Abstract:
The present invention provides a method of using a current transformer for a temperature sensing device. The method determines the temperature of the current transformer&#39;s secondary winding by injecting a DC current into the secondary winding, measuring a voltage across the secondary winding, calculating the resistance of the secondary winding from the voltage induced into the secondary winding by the injected DC current and determining the secondary winding temperature by calculations or a comparison with verified resistance/temperature combinations.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention is generally directed current transformers and particularly to additional sensing that can be derived from a current transformer. 
       BACKGROUND OF THE INVENTION 
       [0002]      FIG. 1  illustrates a typical current transformer (CT) of the prior art, generally indicated by reference numeral  10 . The CT includes a primary winding  14 , which carries current from a power source  18  to a load  22 , and a secondary winding  26 , which receives an induced current from the primary winding  14 . The secondary winding  26  is connected to a measuring/monitoring device  30 , which could be a protection device such as a circuit breaker, overload relay or other device capable of interpreting an induced current signal received from in the secondary winding  26 . If the induced current signal from the secondary winding  26  indicates that current in the primary winding  14  has exceeded a predetermined level the monitoring device  30  can initiate an interruption of current flow in the primary winding  14 . In more sophisticated protection devices the monitoring device  30  can include a memory  34  for storing algorithms  38  used by at least one processor  42  to interpret the induced current signal from the secondary winding  26 , determine if current flow in the primary winding  14  exceeds the predetermined level and initiate the interruption of current flow in the primary winding  14 . 
         [0003]    Current transformers, as described above, are widely used in protection systems to monitor load currents. In many applications CTs are placed in areas where the temperature can vary enough to affect the accuracy of the CT. In critical applications where accuracy of the CT is extremely important a temperature sensing device can be installed near the CT to provide local temperature information. The CT monitoring equipment can use the local temperature information to compensate for the temperature&#39;s effect on the CT. This requires installing the temperature measuring device and connecting it to the CT monitoring equipment, which adds additional cost for the temperature sensing equipment and its installation. In some instances the CTs are installed inside small enclosures that do not have room for additional sensing devices or in locations that are difficult to access. In these situations the temperature sensing device may not be close enough to the CT to provide accurate local temperature readings that can be used to increase the accuracy of the CT. Therefore, having a temperature sensing device that is actually an integral part of the CT would be beneficial. 
       SUMMARY OF THE INVENTION 
       [0004]    The present invention uses certain characteristics of current transformers (CTs) and the addition of a DC current circuit and a voltage measuring circuit to obtain temperature information from the CT. Essentially the CT is modified to function as both a current transformer and a temperature sensor. With the additional temperature information the measuring/monitoring circuit of the CT can perform additional diagnostic functions that are not easily accomplished without temperature information, including, but not limited to, determining if the CT has a temperature related accuracy issue or if there is a possible loose electrical connection or arcing in the primary circuit. These new diagnostic functions can provide an early warning for possible events in the primary circuit that could result in lost time or equipment damage. The modified current transformer of the present invention provides a system for obtaining both current and temperature information comprising:
   a current transformer having a primary winding and a secondary winding;   a circuit for injecting a DC current into the secondary winding;   a circuit for measuring a voltage across the secondary winding;   a processor for calculating a resistance of the secondary winding from the measured voltage;   a memory for storing the calculated resistance of the secondary winding;   determining, by the processor, a temperature of the secondary winding; and   storing the determined temperature of the secondary winding in the memory for diagnostic use by the processor.   
 
         [0012]    The present invention also provides a method of determining the temperature of a current transformer&#39;s secondary winding comprising the step of:
   injecting a direct current (DC) into the secondary winding of the CT;   measuring a voltage across the secondary winding induced by the injected DC current;   calculating, by a processor, a resistance of the secondary winding from the measured voltage;   determining, by the processor, a temperature of the secondary winding; and   storing the determined temperature of the secondary winding in a memory for future diagnostic use by the processor.   
