Abstract:
A shield of a power cable diverts leakage current to ground. During normal operation, the leakage current is steady. An increase in leakage current can lead to a failure mode of the power cable. Systems and methods are described to measure leakage current with a current transducer, which generates a current signal based on the leakage current, and to process the current signal to provide an output of the health of the power cable.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The subject matter disclosed herein relates to health monitoring of power cables. 
         [0002]    In a power cable, a shield is a metallic layer that encloses the conductor and is connected to a ground strip. The shield diverts leakage current to ground. When the leakage current through the shield increases to exceed a certain value, a fault mode occurs in the power cable. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0003]    According to one aspect of the invention, a power cable health monitoring system includes a shield surrounding the power cable and configured to divert leakage current from the power cable to ground; a current transducer configured to measure the leakage current and output a current signal corresponding to the leakage current; and a processor configured to receive the current signal and output an indication of the power cable health based on the current signal 
         [0004]    According to another aspect of the invention, a starter system for a gas turbine engine includes a static starter configured to output three-phase current; a set of three power cables configured to carry the three-phase current from the static starter; a set of three shields, each shield configured to surround a corresponding one of the set of three power cables and to divert leakage current to ground; a current transducer configured to measure the leakage current through each of the set of three shields and output a current signal corresponding to each of the leakage current measurements; a processor configured to receive the current signal corresponding to each of the leakage current measurements and output an indication of a health of each of the set of three power cables based on the corresponding current signal; and a synchronous generator configured to receive the three-phase current from the static starter through the set of three power cables and supply power to a rotor of the gas turbine engine. 
         [0005]    According to yet another aspect of the invention, a method of monitoring health of a power cable includes detecting leakage current through a shield of the power cable; generating a current signal based on the leakage current detected through the shield; processing the current signal to determine the health of the power cable; and outputting a status based on the current signal processed to determine the health. 
         [0006]    These and other advantages and features will become more apparent from the following description taken in conjunction with the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0007]    The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
           [0008]      FIG. 1  is a block diagram of a starting system of a gas turbine power plant according to an embodiment of the present invention; 
           [0009]      FIG. 2  is a block diagram of a power cable health monitoring system of the starting system shown at  FIG. 1  according to an embodiment of the invention; and 
           [0010]      FIG. 3  shows processes included in a method of monitoring the health of a power cable according to an embodiment of the invention. 
       
    
    
       [0011]    The detailed description explains embodiments of the invention, together with advantages and features, by way of example with reference to the drawings. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0012]    Embodiments of the present invention include systems and methods to monitor the health of a power cable to allow action to be taken before a fault condition is reached. Although an exemplary system (starting system of a gas turbine power plant) is detailed below, it is to be understood that the embodiments of the present invention may be applied to any system that would benefit from monitoring leakage current and cable health. 
         [0013]      FIG. 1  is a block diagram of a starting system  100  of a gas turbine power plant according to an embodiment of the present invention. During normal operation, the gas turbine  170  supplies mechanical energy  172  to the synchronous generator  150 . Before the gas turbine  170  can perform its function, it must first reach its required operating speed. A gas turbine  170  typically takes 30 minutes to reach full speed without a load. At the start of operation, the synchronous generator  150  aids the gas turbine  170  in reaching its required speed with the help of a static starter  110 . The gas turbine  170  speed increases from 0 rpm to a required speed (e.g., 3000 rpm or 3600 rpm depending on a frequency of 50 Hz or 60 Hz). The static starter  110  serves as a variable frequency drive. The static starter  110  provides alternating current (AC) to the synchronous generator  150  stator windings  152 . Flow of the AC from the static starter  110  through the stator windings  152  generates a flux. At the same time, high direct current (DC) flowing in the rotor winding  155  also generates flux. The flux linkage  157  of the flux generated at the stator windings  152  and the rotor  155  helps the gas turbine  170  acquire its required speed. The supply of AC from the static starter  110  to the stator windings  152  is via power cables  120 . 
