Abstract:
A long-range wireless phasing voltmeter having a high impedance AC voltmeter in series between two high impedance probes. Shielding surrounds and electrically isolates the voltmeter and probes that communicate wirelessly via a pair of frequency translators, thus eliminating the cable customarily used to connect the probes and enabling measurements of the voltages, voltage differences, phases and phase difference between two conductor that may be miles apart. The signal transmitted from the first probe to the second may be analog or digital and is automatically compensated to correct the synchronization of the transmitted signal for the delay required to send it via the two translators.

Description:
PRIORITY CLAIM 
   Applicant claims priority to U.S. Provisional Application Ser. No. 60/572,510 filed May 19, 2004, which is incorporated herein by reference. 

   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   Not applicable. 
   REFERENCE TO A SEQUENCE LISTING, A TABLE, OR A COMPUTER PROGRAM LISTING COMPACT DISK APPENDIX 
   Not Applicable. 
   BACKGROUND OF THE INVENTION 
   The present invention relates to voltmeters generally and to voltmeters for use in electrical power transmission line servicing and maintenance in particular. 
   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. 
   Presently, high voltage phasing voltmeters use two test probes, which are each high voltage resistors housed in an insulated holder. The probes are electrically connected in series with a meter using a cable to connect them. The holders will have metal hooks or other fittings on their ends to facilitate a quick, 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 up to a transmission line. The meter may be designed to measure either voltage or current, but its display indicates voltage. 
   In addition to concerns for safety, there are a number of factors that introduce inaccuracies in these types of measurements. One of these factors is the existence of capacitive currents. However, the inaccuracies in phasing voltmeters attributable to capacitive currents are eliminated by the design disclosed and described in a commonly owned U.S. Pat. No. 6,459,252 issued Oct. 1, 2002, which is incorporated herein by reference. 
   Other factors that affect accuracy with phasing voltmeters have also been solved. On occasion, more than a few feet may separate power transmission lines. While the alternating current phasing voltmeter disclosed in the related U.S. Pat. No. 6,459,252, practically eliminates capacitive currents regardless of the length of the cable required to make measurements of power lines that are separated, it does not effectively address the problem of the physical problems in dealing with a long cable or the problem of having a cable that is not long enough. Commonly owned U.S. Pat. No. 6,617,840 issued Sep. 9, 2003 and U.S. Pat. No. 6,734,658, issued May 11, 2004, however, solve this problem to some extent by eliminating the cable altogether in favor of an alternating current phasing voltmeter in a master and slave probe combination that communicate voltage information wirelessly. The patent specification of U.S. Pat. No. 6,734,658 is also incorporated herein by reference. This phasing voltmeter also helps to overcome the natural reluctance of workers to apply test probes to power lines carrying very high voltages and it incorporates the accuracy improvements of U.S. Pat. No. 6,459,252. 
   However, there remains a need for a phasing voltmeter that is accurate, easy to read, and can be easily used when the transmission lines are separated by more than a few hundred feet, perhaps even separated by a distance on the order of a few miles. 
   SUMMARY OF THE INVENTION 
   According to its major aspects and briefly recited, the present invention is a wireless phasing voltmeter system in which two intelligent, full-duplex translators enable the transmission of voltage measurement information and comparison of voltages measured by two wireless probes that are several miles apart. The system adjusts the voltage measurement information to compensate for the delays in transmission of the signal from one probe to the other. The phasing voltmeter system, in its preferred embodiment, includes the two probes with associated signal processing electronics, two transceivers, two full duplex translators with internal microprocessors, and a high impedance alternating current (AC) voltage processor with display. The phasing voltmeter system determines the voltages, the voltage difference between measured power lines, the phases and the phase differences. 
   The voltage signal obtained by one of the probes, which includes both instantaneous voltage and phase information, is transmitted using multiple carrier frequencies, one frequency between the first test probes and the first intelligent frequency translator, and two more carrier frequencies between the first and second frequency translators. All three of the carrier frequencies are preferably radio frequency carrier frequencies. The signal from the first or “slave” probe is transmitted to the first intelligent frequency translator by the first transceiver, then forwarded to the second intelligent frequency translator, and finally to the second transceiver where it is compared to the signal from the second or “master” probe by its master processor. The comparison of the signals by the processor allows the display of the voltages of the two power lines, the voltage difference, the phase of each and the phase difference. 
