Abstract:
A system and method for measuring at least one characteristic of a power line supported by a power line pole includes a pin having a first end coupled to the pole and a second end adapted to engage a sensor, wherein the sensor measures at least one characteristic of the power line and an insulator coupled to the second end of the pin supports the power line. The close proximity of the sensing element to the conductor immerses the sensor in the strong magnetic field close to the current carrying conductor to provide low cost accurate current measurement capability.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application claims the benefit of U.S. Provisional Patent Application No. 60/127,487 Provisional application expired, filed Apr. 2, 1999, the contents of which are hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to power distribution systems, and more particularly, to sensors and systems for estimating characteristics of a power line. 
     BACKGROUND 
     Electrical energy plays a central role in industrialized societies. The reliability of electric power systems is a critical factor in the planning, design and operation of power distribution systems. To ensure reliability, automated, real-time control of the system is desirable to allow for rapid respond to the actual demand of electricity and any unforeseen contingencies (equipment outages). 
     Modernized power distribution networks typically utilize energy control centers to monitor and regulate network operation. Although these centers have greatly advanced in technology, their basic control objectives of economy and security remain the same. The economic goal is to minimize the cost of supplying the electrical demand. The security goal is directed to the minimum requirements for reliability and quality of service. Electric utilities desire measurements of line voltage and current to allow for automated customer billing, load and reliability monitoring, and for real time control of the system. 
     Traditionally, current measurements with accuracies to within 0.3% or less are desired for customer billing while less accurate measurements (1 to 10%) typically suffice for other functions such as fault isolation and system control. However, in the past, many electric utilities typically employed highly accurate and very expensive current and or voltage transformers to perform measurements requiring strict accuracy as well as well as measurements to support functions such as fault isolation that require less accuracy. System inefficiencies are further exaggerated by the fact that distribution lines must be cut to accommodate the installation of conventional current transformers, which in most cases, is labor intensive. 
     Recently, the cost of measuring voltage and current in the 1 to 10% accuracy range in electrical power distribution feeders has decreased. Presently, line post current sensors that include a sensing element permanently embedded within an insulator are used as a less expensive and easier to install alternative to current transformers. Some other current sensors are connected at high voltage potential but the output signals must have isolation from the high voltage; frequently this is done via radio or fiber optic communications. There are also economical, temporary current sensors that are held in place with hot line sticks or are simply hung over the line. The installation time and effort of most new measuring devices is less than the time Be and effort to install current transformers. However, electric utility personnel are very familiar with the traditional current transformers and adoption of new devices within the industry has been gradual. 
     Therefore, it would be advantageous to provide a compact, lightweight sensing device that measures at least one operating characteristic of an overhead power line that may be readily attached and removed from a power line. The sensing device should preferably be suitable for economy of manufacture and durable to minimize operation and maintenance cost. 
     SUMMARY OF THE INVENTION 
     In one aspect of the present invention, a system for measuring at least one characteristic of a power line is supported by a power line pole and includes a pin having one end coupled to the pole and a second end adapted to engage a sensor. The sensor measures at least one characteristic of the power line. An insulator coupled to the second end of the pin, supports the power line in a conventional manner. 
