Abstract:
A phasing voltmeter having a dual input AC voltage measuring device in parallel with a series configuration of two high impedance high voltage resistors and two metering resistors. Shielding surrounds and isolates the voltmeter and is connected to the series configuration at a junction between the two metering resistors. The AC voltage measuring device measures the voltage across two voltage lines as well as the exact values of the stray AC capacitive charging currents associated with all high voltage sources. From these previously undesirable stray capacitive charging currents, a math processor in the voltmeter and in contact with two dual input measuring device receives input from between each resistor in the series, determines and displays the actual voltage across the conductors, its origin, the leading phase, lagging phase, phase sequence and phase rotation of the voltages on the AC conductors undergoing testing.

Description:
BACKGROUND OF THE INVENTION 
     Electricity transmitted through power lines destined for commercial, industrial and residential use can involve hundreds of thousands of volts and high currents. To measure the voltage, the utility worker must contact a high voltage line, which presents a significant risk to the worker. Indeed, mere proximity to a high voltage line is extremely dangerous. Nonetheless, in installing, servicing and repairing power lines, measurements of the voltage and phasing parameters such as the phase sequence or rotation must be made. 
     The circumstances and equipment used for measuring voltage and phase sequence of transmission lines vary considerably. For example, prior art phasing voltmeters with long interconnect cords are customarily used for measuring the voltage on individual lines of a multi-phase transmission power lines. This corded phasing volt meter helps to prevent two lines that are not in phase from being connected inadvertently. When other parameters of the AC voltage, such as the phase sequence or rotation of the voltage on the conductors, are to be determined, a completely different instrument or a whole set of instruments and additional equipment costing many thousands of dollars must be used. 
     However, if a prior art type phasing voltmeter such as the one described in U.S. Pat. No. 6,459,252 were improved as described herein, it could be used to measure voltage parameters such as leading phase, lagging phase, voltage phase sequence and phase rotation when a voltage measurement is taken and thereby minimize the number of times the worker approaches or contacts a high voltage line to reduce the risk to the worker and eliminate additional equipment, thus avoiding injuries and saving thousands of dollars in expenses. 
     There are four well-known, distinctly different voltage measurements that may be made on a three-phase power line. The present voltmeter is capable of all four and adds a fifth voltage measurement, heretofore never mentioned. This fifth measurement accurately determines the magnitude of a source of measurement inaccuracy among high voltage phasing meters when making the four standard measurements mentioned above. 
     Historically, high voltage measurements made using a corded type high voltage phasing meter have been plagued with inaccuracies stemming from stray capacitive charging currents. At high voltages, stray charging currents emanate from the surface of every component of the measuring device including the cord itself. The capacitive current is related to the capacitive reactance, Xc, which can range from several thousand ohms up to millions of ohms, depending on the location of the meter and its cable with respect to ground or grounded objects. Under extreme conditions, such as when a long series cable of a corded phasing volt meter is lying directly on the ground between two pad-mounted transformers, the value of the capacitive reactance can be very low resulting in a capacitive current that can equal or exceed the measured current. 
     While at least one prior art phasing voltmeter is able to reconcile the stray AC capacitive charging currents to provide accurate voltage indications under all conditions, even it fails to recognize, analyze and extract the valuable information contained in the charging current. There remains a need for an improved phasing voltmeter that has the ability to measure and analyze the stray AC capacitive charging current in order to provide additional voltage-related information to the user based on a single contact with the high voltage line. 
     SUMMARY OF THE INVENTION 
     According to its major aspects and briefly recited, the present invention is a phasing voltmeter that determines the origin of the capacitive currents, measures and analyzes them to provide to the operator of the voltmeter the actual voltage, the source of the voltage, the leading phase, the lagging phase, the phase sequence, and the rotation of the voltages on the conductors undergoing testing. 
     The present phasing voltmeter includes a pair of high impedance resistors in an electrical series with a cable and two low impedance resistors. This series is connected electrically at a single point of contact to the meter&#39;s electrical shielding, similar to that of the prior art phasing voltmeter cited above. However, unlike circuits of the prior art voltmeter, the present voltmeter has a dual input AC voltage measuring device wherein each input is connected in parallel with one of the of the two low impedance metering resistors. 
