Electrical noise detector for detecting power line gap defects
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to apparatus for detecting faults in electrical transmission and/or distribution apparatus, and more specifically, to apparatus for detecting gap-type defects in electrical power line equipment in response to broadband radio frequency noise generated thereby. 
     2. Description of the Prior Art 
     Radio frequency noise has many causes. Of special interest to electric utilities is noise caused by arcing of charge across small gaps in transmission and distribution power line equipment. These gaps may develop over time due to expansion and contraction around metal hardware, the build-up of corrosion around metal hardware, or occasionally, improper installation. Arcing occurs when charge builds up on one side of the gap and discharges across the gap, most frequently near the peaks of the sinusoidal power curve. A multiplicity of discharges occur near each peak of the power curve. Each discharge generate broadband electrical noise which may cause interference to communication systems. 
     One method to detect such gaps defects is to locate and analyze the electrical noise generated thereby. The analysis must differentiate between gap-type noise and noise due to other sources, such as ignition noise, atmospheric noise, and corona noise. 
     The analysis may require the displaying of signals received by a radio receiver on an oscilloscope. Such analysis requires trained personnel to recognize the pattern of noise generated by gap-type defects and AC power line equipment. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a device for identifying a defect in power line equipment includes an antenna coupled to a radio frequency receiving means to detect broadband radio frequency electrical noise. The receiving means produces first electrical signals indicative of the received radio frequency noise. A means is provided for blocking the first electrical signals below a predetermined magnitude. Circuitry is provided for identifying the time periods in which the first electrical signals are beyond the predetermined magnitude. Further means exist in order to determine if these time periods have a specific periodic frequency. If such periodic frequency is detected, means, such as an alarm light, identifies such detection. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may be better understood, and further advantages and uses thereof more readily apparent, when considered in view of the following detailed description of exemplary embodiments, taken with the accompanying drawings in which: 
     FIG. 1 is a functional block diagram of a radio frequency noise detector constructed according to the teachings of the present invention. 
     FIG. 2 is a graph which illustrates a typical pattern of radio frequency electromagnetic signals over a period of time. 
     FIG. 3 is a graph which illustrates a typical pattern of radio frequency electromagnetic signals over a period of time with signals below a certain magnitude attenuated. 
     FIG. 4 is a graph which illustrates a typical pattern of radio frequency electromagnetic signals over a period of time after passing through a sample and hold circuit. 
     FIG. 5 is a graph which illustrates a typical pattern of single frequency periodic pulses. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, there is shown apparatus 10 constructed according to the teachings of the invention for detecting electrical noise produced by arcing across gaps in electrical power line equipment. Apparatus 10 includes a conventional antenna 12. Antenna 12 may be of the isotropic type, for example, and it may be disposed in an open area to receive electromagnetic radiation emitted from a suspected radiation source. Typically, a gap-type radiation source associated with the transmission and distribution of electrical power produces discrete discharges of radiation, with each discharge being a complex waveform comprised of many different frequency components. Antenna 12 converts local electrical noise as well as electromagnetic radiation from a gap discharge into electrical signals. A conventional radio frequency receiver 14 is coupled to the antenna 12. Receiver 14 may be of the amplitude modulation type, for example. Receiver 14 is tuned to receive signals of a particular frequency and particular bandwith and function to demodulate the electrical signals within the tuned bandwidth. The demodulated electrical signals may, over a period of time, be depicted as the pulse train 16 shown in the graph of FIG. 2, wherein each line of the graph represents a pulse of electrical noise. The abscissa is representative of time, and the ordinate is representative of the voltage magnitude of the demodulated electrical signals. 
     The demodulated pulse train 16 of electrical signals produced by the radio receiver 14 is coupled to an input of a squelch circuit 18. A reference voltage 20 is coupled to a second input of the squelch circuit 18. When the voltage magnitudes of the demodulated electrical signal pulses are greater than that of the reference voltage 20, the squelch circuit 18 allows the demodulated electrical signal pulses to pass therethrough. However, when the voltage magnitudes are less than that of the reference voltage 20, the squelch circuit 18 prevents any signal from passing, thereby preventing passage of any signal pulse which is less than a predetermined voltage magnitude. The resultant pulse train of signals passed by the squelch circuit 18 over a period of time may be depicted by the pulses 21 shown in FIG. 3. Accordingly, the demodulated electrical signal pulses having a voltage magnitude less than a predetermined level are blocked out, while signal pulses having voltage magnitudes greater than the predetermined level are passed without alteration. 
