the change, the computer will search database  40  for a similar occurrence associated with a known event or condition and activate an alarm to alert the operator. It will be appreciated that the hypothetical example represented in  FIG. 3  is only for the purpose of illustration; the actual parameters used to characterize the state of the transmission line may vary from case-to-case depending upon the particular system and will include values that are represented by complex numbers such as impedance or admittance (phasors), or multidimensional quantities (tensors) that include a component for each of a plurality of dimensions. See Arrillaga,  Power System Harmonic Analysis  (John Wiley &amp; Sons, 1996)(chapter 10, describing iterative harmonic analysis). Analysis of the electro magnetic field surrounding a three-phase transmission line, in which the phases are 120° apart, becomes even more complex. 
   System  30  also includes a grid map of the local distribution system along with a national grid map maintained on one or more databases  42 , along with National Means Data relating to the national transmission and distribution system that is maintained in a database  44 . Geo-Spacial data such as the topography of the local distribution area is maintained on yet another database  46  which is linked via a graphic to a dynamic data collection, retrieval and display platform  47  such as Boeing Autometric&#39;s EDGE® system which includes tools for integrating imagery, maps, terrain, models and weather data. 
   Preferably, databases  40 – 46  are maintained on local storage media such an internal hard disk in computer  32 , however, the data bases may be maintained on remote devices, accessed by computer  32  via a wireless or hardwired connection such as a telephone modem or network. In some cases, it may be desirable to distribute the computing function, depending upon the amount of data collected, the size of databases  40 – 46  and the desired output, in which case additional computers located either locally or remotely may be employed in addition to computer  32 . 
   In operation, computer  32  receives collected and processed data from analyzer  26  and incrementally compares the data to historical data for a period corresponding to a portion of the base frequency cycle (60 Hz). For example, data collected for a period of one millionth of second beginning at a selected location in the cycle (between 0 and 360 degrees) can be compared to the corresponding increment from the historical database at step  50 . If an abnormal condition is detected, typically in the form of a transient or a variation from the “normal” condition of transmission line  14 , computer  32  attempts to identify the source of the abnormality by comparing the variation in the waveform to abnormalities associated with historical events stored in database  40 . 
   For example, a lightning strike will generate a very large, very rapid increase in voltage along with an increase in current. On the other hand, the sudden opening of a large circuit breaker may also result in a voltage surge, however the duration of the surge will be different and the current will drop. Computer  32  will distinguish and identify the two different events based upon historical data associated with similar events that occurred in the past. 
   Depending upon the classification and magnitude of the event, computer  32  may also alert the system operator with an alarm. In the event that computer  32  is unable to classify the detected abnormality, computer  32  creates a new event classification for inclusion in the database. When the source or event causing the particular abnormality is identified, the database is updated. 
   In addition to identifying abnormal conditions and the source of such conditions, computer  32  determines the distance of the event or occurrence from the inductive coupler through the use of known mathematical techniques. See Chowdhure,  Electromagnetic Transients in Power Systems,  (1996, Research Studies Press LTD and John Wiley &amp; Sons, Inc., Chapters 2, 8 and 10). For example, in the case of a feeder line connected to several step-down transformers, a lightning strike to one of the stepped down lines will result in voltage and current transients as described above in the feeder line. Using one or more of historical data from database  40 , the distance from the inductive coupler and the local grid map from data base  42 , computer  32  can determine which of the segments was struck. Utilizing the local grid map from data base  42 , computer  32  can then generate a graphical display including the grid map showing the location of the strike and output the map via interface  36   b  to a display such as CRT monitor  54 . Further, in a preferred embodiment, computer  32  accesses Geo-Spacial data base  46  to include the topography of the area where the strike occurred into the graphical display, including access roads and similar information. If desired, computer  32  can also access data platform  47  to incorporate additional real time information such as weather conditions that may be the source of an abnormal condition along with other potentially useful information into the graphical display. All of the forgoing functions are performed on a real time basis. 
   Turning to  FIGS. 1 and 4 , an inductive coupler  12  clamped onto 13 KVA transmission line  14  to detect changes in the electro magnetic field surrounding the line includes first and second windings or coils  62  and  64  formed from a continuous piece of number  2  copper/aluminum alloy wire connected at a first end  63  to the center conductor of microwave cable  11  and terminating at an unconnected second end. Windings  62  and  64  are spaced approximately 10 cm apart and each include five turns  61  with a diameter of approximately 3 cm that are offset from a line perpendicular to a longitudinal axis of coupler  12  at an angle α. Since coupler  12  is mounted with its longitudinal axis substantially parallel to transmission line  14 , windings  62  are  64  are also offset from a line perpendicular to transmission line  14 . As illustrated, windings  62  and  64  are configured and positioned to detect changes in the magnetic field surrounding transmission line  14  reflecting changes in the current flowing through the transmission line. The spacing of the coils relative to each other results in simultaneous measurements corresponding to two points on a waveform propagated along transmission line  14 . 