         [0014]    The health of the power cable  120  must be maintained in order to complete the operation of brining the gas turbine  170  to its required speed. In the exemplary starting system  100 , the power cable  120  is comprised of a set of three power cables  120 . Specifically, three-phase power cables  120  run from the static starter  110  to the synchronous generator  150  stator windings  152 . Thus, the health of each of the power cables  120  must be monitored and maintained to successfully start the gas turbine  170 . 
         [0015]      FIG. 2  is a block diagram of a power cable  120  health monitoring system  200  of the starting system  100  shown at  FIG. 1  according to an embodiment of the invention. As shown, the health monitoring system  200  is part of the static starter  110  in the starting system  100  of  FIG. 1 . The static starter  110  includes converter circuits  215  to convert AC input  213  into direct current (DC) and inverter circuits  217  to convert the DC back to AC at a desired frequency. The AC out of the inverter circuits  217  is supplied as three-phase current through three power cables  120  to the synchronous generator  150 . The health monitoring system  200  includes a shield  210  corresponding to each power cable  120 , a current transducer  230 , an input/output (I/O) board  240 , and a processor  250 . Each shield  210  is connected to a ground strip  220  and diverts leakage current of its corresponding power cable  120  to ground. The shield  210  may be comprised of a layer of copper, for example. 
         [0016]    The leakage current flowing through the shield  210  maintains a stable value during healthy conditions of the power cable  120 . During a fault condition, the leakage current flowing through the shield  210  increases. The current transducer  230  measures the leakage current through the shield  210  and sends a current signal  235  to the I/O board  240 . Although a single current transducer  230  is shown by  FIG. 2 , one or more current transducers  230  may be used with multiple power cables  120  and their respective shields  210 . The current signal  235  from the current transducer  230  may be a 4 to 20 mA current signal, for example. The current signal  235  is forwarded to the processor  250  via the interconnection  245 . The interconnection  245  may be a ribbon cable, for example. The processor  250  detects the current signal  235  sensed by the current transducer  230  and annunciates information to a user via an output  260 . 
         [0017]    The current transducer  230 , I/O board  240 , processor  250 , and output  260  may be housed together or separately. The I/O board  240  functionality may be incorporated in the processor  250 . The processor  250  may include one or more memory devices and may be comprised of one or more processors  250 . The output  260  may provide an audible or visual output or both The output  260  may provide a visual and/or audible alarm if a predetermined threshold leakage current is exceeded or may allow continuous health checks of the power cable  120  through an indication of the current signal  235  from the transducer  230 . If a certain leakage current is exceeded in one of the power cables  120 , an operator may be prompted to shut off the static starter  110 , perform maintenance of the power cable  120 , and then re-start the static starter  110  until the gas turbine  170  is pulled to its required speed. 
         [0018]    As noted, in the exemplary starting system  100  of the gas turbine  170  discussed with reference to  FIG. 1 , the transducer  230 , I/O board  240 , processor  250 , and output  260  may all be part of the static starter  110 . However, in a different system, the current transducer  230 , I/O board  240 , processor  250 , and output  260  may all be housed separately or together and may be separate from or integrated with other parts of the system using the health monitoring system  200 . Further, while three-phase current is supplied to the synchronous generator  150  by three power cables  120  from the static starter  110 , a different system may comprise one to a different number of power cables  120 . In such a system, each power cable  120  would still have a corresponding shield  210 , and one or more current transducers  230  would sense the leakage current for each power cable  120 . 
         [0019]      FIG. 3  shows processes included in a method of monitoring the health of a power cable  120 . At block  5310 , sensing the leakage current includes arranging a current transducer  230  per power cable  120  to measure shield  210  current (leakage current) for that power cable  120  and generating a current signal  235 . Providing the current signal  235  to the processor at  5320  includes the current transducer  230  sending the current signal  235  to the I/O board  240  and the I/O board  240  passing on the current signal  235  through an interconnection  245  to the processor  250 . At the processor, the current signal  235  is processed, and outputting information/alarm at S 330  includes providing a visual and/or audible output to a user at the output  260 . 
         [0020]    While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.