     Referring to FIG. 1, these passed electrical signals 21 are coupled to a sample and hold circuit 22 in order to periodically sample the passed electrical signals 21 during discrete sampling periods. When the sample and hold circuit 22 detects the existance of a passed electrical signal 21 during its sample period, the circuit 22 produces a signal of a magnitude equal to the magnitude of the sampled passed electrical signal and holds the magnitude of the produced signal at this level for the remainder of the sampling period. When the sample and hold circuit 22 does not detect the existance of a passed electrical signal 21 during its sample, the circuit 22 produces a signal of zero magnitude and holds the magnitude of the produced signal at this level for the remainder of the sampling period. The sample and hold circuit 22 thereby effectively envelopes the signal pulses 21. The waveform produced by the sample and hold circuit is shown in FIG. 4. The envelopes 24 now appear as a train of wider pulses in time which may be periodic. 
     The new pulse train of pulse waveforms 24 generated by the sample and hold circuit 22 is passed through a notch filter circuit 26 in order to remove all pulses 24 from the new pulse train except those pulses that repeat at a predetermined particular frequency which is uniquely identified with gap-type noise in powerline equipment, e.g. 120 hertz in a 60 hertz system. 
     In other words, since it is known that gap-type noise on a 60 hz power line occurs in a repetitive manner and is primarily associated with a 120 hz frequency, the notch filter circuit 26 is configured so that only noise that recurs at the predetermined frequency associated with the gap-type noise is passed to the next circuit and electrical noise occurring at other frequencies and for reasons other than a gap defect, can be rejected. 
     If no pulses 24 recur at the predetermined frequency, no pulses 24 are passed. An example of the filtered pulse train output by the filter circuit 26, which includes pulses or waveforms 28 is shown in FIG. 5. 
     The existence of the filtered pulse train is externally indicated to attending personnel. For example, an audio signal may be provided by a speaker 30 after the pulse train is amplified by an amplifying circuit 32 shown in FIG. 1. In addition, or as an alternative, the filtered pulse train may be rectified by a rectifier circuit 34. The rectified signal produced by the rectifier circuit 34 is coupled to one input of a comparator circuit 36. A known reference voltage 38 is coupled to a second input of the comparator circuit 36. When the voltage magnitude of the rectified signal is greater than that of the reference voltage 38, the comparator circuit 36 produces an output signal which may be used to power or drive an alarm device 40 such as a light, for example. Either an indication from the speaker 30 or from the alarm device 40 represents the detection of a gap-type fault in nearby power line equipment. 
     When used to detect sources of gap-type noise, the detector may be placed in the vicinity of functioning power line equipment. From this vicinity, all local radio frequency electrical noise, including noise due to discharges across any gap in the adjacent power line equipment is readily detected. The local electrical noise is detected by the antenna 12 of FIG. 1, producing electrical signals in the antenna 12. The radio receiver 14 receives and demodulate the electrical signals and produces demodulated signals indicative of the local radio frequency noise. The squelch circuit 18 attenuates signals having a magnitude less than a reference voltage 20, and passes only those signals greater than the reference voltage 20. These passed signals are coupled to the sample and hold circuit 22 providing a waveform envelope containing pulses indicative of local electrical noise and any gap noise. This waveform is passed through a notch filter circuit 26, with the filter circuit passing only those pulses which repeat at a predetermined frequency, e.g. 120 hertz, thus separating gap noise from noise due to other sources. The filtered signal, now representative of only gap noise, may be amplified by an amplifying circuit 32 and outputted to a speaker 30 whereby an attendant may be notified of the presence of the gap noise. In addition, or as an alternative, the filtered signal may be rectified by a rectifier circuit 34, and if the rectified signal exceeds a reference voltage 38, an alarm device 40 may be powered. 
     While the present invention has been described in connection with the preferred embodiment shown in FIG. 1, it is understood that other similar embodiments may be used or modifications and additions may be made to the described embodiment for performing the same functions of the present invention without deviating therefrom. Therefore, the present invention should not be limited to any single embodiment, but rather construed in breadth and scope in accordance with the recitation of the appended claims.