Abstract:
A phasing voltage meter comprises first and second probes. Each probe comprises an insulated shield supporting an electrode for contacting a high voltage electrical conductor. The shield houses a high voltage resistor connected in series with the electrode. A capacitance formed by a metallic collar across the resistor compensates for stray capacitance across the resistor. A meter comprises a housing enclosing electrode circuit for measuring phasing voltage. The electrical circuit measures voltage across the electrodes and provides an output representing phasing voltage.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not Applicable. 
     FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable. 
     MICROFICHE/COPYRIGHT REFERENCE 
     Not Applicable. 
     FIELD 
     This disclosure relates to a high voltage phasing meter with electrostatic shielding. 
     BACKGROUND 
     Electrical power distribution systems often include overhead electrical power distribution lines mounted upon poles by a wide variety of mounting structure. Other distribution systems include underground distribution lines in which protected cables run under the ground surface. It is often necessary to take phase-to-phase or phase-to-ground voltage measurements across distribution and transmission lines while testing for induced or live power line or equipment. 
     Known high voltage safety line detectors, meters and testers comprise high resistance probes connected in series with a calibrated panel meter to read the voltage across the phase-to-phase or phase-to-ground terminals. They are designed for use as safety tools by high voltage line maintenance workers to verify the status of the line or equipment as nominal, induced or de-energized voltage. Known devices for providing such measurements include contact type and non-contact type. With contact type a reference probe or transmitter and a meter probe or receiver are connected in series with a cable as the loop is closed with load terminals. 
     The high resistance probes include a high voltage resistor connected in series with an electrode. The voltage distribution along the length of the high voltage resistor is non-uniform due to stray capacitance. This can result in errors in voltage measurement and provide unequal electrical stress distribution along the high voltage resistor. 
     Particularly, the high voltage measurement is carried out by using high voltage resistors in the phasing meter. One type of phasing meter uses a micro ammeter method of measurement. The voltage applied across the resistor leads to current flow in a meter circuit and amount of current flow corresponds to the voltage being measured. Another method is known as the voltage divider method and the phasing meter includes high value resistors producing a high voltage drop and a low value resistor producing a low voltage drop. The line to line or line to ground high voltage is applied across the resistors which are in series and the voltage drop across the low value resistor corresponds to the actual voltage being measured. 
     A high voltage resistor used in a phasing meter may have a non-uniform voltage distribution along the length of the resistor and creates instability due to stray capacitance between resistance portions to ground. The voltage stress is not uniform across the length of the resistor due to stray capacitance. The voltage stress will be more than two times the uniform stress at the resistor end near to the high voltage side. This leads to degradation of the resistor material at these points. Also, due to stray capacitance, the current which is flowing through the circuit is not the same as the theoretically calculated current leading to voltage measurement errors. The total current should be the circuit and stray capacitive current. However, the actual current portion is diverted by the stray capacitance leading to the measurement error. 
     The disclosure is directed to improvements in high voltage phasing voltmeters. 
     SUMMARY 
     As described herein, a phasing voltage meter compensates for stray capacitance across the probe resistor. 
     Particularly, a phasing voltage meter comprises first and second probes. Each probe comprises an insulated shield supporting an electrode for contacting a high voltage electrical conductor. The insulated shield houses a high voltage resistor connected in series with the electrode. A capacitance, formed by metallic collars, across the resistor compensates for stray capacitance from the resistor to ground. A meter comprises a housing enclosing an electrical circuit for measuring phasing voltage. The electrical circuit measures voltage across the electrodes and provides an output representing phasing voltage. 
     It is a feature that the capacitance comprises a first metallic collar at one end of the high voltage resistor and a second metallic collar at an opposite end of the high voltage resistor. The first metallic collar may be of a different size than the second metallic collar. The first metallic collar may be larger than the second metallic collar. The first metallic collar may have a greater diameter than the second metallic collar. The first metallic collar may have a greater thickness than the second metallic collar. 
     It is a further feature that the first metallic collar and the second metallic collar support the high voltage resistor in the shield. 
     It is another feature that each probe comprises a handle portion at a near end of the shield. The meter housing is integral with the handle portion of the first probe. 
     It is still another feature that the first and second metallic collars are secured in the shield with a potting compound. 
     There is also disclosed a high voltage phasing meter comprising a first probe and a second probe. Each of the probes comprises an elongate insulated shield having a handle portion at one end. An electrode extends from a distal end of the insulated shield for contacting a high voltage electrical conductor. A high impedance circuit in the shield comprises a high voltage resistor and a capacitance, formed by metallic collars, across the high voltage resistor to provide uniform current for each section of the high voltage resistor. A meter comprises a housing enclosing an electrical circuit for measuring phasing voltage. The electrical circuit measures voltage across the electrodes and provides an output representing phasing voltage. 
