Abstract:
The present invention relates to a non-contact arc detection, recognition, and warning apparatus for the detection of arcing in an electrical system. The apparatus includes an electric field detector having first and second electric field sensors adapted to sense electric fields; a magnetic field detector having a multi-axis magnetic field sensor adapted to detect a magnetic field; and a digital signal processor. The digital signal processor compares and enhances signals obtained from the electric field detector and the magnetic field detector and creates a response to alert a user of the presence of arcing.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to arc detection, and more particularly, to a non-contact arc detection apparatus and method. 
     The presence of arcing poses significant safety issues to individuals working in and around power distribution components. Historically, arc detection has been performed by level detection schemes from current sensing arc detectors in contact with the power line conductor, level detection by current sensors coupled to a power line conductor, rate-of-change of current or voltage from current transformers connected to the power line conductor, the change in phase relationship of the current on the power line conductor compared to an externally generated signal, and the comparison of the phase plane plot change of the current and voltage on the power line conductor. Each of the above methods and apparatuses have required direct contact, coupled sensing, or direct connection to the monitored power line. Further, methods and prior art have been direct or coupled to only single power lines. 
     In the case of underground vaults, workers often encounter more than a single power line, resulting in the need to monitor each line. In addition to the need to know of potential electrical safety hazards prior to entering the vaults, workers need to be able to monitor potential arcing problems while working with distribution cables and components in these vaults to alert them when arcing conditions occur. 
     Limitations of the above prior art methods include problems associated with unknown vault conditions prior to vault egress and the condition of installed equipment in vaults where vault flooding may compromise electronic equipment and lack of early warning devices and methods during the time work is performed in the vault. Further limitations of the number of cables in the vault requiring monitoring are apparent. 
     BRIEF SUMMARY OF THE INVENTION 
     These and other shortcomings of the prior art are addressed by the present invention, which provides an apparatus and method to perform non-contact arc detection, recognition, and warning on electrical power lines. 
     According to one aspect of the present invention, a non-contact arc detecting apparatus includes an electric field detector having first and second electric field sensors adapted to sense electric fields; a magnetic field detector having a multi-axis magnetic field sensor adapted to detect a magnetic field; and a digital signal processor adapted to compare and enhance signals obtained from the electric and magnetic field detectors and create a response to alert a user of the presence of arcing. 
     According to another aspect of the present invention, a portable non-contact arc detection, recognition, and warning apparatus includes an electric field detector, a magnetic field detector, and a digital signal processor. The electric field detector includes first and second electric field sensors adapted to sense electric fields; first and second AM receiver circuits electrically connected to the first and second electric field sensors to transform signals received from the first and second electric field sensors into LO-band and HI-band signals; and first and second demodulators electrically connected to the first and second AM receiver circuits to receive the LO-band and HI-band signals and resolve audio frequencies of the LO-band and HI-band signals in the 1 Hz to about 20 KHz range to form DEMOD-LO and DEMOD-HI signals. The magnetic field detector includes a multi-axis magnetic field sensor adapted to detect a magnetic field; a first HI-pass filter electrically connected to the multi-axis magnetic field sensor to form a first buffered HI-pass signal; a second HI-pass filter electronically connected to the multi-axis magnetic field sensor; and a series of three 12 th  order notch filters electrically connected to the second HI-pass filter to form a second buffered HI-pass signal. The digital signal processor is adapted to receive the DEMOD-LO, DEMOD-HI, first buffered HI-pass, and second buffered HI-pass signals from the electric field and magnetic field detectors to compare and enhance the signals and create a response to alert a user of the presence of arcing. 
     According to another aspect of the present invention, a method of determining an arcing condition includes the steps of providing an arc detection device having an electric field detector, a magnetic field detector, and a digital signal processor. The method further includes the steps of simultaneously detecting waveform signatures in both electric and magnetic fields using the electric and magnetic field detectors; enhancing the signatures using signal processing techniques; using the digital signal processor to form time coordinated signals of the enhanced signatures; using the digital signal processor to compare the time coordinated signals to a third derivative signal of the magnetic field to determine arcing conditions; and reporting arcing conditions to a user. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject matter that is regarded as the invention may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which: 
         FIG. 1  is a schematic drawing of a non-contact arc detection and warning apparatus according to an embodiment of the invention; and 
         FIG. 2  is a block diagram of a method according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the drawings, an exemplary non-contact arc detection, recognition, and warning apparatus according to the present invention is illustrated in  FIG. 1  and shown generally at reference numeral  10 . 
