Abstract:
A sensor apparatus for detecting leakage current in a suspension-type insulator of an electrical power system includes: a housing having a first half of a leakage current sensor contained therein; a door pivotally connected to the housing by a hinge, and having a second half of the leakage current sensor contained therein, the door pivotable about the hinge between an open position in which the first and second halves of the leakage current sensor are separate from each other, and a closed position in which mating surfaces of the first and second halves of the leakage current sensor join together to define a closed perimeter; and a clamping mechanism connected to the housing comprising a first jaw and a second jaw, wherein the jaws are moveable between an open position and a closed position.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates generally to the monitoring of leakage currents in a transmission system and, more particularly, to a sensor for accurately detecting and monitoring leakage current in suspension type insulators. 
         [0002]    In many countries, including the US, wood is utilized as part of the line insulation to improve the basic insulation level (BIL) of the line, as it has been recognized that the lighting performance of transmission lines can be improved by utilizing the wood support or pole. This has, however, not been without problems as there were, and still are, many instances of fires of the wood poles and cross arms caused by low frequency leakage current and sparking on the wood from sources such as leakage current due to insulator contamination despite mitigation measures being taken. 
         [0003]    Insulators installed on transmission and distribution systems are exposed to contamination, for example from marine salt, road salt, and industrial pollutants. This contamination can result in flashover of the insulator, usually under light wetting conditions, e.g. condensation, when the salts and water mix to become a conductive electrolyte. Flashover is a problem in that it results in an outage which interrupts power to a utility company&#39;s customers. 
         [0004]    When the salts on the surfaces of an insulator become wet they form an electrolyte which is conductive. Since the one end of the insulator is energized, and the other end is grounded, currents flow along the insulator surfaces. If these currents are large enough, arcing will occur (called dry band arcing). This arcing will either extinguish or grow to result in a flashover. The magnitude of leakage currents that occur under dry band arcing conditions provide an indication of the risk of the insulator flashing over. If insulators are at risk of flashover due to contamination build-up, utilities can wash the insulators or take other measures, such as redirect power to other transmission and distribution assets. 
         [0005]    In addition when composite (also called polymer or non-ceramic) or coated insulators are used the leakage currents and arcing on the surface can degrade the rubber material. By monitoring the leakage currents an indication can be obtained as to the level of degradation. 
         [0006]    Some commonly used mitigation measures are listed below; however, none of these mitigation measures provide a means for monitoring and pinpointing potential leakage current problems so that a utility company can take preventative measures. 
         [0007]    1. Wrapping metal bands around the wood pole and connecting it to the insulator hardware. This method has the advantage that the reduction in the “insulated” wood path lengths (used as improvement for the BIL) is limited. The conductor material used for this purpose should be compatible with other hardware not to cause corrosion. 
         [0008]    2. A small guard electrode, in the form of a coach screw or a multi-spiked plate (e.g. gang-nail), is fastened to the wood outside the rain shadow area and bonded to the insulator hardware. This method has a minimal effect on the BIL of the structure. 
         [0009]    3. Application of conducting paint to cover the high resistance zones around metal to wood interfaces. This method has a minimal effect on the BIL of the structure. 
         [0010]    4. Bonding of the insulator hardware together with a conductor. The intension with this bond wire is to “balance” the leakage current so that only a small residual current will flow in the pole. There are two variants to this scheme:
       (a) The insulator bases are connected together but not grounded. The ground lead terminates some distance away to realize the required BIL phase-to-ground for induced lightning surges.   (b) The insulator bases can be bonded together and connected to ground. In this case the wood is not utilized anymore as part of the line insulation against lightning. Also here it is important that the material of the bond wire is selected to be compatible with the other hardware used to prevent corrosion.       
 