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  illustrates a typical current transformer for measuring current in an electrical conductor. 
           [0019]      FIG. 2  illustrates a current transformer of the present invention which senses both current and temperature. 
           [0020]      FIG. 3  is a flow chart illustrating the method of determining the temperature of the current transformer secondary winding. 
           [0021]      FIG. 4  is a flow chart illustrating the method of determining a possible loose electrical connection in the primary circuit. 
           [0022]      FIG. 5  is a flow chart illustrating the method of determining a possible open circuit, phase loss or arcing in the primary circuit. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0023]    Referring now to  FIG. 2 , a CT of the present invention, generally indicated by reference numeral  46  is shown. As in the prior art CT  10  of  FIG. 1 , a primary winding  14  carries current from a power source  18  to a load  22 , and induces current into the secondary winding  26  where it is measured by the measuring/monitoring device  30 . The CT  46  of the present invention includes a DC current circuit  50 , a voltage measurement circuit  54 , consisting of a buffer  58  and an analogue to digital converter (ADC)  62 , a memory  34  for storing reference data, historic data and algorithms  38  and one of more processors  42 . Reference data can include, but is not limited to, CT design/material data, lookup tables, resistance/temperature relationships and verified test data. Historic data can include, but is not limited to, past resistance data, voltage data, temperature data, expected temperatures based on sensed currents, and diagnostic results. The DC current circuit  50  can be controlled by the measuring/monitoring device  30  such that the injected DC current can be turned off, increased or decreased as required to increase measurement accuracy. The processor  42  uses the algorithms  38 , reference data and measured data to determine the temperature of the secondary winding  26 . Using the superposition electrical theorem, the ADC  62  can measure current sensor output current * burden resistance R B  and current source * (burden resistance R B  I current sensor  46  winding  26  resistance). Knowing the burden resistance R B  and the output of the current source  50 , the resistance of the current sensor  46  winding  26  can be calculated. Alternatively, the resistance of the secondary winding  26  can be calculated from information about the voltage induced by the DC current circuit  50 . Once the resistance of the secondary winding  26  is known the temperature can be determined based on characteristics of the CT  46  design (relationships between resistance and temperature based on materials used in the CT  46  and stored in memory  34 ). A resistance/temperature relationship table created by measuring secondary winding  26  resistances at verified temperatures can also be used to determine the temperature of secondary winding  26 . The resistance/temperature table can also be used as a calibration parameter if there is a temperature initiated CT  46  sensing error. 
         [0024]    Once the induced voltage, resistance and temperature of the secondary winding  26  have been determined and stored in memory  34 , the processor  42  can evaluate the accuracy of the induced current signal from the secondary winding  26  by comparing the determined secondary winding  26  temperature with previously determined secondary winding  26  temperatures known to produce CT sensing errors and compensate accordingly for any temperature related error. Compensating can be accomplished by comparing the sensed current believed to be in error with historic resistance/temperature data or verified resistance/temperature test data and other stored characteristics of the CT  46  to determining a correction factor that can be used to compensate for the temperature related current sensing error. The ability to compensate for temperature related errors permits the CT  46  to maintain dependable accurate current readings. 
         [0025]    With the secondary winding  26  temperature information the monitoring device  30  can also determine potential electrical problems in the primary winding  14  circuit by comparing the determined temperature of the secondary winding  26  with historic data of the CT  46  stored in memory  34 . A determined temperature of the secondary winding  26  and its associated sensed current can be compared, by the processor  42 , with an expected range of secondary winding  26  temperatures for the same sensed current, stored in the memory  34 . If the determined secondary winding  26  temperature is greater than the expected range of secondary winding  26  temperatures for the same sensed current a possible loose electrical connection in the primary winding  14  circuit can be detected by the processor  42 . If a comparison, by the processor  42 , of several recently stored temperatures of the secondary winding  26  is cyclic between an expected range of temperatures for the sensed currents and temperatures greater than the expected range of temperatures for the sensed current, an indication of an open circuit, phase loss or arcing in the primary winding  14  circuit can be detected by the processor  42 . Comparing the measured induce voltage, sensed current and derived temperature with historic data stored in memory  34  can also detect subtle changes over time that could indicate a deteriorating performance of the CT  46  or a need for preventive maintenance. Comparisons to historic data can also detect sudden deviations for the established characterization of the CT  46 , indicating a need for more urgent corrective action. 
