Abstract:
A capacitor component is adapted for capacitive coupling to an electrical power apparatus in which a PD is suspected, such as a shielded power cable; and an impedance transformance device has an input side connected directly to the capacitor component. Together the two components are used as a sensor to pick up and convey PD signals to a PD signal-analyzing system. Although the two components inherently form a classic high-pass RC filter, in accordance with the invention the input impedance at the capacitor side is much greater than the output impedance at the signal-analyzing system side. The effect is to pass much more of the lower frequency components of the PD signal for analysis by the PD signal-analyzing system than if the tranformance device were not included.

Description:
BACKGROUND 
       [0001]    As electrical systems age, defects such as: cavities inside of insulating materials; thinning of insulation in motor and transformer windings; contamination across insulating surfaces; incorrect voltage to ground spacing; etc., can begin to discharge. The presence of these electrical discharges is an indicator of hidden defects which, if left unattended, can lead to system failure. In fact, the discharges themselves will, over time, degrade the material that is sustaining them also leading to system failure. Because these discharges may occur within the interior of an insulating material and because these discharge events can be very small in absolute magnitude, their presence can be unnoticeable to human senses. 
         [0002]    A partial discharge (PD) is a flow of electrons and ions which occurs in a gas over a small volume of the total insulation system. The defects concentrate the electric stress and can degrade the dielectric. The degradation initiates within a limited portion of the dielectric and typically does not lead to acute failure immediately. Instead, the PD in an insulation system performs as a chronic symptom which develops and eventually causes failure. The characteristics of a PD are “health indicators” which represent the performance of the insulation system in an electrical apparatus. Attempts to detect, analyze, and locate PDs have been made for a long time, including systems designed to measure and analyze acoustic energy emitted by PD pulses, optical energy emitted by PD pulses, and/or electromagnetic energy emitted by PD pulses. In electromagnetic detection systems, induction coupled (magnetic) sensors and capacitive coupled (electrical) sensors have been used. 
         [0003]    In electromagnetic terms, a PD pulse is characterized by a fast rise time in the range of sub-nanoseconds (sub-ns) and a signal bandwidth up to the gigahertz (GHz) range at its origin. The initial ns or sub-ns spike is followed by an “ionic portion,” which may have a longer duration (about  100  ns). In a shielded power cable, the PD signal propagates along the power cable from its origin, but high frequency components of the signal attenuate more significantly (i.e., over shorter distances) than lower frequency components. Sometimes the signal bandwidth at a detection point is taken as a measure of the distance of propagation in an attempt to locate the origin of the PD. 
         [0004]    Aspects of PD measurements are described in:
       “Fundamental Limitations in the Measurement of Corona and Partial Discharge,” IEEE Transactions on Electrical Insulation, Vol. EI-17, No. 2, April 1982;   “Partial discharge detection in cables using VHF capacitive Couplers,” IEEE Transactions on Dielectrics and Electrical Insulation, Vol. 10, No. 2, April 2003;
 
and the references cited therein.
       
 
       SUMMARY 
       [0007]    This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This summary is not intended to identify key features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. 
         [0008]    The present invention provides a partial discharge (PD) sensor for a PD signal-analyzing system that has a known coupling impedance. The sensor has a capacitor component for capacitive coupling to an electrical power apparatus in which a PD is suspected, such as a shielded power cable. The sensor includes an impedance transformance device having an input side connected directly to the capacitor component and an output side configured for connection to the PD signal-analyzing system. The impedance transformance device is effective to convey a PD signal from the input side to the output side. 