     The apparatus  10  performs non-contact arc detection, recognition, and warning of arcing on electrical power lines by simultaneously evaluating time coordinated characteristic waveform signatures in both the Electric (E) and Magnetic (H) fields and further comparing those time coordinated electric and magnetic field signatures with a time coordinated third derivative signal of the Magnetic field (H). 
     The apparatus  10  includes an electric field (E) detector  11 , a magnetic field (H) detector  12 , and a digital signal processor  13 . The electric field detector  11  includes first and second electric field sensors  14  and  16 , respectively, for sensing electric fields (E). The first and second electric field sensors  14  and  16  may be of helical construction having a resonant frequency of 300 MHz. Other suitable constructions may be used. The first electric field sensor  14  is electrically connected to a first AM receiver circuit  17  which receives a signal from the first electric field sensor  14 . The first AM receiver circuit  17  operates in a 100 kHz to 300 kHz LO-band and transforms the signal into a LO-band signal. A first demodulator  18  is electrically connected between the first AM receiver circuit  17  and the digital signal processor  13 . The first demodulator  18  demodulates the LO-band signal into a DEMOD-LO signal to resolve audio frequencies in the 1 Hz to 20 KHz range and sends it to the digital signal processor  13  to be processed and compare even harmonics and inter-harmonics of a 60 Hz fundamental frequency to a pre-determined threshold. 
     The second electric field sensor  16  is electrically connected to a second AM receiver circuit  19  which receives a signal from the second electric field sensor  16  and transforms the signal into a HI-band signal. The second AM receiver circuit  19  operates in a 1 MHz to 5 MHz HI-band. A second demodulator  20  is electrically connected between the second AM receiver circuit  19  and the digital signal processor  13 . The second demodulator  20  demodulates the HI-band signal into a DEMOD-HI signal to resolve audio frequencies in the 1 Hz to 20 KHz range and sends it to the digital signal processor  13  to be processed and compare even harmonics and inter-harmonics of a 60 Hz fundamental frequency to a pre-determined threshold. 
     The magnetic field (H) detector  12  includes a multi-axis magnetic field sensor  21  electrically connected to first and second HI-pass filters  22  and  23 . The magnetic sensor  21  includes five coils wound on five ferrite rods in series connection and physically placed in a 45 degree orientation. It should be appreciated that other appropriate orientations or combinations may be used. 
     The first HI-pass filter  22  is a 10 KHz HI-pass filter. The first HI-pass filter  22  is electrically connected to a gain amplifier  24  and forms a buffered HI-pass signal (H 10K). The HI-pass signal is amplified by the amplifier  24  and sent to the digital signal processor  13 . 
     The second HI-pass filter  23  is a 200 Hz HI-pass filter. The second HI-pass filter  24  is electrically connected to a series of three 12 th  order, notch filters  26 ,  27 ,  28  and a gain amplifier  29 . Notch filter  26  is a 300 Hz notch filter connected in series to the 200 Hz Hi-pass filter output. Notch filter  27  is a 420 Hz notch filter connected in series to the 300 Hz notch filter  26  output. Notch filter  28  is a 540 Hz notch filter connected in series to the 420 Hz notch filter  27  output. The gain amplifier  29  is electrically connected to the output of the 540 Hz notch filter  28 . A buffered HI-pass signal (H CONDX) is formed by the filters  23 ,  26 ,  27 ,  28 , and gain amplifier  29 , and is sent to the digital signal processor  13 . 
     Referring to  FIG. 2 , a method according to an embodiment of the invention is shown generally at reference numeral  30 . It should be appreciated that the present invention may apply to single power line conductors, multiple power line conductors, arcing to grounded objects, or arcing to other proximate power line conductors. 
     In general, the method includes the steps of (1) sensing the electric field (E) and magnetic field (H); (2) forming two signals from the sensed electric field into two bands (LO-band and HI-band); (3) demodulating the two LO-band and HI-band signals; (4) presenting the two demodulated signals DEMOD-LO and DEMOD-HI to the digital signal processor  13 ; (5) sensing the magnetic field using a multi-axis magnetic field sensor  21 ; (6) forming an additional two signals H 10K and H CONDX from the sensed magnetic field sensor  21 ; (7) presenting the two signal conditioned signals H 10K and H CONDX to the digital signal processor  13 ; (8) using the digital signal processor  13  to compare and analyze frequency components from signals DEMOD-LO and DEMOD-HI; (9) comparing the magnitude of the third derivative of signal H CONDX to a preset level; (10) comparing the magnitude of the signal H 10K to a preset level; (11) determining a result as to arcing, voltage, or other conditions from the time coordinated four signals; and (12) enunciating the result via tone alerts and indicator status lights. 