   An important feature of the present invention is the use of intelligent frequency translators. Both translators have on board microprocessors, hence the designation “intelligent” translators. In addition to allowing the signal to be transmitted over a distance on the order of several miles, the intelligent frequency translator automatically adjusts the voltage signal information to compensate for the delay in transmission, regardless of distance, so that the displayed information is as accurate as if the probes were close together. 
   An important feature of the present phasing voltmeter system is the use of multiple carrier frequencies to transmit the voltage signal information from the first test probe to a processor in the second probe potentially miles away. Such an arrangement—and using a full duplex intelligent frequency translator to receive a signal on a first frequency carrier and retransmit it while receiving a synchronizing return pulse on a frequency carrier pair in particular—permits measurements over extended distances quickly and accurately. Although full duplex frequency translators themselves are well known in general, they are believed to be unknown in voltmeters. 
   Another important feature of the present invention is the use of radio frequency carriers to transmit the signal from one of the high impedance probes to the voltmeter. Radio transmission eliminates the need for a cable between the probes, which is completely impractical when power lines, and thus the probes, are separated by more than a very few hundred feet. Although any electromagnetic carrier frequency (visible, infra-red, radio-frequencies, microwave, for example) can be used, radio frequencies are preferred because they are not restricted to line of sight transmissions. Preferably the signals are transmitted on the carriers digitally and in such a way, as will be described in more detail herein, so as to minimize the effects of electrical noise on the transmission, such as for example by frequency shift keying. 
   Another feature of the present invention is the use of a low frequency for obtaining and transmitting digital data (as opposed to the carrier frequencies) from the probes. The low frequency improves the signal to noise ratio because the data can be sampled by the translators often enough to verify each bit. 
   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 SEVERAL VIEWS OF THE DRAWINGS 
     In the figures, 
       FIG. 1  illustrates the retransmission of the signal from the probe to the meter through a translating transmitter and receiver set using a different frequency carrier and greater power; 
       FIG. 2  illustrates an embodiment of the wireless phasing voltmeter, according to an alternative preferred embodiment of the present invention; and 
       FIG. 3  illustrates the components of the two translators, according to a preferred embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention is a phasing voltmeter system that is an improvement over existing phasing voltmeters. The improvement lies in its ability to make accurate measurements of voltage information extracted from power lines that may be separated by a distance of several miles, extends the range of the master/slave probes described in U.S. Pat. No. 6,734,658, when desired. 
   Referring now to  FIGS. 1–3 , there is illustrated a first electric utility vehicle  500  and a second electric utility vehicle  510  spaced a distance apart, such as a distance of several miles. First and second electric utility vehicles are each parked near transmission lines. First vehicle  500  is under lines A, B and C. The phases of the power lines near second vehicle  510  are unknown. 
   First vehicle  500  has a first intelligent frequency translator  520  in radio communication on a first carrier frequency with a first test probe  530  and in radio communication in full duplex on a second and third carrier frequency with a second intelligent frequency translator  540  that is in turn in radio communication on the first carrier frequency with a processor  550  inside a second test probe  560 . The signal from test probe  530  is transmitted on the first frequency carrier to first frequency translator  520 . First frequency translator  520  translates the signal it receives from the first frequency carrier to the second frequency carrier and forwards the translated signal to second frequency translator  540  using a second and third carrier frequency. Second intelligent frequency translator  540  translates the signal from the second frequency carrier to the first frequency carrier and transmits it to processor  550  where it is processed along with a second signal from second test probe  560 . 
   Prior to sending the voltage signal information to second intelligent frequency translator, first frequency translator automatically determines the length of the delay in the signal that is due to the transmission time, as will be explained below in more detail. The length of the transmission time is used by first frequency frequency ranslator  540  to advance the signal&#39;s transmission so that, when it arrives, it will be synchronized with the signal from the first probe as if the two were located in the same instrument. 