     By using this arrangement, the present voltmeter sets up three current paths in three different voltage divider networks which enables it to measure (1) the desired metering current representing the applied input voltage, (2) the previously undesirable stray capacitive charging current originating from a first source voltage or ground, and (3) the previously undesirable stray capacitive charging current originating from second source voltage or ground. The magnitudes, origins, and phases of the stray capacitive charging currents in the present invention are now known and are used to calculate an accurate line voltage and to indicate the source of the voltage, the leading phase, the lagging phase, the phase sequence, and the phase rotation or ground. 
     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 
       In the figures, 
         FIG. 1  illustrates the use of a phasing voltmeter by a utility worker; 
         FIGS. 2A-2D  illustrates the four basic types of measurements for which a phasing voltmeter may be used; 
         FIG. 3  illustrates a fifth measurement that the present phasing voltmeter may make in combination with those of  FIGS. 2A-2D ; 
         FIG. 4  illustrates schematically a prior art phasing voltmeter; 
         FIG. 5  illustrates the present phasing voltmeter schematically, according to a preferred embodiment of the present invention; 
         FIG. 6  is an alternate schematic illustration of the present voltmeter showing the three measurements taken by meters M 1 , M 2  and M 3 , according to an embodiment of the invention; and 
         FIG. 7  is a graph illustrating the asymmetrical relationship between the voltage drops across metering resistors R 3  and R 4  in the present invention when measuring voltages across phases A to B and phases B to A. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention is a phasing voltmeter that is an improvement over prior art phasing voltmeters. Externally, and to a significant extent internally, the present phasing voltmeter has features similar to the prior art voltmeter of U.S. Pat. No. 6,459,252 and operates in much the same way.  FIG. 1  illustrates a high voltage phasing meter  16  such as that of U.S. Pat. No. 6,459,252 in use by a utility worker. The present meter is used in the same way. 
     The overhead lines designated A, B, and C represent electrical power lines carrying alternating current. Each line is 120 degrees out of phase with the other two lines. Voltage transmission lines are of course not part of the present invention. The measurement of the voltage is shown in  FIG. 1  as being made across the A and B lines rather than, say, the A and C lines for convenience only. 
     Presently, and as illustrated in  FIG. 1 , high voltage phasing voltmeters use two insulated, high voltage resistors in probes  12  and  14  in series with each other and with the meter  16  and a cable  20 . Probes  12 ,  14 , have metal hooks or other fittings on their ends for making electrical contact with transmission lines quickly and easily. Meter  16  may be mounted to one of the two insulated probes, such as probe  12  as seen in  FIG. 1 , and oriented so that the electric utility worker can read the voltage displayed on meter  16  from below. “Hot sticks” (not shown) may be used to extend probes  12 ,  14 , so that the entire assembly can be held aloft. Meter  16  may be designed to measure either voltage or current, but its display indicates voltage. 
     Referring now to  FIGS. 2A, 2B, 2C and 2D , each of which illustrates one of four different voltage measurements that may be made on a three-phase power line. They are: phase to phase ( FIG. 2A ), phase to ground ( FIG. 2B ), ground to phase ( FIG. 2C ), and a “zero reference” test ( FIG. 2D ). This last measurement should indicate very nearly zero volts when measuring the voltage difference between two conductors of the same phase and the voltage between two electrical connections on the same conductor. 
     Referring now to  FIG. 3 , the present invention illustrates a fifth voltage measurement that is the reverse of that illustrated in  FIG. 2A . It is generally assumed that reversing the probes provides no new information. Of course, neither the phase A to phase B voltage measurement as shown in  FIG. 2A  nor its reverse as shown in  FIG. 3 , do not define a direction. However, when the leads are reversed, however, prior art phasing meters respond differently to B phase to A phase voltage measurements than the A phase to B phase voltage measurement. Unlike prior art phasing voltmeters, the present voltmeter includes circuitry to measure that response difference and, moreover, to extract information from it that enables the present phasing voltage meter to display the source of voltage, leading phase, lagging phase, phase sequence and phase rotation of the voltage on the conductors each time a voltage measurement is taken. 
     Referring now to  FIG. 4 , which shows schematically a prior art high voltage phasing meter  16  such as that taught by U.S. Pat. No. 6,459,252. Prior art meter  16  removes the inaccuracies of the voltage readings stemming from stray AC capacitive charging currents when used to measure voltage, as shown in  FIG. 2A  for example. 
