Abstract:
A system and method for testing an on-line current transformer is provided. The current transformer includes a primary winding and a secondary winding. An operating current continues to flow through the primary winding during testing of the current transformer. A controllable load is applied to the current transformer secondary winding. The controllable load is varied over a range of load settings including a maximum current setting and a maximum voltage setting. At a plurality of load settings within the range of load settings, a current flowing through the current transformer secondary winding is measured. Also, a voltage across the current transformer secondary winding is measured. An actual excitation curve is generated from the measured currents and voltages corresponding to the plurality of load settings.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application No. 60/269,196, filed Feb. 15, 2001. 
    
    
     BACKGROUND AND SUMMARY OF THE INVENTION 
     The present invention relates generally to current transformers, and in particular to in-service testing of current transformers used in power generation and distribution systems. 
     Power generation and distribution systems use current transformers at numerous locations throughout the system as the primary sensor to monitor the current flowing through critical assemblies. The output of the current transformer provides a representation of the current flowing through the assembly that is being monitored. Associated monitoring and control instrumentation in combination with the current transformer may provide critical system functions such as overload protection and power usage monitoring. The importance to plant operation and reliability of current transformers cannot be over-emphasized. Problems with current transformers and associated monitoring and control instrumentation may cause very expensive and costly outages that are usually avoided at all costs. Therefore, it is imperative that these units be reliable and perform as designed at all times, and that the operating condition of a current transformer be known throughout its operating life. 
     However, using conventional techniques for evaluating the condition of a current transformer requires extensive downtime and associated expense. To evaluate a current transformer using conventional techniques, the primary conductor of the transformer must be disconnected from the assembly it is monitoring so that current flow through the primary is interrupted. Then, after the primary current is interrupted, a test current in accordance with IEEE specifications (IEEE C57.13 (1993) and ANSI C57.13.1 (1981)) is used to construct an excitation curve that reflects the operating characteristics of the current transformer. The excitation curve is then evaluated to determine if there are problems with the current transformer. 
     Since disconnecting the current transformer is expensive and requires extensive downtime, testing is rarely performed except when problems occur. However, waiting for a problem to occur results in unexpected outages which are very expensive in terms of lost generation capacity, labor, and materials, and of course substantial inconvenience to customers. 
     The present current transformer test system provides a method of testing a current transformer that has a primary winding and a secondary winding. An operating current continues to flow through the primary winding during testing of the current transformer. A controllable load is applied to the current transformer secondary winding. The controllable load is varied over a range of load settings including a maximum current setting and a maximum voltage setting. At a plurality of load settings within the range of load settings, a current flowing through the current transformer secondary winding is measured. Also, a voltage across the current transformer secondary winding is measured. An actual excitation curve is generated from the measured currents and voltages corresponding to the plurality of load settings. 
     For a more complete understanding of the invention, its objects and advantages, reference may be had to the following specification and to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a block diagram of a test instrument panel connected to a unit under test; 
     FIG. 2 illustrates a schematic diagram of a current transformer test system in accordance with the teachings of the invention; 
     FIG. 3 illustrates a presently preferred embodiment of a process for testing a current transformer in accordance with the teachings of the invention; 
     FIG. 4 is a graphical representation of an active performance curve; and 
     FIG. 5 is a graphical representation of an actual excitation curve. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, an on-line current transformer test system  10  according to the present invention is shown connected to a current transformer  12 . Internal instrumentation  14  associated with the current transformer  12  is preferably disconnected. However, the scope of the invention includes testing the current transformer with the internal instrumentation  14  connected. The on-line current transformer test system  10  is particularly suitable for evaluating current transformers used in power generation and distribution systems, since the system  10  does not require the disconnection of the current transformer  12  that is under test. Current transformers used in power generation and distribution systems are generally massive devices in which disconnection of the device requires extensive labor and down time. The test system  10  provides for on-line and in-service of a current transformer and associated monitoring and control loops. This is especially advantageous in power generation and distribution systems, where the down time for testing with conventional systems may result in millions of dollars in lost generation revenue. 
     Conventional current transformer testing methodologies in the power generation business, universally require the primary winding of the transformer to be disconnected before testing the current transformer. The primary winding is disconnected so that a voltage may be impressed across the secondary winding of the current transformer to evaluate the performance characteristics of that current transformer. 