   Frequency translators are well known. See, for example, the description in U.S. Pat. No. 6,272,329, incorporated herein by reference, and frequency translator products sold by a number of suppliers including Conner Winfield, Raltron Electronics Corporation and Vectron International. Transceivers, which are devices capable of transmitting and of receiving, are also known. In the present specification, although the term transceiver will be used, it is intended that using two separate components, one to transmit and one to receive, are equivalent of one transceiver and, if two separate components are used, together they are a transceiver. Also, in the present description, transmissions in only one direction may be described but in practice certain “check back” information for confirming receipt or synchonization may be transmitted in the opposite direction. Therefore, transceiver may be used when a transmitter or a receiver may be sufficient if the check back information is handled in a different way. In this event, transceiver is also equivalent to either a transmitter or a receiver. 
     FIG. 2 , illustrates schematically a circuit diagram of an embodiment of the present invention being used to measure the voltage differences between conductors A and an unknown phase of an energized, three-phase transmission line having conductors A, B and C. 
   In the present invention, illustrated in  FIG. 2 , a first probe  70 , namely, the slave probe, 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 is tied electrically to a grounded shield  72  on first probe  70 . 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 slave processor  76  as a voltage information signal. Slave processor  76  manages the flow of digital data between two components, namely, A/D converter  74  and a modem  108 , described below, and processes the voltage information signal from these two sources and forwards them to a transceiver  106  via modem  108  on a first frequency carrier. 
   A first intelligent frequency translator  78  with an antenna  80  receives voltage information signals from slave processor  76  on first frequency carrier. A second intelligent frequency translator  82  and another antenna  84  are located remotely from first intelligent frequency translator  78 . First frequency translator  78  can transmit to and receive signals from second frequency translator  82  in full duplex on two different frequency carriers, a first frequency carrier for transmitting by intelligent frequency translator  78  and a second frequency carrier for receiving. First and second intelligent frequency translators  78 ,  82 , and their respective antennae  80 ,  84 , may be miles apart, potentially to the limits of the signal frequency with due allowances for terrain and intervening structures. 
   A second probe  90 , the master probe, spaced potentially miles apart from first probe  70 , detects the voltage carried by C and drops that voltage significantly across R 12 , a high impedance resistor. The voltage is dropped further across resistor R 13 , which is tied electrically to a grounded shield  92 . As in first probe  70 , the ratio of the resistance of R 12  to R 13  is preferably approximately 1,000,000 to one. An analog-to-digital (A/D) converter  94  converts the analog voltage drop across R 13  to a digital voltage signal and passes it to a master processor  96 . The digitized voltage signal information from second probe  90  will be compared to that of first probe  70  once received. The carrier frequencies of first and second probes  70 ,  90 , may be different but, when the power lines are close enough, probes  70  and  90  can communicate directly if they operate on the same, i.e, first, frequency carrier. The digitized voltage signal information may be transmitted by frequency modulation, amplitude modulation, phase modulation, or by frequency shift keying, the last of these being preferred, but all of which are well known techniques for transmitting digital signals. 
   The signal detected by antenna  80  of first frequency translator  82  is forwarded by first frequency translator  78  and antenna  80  to second frequency translator  82  via antenna  84 , and then to master processor  96  via transceiver  100  and modem  98 , which receives and demodulates the voltage signal information before forwarding it to master processor  96 . The signals from probes  70  and  90  are processed by master processor  96  to compare voltage signal information. The compared voltage information signals may be converted to analog signals by a digital-to-analog (D/A) converter (not shown) for analog display on a display  112  or left in digital form for display 
   Slave processor obtains two pieces of information from the measurement by probe  70  of the voltage of power line A. It obtains the instantaneous voltage with respect to ground and can extract from that data the zero crossing time which marks the beginning of a cycle and thus marks phase of the electrical power on the line. The zero crossing time occurs as the voltage moves from negative to positive. Phase information can be extracted by synchronizing the phase information between the voltage measurements of the two power lines. 
   Because the distance separating the two probes and the two intelligent frequency translators of the present invention, significant delays will likely be introduced in the receipt of the voltage and phase information received from first probe  70  that make synchronizing the phase information more important and more difficult. In order to compensate for this delay in the synchronization process, it is especially important to determine for each measurement the amount of delay being incurred, and then to adjust the one of the two signals accordingly. 