     In another aspect of the present invention, a method for measuring at least one characteristic of a power line supported by a power line pole includes engaging a sensor with a first end of a pin, coupling an insulator to the first end of the pin, coupling a second end of the pin to the power pole, coupling the power line to the insulator, and measuring a characteristic of the power line. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where: 
     FIG. 1 is a system level diagram of a power distribution system, with three phase power lines supported with pin insulators that house sensors which forward information to a unit housing for processing and display; 
     FIG. 2 is a perspective view of a preferred current sensor installed in the pin of pin type insulator that is used to support a power line; 
     FIG. 3 is a perspective view, partly in cross section, of the preferred pin housing a current sensor with a water tight plug for electrically coupling the sensor to a control unit; 
     FIG. 3A is a top plan view of a circular housing for tapping and housing the current sensor in the top of the pin of the pin type insulator; 
     FIG. 4 is a cross-sectional view of a pin type insulator that includes a capacitor formed by a semiconductive glaze applied to the upper surface and threaded aperture of the insulator, which is coupled to the electrical conductor of a voltage sensor for measuring the voltage on the power line; 
     FIG. 4A is a schematic diagram of a voltage divider circuit implementation of the voltage sensor of FIG. 4; 
     FIG. 5 is a cross-sectional view of a combined voltage/current sensor integrated into the pin of a pin type insulator; 
     FIG. 6 is a cross-sectional view of an alternate voltage sensor combined with a current sensor integrated into the pin of a pin type insulator; and 
     FIG. 7 is a cross-sectional of a further alternate voltage sensor combined with a current sensor integrated into the pin of a pin type insulator. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In a preferred embodiment of the present invention, a sensing device is employed to interface with an overhead power line to assist in the performance of a variety of functions, such as, for example, tracking customer usage, monitoring system load and reliability as well supplying data for system control. Referring to FIG. 1, in the field of power transmission and distribution, generating systems produce electrical power which is transmitted through a grid of electrical high voltage alternating-current (AC), three-phase power lines ( 20 ,  22  and  24 ). Pin type insulators  26 ( a ),  26 ( b ) and  26 ( c ) (hereinafter referred to as pin insulator), as described by ANSI C29.5 and C29.6 standards, are universally accepted as the standard insulator used for supporting and insulating the high voltage (4 to 69 kV) power lines  20 ,  22  and  24 . 
     A preferred embodiment of the present invention integrates a sensor within the supporting pin insulator  26 . The commercially available pin insulator  26  provides the insulation and protection for the sensing device (not shown) and the signal wires located under it. The entire assembly is fastened to a wood cross-arm or pole bracket  28 . 
     The sensor measures at least one operating characteristic of the overhead power line and outputs that characteristic to a control unit housing  30  located at the base of the pole via an external cable  32 . The characteristic may relate to any aspect of current or voltage on the line, or any other characteristic of the line. The control unit housing  30  preferably contains a control unit  34  that receives an output from the sensor and forwards the received power line characteristics to a remote terminal unit  36 . The remote terminal unit  36  is preferably coupled to a radio (not shown) located inside the control unit housing  30  which is coupled to an antenna  38  located just outside the control unit housing  30  for sending, via RF radio signals, data output by the sensor concerning power line characteristics to a remote control station (not shown). In addition, the remote terminal unit  36  may receive control or reprogramming signals transmitted from the remote ground monitoring and control station (not shown). The control unit  34  may be used to calibrate the sensor, either remotely via commands sent from the control station or locally at the control unit  34 . 
     Referring to FIG. 2, the pin insulator  26  assembly includes a standard pin  40  that supports an insulator  42  and fastens the pin insulator  26  to the wood cross-arm  28  or pole-mounting bracket. The pin  40  and insulator  42  preferably comply with the requirements set forth in various standards such as for example ANSI C29.5 and C29.6, the contents of which are incorporated herein by reference as if set forth in full. In the preferred embodiment, the top of the pin  41  is in the range of about ⅝″ to 2″ below energized conductor  44 . The top of pin  41  is separated from the conductor  44  by the industry standard insulator  42 , which is preferably screwed onto the pin  40 . A tie wire  46  preferably secures the conductor  44  to the insulator  42 . One of ordinary skill in the art will appreciate that the present invention may be adapted for use in any pin type insulator assembly, including those fabricated in accordance with various international standards, such as, for example, British standard B.S.  137-1960.  Therefore, the preferred pin type insulator is by way of example and not limitation. 
     The insulator  42  is preferably commercial grade wet-process porcelain. The electric field around the interface of conductor  44  and insulator  42  is highly stressed, which can result in corona discharge. Therefore, the upper portion and pin hole threaded aperture  48  of the exemplary insulator  42  are preferably coated with a semiconductive glaze to minimize radio interference. 
     In accordance with the applicable standards, the preferred pin  40  should be either one inch or one and three-eight&#39;s inches in diameter. The threads of pin  40  preferably comply with ANSI C29.5 and C29.6 for either one inch or one and three-eights inch diameter threaded pin, preferably having four threads per inch for domestic applications in the United States. The body of the pin  40  is preferably made from an aluminum casting. The aluminum casting is preferably cored to allow electrical attachments internal to the pin  40 . One of ordinary skill in the art will appreciate that the present invention may be adapted to accommodate various pin characteristics so that the preferred pin characteristics are by way of example and not limitation. 