     In contradistinction, the present invention impresses a range of burden impedances across the secondary winding of the current transformer  12  to evaluate the performance characteristics of the transformer  12 . The primary winding preferably remains connected throughout the test. The secondary winding voltage and current values corresponding to increments in the burden impedance are used to generate an active performance curve representing the inverse excitation characteristics of the current transformer  12 . A regressive analysis algorithm is applied to the inverse excitation characteristic to generate an actual excitation curve. 
     Referring to FIG. 2, a schematic diagram of a current transformer test box  16  connected to a current transformer to be evaluated  12 , is illustrated. The primary winding of the current transformer  12  preferably remains connected to the circuit for which the transformer  12  normally monitors current. Examples of typical current transformers that may be evaluated by the test system  10  include generator current transformers (GCTs) and bushing current transformers (BCTs). GCTs are typically used for protecting power generators and monitoring power generation output currents. BCTs are typically used for the monitoring and protection of substation transformers. 
     The current transformer test box  16  includes a burden impedance  22  connected in parallel with the secondary winding of the current transformer  12 . Preferably, the burden impedance is a string of resistors. Two selector switches  18  and  20  are connected to several points within the string of resistors  22  so that a wide range of resistance values may be applied across the current transformer  12 . The selector switches  18  and  20  are configured so that different combinations of resistors within the string of resistors  22  are shorted out depending on the position of the switches  18  and  20 . An ammeter  24  is placed in series with the current transformer secondary winding to measure the secondary current. A voltmeter  26  is connected across the secondary winding to measure the secondary voltage. Although the test box  16  preferably includes discrete resistors in combination with manual switches, it is within the scope of the invention to automate the circuitry of the test box  16 . For example, in an alternative embodiment of the presently preferred invention, a processor  21  is coupled to controllable switches and the string of resistors  22  to generate the actual excitation curve. In addition, an electronic load alone or in combination with the processor  21  may be used to generate the actual excitation curve. The scope of the invention also includes using a burden impedance such as a programmable current load or resistor-capacitor network instead of the string of resistors  22 . 
     Referring to FIG. 3 in addition to FIG. 2, a process for generating an actual excitation curve that characterizes the current transformer  12  is shown. At step  30 , the burden impedance, Zb,  22  is connected to the secondary winding of the current transformer. The value of the burden impedance  22  is initially set to a minimum burden impedance. Preferably, the minimum burden impedance is zero ohms. Preferably, internal instrumentation circuits are disconnected to ensure that current flowing from the current transformer  12  flows into the test box  12 . At step  32 , the short circuit secondary current, Im(0), and voltage of the current transformer  12  are measured and recorded. At steps  34  and  36 , the burden impedance  22  is increased and the secondary winding current, Im(n), and voltage, Vm(n), are measured and recorded. Steps  34  and  36  are repeated, with the impressed impedance being incrementally increased up to a maximum burden impedance that corresponds to either a selected secondary voltage or an impedance at which further increases in impedance result in no further increases in voltage. The selected secondary voltage may be chosen, for example, based upon the withstanding voltage of the current transformer and interconnect wiring. Although the minimum value of burden impedance is preferably zero ohms, the scope of the invention includes non-zero values of impedance so long as it is possible to interpolate Im(0) from the measured Im(n) at those impedance values. 
     At step  38 , the burden impedance  22  is decreased and the secondary winding current, Im(n), and voltage, Vm(n), are measured and recorded. Step  38  is repeated, with the impressed impedance being incrementally decreased until the burden impedance is zero. Step  34  serves to demagnetize the current transformer  12  if the transformer  12  was magnetized at the start of the process. Step  39 , determine whether there is a difference in readings between steps  34  and  38 . If there is no difference in the readings, then the data provided by either of steps  34  and  38  may be used as the true transformer excitation data. Step  41 , if there is a difference in readings, then the current transformer  12  was magnetized, and the true transformer excitation data is provided by step  38 . Any difference in the readings represents measurement inaccuracies of the transformer current due to magnetization of the current transformer  12 . The difference in readings may be used for manipulating previously obtained data associated with the current transformer  12 . A process using differences in readings is presented in a later section of this specification. 
