Abstract:
A wireless phasing voltmeter having a high impedance AC voltmeter in series with two high impedance probes. Shielding surrounds and electrically isolates the voltmeter and probes which communicate via wireless, FM transmission, thus eliminating the cable customarily used to connect the probes. The signal transmitted may be analog or digital. The transmitter and receiver may be plugged into the same jacks on the probe that were used for the cable. In parallel with the voltmeter and connected electrically with the shielding is an electrical circuit designed to add the capacitive current to a current detected by the probes in such a way that the net effect on the measured current is zero.

Description:
1. FIELD OF THE INVENTION  
         [0001]    The present invention relates to voltmeters generally and to voltmeters for use in electrical power transmission line servicing and maintenance in particular.  
         2. BACKGROUND OF THE INVENTION  
         [0002]    Electricity transmitted through power lines destined for commercial, industrial and residential use can involve hundreds of thousands of volts and high currents. Inevitably, there is an element of danger in measuring the voltage on a transmission line because of the need to make contact with the line. Indeed, even the proximity to a high voltage line may be sufficient to cause a spark to jump through the air to the nearest object. Nonetheless, in installing, servicing and repairing power lines, there are various occasions when contact is made, such as when the voltage carried by a line must be measured.  
           [0003]    The circumstances and equipment used for measurements of the voltage of transmission lines varies considerably. For example, the absolute voltage carried by a line may be measured by a “high line resistive voltmeter.” As another example, in servicing or repairing voltage regulators, an “off neutral detector” is used to determine if the regulator is passing current or has been effectively isolated from the power source. In still another application, a “phasing voltmeter” is customarily used in connecting individual lines of the multi-phase transmission power lines. The phasing voltmeter helps to prevent two lines that are not in phase from being connected inadvertently.  
           [0004]    Presently, high voltage phasing voltmeters use two test probes, which are each high voltage resistors housed in an insulated holder, in series with each other and a meter and a cable. The test probes are connected to the series cable and the series meter. The holders will have metal hooks or other fittings on their ends for good electrical contact with transmission lines. Often the meter is mounted to one of the two test probes and oriented so that the electric utility worker can read the voltage displayed on the meter. “Hot sticks” may be used to hold and elevate the entire assembly. The meter may be designed to measure either voltage or current, but its display indicates voltage. However, the indicated voltage is not always the true voltage difference for the four types of measurements listed above.  
           [0005]    High voltage measurements are plagued with inaccuracies stemming from stray capacitive charging currents. At high voltages, these stray currents emanate from the surface of every component of the measuring device including the cable. The capacitive current is related to the capacitive reactance, Xc, which can range from several thousand ohms on up, depending on the position of the meter and cable with respect to the ground. Under extreme conditions, such as when the series cable is lying directly on the ground between two pad-mounted transformers, the value of the capacitive reactance can be very low. The resulting capacitive current can then equal or exceed the measured current. Moreover, the voltage measured by the meter varies depending on the location of the meter and cable.  
           [0006]    However, the inaccuracies in phasing voltmeters attributable to capacitive currents are eliminated by the design disclosed and described in a commonly owned U.S. patent application, Ser. No. 09/766,254, filed Jan. 18, 2001, which will be referred to herein as the “companion specification.” 
           [0007]    Other problems with phasing voltmeters have not been solved. On occasion, the two power transmission lines are separated by a considerable distance. While the alternating current phasing voltmeter disclosed in the related application practically eliminates capacitive currents regardless of the length of the cable, it does not effectively address the problem of the logistics in dealing with a long cable or the problem of having a cable that is not long enough.  
           [0008]    There are other problems with phasing voltmeters. When very high voltages are being measured, the inherent dangers of applying the insulated test probes with a connecting cable to the lines create a natural reluctance to proceed.  
           [0009]    Finally, accuracy in measurement does not always translate to accuracy in reading of the meter. These device are used in the out of doors, during all types of weather and at all times of the day. Being able to extract the indicated voltage is not always easy to do or, in fact, done accurately.  
           [0010]    Thus there remains a need for a phasing voltmeter that is accurate regardless of the capacitive current, easy to read, and can be easily used when the transmission lines are separated by more than a few feet.  
         SUMMARY OF THE INVENTION  
         [0011]    According to its major aspects and briefly recited, the present invention is a wireless phasing voltmeter where the capacitive currents are combined with the primary voltage measurement of the electrical transmission lines in such a way that the capacitive current has no net affect on the voltage measured regardless of the magnitude of the capacitive current. The voltage signal is transmitted using radio frequencies from one test probe to the voltmeter and the output of the voltmeter may be displayed digitally. Thus, the meter can be applied to transmission lines a considerable distance apart without being limited by the length of a cable.  