     More particularly, the method includes sensing electric fields (E) from two distinct electric field sensors  14  and  16 , Block  31 . Electric field signals representative of the sensed electric fields from the sensors  14  and  16  are received by AM receiver circuits  17  and  19 , respectively, Block  32 . The output of each of these receivers  17 ,  19  (signal LO-band and signal HI-band) is separately demodulated (signal DEMOD-LO and signal DEMOD-HI) by a respective one of demodulators  18  and  20  and provides an audio range signal for further processing by digital signal processor  13 , Block  33 . These two signals, DEMOD-LO and DEMOD-HI, are independently connected to analog inputs of the digital signal processor  13 . The digital signal processor  13  processes the demodulated frequencies to compare even harmonics and inter-harmonics of 60 Hz, Block  34 , and forms time coordinated signals of the signals, Block  36 ,—DEMOD-LO and DEMOD-HI using digital signal processing and analog techniques. 
     The method further includes the steps of simultaneously sensing magnetic fields (H) from multi-axis magnetic field sensor  21 , Block  37 , buffering the signal representing the sensed magnetic fields (H), and forming two distinct signals, Blocks  38 - 40 . The first signal (H 10K) is enhanced from the output of a Hi-pass filter stage (HI-pass filter  22  and gain amplifier  24 ) series connected to the magnetic field sensor  21 , Block  39 . The second signal (H CONDX) is enhanced from the output of a signal conditioning stage (filters  23 ,  26 ,  27 ,  28  and amplifier  29 ) series connected to the magnetic field sensor  21 , Block  40 . The first and second signals (H 10K and H CONDX) are independently connected to analog inputs of the digital signal processor  13 . The digital signal processor  13  compares the non-harmonically related frequencies of a 60 Hz fundamental frequency to a pre-determined threshold, Block  41 , and forms time coordinated signals of the signals, Block  42 . 
     It should be appreciated that the high order filter designs and the use of specific notch frequencies are determined as harmonics of a 60 Hz fundamental frequency. It should also be appreciated that the signal conditioning stage may be implemented as a series of analog filters or a digitally processed signal in a digital signal processor. The choice of notch frequencies represents the 5 th , 7 th , and 9 th , harmonic of the 60 Hz fundamental in the preferred embodiment. It should be appreciated that other fundamental frequencies and harmonic frequencies may be chosen for the signal conditioning stage. 
     The method also includes the steps of using the digital signal processor  13  to (a) evaluate the components of even harmonics found in signals DEMOD-LO and DEMOD-HI, Block  34 , (b) compare the second harmonic magnitude found in signal DEMOD-LO to a reference signal, Block  34 , (c) compare the second harmonic magnitude found in signal DEMOD-HI to a reference signal, Block  34 , (d) form a third derivative of the signal H CONDX, Block  43 , (e) compare the status of time coordinated electric (E) and magnetic (H) field signals with the time coordinated third derivative signal of the magnetic field (H) to determine if an arc is present, Block  44 , and (f) send a signal to report via distinct tones and indicators the status of arcing conditions thus determined, Block  46 . 
     It should be appreciated that the magnetic and electric field sensors may be combined into a single package sensor such that it may be safely lowered into an underground vault to determine the safety conditions in the vault prior to workers entering the vault. It should be appreciated that the apparatus  10  may be a hand held apparatus to allow use of the apparatus  10  in tight confines and to allow easy transport. Further, once in the vault, the apparatus will continue to monitor arcing conditions to alert workers to arcing conditions. 
     It should be appreciated that the digital signal processing of signals may be accomplished using analog circuit elements in other embodiments of the invention. 
     The foregoing has described a non-contact arc detecting apparatus and method. While specific embodiments of the present invention have been described, it will be apparent to those skilled in the art that various modifications thereto can be made without departing from the spirit and scope of the invention. Accordingly, the foregoing description of the preferred embodiment of the invention and the best mode for practicing the invention are provided for the purpose of illustration only and not for the purpose of limitation.