         [0013]    5. An extension of the previous method is to use steel cross-arms to bond the insulator bases together. The steel cross-arm can either be grounded or be left floating depending on whether or not the utility wants wood as part of the line insulation for lightning performance. 
         [0014]    6. Finally the insulators used can be upgraded to those with an improved contamination performance. For example porcelain insulators can be replaced by hydrophobic silicone rubber composite insulators. Other options include regular insulator cleaning or the application of silicone grease to insulators. 
       BRIEF SUMMARY OF THE INVENTION 
       [0015]    These and other shortcomings of the prior art are addressed by a leakage current monitoring system that continuously monitors and reports potential leakage current issues to allow an action to be taken to mitigate any potential problems that may arise from the leakage current. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The subject matter that is regarded as the invention may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which: 
           [0017]      FIG. 1  is a schematic side view of a portion of a utility pole carrying a suspension-type insulator and a sensor unit constructed according to an aspect of the present invention; 
           [0018]      FIG. 2  is a perspective view of the sensor unit shown in  FIG. 1 ; 
           [0019]      FIG. 3  is another view of the sensor unit shown in  FIG. 1 ; 
           [0020]      FIG. 4  is a view of the sensor unit of  FIG. 1  with a cover removed to show the internal components; 
           [0021]      FIG. 5  shows a door of the sensor unit of  FIG. 1 ; and 
           [0022]      FIG. 6  is a block diagram showing the operation of the sensor unit of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0023]    Referring now to the drawings, wherein identical numerals indicate the same elements throughout the figures,  FIG. 1  schematically depicts an insulator  10  which is suspended from a cross-arm  11  of a utility pole  12  and which supports an electrical conductor  13 . The insulator  10  is a known suspension-type insulator having a generally cylindrical exterior shape with an upper end or grounded end fitting  14  and a lower end  16  connected to the electrical conductor  13 , and is made from an electrically insulating (i.e., non-electrically-conductive) material. An exemplary sensor unit for accurately detecting and monitoring leakage currents according to an embodiment of the invention is mounted on the upper end  14  and shown generally at reference numeral  20 . 
         [0024]    In summary, the sensor unit  20  is an RF sensor which attaches to the insulator&#39;s grounded end  14 , measures the leakage current flowing in the insulator  10 , processes the current and assigns them to specific ranges. The sensor unit  20  then transmits the information wirelessly to either a local base station or a handheld device. 
         [0025]    Referring to  FIGS. 2 and 3 , the sensor unit  20  includes a housing  21  for containing a leakage current sensor  22  and electronic module  70  (shown in  FIG. 4 ), a door  23  hinged to the housing  21  to allow the sensor unit  20  to be positioned onto the insulator  10 , and a clamping mechanism  24  for clamping the sensor unit  20  to the insulator  10 . The housing  21  is a metal housing which encloses the sensor unit&#39;s functional components (described in detail below) and protects them from electromagnetic influences. 
         [0026]    The door  23  is pivotally hinged to the housing  21  at hinge  26  to allow the door  23  to pivot away from the housing  21  to an open position and allow the sensor unit  20  to be installed on the insulator  10 . Fastener  27  secures the door  23  to the housing  21  in a closed position once the sensor unit  20  has been installed. A hot stick adapter  28  is connected to the housing  21  and positioned next to fastener  27  to allow a user to use a hot stick for placement of the sensor unit  20  on the insulator  10 . The hinge  26  is resistive in nature so that it moves freely, making it easier for the installer—especially under live line conditions. 
         [0027]    Clamping mechanism  30  is secured to the housing  21  and includes first and second adjustable clamping jaws  31  and  32  connected to first and second pivotable arms  33  and  34 , respectively. The jaws  31  and  32  are used to mechanically attach the sensor unit  20  to the grounded end  14  of the insulator  10  and are adjustable to account for different insulator end fitting diameters. As shown in  FIG. 2 , the jaws  31  and  32  include apertures  36  for receiving fasteners therethrough and to allow for adjustment of the jaws  31  and  32  with respect to arms  33  and  34 . The jaws  31  and  32  are adjusted by removing fasteners  37 , moving the jaws  31  and  32  relative to arms  33  and  34  until a desired aperture  36  (representative of a specific diameter) aligns itself with apertures in the arms  33  and  34 . The fasteners  37  are then re-installed through the apertures in the arms and apertures  36  to secure the jaws  31  and  32  in position. As illustrated, the jaws  31  and  32  have a V or U shaped profile for mating engagement with the grounded end  14  of the insulator  10 . 
         [0028]    The jaws  31  and  32  are moved between opened and closed positions by a bolt mechanism  40  connected to threaded collars  41  and  42  disposed at an end of the arms  33  and  34 , respectively. The bolt mechanism  40  includes a pair of opposing bolts  43  and  44  coupled together at their ends by a coupling  46 . Each of the bolts  43 ,  44  extend through a respective one of the threaded collars  41 ,  42 . As the bolts  43  and  44  are turned, the threaded collars  41  and  42  move along the length of the bolts  43  and  44 , which in turn causes the arms  33  and  34  to move about pivots  47  and  48  such that they move the jaws  31  and  32  between opened and closed positions. The key feature of the bolt mechanism  40  is that it allows a user to turn either one of the bolts  43 ,  44  to open and close the jaws  31  and  32 , thereby eliminating any issues with the user being on a specific side of the sensor unit  20 . When installing the sensor unit  20  onto the grounded end fitting  14 , the door  23  and jaws  31 ,  32  are moved to the open position for receiving the end fitting  14 . The jaws  31 ,  32  are then moved to the closed position to clamp onto the end fitting  14 . With the sensor unit  20  firmly clamped to the end fitting  14 , the door  23  is then moved to the closed position and secured in place by fastener  27 . 