           [0012]    The present phasing voltmeter includes a pair of test probes in series with a high impedance alternating current (AC) voltmeter, a radio frequency transmitter, and a radio frequency receiver. In parallel with the voltmeter is a low impedance electrical circuit, tied electrically at a single point of contact to electrical shielding surrounding and electrically isolating the resistors, AC voltmeter, transmitter and receiver. The shielding picks up the capacitive currents in the vicinity of the phasing voltmeter.  
           [0013]    The purpose of the electrical circuit is described in the companion specification. Furthermore, a number of different embodiments of this electrical circuit are described in the companion specification, but, for convenience, only a gain and a balance resistor will be illustrated and described for use with the present invention, however, it will be understood that any of those described in the companion specification may be used with the present invention,  
           [0014]    In addition to the high impedance resistors, voltmeter, transmitter, receiver and electrical circuit, a phase shift network may optionally, but preferably, be added to account for the shift in the phase that occurs. Also, the present wireless AC phasing voltmeter may process and transmit the measured voltage digitally to provide an alternative way to transmit the signal from the second probe.  
           [0015]    An important feature of the present invention is the use of radio frequency transmitter and receiver to transmit the signal from one of the high impedance resistors to the voltmeter. This feature has several advantages. First, it eliminates the cable, which, in addition to the cost and the requirement to manage it as part of the voltmeter, imposes a significant physical limitation on the distance between the two high impedance resistors. Second, it allows measurements of the voltage difference between two power transmission lines that may be very far apart. Third, it reduces the natural trepidation of workers who are responsible for making measurement on transmission lines carrying very large voltages. Although any electromagnetic waves (visible, infra-red, radio-frequencies, microwave, for example) can be used, radio-frequencies are preferred because they allow for other objects to be in the line of sight between transmitter and receiver without loss of signal. Preferably the signals are transmitted digitally and in such a way as to minimize the effect of electrical noise on the transmission, such as by frequency shift keying.  
           [0016]    Another feature of the present invention is that current phasing voltmeters can be backfitted to use a transmitter and a receiver instead of a cable. Currently, the cable has jacks on each end that plug into the voltmeter and the other test probe. The present transmitter and receiver can be made with jacks that plug into the same plugs the cable would use.  
           [0017]    Still another feature of the present invention is the inclusion of a phase shift network to eliminate a small error that is introduced by the modulation and demodulation of the electrical signal.  
           [0018]    These and other features and their advantages will be apparent to those skilled in the art of transmission line voltage measurement from a careful reading of the Detailed Description of Preferred Embodiments accompanied by the following drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    In the figures,  
         [0020]    [0020]FIG. 1 is a schematic illustration of a wireless phasing voltmeter according to a preferred embodiment of the present invention;  
         [0021]    [0021]FIG. 2 is a detail of the wireless phasing voltmeter of FIG. 1 showing the phase shift network; and  
         [0022]    [0022]FIG. 3 is another embodiment of the wireless phasing voltmeter, according to an alternative preferred embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0023]    The present invention is a phasing voltmeter that is an improvement over existing phasing voltmeters. Preferably, it employs the electrical circuit in the companion specification identified above, which specification is hereby incorporated herein by reference.  
         [0024]    Referring now to the figures, FIG. 1 illustrates schematically a circuit diagram of an embodiment of the present invention being used to measure the voltage difference between conductors A and C of an energized, three-phase transmission line. A first high impedance resistor R 1  is located in a first test probe  10  and connected in series with an alternating current (AC) voltmeter  12 . Preferably, a phase shift network  14  is connected in series with voltmeter  12 , and located between voltmeter  12  and a radio frequency receiver  16 .  
         [0025]    Shielding  18  surrounds and electrically isolates voltmeter  12 , phase shift network  14  and receiver  16  and much of first test probe  10 . A ground lead  20  preferably is used to ground shielding  18 .  
         [0026]    In addition, shielding  18  is also connected to a point  22  between two resistors R 3  and R 4  that are in series with each other and in parallel with voltmeter  12 . These are, respectively, the gain and balance resistors, whose function is described in detail in the companion specification and serve to add the capacitive current to the voltage difference being measured in such a way that the vector addition of the capacitive current has no impact on the magnitude of the measured voltage difference.  
         [0027]    Receiver  16  includes an antenna  24  to detect radio frequency signals, a radio frequency amplifier  26  to amplify the detected signals, and frequency demodulator  28  to demodulate the detected signal, a 60 hertz amplifier  30  and a resistor R 6 .  
         [0028]    At an arbitrary distance from first test probe  10 , but preferably within range of receiver  16 , is a second test probe  40  with a second high impedance resistor R 2  connected to a radio frequency transmitter  44 . Transmitter  44  and receiver  16  are able to transmit and receive the same carrier frequency and modulate the carrier, preferably using a technique of applying a signal to a carrier wave that can be detected in spite of electrical noise, such as frequency modulation (FM).  
