Abstract:
The present invention is a voltage detector that is capable of measuring AC voltages, especially the high voltages encountered by electric utility linemen, with improved accuracy. To obtain these improvements in accuracy the present invention includes a novel circuit, which can be incorporated into a standard digital voltage detector. This circuit is able to accurately determine the magnitude of external capacitive reactance, which allows the voltage detector to compensate for variances in the external capacitive reactance that, if uncompensated, could adversely affect the voltage measurement; consequently, this invention provides the user with an AC voltage detector having improved accuracy characteristics.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/327,481, filed on Oct. 5, 2001, which is incorporated herein by reference. Applicant claims the priority date benefits of that application. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a high voltage measuring device and, more particularly, to a high voltage detector that can be used for measuring voltages in high voltage AC circuits or systems with improved accuracy. 
     Voltage detectors are a form of voltmeter that measure voltages without the use of a ground lead. Typically voltage detectors are used for measuring high alternating current (“AC”) voltages such as those encountered by electrical power utility linemen. To make measurements, voltage detector designers know that they need to determine the magnitude of the alternating current flowing through the voltage detector, and they also know that the magnitude of the alternating current being measured is a function of three things: (1) the internal impedance of the device, which is a known quantity; (2) the external capacitive reactance between the device and electrical ground; and (3) the magnitude of the voltage source being measured, e.g., the voltage carried through a high voltage conductor. To be able to determine the unknown voltage magnitude, the detector designers need to determine or assume a value for the external capacitive reactance. To the best of Applicant&#39;s knowledge, all voltage detectors manufactured today assume a nominal value for the external capacitive reactance, which results in the detector providing a nominal voltage reading based on the capacitive reactance value assumed. However, since external capacitive reactance is a variable that is based on the many conditions under which the detector may be used, e.g., the external capacitive reactance is greatly affected by the height of the detector above ground, this assumed value for external capacitive reactance can cause an accuracy problem, i.e., the voltage measurements taken will be in error whenever the external capacitive reactance is above or below the assumed value. 
     SUMMARY OF THE INVENTION 
     According to its major aspects and broadly stated, the present invention is a voltage detector that is capable of measuring AC voltages with improved accuracy. To obtain these improvements in accuracy the present invention includes a novel circuit, which can be incorporated into a standard digital type voltage detector. Instead of relying on an assumed, i.e., fixed, external capacitive reactance value, which will lead to AC voltage measurement errors whenever the actual external capacitive reactance differs from the assumed value, this detector is designed to determine the phase angle difference between the applied voltage, e.g., the electrical transmission line voltage being measured, and the resulting current in an alternating current system. 
     Generally stated, in a purely resistive AC circuit the phase angle difference between the voltage and current waveforms is zero degrees (0°); in a purely capacitive AC circuit the phase angle difference between the voltage and current waveforms will be ninety degrees (90°), with the current waveform leading the voltage waveform by this angular amount; and, in a combination resistive-capacitive circuit, the phase angle difference will be in the range between 0° and 90°. By using this phase angle difference concept, i.e, that the phase angle between the voltage and current waveforms is a function of the impedance characteristics of the circuit or system, the present invention is able to determine the actual value of the external capacitive reactance instead of relying on an assumed value for this characteristic and, therefore, is able to provide voltage measurements with improved accuracy by compensating for the variances in the magnitudes of the external capacitive reactance from measurement to measurement. 
     A major advantage of the present invention is that its design can be used, and/or modified, to measure AC voltages with improved accuracy over a wide range of magnitudes. 
     Another advantage of the present invention is that the simplicity of design of the novel circuit allows the circuit to be substituted and, therefore, used in a wide variety of meter configurations. 