         [0029]    Referring to  FIG. 4 , the leakage current sensor  22  includes a toroidal current transformer  50  with a frequency response from &lt;5 Hz to &gt;100 kHz and a sensitivity of lower than 10 mA. The transformer  50  is formed by a leakage current toroidal ferrite core  51  and winding  52  wound around the core  51  with multiple turns. The core  51  is formed of two halves  51 A and  51 B so that it can surround the grounded end  14  of insulator  10 . When the two halves  51 A and  51 B of the core  51  are put together, the voltage output from the windings  52  is proportional to the current flowing through the middle of the core  51 . 
         [0030]    A sheet metal housing  53  (steel which is ferromagnetic for magnetic field shielding) surrounds the core  51 . As shown, the housing  53  is also formed by two halves  53 A and  53 B to surround respective core halves  51 A and  51  B. The metal housing  53  is split along an inside with a metal slot  54 , See  FIG. 5 , so that the core  51  can still couple with a magnetic field from current flowing in an insulator&#39;s metal end fitting. This enables the core  51  to be shielded from stray magnetic fields which are not due to current flowing through an insulator end fitting but still measure the currents flowing through the insulator metal end fitting. This is very important as there are large magnetic fields due to the currents flowing in the conductors which are energized at ends of the insulators. 
         [0031]    As shown in  FIG. 5 , core  51 B and housing  53 B are installed in the door  23  so that the sensor unit  20  can be installed in the field on insulators without removing them from service. The core  51 B and housing  53 B are connected to a plate  56  which is connected to the door  23 . The plate  56  is moveable relative to the door  23  and is biased towards the housing  21  by springs  57  and  58  to ensure that mating surfaces  59  and  60  of cores  51 A and  51 B are properly mated together. The mating surfaces  59  and  60  of the cores  51 A and  51 B are machined with key patterns so that alignment is ensured between the two cores. Alignment is vital so that the unit consistently measures the leakage currents flowing through the end fitting of the insulator, and it reduces the influence of stray magnetic fields. 
         [0032]    Referring again to  FIG. 4 , an output  61  from the sensor  22  operably connects the sensor  22  to the electronics module  70 . The electronics module  70  is imbedded in potting compound to protect it from both environmental and electromagnetic influences and includes analog, digital, and radio frequency (RF) electronics which function to receive, process, and store signals from the sensor  22 , to receive external commands, and to transmit data to an external source. The electronics module  70  may include, for example, a printed circuit board incorporating analog, digital and/or radio-frequency (“RF”) electronic components or may incorporate discrete components and/or one or more microprocessors. 
         [0033]    In addition to the electronics module  70 , the housing  21  includes an electric power source for the electronics module  70 , such as the illustrated batteries  72 . The housing  21  also includes one or more RF antennas  63  which protrude from the exterior of the housing  21  and are used to transmit signals generated by the electronics module  70  to a remote receiver (not shown), and/or to receive RF signals from a remote receiver (not shown). The sensor unit  20  incorporates a communication system that may be based on the IEEE 805.15.4 architecture. The communication protocol is customized to allow two-way communications. 
         [0034]    In the illustrated example, one or more magnetically-operated switches  71  are mounted inside the housing  21  and coupled to the electronics module  70 . The switches  71  may be tripped by placing a magnet in the near vicinity of the switch  71  on the outside of the housing  21 . In the illustrated example, the sensor unit  20  may include a power switch which toggles the sensor unit  20  between the on and off state, and a reset switch which signals the sensor unit  20  to erase any stored data. 
         [0035]    The electronics module  70  may include a temperature sensor, in order to assist in assessing condensation conditions. The electronics module  70  may also include a  3 D accelerometer, in order to assess whether the insulator or structure is experiencing vibration issues. 
         [0036]    The operation of the electronics module  70  and the sensor unit  20  will now be described with reference to the block diagram in  FIG. 6 . In block  200 , the electronics module  70  uses a peak detect circuit of a known type to measure and hold a voltage signal from the sensor assembly described above. The voltage signal is proportional to the highest leakage current measured in a predetermined first time interval, e.g. 60 seconds. This peak detect circuit is reset at the first interval, e.g. 60 seconds, based on a digital signal from the microprocessor. At block  202 , an Analog to Digital (A/D) converter (which may be part of a microprocessor of the electronics module  70 ) measures the value from the peak detect circuit, repeating as the first interval. At block  204 , the microprocessor evaluates the digital value and assigns the value to membership in a category or “bin”. The bin represents a range in which the measured value lies. For example, there may be six numbered bins which account for different leakage current magnitudes. Examples of two different settings for the leakage current threshold for the bins are listed in Table 1 below, in which “regular” indicates a classification that is relatively less sensitive to leakage currents and “sensitive” indicates a classification that is relatively more sensitive to leakage currents. 
         [0000]    
       