         [0029]    Transmitter  44  includes a 60 hertz amplifier  46 , a frequency modulator  48 , a radio frequency amplifier  50  and an antenna  52 . Transmitter  44  also preferably has a ground lead  54  and is tied electrically to shielding  56  covering and isolating much of second test probe  40  and transmitter  44  through an adjustable resistor R 5 .  
         [0030]    The signal detected by second probe  40  from 60 cycle power transmission conductor C using resistor R 2  and R 5  in a voltage divider network is amplified by 60 hertz amplifier  46  and used by modulator  48  to modulate a radio frequency carrier wave supplied by modulator  48 . As in any frequency modulated signal, the peak frequency swing of the transmitter is a function of the peak voltage applied and may be expressed in terms of volts per kilocycle of deviation. The rate at which the peaks of deviation occur is a function of the applied frequency (60 Hz). The 60 Hz modulating frequency would be phase locked to the applied voltage from the input voltage divider network (R 2  and R 5 ). The modulator output is fed to radio frequency amplifier  50  and then to antenna  52 .  
         [0031]    Antenna  24  picks up the signal from antenna  52 , amplifies it with radio frequency amplifier  26 , removes the 60 hertz signal from the radio frequency carrier wave using demodulator  28 , and amplifies the extracted 60 cycle signal with 60 hertz amplifier  30 . The gain of amplifier  30  may be set to establish a new voltage divider network consisting of R 6  and R 4 , to cause the original voltage signal from voltage divider network R 2 , R 5 , to be reproduced across R 4 .  
         [0032]    Optionally, but preferably, phase shift network  14  is inserted between voltmeter  12  an receiver  16 .  
         [0033]    Phase shift network  14 , illustrated in FIG. 2, is composed of two resistor/capacitor (RC) combinations in series. In a typical high voltage AC phasing voltmeter for use in measuring voltage differences in 60 Hz conductors, both have 0.1 microfarad capacitors  60  and  62 , but the first has a 30K resistor and the second has an adjustable 50K resistor. The purpose of phase shift network is to correct for a shift in phase, by about 70 degrees. While not wishing to be bound by theory, it is believed that the cause of the phase shift is the modulation and demodulation of the signal. The distance between receiver  24  and transmitter  44  does not seem to be a factor in causing the phase shift.  
         [0034]    In an alternate preferred embodiment of the present invention, illustrated in FIG. 3, a first probe  70  is used to detect voltage from conductor A of a three phase power transmission system having conductors A, B, and C. As before, a high impedance resistor R 10  is used to drop the voltage. A second, preferably adjustable resistor R 11  drops the voltage still further and tied electrically to a grounded shield  72 . The ratio of the resistances of R 10  to R 11  is preferably approximately 1,000,000 to one. In a 100,000 volt power line, the voltage across R 11  would be 0.1 volts. The voltage across R 11  is converted from an analog signal to a digital signal by analog-to-digital converter  74  and fed to master processor  76 . Master processor  76  manages the flow of digital data from two sources, converter  74  and a modem  108 , described below, processes the signals from these two sources and forwards them for display.  
         [0035]    A second probe  90 , spaced apart from first probe  70 , but within range, detects the voltage signal carried by C and drops that voltage significantly across R 12 , a high impedance resistor. As in first probe  70 , the ratio of the resistance of R 12  to R 13  is preferably approximately 1,000,000 to one. The voltage is dropped further across resistor R 13  which is tied electrically to a grounded shield  92 . An analog-to-digital converter  94  converts the analog voltage drop across R 13  to a digital signal and passes it to a slave processor  96 . The digitized signal from slave processor is transmitted using a modem  98  to transmit the digitized signal and transceiver  100  to transmit it through an antenna  102 . The digitized signal may be transmitted by frequency modulation, amplitude modulation, phase modulation, or by frequency shift keying, the last of these being preferred and all of which are well known techniques for transmitting digital signals.  
         [0036]    The signal is picked up by an antenna  104  and forwarded to master processor  76  via a transceiver  106  and modem  108  which demodulates the signal. The signals are processed to extract the voltage difference, which may be converted back to analog by a digital-to-analog converter  110  for analog display on display  112 , or left for digital display.  
         [0037]    Ground leads of the embodiments of FIGS. 1 and 2 may be eliminated but with an attendant loss in accuracy. With them, the present phasing voltmeter is accurate to ±1%; without them, the voltmeter is accurate only to ±10%.  
         [0038]    The use of radio frequencies is convenient and makes it possible to transmit through or around obstacles that might hinder line-of-sight transmissions such as visible light, infra-red and possibly microwave transmissions. However, these other forms of electromagnetic waves could also be used. Also, the use of frequency modulation is preferred because of its resistance to noise but amplitude modulation is also possible. Finally, it will be apparent to those skilled in the art of electrical power transmission line servicing and maintenance equipment design that many other modifications and substitutions can be made to the foregoing preferred embodiments without departing from the spirit and scope of the present invention.