         [0009]    In one aspect of the invention the transformance device has a first effective impedance at the input side and a second effective impedance at the output side, the first effective impedance being much larger than the second effective impedance, such as at least two orders of magnitude larger. The capacitance of the capacitor component in combination with the first effective impedance inherently forms a classic high-pass filter, but with a low frequency cutoff much lower than would be caused by the capacitance of the capacitor component in combination with the known coupling impedance of the signal-analyzing system. For example, for a PD signal-analyzing system having a coupling impedance of 50 ohms, the impedance transformance device can have an output impedance that matches the coupling impedance, but with an input impedance at the capacitor component side of at least 5 KΩ. The effect on the low frequency cutoff is to pass much more of the lower frequency components of the PD signal for analysis by the PD signal-analyzing system. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0010]    The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
           [0011]      FIG. 1  (prior art) is a diagrammatic representation of a HV shielded power cable and a known capacitive PD sensor and signal transmission line coupled thereto; 
           [0012]      FIG. 2  (prior art) is an enlarged diagrammatic plan view of aspects of the known sensor and line of  FIG. 1 ; 
           [0013]      FIG. 3  (prior art) is an even more diagrammatic representation of the sensor and transmission line of  FIG. 1 , as used in connection with PD signal-analyzing equipment; and  FIG. 3A  is a simplified circuit diagram of the system represented in  FIG. 3 ; 
           [0014]      FIG. 4  is a diagrammatic representation of a first embodiment of an improved capacitive sensor in accordance with the present invention illustrating its use in a PD signal-analyzing system; and  FIG. 4A  is a simplified circuit diagram of the sensor of  FIG. 4 ; 
           [0015]      FIG. 5  is a graph Illustrating the frequency filtering aspects of the prior art sensor represented in  FIG. 3A , and the filtering aspects of the improved sensor represented in  FIG. 4A ; 
           [0016]      FIG. 6  is a diagrammatic representation of a second embodiment of an improved capacitive sensor in accordance with the present invention illustrating its use in a PD signal-analyzing system; and  FIG. 6A  is a simplified circuit diagram of the sensor of  FIG. 6 ; 
           [0017]      FIG. 7  is a diagrammatic representation of a third embodiment of an improved capacitive sensor in accordance with the present invention illustrating its use in a PD signal-analyzing system; and  FIG. 7A  is a simplified circuit diagram of the sensor of  FIG. 7 ; and 
           [0018]      FIG. 8  is a diagrammatic representation of a fourth embodiment of an improved capacitive sensor in accordance with the present invention illustrating its use in a PD signal-analyzing system. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]      FIGS. 1 ,  2 ,  3 , and  3 A are diagrammatic representations of a capacitive sensor and related components used in the “CableWISE™” service provided by UtilX Corporation of Kent, Wash., (www.utilx.com). That system is used to evaluate the condition of medium voltage (MV) and high voltage (HV) shielded cable systems, transformers and switch gear, by detection and analysis of partial discharge (PD) signals. The system is designed for “online” detection, i.e., it is not necessary to take the system down for testing or to introduce external signals. With reference to  FIG. 1 , the system uses a capacitive sensing component  10  with a fitting  12  for connection to a pole (“hot stick”) that can be used to position the sensor with a U-shaped recess  14  transversely receiving a small segment of the cable  15  under test. As seen in  FIG. 2 , the cable-receiving recess  14  has an inner U-shaped insulative or dielectric member  16  with a large capacitor plate  18  extending along its outer periphery. A longer U-shaped dielectric element  20  has the other arcuate metallic capacitor plate  22  along its inner periphery. Typically, plates  18  and  22  will be copper bands separated by an air gap or another dielectric. PD pulses conveyed along the cable cause detectable fluctuations in the voltage between the capacitor plates. The voltage fluctuations have the signal that conveys the periodic PD signature to be analyzed. They are picked up by respective conductors  26  and  28  that lead to a standard coupling  30  for a signal transmission line  32  in the form of a coaxial cable. Conductor  26  connects to the coax core, and conductor  28  to the coax sheath. In representative installations, the MV/HV cable to be evaluated may range from 2.4 kV up to 345 kV cable. Sensors are provided in at least 1″-5″ sizes for the different cables. In representative embodiments the capacitance achieved by the cable shield  15  and the plate  18  is a significant value, in the range of 10-80 picofarads. 
         [0020]    The relationship of the sensor  10  to the signal-analyzing apparatus D is represented in  FIG. 3 . Cable  15  carries the HV AC current. A PD discharge signal is represented at P, which typically is generated at some distance from the sensor. The capacitor plates  18 ,  22 , are connected, respectively, to the core and sheath conductors of the coaxial cable  32  through the standard coupling  30 . The length of the cable can be quite long. For example, the hot stick itself may be 6 feet to 30 feet long. The other end of the signal transmitting coaxial cable is coupled to the PD signal-analyzing apparatus D, such as by a standard coupling  34 . As described herein, the signal transmitting component  32  (in this case the standard coaxial cable) is part of the signal-analyzing system and has a known characteristic impedance. In a representative embodiment, a cable with characteristic impedance of 50 ohms is used. This is the coupling impedance presented to the output side of the sensor. It is important to match the impedance of the cable to the input impedance of the signal-analyzing equipment to prevent reflections. 