     These and other features and their advantages will be apparent to those skilled in the art of from a careful reading of the Detailed Description of a Preferred Embodiments accompanied by the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the figures, 
     FIG. 1 is a diagram of a purely resistive AC circuit having a 60 Hertz voltage source and a resistive load, and the resultant voltage and current waveforms measured relative to the load; 
     FIG. 2 is a diagram of a purely capacitive AC circuit having a 60 Hertz voltage source and a capacitive load, and the resultant voltage and current waveforms measured relative to the load; 
     FIG. 3A is a diagram of a meter circuit having a known internal resistance and external capacitive reactance; 
     FIG. 3B is a diagram of a meter circuit having a known internal capacitive reactance and external capacitive reactance; 
     FIG. 4A is a diagram of the detector showing the internal resistance path signal being developed, according to a preferred embodiment of the present invention; 
     FIG. 4B is a diagram of the detector showing the internal capacitive reactance path signal being developed, according to a preferred embodiment of the present invention; and 
     FIG. 5 is a diagram of the detector showing two internal resistance paths, which may have different resistance values, according to another preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the drawings, the present invention is a voltage detector having improved AC circuit voltage measurement accuracy. The voltage detector, is generally referred to by reference number  10  and the novel detector circuit is generally referred to by reference number  20 . 
     Referring now to FIGS. 1-3, representations of typical AC circuits and their associated voltage and current waveforms are shown. It is well known that in a purely resistive AC circuit the resultant voltage and current waveforms are “in-phase” with each other or, in other words, the phase angle difference between the two waveforms is zero degrees (0°) (as shown in FIG.  1 ). In the purely capacitive AC circuit the current waveform leads the voltage waveform and the phase angle difference between the two waveforms is ninety degrees (90°) or, in other words, the waveforms are “out-of-phase” by ninety degrees (90°) (as shown in FIG.  2 ). It is also well known that a combination resistive-capacitive AC circuit will have a phase angle difference in the range between 0° and 90°. 
     Referring now to FIGS. 3A and 3B, representations of typical meter circuits used for measuring AC circuit voltages are shown for illustrative purposes. In FIG. 3A, the magnitudes of the internal resistance  12  and the external capacitive reactance  14  are equal, which results in a forty-five degree (45°) phase angle difference between the applied voltage, e.g., the electrical transmission line voltage being measured, and the resulting current waveforms. In FIG. 3B, the internal resistance  12  is replaced with an internal capacitive reactance  16  of equal magnitude, which makes it equal in magnitude to the external capacitive reactance  14 , and which results in a ninety degree (90°) phase angle difference between the applied voltage and the resulting current waveforms. 
     Based on this concept, by inserting a known internal capacitive reactance  16  into the detector circuit  20 , as shown in FIG. 4B, a known phase angle difference between the applied voltage waveform and the resulting detector  10  current waveform of effectively ninety degrees (90°) will result. Relatedly, if the internal resistance  12  is substituted for the internal capacitive reactance  16  in the detector circuit  20 , the phase shift differences between the applied voltage waveform and the resulting current through the detector  10  waveform will follow an exact relationship corresponding to the variances in the external capacitive reactance  14 . Therefore, if the phase shift differences between the resulting current through the detector  10  waveforms can be determined for the above-described circuit changes then the external capacitive reactance  14  can likewise be determined based on these differences. 
     Referring now to FIGS. 4A and 4B, the present invention is a voltage detector  10  that is based on the foregoing principles. The detector  10  includes detector circuit  20 . Preferably, the voltage detector  10 , with the exception of the novel detector circuit  20 , is a standard digital voltage detector capable of being used for measuring AC voltages, but the voltage detector  10  can be any other type of measuring instrument or device that would be suitable for measuring AC voltages. Since voltage detectors are well known in the art, and since the inventive concept of the present invention lies in the detector circuit  20  and its combination with the voltage detector  10 , and not in the voltage detector  10  itself, with the exception of the novel detector circuit  20 , the circuitry used in, and the fabrication and the operation of, the voltage detector  10  used in combination with the novel detector circuit  20  will not be described herein. 
     More specifically, the detector circuit  20  is a means for determining the phase angle difference between an applied voltage and the resulting current through the detector circuit  20  comprising: (1) a switching and sensing means  21 , for switching between the internal resistance input path  22  and the internal capacitive reactance input path  24 , and for sensing the analog current signal  23  through an input path ( 22  or  24 ), which is proportional to the voltage in the circuit being measured; (2) a high speed analog to digital converter  28  for converting an analog current flow signal  23  into a digital signal ( 25  or  26 ) for digital signal processing; (3) a computer processor  30 , which is used to analyze the magnitude and phase of each voltage based on the digital signals ( 25  and  26 ), and to calculate and/or determine the external capacitive reactance  14  based on this analysis; and (4) a display means  32  for visually providing the improved accuracy measurement to a user. 