     Voltmeter  16  has a first probe  12  is shown in contact with the A line; a second probe  14  is shown in contact with the C line. First probe  12  includes a resistor R 1 ; second probe  14  includes a resistor R 2 . 
     Between first and second probes  12  and  14 , and electrically in series with them, is a cable  20  and two metering resistors R 3  and R 4 . 
     Electrical shielding  24  is electrically continuous and extends from first and second probes  12 ,  14 , to cable  20 , and to other non-ferrous parts of voltmeter  16 . 
     Shielding  24 , because it extends over substantially the whole of phasing voltmeter  16 , assures that the capacitive reactance, X c , between the ground and every part of phasing voltmeter  16  is the same, which is important because of the location-specific nature of capacitive reactance. Without the continuous shielding, the capacitive reactance would vary depending on the physical relationship between each separately shielded part of phasing voltmeter  16  and ground, but wherever phasing voltmeter  16  is, X c  will be the same throughout shielding  24 . 
     In  FIG. 4 , meter  16  is shown measuring voltage across the A phase power source at electrical connection T 1  and the C phase power source at electrical connection T 2 .  FIG. 4  shows probe resistors R 1  and R 2  and meter resistors R 3  and R 4  in a four-resistor series. Cable  20  connects R 2  to AC meter  16  and R 1 . Shielding  24  surrounds and isolates cable  20 , probes  12 ,  14  and meter  16  as well as resistors  3  and  4 . Shielding  24  ties to a common electrical connection  26  between meter resistors R 3  and R 4 . Voltmeter  16  may then read the voltage across electrical connections T 3  and T 4  without the impact of X c . By tying the shielding to the common electrical connection  26 , the voltage across the meter remains the same for all values of X c  resulting from capacitive currents. 
     Turning now to the present voltmeter  40 , illustrated schematically in  FIG. 5 , the improved meter  40  is shown measuring the voltage across power sources A and C. Meter  40  includes two probes  44 ,  48 , a cable  20 , resistors R 1  and R 2  that make contact at electrical connections T 1  and T 2 , respectively, with power sources A and C. Ideally, R 1  and R 2  have large resistances and are matched so that the resistances of R 1  and R 2  are the same or very close in magnitude. The resistances of R 1  and R 2  may be tens of millions of ohms, such as, for example, 50,000,000 ohms. 
     Resistors R 1  and R 2  should have sufficient resistance to keep the current passing through them very low, on the order of a milliamp or preferably less, such as approximately 0.5 milliamps so as to limit resistive heat. In phasing voltmeter  40 , prolonged contact with transmission lines without generating appreciable resistive heat by either the probe resistors R 1  and R 2  or the components of voltmeter  40  is important as resistive heat adversely affects the accuracy of voltage measurements. 
     Meter  40  also has two metering resistors R 3  and R 4  in series with and between resistors R 1  and R 2 , to form a four-resistor series R 1 , R 3 , R 4 , and R 2 . Voltmeter  40  also has shielding  56  surrounding its components including probe resistors R 1  and R 2 , metering resisters R 3  and R 4 , and cable  52 . Metering resistors R 3  and R 4  should also be matched to each other, that is, having the same or very nearly the same resistance. 
     Shielding  56  is electrically isolated from resistors R 1  and R 2  of probes  44 ,  48 , as well as the electrical conductor in cable  20 , the electronics of meter  40 , and resistors R 3  and R 4  except for an electrical connection at common point  60 . Metering resistors R 3  and R 4  may be adjustable resistors for calibration. 
     Voltmeter  40  has electrical connections T 3 , between R 1  and R 3 , and T 4 , between R 4  and R 2 , from which to measure the voltage across source A and source C or perform any of the measurements shown in  FIGS. 2A-2D  or  FIG. 3 . An electrical contact point T 5  is provided between R 3  and R 4  which coincide with common point  60 . In the present specification including claims, a power source may be used to mean an electrical power line or ground or any potential source of electrical power. 
     Unlike the present voltmeter  40 , prior art meter  16  has but one connection from T 3  and one connection from T 4  for measuring voltage differences. In FIG.  5 , by contrast, meter  40  has a processor  76  that receives digital input from a first and second, dual input, analog-to-digital converter  68 ,  72 . Converters  68 ,  72  receive analog inputs from T 3 , T 4 , and T 5  and provide digital signals to processor  76  for three voltage measurements: T 3 -T 4 , T 3 -T 5 , and T 5 -T 4 . 