         [0021]    A simplified representation of the traditional capacitive sensor circuit is shown in  FIG. 3A . V IN  represents the effective PD pulse conveyed along the cable ( 15  in  FIG. 3 ). C 2  represents the value of the inherent capacitance between the outer periphery of the shielded cable and the adjacent plate (plate  18  in  FIG. 3 ) of the sensor capacitor, and C 1  represents the capacitance of the spaced sensor plates ( 18  and  22  in  FIG. 3 ). In this system, the signal to be transmitted to the signal-analyzing equipment D consists of the PD characterizing voltage across the sensor capacitor C 1 . The characteristic impedance R 0  of the signal transmission component (which is matched to the input impedance R IN  of the signal-analyzing equipment) is the coupling impedance and results in a resistive load to the sensor, e.g., 50 ohms, which forms an inherent classic high-pass filter, the lower cutoff frequency being a function of 1/2πRC. In a representative embodiment, the 3 dB cutoff point is at approximately 100 MHz (using the example of a 50 ohm coaxial cable for transmission of the signal to signal-analyzing equipment with an input impedance of 50 ohms and a sensor capacitance of approximately 30 picofarads). 
         [0022]    The present invention provides a capacitive sensor that still may be used with conventional signal transmission and signal-analyzing equipment, but which provides much greater performance for lower frequency signals. With reference to  FIG. 4 , in a first embodiment of an improved sensor  100  in accordance with the present invention, an impedance transformance component  102  is provided between the capacitor defined by the plates  18 ,  22  and the signal transmission component  32 . The impedance transformance occurs directly adjacent to the sensor capacitor. For example, the distance between the coupling capacitor and the input side of the transformance component should be less than  10  cm to avoid significant stray capacitance, conductance, and signal reflection. In a preferred implementation, the input impedance at component  102  is at least two orders of magnitude greater than the output impedance. As noted above, the output impedance is equal to the characteristic impedance of the transmission line  32  which, in turn, matches the input impedance of the signal-analyzing system D. 
         [0023]    One implementation is shown in more detail in  FIG. 4A . Starting at the left, conductors  104  and  106  extend from the sensor capacitor plates  18  and  22 , respectively. These connect to a voltage divider network having resistors R D1  and R D2  from a DC power source +V (a 9V battery in a representative implementation). The voltage divider provides a DC bias to the input pin of an integrated circuit  103  (for example, Analog Devices AD8079 buffer amplifier). The voltage divider and integrated circuit provide the desired high input impedance which is about 500 KΩ in this embodiment. The buffer amplifier is configured with unity gain. A resistor R O  is connected to the output to match the coupling impedance, in this case the characteristic impedance of the signal transmission line (coax cable), and an output capacitor C O  for DC isolation. 
         [0024]      FIG. 5  shows the effect of the impedance transformance component on the frequency response for the signal supplied along the transmission line. The dash line  110  on the graph represents the frequency response for the known sensor described with reference to  FIGS. 1 to 3A . The 3 dB cutoff occurs at about 100 MHz although in some systems it may be 40 MHz depending on the capacitance of the sensor in use. The solid line  111  represents the frequency response achieved when a sensor in accordance with the present invention is used. Incorporating the impedance transformation component adjacent to the coupling capacitor results in the low cutoff frequency being reduced tremendously, to approximately 10 KHz in the representative example. This provides a significant advantage for the improved sensor as compared to the prior art sensor, particularly in terms of the distance at which a PD can be detected. In a typical 15 KV cable, because attenuation is greater at high frequencies than at lower frequencies, the PD signal may have a bandwidth of up to 1 GHz at its origin, but 20 MHz at a distance of 100 m, and 4 MHz at a distance of 500 m. Since the sensor in accordance with the present invention passes much more of the lower frequency bandwidth, it is effective at greater distances for PD detection using otherwise conventional signal-analyzing systems. 