     After calculating and/or determining the external capacitive reactance  14 , two of the parameters in an equation having three parameters are known, i.e., the internal impedance, which is either the magnitude of the internal resistance  12  or the internal capacitive reactance  16 , and the external capacitive reactance  14 . Consequently, it is mathematically possible to determine the unknown third parameter, e.g., the exact AC line voltage. The equations that can be used for determining the AC line voltage include, but are not limited to, the following: 
     Voltage (ac) =Z total *I; Z total =(R 2 +X c   2 )½; θz=θv−θi; and θz=tan  −1 (X c /R). 
     Where Voltage (ac) , Z total , and I are phasors; and where Voltage (ac)  is the AC line voltage being sensed and/or measured; Z total  is the total impedance of the circuit; I is the resulting current being sensed and/or measured; R is the internal resistance; X c  is the total capacitive reactance, i.e., internal and external capacitive reactance of the system; θz is the phase angle difference between the voltage and current waveforms; θv is the phase angle of the voltage waveform; and θi is the phase angle of the current waveform. 
     As mentioned above, it is known that the current waveform in a purely capacitive AC circuit leads the voltage waveform by ninety degrees (90°). Referring now to FIG. 4B, the computer processor  30  will use a reference digital signal  25  developed by the analog to digital converter  28  that corresponds to the applied voltage and the resulting current flow through the detector circuit  20  when the internal capacitive reactance  16  is in the detector circuit  20 , i.e., when the switching means  21  selects the internal capacitive reactance input path  24 . When the switching means  21  alternately selects the internal resistance input path  22 , the analog to digital converter  28  will develop a comparison digital signal  26 , and will send that signal  26  to the computer processor  30 . The computer processor  30  will then analyze the differences between the reference digital signal  25  and the comparison digital signal  26 . Since the phase angle difference when the internal resistance input path  22  is selected by the switching means  21  will be some angular amount less than ninety degrees (90°), and since the angular amount of the phase angle difference can be determined by the detector circuit  20 , the exact value of the external capacitive reactance can be ascertained. Preferably, this can be accomplished by having a data table programmed into the circuitry and/or memory of the computer processor  30 . This data table is preferably constructed from field data that corresponds to the exact relationship between various phase angles and/or phase angle differences, and the external capacitive reactance values associated with these various phase angles and/or phase angle differences. The computer processor  30  will be programmed to use this information and will adjust the displayed voltage reading either higher or lower to appropriately compensate for variances in the magnitudes of the external capacitive reactance associated with these phase angle differences for a more accurate voltage measurement reading. 
     In another embodiment, the computer processor  30  could be programmed to use an algorithm based on an equation, or some other similar mathematical construct, that uses the capacitive coupling of the device based on the distance that the device is suspended above the earth. 
     The detector  10  and the components and/or devices used in the detector circuit  20  of the present invention are not limited to any specific configuration or design. In this regard, those skilled in the art of AC voltage measuring devices will find that the detector  10  and/or the detector circuit  20  may be configured and/or designed in a variety of similar ways. For example, in another embodiment, as shown in FIG. 5, the detector circuit  20 ′ may substitute another internal resistance input path  24 ′ for the internal resistance input path; therefore, providing two internal resistance input paths  24 ′ and  24 ″. In this embodiment, each resistance input path would be comprised of a known internal resistance  12 ′ and  12 ″, but with the magnitude of each of the resistance differing between the separate resistance input paths  24 ′ and  24 ″. Similarly, two internal resistance input paths comprised of differing resistance magnitudes can be used, as well. 
     Therefore, while the preferred embodiments and the best mode of the present invention are described herein, it should be understood that the best mode for carrying out the invention as described herein is by way of illustration and not by way of limitation. It is intended that the scope of the present invention includes all modifications that incorporate its principal design features, and that the scope and limitations of the present invention are to be determined by the scope of the appended claims and their equivalents.