     Following step  39 , the process splits, following one of two alternative paths. The preferred path follows steps  42  and  44 . At step  42 , an active performance curve is constructed from Vm(n) and Im(n) (see FIG.  4 ). The active performance curve illustrates the relationship between Vm(n) and Im(n) throughout the range of burden impedances. At step  44 , an actual excitation curve is generated from the active performance curve by subtracting the measured current value, Im(n), corresponding to each burden impedance from the measured current value at zero burden impedance, Im(0). Using the derived current differences in combination with the values for Vm(n), the actual excitation curve is constructed (see FIG.  5 ). Although, Im(0) is preferably measured with a burden impedance of zero ohms, it is within the scope of the invention to interpolate a value for Im(0) such as by measuring the current at two or more different burden impedance values greater than zero, and then using a straight line approximation or curve fitting algorithm to compute Im(0). 
     The alternative path includes steps  46  and  48 , and differs from the preferred path by eliminating the step of generating an active performance curve. Step  46  is integrated into step  34 , so that as the current value, Im(n), corresponding to each burden impedance is measured, that value Im(n) is subtracted from the measured current value at zero burden impedance, Im(0), to generate the value of the actual excitation current, Iae(n). Here, the actual excitation curve is generated without the intermediate step of constructing an active performance curve. 
     At step  50 , the actual excitation curve is compared to a baseline excitation curve to determine the operating condition of the current transformer  12  and the associated monitoring and control circuitry. The baseline excitation curve may be an excitation curve provided by the current transformer manufacturer, a previous actual excitation curve generated by the test system  10 , or a theoretical magnetic materials performance curve for a similar current transformer. 
     In addition, the baseline excitation curve may be in a format that is comparable to the active performance curve. For example, previous active performance curves of the current transformer or similar current transformers may be stored as a baseline excitation curve for later comparison. As another example, the manufacturer&#39;s excitation curve may be converted into a compatible format to be compared directly with the active performance curve. The conversion may be made in a similar manner to constructing the actual excitation curve from the active performance curve. More specifically, the baseline curve may be obtained by subtracting the excitation current at all points within the manufacturer&#39;s excitation curve from Im(0) of the active performance curve. 
     Referring to FIG. 3, if at step  39  it is determined that there is a difference in readings between steps  34  and  38 , then the current transformer  12  has been in a magnetized state. A magnetized current transformer provides an inaccurate measurement of the current levels flowing through the primary. Depending on the application of the current transformer  12 , the inaccurate current representation will have a varying impact. For example, a magnetized current transformer that is used in a metering application to measure the quantity of current that is delivered to an entity will result in either undercharging or overcharging that entity for power. A magnetized current transformer that is used in a relaying application to protect a power system from the harmful effects of an overcurrent condition may indicate false overcurrent events, or even worse not provide an overcurrent indication at a current level that protects the system. 
     The difference in readings is used to manipulate previously obtained data associated with the current transformer  12 . An inaccuracy number that is generated from the difference in readings is used to correct prior data. To generate the inaccuracy number, actual excitation curves for the magnetized and unmagnetized current transformer are generated and compared. The inaccuracy number may be computed as an average of the error over all operating currents, a subset of the operating currents such as the linear portion of the excitation curve, or as an error function that varies depending on the operating current. The inaccuracy number may be computed from either the active performance curves or the actual excitation curves. The inaccuracy number is then applied to previously obtained generator current data to extrapolate an estimated actual current corresponding to the prior data. In a metering application, the estimated actual current is used to adjust a previously determined power generation or power consumption. 
     While the present invention is shown and described as comprising a series of discrete manual steps, it will be appreciated that the on-line current transformer test system  10  can be implemented within an automated system such as a programmable load controlled by a microprocessor. 
     Thus it will be appreciated from the above that as a result of the present invention, an on-line current transformer test system for testing on-line current transformers is provided by which the principal objectives, among others, are completely fulfilled. It will be equally apparent and is contemplated that modification and/or changes may be made in the illustrated embodiment without departure from the invention. Accordingly, it is expressly intended that the foregoing description and accompanying drawings are illustrative of preferred embodiments only, not limiting, and that the true spirit and scope of the present invention will be determined by reference to the appended claims and their legal equivalent.