         
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 CURRENT RANGE, 
                 CURRENT RANGE, 
               
               
                 BIN 
                 REGULAR 
                 SENSITIVE 
               
               
                 NUMBER 
                 (PEAK mA) 
                 (PEAK mA) 
               
               
                   
               
             
             
               
                 1 
                  0-10 
                 0-1 
               
               
                 2 
                 10-20 
                 1-2 
               
               
                 3 
                 20-50 
                 2-5 
               
               
                 4 
                  50-200 
                  5-20 
               
               
                 5 
                 200-500 
                 20-50 
               
               
                 6 
                 500+ 
                 50+ 
               
               
                   
               
             
          
         
       
     
         [0037]    The electronics module  70  maintains a counter for each of the bins. When the digital value is assigned to a bin, the counter for that bin is incremented (see block  206 ). At block  208 , the number of counts in each bin is then transmitted using RF to a remote receiver. The transmission repeats at a second interval which is preferably shorter than the first interval described above. 
         [0038]    The sensor unit  20  only stores the statistical parameters (i.e. the bin counts) of the leakage current peaks that occur. No other leakage current parameters need be recorded. Using the communications system described above, a remote user can reset the bins or change the ranges of the bins remotely. 
         [0039]    The sensor unit  20  also keeps track of the time since the last reset. This limits the data message sent by the device to the bare minimum. It also limits the internal circuit complexity and power consumption for the device. Based on the battery characteristic and low power consumption of the sensor unit  20 , battery life is estimated at more than 10 years. 
         [0040]    The sensor units  20  can be employed in different modes. For example, when installed on transmission lines the sensor units  20  may be polled a only few times per year when line maintenance crews do inspections or maintenance, for example using handheld receivers (not shown). 
         [0041]    Alternatively, in substations or on specific transmission line structures a more sophisticated approach may be followed. A dedicated base station system (not shown) installed at the substation/structure would poll the nearby sensor units  20  at a short time interval. This base station stores leakage current data together with weather parameters from attached sensors. The data is then transmitted from the base station using a variety of methods including the use of GPRS modems or connecting to a utility data management system. The data is stored and processed on a remote server. Alarms can be generated based on algorithms and data can be viewed using visualization tools. 
         [0042]    If the leakage current characteristics of the insulators are known, alarms can be generated automatically based on preset leakage current alarm levels. Warnings or alarms can be raised to initiate insulator maintenance (e.g. washing) if certain pre-set leakage current thresholds are exceeded. Leakage current information can also be used select appropriate mitigation measures. 
         [0043]    It should be noted that the sensor units  20  are connected between the insulator and the grounding system. With this configuration the insulator leakage current is directly shunted to ground and it will therefore not pass through the wood cross arm. The leakage currents measured can however be used to raise warnings that conditions and insulator contamination levels are sufficiently high to cause wood pole fires. 
         [0044]    The sensor unit  20  described above has several advantages. The sensor units  20  are suitable for wide spread deployment which makes them practical for installations on overhead lines and substations. Some of the specific advantages of the sensor are low cost; absence of wiring to either power the sensor unit  20  or communicate with the sensor  20 , leading to improved reliability compared to wired units; the ability to quickly deploy many sensor units  20 ; on-board processing of data, providing a user with processed information on which he can make a decision; and a combination of analog and digital electronics, ensuring that no current pulses are ever missed, as compared to previous technologies that used digital measurement only. 
         [0045]    The foregoing has described a sensor apparatus for detecting and monitoring leakage current in suspension type insulators. While specific embodiments of the present invention have been described, it will be apparent to those skilled in the art that various modifications thereto can be made without departing from the spirit and scope of the invention. Accordingly, the foregoing description of the preferred embodiment of the invention and the best mode for practicing the invention are provided for the purpose of illustration only and not for the purpose of limitation.