     Other features and advantages will be apparent from a review of the entire specification, including the appended claims and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of use of a high voltage phasing voltmeter as disclosed herein; 
         FIG. 2  is a perspective of the individual components of the high voltage phasing voltmeter of  FIG. 1 ; 
         FIG. 3  is an electrical schematic of a high voltage phasing meter of the micro ammeter type; 
         FIG. 4  is an electrical schematic of a portion of a high voltage phasing meter of the voltage divider type; 
         FIG. 5  is an electrical schematic illustrating stray capacitance effect on the high voltage resistors in an equivalent circuit for the embodiment of  FIG. 3 ; 
         FIG. 6  is a perspective view of a high voltage resistor with a metallic collar attached for a probe as described herein; 
         FIG. 7  is a perspective view similar to  FIG. 6  showing alignment of the high voltage resistor with a metallic collar attached to be inserted into a shield; 
         FIG. 8  is a perspective view, similar to  FIG. 7 , showing the assembled probe; and 
         FIG. 9  is an electrical schematic, similar to  FIG. 5 , illustrating an electrostatic shielding technique as described herein. 
     
    
    
     DETAILED DESCRIPTION 
     Referring initially to  FIG. 1 , a portable high voltage phasing voltage meter  10  is shown for measuring phase-to-phase voltage between lines in a high voltage distribution and transmission line system  12 . The distribution and transmission line system  12  includes three lines in the form of conductors  14 ,  16  and  18  carrying high voltage power of alternating current with each line being 120° out of phase with the other lines, as is conventional. The phasing meter  10  may be used by a maintenance worker W for measuring phase-to-phase voltage such as between the electrical conductors  14  and  16  as illustrated in  FIG. 1 . The phasing meter  10  may also be used to measure other voltages, such as phase-to-neutral or phase-to-ground. 
     Referring also to  FIG. 2 , the meter  10  comprises a first probe  20 , a second probe  22 , a meter  24 , a first electrode  26 , and a second electrode  28 . 
     The first probe  20  comprises an elongate insulated shield  32  connected to a handle portion  34  at a near end and a terminal  36  at an opposite distal end. An insulating hot stick  35  is selectively attached to the handle portion  34  to extend length of the first probe  20 . The terminal  36  is adapted to threadably receive one of the electrodes  26  or  28 . A coaxial connector  38  is provided in the handle portion  34  for receiving a first coaxial connector of a cable  30 , see  FIG. 1 . 
     The second probe  22  comprises an elongate insulated shield  42  connected to a handle portion  44  at a near end and a terminal  46  at an opposite distal end. An insulating hot stick  45  is selectively attached to the handle portion  44  to extend length of the second probe  22 . The terminal  46  is adapted to threadably receive the other of the electrodes  26  or  28 . A coaxial connector  48  is provided in the handle portion  44  for receiving a second coaxial connector of the cable  30 . 
     The meter  24  comprises a housing  52  integrally formed with the first probe handle portion  34 . The housing  52  is frustoconical in shape including a bottom bezel  54  through which a display  56  is visible. The display  56  can be an analog display or a digital display, as preferred. 
     As shown in  FIG. 1 , the worker W can grip the hot sticks  35  and  45  to contact the lines  14  and  16  with the electrodes  26  and  28 , respectively. The meter display  56  is visible to read the measured phase to phase voltage. 
     Referring to  FIG. 3 , an electrical circuit  60  for the meter  24 , see  FIGS. 1 and 2 , is illustrated. The electrical circuit  60  uses the micro ammeter method for measuring voltage. The first probe  20  comprises a first high voltage resistor R 1 . The second probe  22  comprises a second high voltage resistor R 2 . The voltage being measured, such as across the lines  14  and  16 , see  FIG. 1 , is represented by V 1 . The probes  20  and  22  are connected via the cable  30 , see  FIG. 1 , in a conventional manner, to the electrical circuit  60 . The electrical circuit  60  includes a rectifier circuit  62  connected to the probe resistors R 1  and R 2 . The electrical circuit  60  is powered by a battery V 2 . The battery V 2  powers a measurement circuit  64  including a node D which drives the display  56  in a conventional manner. 
       FIG. 4  illustrates a voltage divider type voltage measurement in which a low voltage resistor R 3  is connected in series with the high voltage resistors R 1  and R 2 . A meter electrical circuit, not shown, measures the voltage V 0  across the low value resistor R 3  in a conventional manner. 
     The probes described herein can be used in connection with either type of measurement circuit. In fact, the particular measurement circuit shown in  FIGS. 3 and 4  are for illustration only as other types of circuits can be used, as will be apparent. 