     In the preferred embodiment, the top of the pin  41 , that portion nearest the electrical conductor  44 , is recessed to house a current sensor  50 . The protected proximity of the top of the pin  40  to the high voltage conductor  44  immerses the current sensor  50  in the strong magnetic field generated by the current carrying high voltage conductor  44 . Advantageously, the high voltage conductor  44  does not have to be cut for installation and the standard insulator  44  may be easily adapted, if necessary, for maintenance or upgrading to a higher voltage conductor  44 . 
     Referring to FIG. 3, the bottom of the pin  40  is preferably machined in a plurality of places. The exemplary pin  40  preferably includes a threaded aperture  51  directly below the head of the pin. A support rod  52  with threads on both sides is then screwed into the pin  40 . The support rod  52  may then be inserted through the wood cross-arm  28  and attached thereto with a flat washer  54  a lock washer and a nut  56  (see FIG.  2 ). In the exemplary embodiment, the support rod  52  is preferably aligned with the head of pin  40  to optimize the transfer of loads from the pin insulator  26  to the cross-arm  28  (see FIG.  2 ). The support rod  52  is preferably galvanized steel and on the order of about five eights inches in diameter. In addition, the end of the support rod  52  threaded into pin  40  is preferably slant cut at an angle in the range of about 30-60 degrees. One of ordinary skill in the art will appreciate that the pin insulator  26  may be attached to the wood cross-arm  28  by a variety of techniques including, for example, clamps, rivets, tie wraps or any other suitable means. Therefore the exemplary support rod is by way of example and not by way of limitation. 
     In the exemplary embodiment, the current sensor  50  is preferably a programmable gain linear Hall effect sensor, such as for example, the MLX90215 manufactured by Microelectronic Integrated Systems, located in Webster, Mass. The core of the Hall effect sensor is a Hall effect element. When a magnetic field is in the near vicinity of the hall effect element, a current flows within this material that is proportional to the strength of the magnetic field. 
     A Hall effect sensor is basically a Hall effect element with a terminal wired to each end of the Hall effect element. The current produced in the Hall effect element creates a potential difference between the two terminals that is proportional to the strength of the magnetic field and indirectly the current flowing through the electrical conductor. The Hall effect sensor preferably includes a feedback loop that allows the sensor to be externally calibrated. External calibration adjusts the ratio of the voltage output and the strength of the external magnetic field created by the high voltage power line as a function of a secondary measurement. The preferred current sensor measures the current in the electrical conductor to an accuracy of approximately 1-10%. One of ordinary skill in the art will appreciate that alternate current sensors such as, for example, air core or lead core transformers with multiple conductive windings may be used in place of the preferred Hall effect sensor. Therefore, the Hall effect sensor is set forth by way of example only. 
     Referring to FIG. 3A, the current sensor  50  (hereinafter referred to as the Hall effect sensor) is located inside a round aluminum or epoxy enclosure  54  preferably with a slot  56  and an external punch mark  58  to reference the location of the slot  56 . The Hall Effect sensor  50  is bonded into the slot  56  on the lower surface of the enclosure. The direction of the Hall effect sensor corresponds to the direction of current flow in the electrical conductor and is indicated by the punch mark  58  at the top of the round enclosure  54 . Referring back to FIG. 3, in the exemplary embodiment the Hall effect sensor has four wires  60   a-d  which are connected to internal cabling  62  that is threaded through the cavity of the aluminum pin  40 . The four wire electrical cable is then attached to a four-pin watertight connector  66 . Connector  66  is preferably coupled to the control unit  34  located at the base of the pole via the external cable  32  (see FIG.  1 ). 
     A DC power supply, preferably 5 volt, for the current sensor is located in the control unit housing  30 . Preferably the DC power supply is connected to a standard 120 VAC circuit, with back up battery power. In an alternate embodiment, the DC power supply may be completely battery powered. In the preferred embodiment the DC power supply is used to power a set of three current sensors, (i.e. one for each phase). 