   In the present invention, the voltage information is digitized at a low frequency, namely 60 Hz, using pulse width modulation. The bits of information each begin on the zero crossing point but their duration varies depending on their representation of the two digital values. Preferably, one bit value is set at one third of a cycle and the other bit value is set at two thirds of a cycle. The voltage is thus represented digitally in a byte comprising a set of eight pulses of widths of one-third or two-thirds to represent, for example, 0s and 1s, respectively. This digital signal is imposed on first carrier frequency, preferably by frequency shift keying, and is then transmitted by first transceiver  106  to first intelligent frequency translator  78 . 
   The components of first intelligent frequency translator  78  and second intelligent frequency translator  82  are shown in  FIG. 3 . For simplicity, the antennas of both are split in this figure and shown as two separate antennas but one antenna for each translator is all that is required with the use of radio frequency combiners. First intelligent frequency translator  78  includes a half duplex receiver  120  that receives the digitized signal from first transceiver  106  via antenna  80  and forwards it to processor  122 . Processor  122  contains a first modem  124  and a second modem  126 . First modem  124  demodulates the digital signal from first carrier frequency (or alternatively, processor  122  may be programmed to extract the digital voltage signal information from first carrier frequency directly). Second modem received the now demodulated voltage signal information from processor  122  and modulates the second carrier frequency with the voltage signal information for transmission via a full duplex transceiver  130  through antenna  80 . The signal, when sent from full duplex transceiver  130  is sent more rapidly than when sent to half duplex receiver  120 , preferably in quick bursts and preferably using frequency shift keying to improve the quality of the signal. 
   Before transmitting the digital voltage signal information on the second carrier frequency, a synchronizing pulse is sent to second intelligent frequency translator  82 . This pulse is synchronized to the start of the alternating current cycle from power line A as detected by first probe  70 , prepares second intelligent frequency translator  82  for receiving the digital voltage signal information, and precipitates a synchronizing return pulse from second intelligent frequency translator  82  as soon as the latter receives the initial synchronizing pulse. The time between these two pulses is used by processor  122  to determine the round trip delay time that occurs when sending a signal through full duplex transceiver  130  to second intelligent frequency translator  82  at whatever distance away it is located. Half of that time is the one-way delay or the transmission delay. The timing of the transmission of the digitized voltage signal from first intelligent frequency translator  78  to second intelligent frequency translator  82  is advanced by the transmission delay time, as determined by processor  122 , thereby compensating fully for the delay time so that, when the digitized voltage signal information is received by second intelligent frequency translator  82 , the digital voltage information is fully synchronized with, and can be properly compared to, the voltage information provided by second probe  90 . Thus, not only can the instantaneous voltages be compared but also the difference in the timing of the zero crossover points of the two signals can be compared to yield complete phase information. 
   The two signals can be compared and analyzed to determine the voltages of the two power lines, the voltage difference, the phase of each and the phase difference. 
   The components of second intelligent frequency translator  82  are similar to those of first intelligent frequency translator  78 . The voltage signal information coming from antenna  80  of full duplex transceiver  130  is received by an antenna  84  of a full duplex transceiver  134 . Full duplex transceiver  134  forwards the received voltage signal information to a processor  136  that, like processor  122 , has two modems  138 ,  140 . Modem  138  demodulates the second carrier frequency to extract the voltage signal information and forwards it to modem  140 , which modulates the first carrier frequency with the voltage signal information and sends the modulated carrier to a half duplex transceiver  142  for transmitting via antenna  84  to probe  90 . 
   Ground leads  110 ,  112  may be eliminated but with a loss in accuracy. With them, the present phasing voltmeter is accurate to ±1%; without them, the voltmeter is accurate only to ±10%. 
   It is also possible to use half duplex transceivers rather than full duplex particularly if the half duplex transceivers are of the instant on variety so that the transmission of signals is not unduly delayed. 
   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. Most importantly, the use of frequency translators allows the signals from the first to the second probes to be transmitted over much larger distances, such as miles. 
   It is intended that the scope of the present invention include 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. It also should be understood, therefore, that the inventive concepts herein described are interchangeable and/or they can be used together in still other permutations of the present invention, and that other modifications and substitutions will be apparent to those skilled in the art of phasing voltmeters and power line measurements from the foregoing description of the preferred embodiments without departing from the spirit or scope of the present invention.