         [0026]    Referring now to  FIG. 3 , the process for determining the temperature of the secondary winding  26  and if necessary compensating for a temperature induced current sensing error using the determined temperature are described. The following step from an algorithm  38  stored in memory  34  are performed by the processor  42  of the measuring/monitoring device  30 . To initiate the temperature sensing function of CT  46  the DC current circuit  50 , which can be controlled by the measuring/monitoring circuit  30 , injects a known DC current into the secondary winding  26  at step  100 . The injected DC current induces a voltage in the secondary winding  26 . The induced voltage is measured by the voltage measurement circuit  54  at step  105 . At step  110  the resistance of secondary winding  26  is calculated. The secondary,winding  26  resistance can be calculated using information about the voltage induced by the DC current circuit  50  or by using the superposition electrical theorem, where the ADC  62  can measure current sensor output current * burden resistance R B  and current source * (burden resistance R B  current sensor  46  winding  26  resistance). Knowing the burden resistance R B  and the output of the current source  50 , the resistance of the current sensor  46  secondary winding  26  can be calculated. The calculated resistance of the secondary winding  26  is stored in memory  34  at step  115 . At step  120  the calculated resistance of the secondary winding  26  is used to determine the temperature of the secondary winding  26 . The calculated resistance of the secondary winding  26  is compared, by the processor  42 , with a resistance/temperature relationship table stored in memory  34 . The resistance/temperature relationship table can be based on design characteristics such as the materials used in CT  46  or on a series of resistance measurement of secondary winding  26  taken at controlled temperatures. The resistance/temperature relationship table can also be used as a calibration parameter. At step  125  the determined temperature of secondary winding  26  is stored in memory  34 . At step  130  the processor  42  compares the determined temperature of secondary winding  26  with a threshold temperature or range of temperatures previously determined to produce CT  46  current sensing errors. The threshold temperatures can be determined by actual testing or by characteristic of the CT  46 , such as materials use in making the CT  46 . If the determined temperature of the secondary winding  26  exceeds the previously determined error threshold temperature the processor  42  will determine if there is a current sensing error and compensate as necessary for a detected error at step  135  and then return to step  100 . If the determined secondary temperature does not exceed the previously determined CT  46  error threshold temperature at step  130 , the processor  42  can return directly to step  100 . 
         [0027]    Once the temperature of the secondary winding  26  has been determined and stored in memory  34 , and any current sensing errors have been resolved, the processor  42  can use the secondary temperature for other diagnostic test. In  FIG. 4 , using steps  100 - 125  of the temperature determining algorithm, the processor  42  can compare, at step  130 , the determined secondary temperature with a previously determined secondary temperature expected for the current being sensed by the CT  46 . If the determined secondary temperature exceeds the expected secondary temperature a possible loose electrical connection can be reported to the monitoring device  30  at step  135 . In  FIG. 5 , using steps  100 - 125  of the temperature determining algorithm, the processor  42  can compare, at step  130 , the determined secondary temperature with recently recorded secondary temperatures to determine if there is a cyclic pattern. If a cyclic temperature pattern of expected secondary temperatures for the current being sensed and secondary temperatures higher than expected is observed a possible open circuit, phase loss or arcing in the primary circuit can be reported to the monitoring device  30  at step  135 . 
         [0028]    Although specific example embodiments of the invention have been disclosed, persons of skill in the art will appreciate that changes may be made to the details described for the specific example embodiments, without departing from the spirit and the scope of the invention.