     In addition, control unit  34  preferably includes a multi-purpose readout that may be used to calibrate the current signals and to display the readings. Each sensor may be calibrated independently. Coarse current calibration is preferably performed by measuring the distance (in inches) from the center of the conductor to the base of the insulator support pin and entering this information into the readout device. Fine calibration may be performed by measuring the current carried by the power line with an auxiliary meter, such as, for example, a hot stick current meter and entering this calibration data into the readout device. The calibrated output of the control unit  32  may then be transmitted to a remote control center via remote terminal unit  36 . Alternatively, the power supply and readout device may provide analog output signals or alarm contacts which are forwarded to other data collection or communication equipment. 
     The preferred pin  40  includes an offset leg  68  that extends below the wood cross-arm to which the pin insulator  26  is attached. The offset leg is preferably installed to indicate the direction of current flow in the electrical conductor. The round enclosure  54  that engages the Hall effect sensor is preferably press fit into the top of the pin  40  with the direction of the Hall effect sensor (the punch mark  58 ) corresponding to the direction of the offset leg  68  of the pin  40 . This gives an external indication of the direction of current flow being measured. 
     In the preferred embodiment, the cavity  64  of pin  40  is filled with epoxy  70  to within approximately one-two inches of the top of the four-pin water tight connector  66 . The epoxy internally seals the Hall effect sensor and substantially prevents any moisture intrusion. In addition, the epoxy substantially secures the support rod  52  in place due to the slanted cut on the end of the pin that was threaded into the pin  40 . After the epoxy dries, the four pin water tight connector  66  is screwed onto the pin along with appropriate sealing measures such as an o-ring  67  to prevent moisture intrusion at the lower end. 
     In the preferred embodiment, pin  40  further comprises a machined surface  72  and a threaded aperture  74  which engage a ground clamp connection  76 . The ground clamp  76  is preferably secured to and grounds the machined surface via a bolt  77  that is screwed into threaded aperture  74 . The pin  40  preferably has a semi-conductive heat shrink  78  or memory retention sleeve applied to the top of the aluminum threads to allow proper sealing/seating of the porcelain pin type insulator. 
     Referring to FIG. 4, in another aspect, the present invention comprises a voltage sensor that detects the presence of a voltage on the high voltage electrical conductor  44  as well as estimates the phase of the voltage. The periodic detection of a voltage on an electrical conductor is useful when performing fault isolation on the electrical conductor. 
     In one aspect, the semiconductive glaze applied to the upper surface  102  and threaded aperture  104  of the insulator  42  is separated by the dielectric porcelain that forms the body of insulator  42 . The semiconductive glaze coated surfaces  102  and  104  form a capacitor (C 1 ) with electrical conductor  44  acting as an input voltage applied to C 1 . In the preferred alternate embodiment an electrical conductor  106 , preferably stainless steel bonding wire is electrically coupled with the semiconductive glaze applied to the threaded aperture  104 . In the alternate embodiment, insulator  108  isolates pin  40  from the semiconductive glaze applied to the threaded aperture  104 . Insulator  108  is preferably an insulating heat shrink wrap. 
     In the alternate embodiment, electrical conductor  106  is preferably coupled to a cable  110  having two outputs  112  and  114 . A grounded shunt  116  is connected between the electrical conductor  106  and the output  114 . Cable  110  is preferably installed in cavity  64  of pin  40 . A second capacitor C 2   111  is preferably connected between electrical conductor  106  and the grounded shunt  116 . The second output  112  is a wire  113  connected in shunt between capacitor  111  and the electrical conductor to provide the voltage divider circuit shown in FIG.  4 A. In operation the high voltage electrical conductor  44  provides the input voltage to C 1 . The differential output voltage V AB  is preferably in the range of about 1-10 volts and is determined by the product of the input voltage V In  and the ratio C 2 /C 1 . 