   Since windings  62  and  64  are angled relative to a line perpendicular to transmission line  14 , the current induced in windings  62  and  64  will be phase shifted relative to the current flowing through transmission line  14 . Angle α may be varied within a range greater than 0 up to the limits imposed by the geometry of the other components of coupler  12 ; however, in a preferred embodiment, α is in the range of from 10 to 14 degrees and most preferably about 12 degrees (between 11 and 13 degrees). 
   Disposed within windings  62  and  64  is a millimeter band radio wave antenna  66  comprising a set of parallel grading windings  65  formed from a continuous length of number 2 copper/aluminum alloy having a first end  67  connected to the shield of microwave cable  11  and terminating at a second unconnected end. As illustrated, grading windings  65  of antenna  66  are positioned parallel to transmission line  14 . Antenna  66  is configured by fractional wavelengths of harmonics based upon a 60-Hz transmission frequency with the length of the longest grading winding  65  being approximately 7.5 inches and the length of each of the shorter windings determined by parametric estimation of the differences between the wavelength of the harmonics. Antenna  66  is capable of detecting signals having a frequency of up to approximately 300 Ghz. As will be appreciated, antenna  66  is designed and positioned to detect the frequency and magnitude of voltage changes in transmission line  14 . Further, since windings  62  and  64  are angled relative to antenna  66 , the signal from windings  62  and  64  will also be phase-shifted relatively to the signal from antenna  66 . Generating phase-shifted signals in this fashion provides for ease of computation when employing the above-referenced mathematical techniques used to process the signals from the windings and the antenna. 
   Windings  62  and  64  along with antenna  66  are enclosed in a non-conductive body or housing  70  such as a length of PVC pipe. In a preferred embodiment, the interior surface of housing  70  is coated with a conductive paint  72  in order to protect coupler  12  from lightning strikes. During the assembly of coupler  12 , housing  70  is filled polycarbonate filler material  74  that serves to maintain windings  62  and  64  and  66  in position within the housing and isolate the windings and antenna from each other and from conductive coating  72 . Housing  70  is also provided with end caps  76 , (one shown) that are glued or otherwise secured onto housing  70 . First end  63  of the wire forming windings  62  and  64  and first end  67  of the wire forming antenna  66  extend through a hole (not shown) in one of end caps  76  and are connected to microwave cable  11  in the manner described above. 
   As will be appreciated, currents and voltage fluctuations in transmission line  14  will induce corresponding currents and voltages in coils  62 ,  64  and antenna  66 . Power management system  10  measures the magnitude, frequency and duration of these currents and voltages to identify conditions and events occurring on transmission line  14 . Further, power management system  10 , utilizing the appropriate mathematical techniques, can determine values for time and frequency dependent parameters such as impedance and reactance, as well as identifying changes in inductance and capacitance by analyzing changes or shifts in phase of voltage and current. Additionally, even more complex parameters, represented as tensors, may be determined using the data obtained via magnetic coupler  12 . Each of such parameters provides a means of characterizing or modeling the condition of transmission line  14  on a real time basis that individually or when combined with other parameters, provides a means of identifying and characterizing events and conditions on transmission line  14  that has not been previously been available. 
   Referring now to  FIG. 5 , a problem encountered in “wheeling” power between power grids is the lack of sufficient real time information needed to efficiently distribute and allocate available power over a distribution system. As described below, in one embodiment, the invention addresses this problem by supplying real time information that enables power distribution and redistribution between power grids on a real time basis.  FIG. 5  illustrates a system according to the invention for monitoring a number of power transmission lines forming all or part of power distribution system or grid  80  includes a plurality of inductive couplers  82  positioned proximate power transmission lines  84  at different locations within for detecting voltage and current fluctuations in the power transmission lines. Each of couplers  82  is connected to a transmitter  86  that transmits a signal corresponding to the current and voltage fluctuations detected with the coupler to a receiving and processing station  90 . Transmitters  86  may be wireless units such as a radio frequency transmitter or a hardwired unit such as a telephone modem and may be provided with any required signal processing equipment such as clocks, filters, converters and oscilloscopes as needed to process the collected data into a transmittable signal. The signal from each transmitter  86  is received at a central processing station  90 , decoded, processed and integrated with the signals from other transmitters  86  to provide real time or near real time data reflecting the condition of the distribution system  80 . The availability of this information on a real time basis enables efficient distribution and allocation of power over distribution system  80 . 
   While certain embodiments of the invention have been illustrated for the purposes of this disclosure, numerous changes in the method and apparatus of the invention presented herein may be made by those skilled in the art, such changes being embodied within the scope and spirit of the present invention as defined in the appended claims.