         [0025]    There is a considerable range for the preferred lower cutoff frequency for the sensor in accordance with the present invention. In an AC system it is preferred that the lower cutoff frequency be high enough to reject the power frequency and its measurable harmonics, which typically includes 60 Hz to approximately 2000 Hz. In the preferred embodiment, the lower cutoff frequency is at least 4000 Hz to achieve the desired rejection with a considerable margin of safety. In a different implementation the lower cutoff frequency could be higher. For example, even a lower cutoff frequency of 1 MHz provides a 100 fold improvement as compared to the 100 MHz cutoff of the known sensor. In terms of the input impedance, these cutoff frequencies correspond to about 100 to 10,000 times the output impedance (the characteristic impedance R 0  of the signal transmission line). For a characteristic impedance R 0  of 50 ohms, the preferred range for the input impedance is 5 KΩ to 500 KΩ. 
         [0026]    In the embodiment of the present invention represented in  FIG. 6 , for which the circuit is shown in  FIG. 6A , the sensor  200  in accordance with the present invention uses a single capacitor plate  18  and utilizes the inherent capacitance between that plate and the sheathed cable  15 . As seen in  FIG. 6A , the circuit for the impedance transformation device is very similar to the circuit shown in  FIG. 4A . The IC buffer, output impedance R O , output capacitance C O , and input voltage bias provided by the 9V battery and voltage divider are the same as previously described. Rather than coupling a two-plate capacitor across the lower resistor of the voltage divider (R D2 ) as in the embodiment shown in  FIG. 4A , the voltage signal at the inner plate  18  is coupled to the IC input. The system ground completes the circuit. The frequency response is very close to the frequency response for the embodiment of  FIG. 4A . The low frequency cutoff is greatly improved as compared to the prior art sensor but somewhat higher than for the embodiment of  FIG. 4A  because the capacitance component of the defining ratio 1/2πRC is of lesser magnitude than for the embodiment of  FIG. 4A . Such cutoff still is far below cutoff inherent in the prior art capacitive sensor. 
         [0027]    In the embodiment of the present invention shown in  FIGS. 7 and 7A , the improved sensor  300  in accordance with the present invention is implemented with a differential amplifier for which the circuit is shown in  FIG. 7A . The coupling capacitor plates  18  and  22  provide the inputs to the top and bottom bands of the IC buffer  103 . The differential outputs then are coupled to a single band of a second IC buffer as shown for a differential to single-ended conversion. Since the apparent input impedance still is very high (much greater than the preferred lower limit of 5 KΩ) and the output impedance is set to match the coupling impedance of the signal-analyzing system, the frequency response is still enhanced greatly with respect to the frequency response of the prior art sensor. By utilizing a differential input, the ground loop noises are cancelled out such that they are not transmitted to the signal-analyzing apparatus D over the signal transmission line  32 . Only the signal appearing between the plates  18  and  22  (across C 1 ) is detected. The benefit is an enhanced signal-to-noise ratio (SNR). 
         [0028]    In the embodiment of the present invention shown in  FIG. 8 , the improved sensor  400  is implemented by incorporating a coupling transformer  402 . The primary winding of the transformer is connected to the sensor plates  18  and  22 , and the secondary winding is connected to the signal transmission line  32 . The transformer provides two benefits over the prior art. Firstly, it presents a relatively high impedance load to the sensor, extending the low frequency cutoff frequency providing benefits as described earlier. The impedance is determined by the equation: 
         [0000]    
       
      
       Z 
       transformer 
       =N 
       2 
       *R 
       0  
      
     
         [0000]    where N is the transformer turns ratio and R 0  is the characteristic impedance of the signal transmission line. For example, a transformer turns ratio of 10:1 and a coaxial cable characteristic impedance of 50 ohms would present a 5000 ohm impedance to the sensor. This would then provide a low frequency cutoff of approximately 400 KHz, more than a 100 fold improvement over the prior art. Even a turns ratio of 5:1 would provide a 25 fold improvement which may be useful for some applications. 
         [0029]    The second benefit is that the transformer converts from a differential input to a single ended output which is then connected to the signal transmission line  32 . This helps to isolate the ground loop noise from the signal-analyzing apparatus D. This benefit is similar to that described for sensor embodiment  300 . However, the transformer will attenuate all signals, but a very low noise preamplifier can be used to raise the signal level back to a useful level. 
         [0030]    While illustrative embodiments have been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.