     For an exemplary 15 kilo volt pin isolator, an input voltage V In  will be on the order of about 10 kV. C 1  is typically in the range of 20 picofarad and C 2  is preferably on the order of about 50 nanofarad, to provide a ratio of approximately 1:2500 and an output voltage of approximately 4 volts. The exact value of the output voltage need not be constant with time. In fact, the capacitance C 1  of the isolator  42  will vary with time depending upon such factors as erosion of the semiconductive glaze, contamination, precipitation or other factors. However, the system monitoring and fault isolation functions may be satisfied by accurate detection of the presence of a voltage on the electrical conductor, i.e. a voltage on voltage off detection. Because accurate measurement of actual voltage level is not required, the preferred system need not be time invariant and variation in the value of capacitor C 1  does not diminish system performance. One of ordinary skill in the art will appreciate that C 2  may be a variable capacitor that may be externally calibrated to provide a voltage in the preferred range so that the described exemplary capacitive values are by way of example and not limitation. 
     Outputs  112  and  114  are preferably routed to a watertight connector (not shown) and coupled to the control unit  32  (see FIG. 1) via the external cable  32 . The control unit  32  preferably measures and displays the differential voltage across terminals  112  and  114 . The control unit  34  forwards the measured voltage to the remote terminal unit  36  for transmission to a remote control center. In the preferred embodiment, capacitor C 2  is variable and under direction of control unit  32 , which may calibrate the voltage sensor by adjusting the output voltage across terminals  112  and  114  to correspond with the approximate known voltage at the sensor location. 
     In the preferred voltage sensor embodiment, support rod  52  again couples pin  40  to the wood cross-arm of the power line pole. In addition, pin  40  is preferably grounded with the ground clamp as previously described for the current sensor embodiment. 
     The described exemplary voltage sensor may utilize a variety of techniques to electrically couple to the semiconductive glaze applied to the threaded aperture  104 . For example, referring to FIG. 5, a conductive spring  120  may be used to couple the cable  110  with the semiconductive glaze applied to the threaded aperture  104 . When installed the conductive spring  120  is compressed by the semiconductive glazed threaded aperture to ensure adequate electrical conductivity. 
     Alternatively, conductive pin  40  may be terminated with molded conductive neoprene  122  (i.e. neoprene loaded with carbon black)(see FIG.  6 ), or other similar materials. Cable  110  may then be embedded into the conductive neoprene  122  to provide electrical conductivity with the semiconductive glaze applied to the threaded aperture  104  of the isolator. In addition, an insulator  124  should preferably be applied to pin  40  below the conductive neoprene to ensure that the pin  40  is properly isolated from the semiconductive glaze applied to the threaded aperture  104  of the isolator. 
     In another aspect of the present invention, the current sensor and voltage sensor may be combined in a single embodiment. Referring to FIG. 7, the stainless electrical conductor  106  is preferably used to terminate cable  110  so as to provide electrical continuity with the semiconductive glaze applied to the threaded aperture (see FIG.  4 ). As with the preferred voltage sensor embodiment disclosed in FIG. 4, the electrical conductor would again be combined with a capacitor (not shown) to provide a voltage divider circuit for detecting the presence of a voltage on the high voltage electrical power line. 
     In addition, current sensor  50  may be bonded into the upper tip of pin  40  preferably with epoxy. A watertight connector (not shown) would again be used to couple the output of the voltage and current sensors to the control unit via the external cable. The conductive pin  40  is again treated with an insulator  108  to isolate the pin  40  from the semiconductive glaze applied to the threaded aperture. Insulator  108  is preferably an insulating heat shrink wrap. In the alternate embodiment, the phase of the current as estimated by the current sensor and the phase of the voltage on the electrical conductor  106  may be used to estimate a power factor to estimate the optimal capacitive load to balance the high voltage power line. 
     Although a preferred embodiment of the present invention has been described, it should not be construed to limit the scope of the appended claims. Those skilled in the art will understand that various modifications may be made to the described embodiment. For example, the exemplary current sensor may be integrated with a variety of pin type insulators as required to support various electrical conductor voltages and loads. In addition, the voltage sensor may be realized by any of a variety of techniques which provide electrical conductivity to the semiconductive glaze applied to the threaded aperture of the isolator. Moreover, to those skilled in the various arts, the invention itself herein will suggest solutions to other tasks and adaptations for other applications. It is therefore desired that the present embodiments be considered in all respects as illustrative and not restrictive, reference being made to the appended claims rather than the foregoing description to indicate the scope of the invention.