Abstract:
A RF current transformer sensor includes a first sensor portion and a second sensor portion. The first and second sensor portions are configured to define a fixed opening for receiving a test object. The RF current transformer sensor is capable of detecting current pulses between the first sensor portion and the second sensor portion for sensing partial discharges from the test object. Further disclosed is a method of partial discharge sensing.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to sensors for partial discharge sensing. In particular, the invention relates to RF current taansformer sensors used for detecting partial discharges in power cables.  
         [0002]     Partial discharge sensing can be used to assess the condition of power cables. More particularly, partial discharge sensing can be used to detect deterioration of insulation of power cables by detecting high frequency currents that are created by small gaps, voids or other deterioration in power cable insulation.  
         [0003]     Typically, a partial discharge sensing system includes a partial discharge sensor, a spectrum analyzer, and a signal cable for carrying electrical signals between the sensor and the spectrum analyzer. In order to conduct an online partial discharge test, an operator secures the sensor at a test location on a “live” power cable (i.e., one with electrical current flowing through it). Then, the operator manually conducts a partial discharge test using the spectrum analyzer.  
         [0004]     Some partial discharge sensors are split core RF current transformer sensors. These sensors are clam-shaped and hinged, with a generally circular central opening formed when the sensor is in a closed position. In use, these sensors are closed around the power cable to be tested in order to conduct tests. Operators must touch these split core sensors in order to secure them to the power cable. With online testing, the power cable is energized or “live”. Therefore, the operator&#39;s hands must come in close proximity to the “live” power cable in order to secure the split core sensor to it. This presents a safety hazard to operators, who risk electrical shock, electrocution, and other serious injury, as the power cables can carry current at a high voltage (e.g., 15,000 volts).  
       BRIEF SUMMARY OF THE INVENTION  
       [0005]     A RF current transformer sensor according to the present invention includes a first sensor portion and a second sensor portion. The first and second sensor portions are configured to define a fixed opening for receiving a test object. The RF current transformer sensor is capable of detecting current pulses between the first sensor portion and the second sensor portion for sensing partial discharges from the test object. The present invention also includes a method of partial discharge sensing. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]      FIG. 1A  is an exploded perspective view of a RF current transformer sensor system according to the present invention.  
         [0007]      FIG. 1B  is a perspective view of a RF current transformer sensor having a “V” or “Y” shape.  
         [0008]      FIG. 1C  is a perspective view of a RF current transformer sensor having a “C” shape.  
         [0009]      FIG. 2  is a block diagram of a partial discharge sensing system utilizing the RF current transformer system of FIG. A. 
     
    
     DETAILED DESCRIPTION  
       [0010]      FIG. 1A  is an exploded perspective view of a portable RF current transformer sensor system  10  that includes a RF current transformer sensor  12  and a universal hot stick  14 . The sensor  12  has a first sensor portion  16  and a second sensor portion  18 . The first and second sensor portions  16  and  18  are arranged in a substantially parallel configuration. Each of the first and second sensor portions  16  and  18  has a first end  20  that is connected to a base portion  22  of the sensor  12  and an opposite second end  24 . The second ends  24  of the first and second sensor portions  16  and  18  can be tapered to facilitate positioning the sensor  12 . A fixed opening  26  is defined between the first and second sensor portions  16  and  18 , and the opening  26  extends from the second ends  24  of the first and second sensor portions  16  and  18  to the base portion  22  of the sensor  12 . A connector port (not shown) for connecting a RF signal cable to the sensor  12  is provided on the base portion  22 .  
         [0011]     A support protrusion  28  extends from the base portion  22  of the sensor  12 . A universal hot stick mount  30  is provided on the support protrusion  28  for mounting the sensor  12  on the hot stick  14  (or a similar support device).  
         [0012]     As shown in  FIG. 1A , the sensor  12  is generally U-shaped. The sensor  12  includes internal windings that function as the secondary winding of the transformer (the power cable to be tested functions as the primary winding of the transformer). The internal windings of the sensor  12  can be roughly equally divided between the first and second sensor portions  16  and  18 . In one embodiment, the sensor  12  has a frequency detection range of about 100 kHz to about 400 MHz or to at least about 200 MHz. In a preferred embodiment, the sensor  12  has a frequency detection range of about 2 MHz to about 60 MHz, which has been found to be a suitable range for online partial discharge testing. Sensors of such desired characteristics are available from Fischer Custom Communications, Inc., Torrance, Calif.  
         [0013]     In one embodiment, an outer surface  32  of the sensor  12  can comprise a metallic material. In an alternative embodiment, the outer surface of the sensor  12  can comprise a polymer material, such as polytetrafluoroethylene (PTFE).  
         [0014]     The universal hot stick  14  includes a universal mount  34  (which includes a threaded fastener) for cooperative engagement with the hot stick mount  30  on the sensor  12 . The respective mounts  30  and  34  permit the sensor  12  to be secured to the hot stick  14  at a desired orientation. The universal mount  34  is secured to a rigid insulative handle  36  of a desired length, for permitting a technician to position the attached sensor  12  from a distance.  
         [0015]     In further embodiments, the sensor can have other shapes.  FIG. 1B  is a perspective view of a RF current transformer sensor  60  having a “V” or “Y” shape.  FIG. 1C  is a perspective view of a RF current transformer sensor  70  having a “C” shape. Sensors  60  and  70  are generally similar to sensor  12 , as shown and described with respect to  FIG. 1A , with modified shapes.  
         [0016]      FIG. 2  is a block diagram of a partial discharge sensing system  100  utilizing the RF current transformer system  10  described above. The sensing system  100  includes a signal cable  102  and a spectrum analyzer  104 . The signal cable  102  can be a conventional RF coaxial cable, and can be connected between the spectrum analyzer  104  and the sensor system  10  in any suitable manner, as will be appreciated by those skilled in the art. Moreover, it should be recognized that additional components, such as preamplifier, can be included with system  100 , that are not shown in  FIG. 2 .  
         [0017]     In operation, the partial discharge sensing system  100  permits online testing of energized insulated cables (i.e., power cables carrying a “live” voltage) or other similar energized objects. A portion of a cable to be tested can be positioned in the opening  26  of the sensor  12  (or the sensor  60 ) by moving the sensor  12  with the attached hot stick  14 , such that the cable to be tested contacts the sensor  12  at two or more points. An operator can position the sensor  12  while keeping his or her body and appendages spaced from the sensor  12  and, in addition, spaced from the energized cable to be tested. Online partial discharge testing, to sense any surface currents that may be present on the cable to be tested due to cracks in its insulation, can then be conducted with the spectrum analyzer  104  according to known analysis procedures (e.g., using frequency domain test procedures). Then, the sensor  12  can be moved away from the tested cable by moving the hot stick  14 . In that way, the operator can position the sensor  12 , conduct test routines, and move the sensor  12  away from the test location without having to ever place his or her hands near the sensor  12  or the energized cables to be tested. This reduces the risk of harm to the operator due to electrocution and shock.  
         [0018]     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention. For instance, the manner of connecting a sensor according to the present invention to a suitable spectrum analyzer can include any means of transmitting signals therebetween.