     Phasing voltmeter  40 , as described, thus has three precision voltage divider networks. The first of these three precision voltage divider networks divides the voltage between transmission sources A and C (or other voltage source or ground) across the inputs from resistor R 1  and resistor R 2  by an exact amount (such as a factor of 1,000,000) and provides two precise voltages, one to first converter  68  from T 3  and one to second converter  72  from T 4 . 
     The second of the precision voltage divider networks divides the voltage between transmission line A measured at T 3  and shielding  56  at T 5 . R 3  serves as the metering resistor in this second voltage divider network and supplies a complex voltage waveform to the first input to first converter  68 . The current in this voltage divider network represents the desired metering current plus the charging current supplied from transmission source A. 
     Finally, the third precision voltage divider network divides the voltage between transmission line C measured at T 4  and T 5 . Resistor R 4  serves as the metering resistor in this third voltage divider network and supplies a complex voltage waveform to the second input of second converter  72 . The current in this voltage divider network represents the desired metering current plus the charging current supplied from transmission source C. 
     Processor  76  receives the two complex voltage waveforms from the output of dual converters  68 ,  72 , and processes voltage and phase angle information to calculate and display accurate voltage indications on display  80 . 
     Optionally or in addition to indicating voltages on display  80 , a first and a second light-emitting diode, or LED  84 ,  88 , may be used to indicate results. Each of first and second LEDs  84 ,  88  may be capable of red, green and yellow light. First LED  84  may correspond to the power source with which first probe  44  is in contact, and second LED  88  may correspond to the power source with which probe  48  is in contact. On a single-phase, two-conductor AC electric system, a red indication on first LED  84  and green indication on second LED  88  can be used to indicate the source of voltage is on first probe  44  and ground is on second probe  48 . A reverse of these colors would of course indicate a reverse of the voltage source and ground on first and second probes  44 ,  48 , respectively. 
     On a standard three-phase AC electric system with a fourth ground conductor, the red/yellow/green first and second LEDs  84  and  88  can be used to indicate which conductor is the leading phase and which conductor is the lagging phase. A red indication on first LED  84  and yellow indication on second LED  88  indicates the leading phase is represented on first probe  44  and the lagging phase is represented on second probe  48 . 
     First and second LEDs  84 ,  88 , may also be set up to indicate and identify phase sequence or phase rotation by color. 
       FIG. 6  is an alternative schematic of voltmeter  40  in which M 1 , M 2 , and M 3  are shown instead of dual input A/D converters  68 ,  72 , and processor  76 . M 1  measures the voltage across T 3  and T 4 , M 2  measures the voltage across T 2  and T 5 , and M 3  measures the voltage across T 5  and T 4 . 
       FIG. 7  is a graphic representation of an example of voltages measured by M 1 , M 2  and M 3  over a range of X c  ranging from very near zero to infinity. The graph in  FIG. 7  demonstrates that the voltages for M 2  and M 3  are asymmetrical and that the leading phase provides the largest share of the stray AC capacitive charging current(s). This graph also contrasts the difference in magnitude of the voltage indications from M 1  at the top of the graph as would also be determined by the prior art meter of  FIG. 4 , with the voltages measured by M 2  and M 3  by the voltmeter of  FIGS. 5 and 6 , according to the present invention. The two additional voltages measured by M 2  and M 3  provide complete information about phase from the reactive capacitance. 
     Accordingly by placing first and second probes  44 ,  48 , of meter  40  in contact with two power sources, not only will the user be able to accurately make all five measurements of  FIGS. 2A-2D  and  FIG. 3 , but will also determine the origin of the voltage, the leading phase, lagging phase, phase sequence and the phase rotation of the voltages on the conductors at the same time. Additional equipment will not be needed and additional contacts with the power lines are avoided. 
     Those skilled in the art of voltage measurement, particularly high voltage measurement will appreciate that substitutions and modification may be made in the specific design of the voltmeter described herein, such as in the magnitudes of the probe resistances and meter resistances, without departing from the spirit and scope of the present invention, which is defined by the appended claims.