     Referring to  FIG. 5 , the circuit of  FIG. 3  is shown with the circuit  60  represented in equivalent form by a resistance RM. While each probe includes a respective high voltage resistor R 1  and R 2 , each high voltage resistor has a given length in which it can utilize a discrete number of resistors of the same resistance value. These are referred to herein as resistor sections. In the illustrated schematic in  FIG. 5 , the resistor R 1  is illustrated as discrete resistor sections R 11 , R 12 , R 13  and R 14  all in series. Similarly, the second high voltage resistor R 2  is illustrated as discrete resistor sections R 21 , R 22 , R 23  and R 24 , again all in series. Ideally, the current flowing through each resistor section should be equal to the current flowing through the resistance RM. However, this is not always the case due to stray capacitance represented by the capacitors C 1 , C 2 , C 3  and C 4  across the resistor sections to ground associated with the first high voltage resistor R 1 , and similarly, capacitance C 5 , C 6 , C 7  and C 8  across the resistance sections to ground of the second high voltage resistor R 2 . While the resistor sections R 11 -R 14  and R 21 -R 24  are assumed to be of equal values, the current through each resistor section is not equal due to the effect of the stray capacitance. This causes unequal voltage distribution along the length of the respective high voltage resistors R 1  and R 2  making electrical stresses non uniform. There will be greater stress at the side closest to the supply V 1  and less at the side closest to the resistance RM. This stress distribution can lead to degrading of the resistors R 1  and R 2  and lead to failure over a period of time. 
     Referring to  FIG. 6 , the first high voltage resistor R 1  is illustrated as comprising an elongate resistor element  70  between opposite leads  72  and  74 . As described herein, a capacitance  76 , formed by metallic collars, is placed across the resistor R 1  to compensate for stray capacitance and provide uniform current through each section of the high voltage resistor R 1 . The capacitance  76  is formed by a first metallic shield or collar  78  at one end of the resistor element  70  and a second metallic shield or collar  80  at an opposite end of the resistor element  70  to provide better voltage distribution. The first collar  78  is of a different size than the second collar  80 . More particularly, the first collar  78  is larger than the second collar  80 . In the illustrated embodiment, the first collar  78  has a diameter on the order of 22.5 mm and a thickness of 10 mm. The second collar  80  has a diameter of about 12 mm and a thickness of about 4 mm. As is apparent, other sizes could be used. The collars  78  and  80  sandwich the resistor element  70  with the leads  72  and  74  extending respectively therethrough. 
     Referring to  FIG. 7 , the insulated shield  32  comprises an elongate cylindrical housing  82  of a size corresponding to the diameter of the first collar  78 . The first high voltage resistor R 1  with the collars  78  and  80  is inserted into the shield housing  82 . A potting compound, represented at  84 , is used to fill the housing  82  to secure the collars  78  and  80  in the shield housing  82  with the terminal  36  being secured to the second lead  74  in a conventional manner and a terminal  86  being secured to the first lead  72  in a conventional manner. 
     The second probe  22  is of similar design to the first probe  20  including the electrostatic shielding across the second high voltage resistor R 2 . 
     The use of the metallic collars  78  and  80  having different diameters and thickness and placed at both ends of the high voltage resistors R 1  and R 2  forms capacitance from the collars  78  and  80  to the resistor sections of the high voltage resistors in the form of compensative capacitance. This is illustrated in  FIG. 9  as capacitance components CC 1 -CC 8  across the resistor elements R 11 -R 14  and R 21 -R 24 . The current due to stray capacitance represented as C 1 -C 8  will be approximately compensated by the metallic shielding compensative capacitive current. Thus, the voltage drop across each resistor section of the length of the resistors will be about equal. 
     Additionally, the collars  78  and  80  provide better mechanical support for the high voltage resistors R 1  and R 2  mounted in the respective probe shields  32  and  42 . The epoxy potting of the shield housings makes it simpler for assembling with the shielded collars. 
     Thus, in accordance with the disclosed probes, there is a reduction in high voltage measurement error owing to stray capacitance effect being compensated. A better electrical stress distribution is provided along the length of the high voltage resistors R 1  and R 2  ensuring reliability of the resistors. Particularly, the stray capacitance effect is minimized by use of the metallic circular collars  78  and  80  placed at opposite ends of the high voltage resistance element  70 . This makes the voltage distribution more linear and reduces the voltage error. 
     It will be appreciated by those skilled in the art that there are many possible modifications to be made to the specific forms of the features and components of the disclosed embodiments while keeping within the spirit of the concepts disclosed herein. Accordingly, no limitations to the specific forms of the embodiments disclosed herein should be read into the claims unless expressly recited in the claims. Although a few embodiments have been described in detail above, other modifications are possible. Other embodiments may be within the scope of the following claims.