Abstract:
Methods, computer program segment and apparatus for analyzing high voltage circuit breakers are provided. The method includes electrically coupling each circuit breaker contact to ground, applying a test voltage across a circuit breaker contact, measuring an output voltage signal that is proportional to a capacitance of the circuit breaker contact, changing a state of the circuit breaker contact pair from at least one of an open position to a closed position and the closed position to the open position, and detecting a step change of the output voltage that corresponds to the change of state of the circuit breaker. A circuit breaker test device is provided. The device includes a test voltage source, a filter circuit, and an output circuit for measuring the circuit breaker contact pair capacitance, wherein the test voltage source and filter circuit are configured to be coupled to the circuit breaker contact pair during testing.

Description:
BACKGROUND OF THE INVENTION 
   This invention relates generally to high voltage circuit breakers, and more specifically to methods and systems for analyzing circuit breaker contacts. 
   During testing of at least some known circuit breakers, a plurality of circuit breaker parameters may be monitored to facilitate determining that the circuit breaker is operating as designed. One such parameter may be a circuit breaker contact pair status, which may indicate whether the contacts are opened or closed, and an analog position of the circuit breaker contacts. 
   Timing the main contact and auxiliary contacts may also be indicative of each contacts&#39; state. At least some known contacts are timed using a small DC current induced into a first of the pair of contacts and detecting the current at a second of the contact pair. In one embodiment, the DC current may be recorded such that a current trace may be used to determine the timing of each contact. In an alternative embodiment, the presence or absence of the DC current may be used to start and stop timers to facilitate directly measuring the contact timing. Circuit breaker contact analog position and contact motion may be determined applying a mechanical transducer to the circuit breaker contact mechanism to transfer a motive force to a movable contact of the contact pair. 
   Circuit breaker contact timing may be affected by induced currents, voltages, or other disturbances in a high voltage environment where circuit breaker typically is performed. Such disturbances may put a demand on the test equipment that limit the effectiveness and/or portability of the test equipment. Motion measurement may be complicated by mechanical difficulties when mounting the transducer to the circuit breaker and when measuring rapid mechanical acceleration during circuit breaker operation. Additionally, a material the circuit breaker contact is constructed from may adversely affect the timing result. At least some known circuit breaker designs use contact materials with a relatively higher contact resistance, such as, for example, graphite, to protect the contact surface from wear during contact arcing. Furthermore, present timing techniques require removal of grounding cables from the circuit breaker under test to receive accurate results. 
   BRIEF DESCRIPTION OF THE INVENTION 
   In one aspect, a method for analyzing circuit breakers is provided. The method includes electrically coupling each circuit breaker contact to ground, applying a test voltage across a circuit breaker contact, measuring an output voltage signal that is proportional to a capacitance of the circuit breaker contact, changing a state of the circuit breaker contact from at least one of an open position to a closed position and the closed position to the open position, and detecting a step change of the output voltage that corresponds to the change of state of the circuit breaker. 
   In another aspect, a computer program embodied on a computer readable media for analyzing circuit breakers is provided. The program includes a software code segment programmed to control a test source to apply a test voltage across a circuit breaker contact, measure an output voltage signal derived from the applied test voltage that is proportional to a capacitance of the circuit breaker contact, control operation of the circuit breaker under test to change a state of the circuit breaker contact from at least one of an open position to a closed position and the closed position to the open position, and detect a step change of the output voltage that corresponds to the change of state of the circuit breaker. 
   In yet another aspect, a test device for analyzing circuit breakers is provided. The device includes at least a test voltage source, a filter circuit, and an output circuit for measuring the circuit breaker contact capacitance, wherein the test voltage source and filter circuit are configured to be coupled in electrical series with the circuit breaker contact during testing. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic illustration of an exemplary high voltage circuit breaker phase; 
       FIG. 2  is a schematic illustration of an exemplary equivalent circuit of a contact pair that may be used in the circuit breaker phase shown in  FIG. 1 ; 
       FIG. 3  is a schematic illustration of an exemplary testing circuit that may be used to test a circuit breaker that is represented by the equivalent circuit shown in  FIG. 2 ; 
       FIG. 4  is a schematic illustration of an equivalent circuit of the testing circuit shown in  FIG. 3 , illustrated at the time of first contact touch; and 
       FIG. 5  is a schematic illustration of an exemplary testing circuit that may be used to time the contacts of the circuit breaker phase shown in  FIG. 1 ; 
       FIG. 6  is a graph of an exemplary trace of the output voltage of the testing circuit during a test procedure; and 
       FIG. 7  is a flow diagram of an exemplary method  700  of measuring timing of a circuit breaker while grounding each circuit breaker contact. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Periodic testing of circuit breakers may include a contact timing test. The timing test continuously measures the circuit breaker contact capacitance, from which the moment of first contact touch, and when the maximum capacitance between the contacts is reached may be determined. Further, the maximum capacitance value, in the function of time, may be used as a start or a stop value in the total operating time measurement. 
   Additionally, although the herein described methods are described with regard to circuit breaker contacts, it is contemplated that the benefits of the invention accrue to non-circuit breaker contacts such as those contacts typically employed in, for example, but not limited to, relays or switches. 
     FIG. 1  is a schematic illustration of an exemplary high voltage circuit breaker phase  100 . A high voltage circuit breaker (not shown) may include a pre-insertion resistor  102  and a moving resistor contact  104  electrically in parallel with a moving main contact  106 . In the exemplary embodiment, phase  100  includes two breaks  108  each break  108  includes a pre-insertion resistor (only one shown in FIG.  1 ). 
   In operation, from an open position, the circuit breaker receives a command to close, linkages within the circuit breaker cause movable portions of contacts  104  and  106  to move toward engagement of the respective non-movable portions of contacts  104  and  106 . During a testing sequence, movement of the movable portion of contacts  104  and  106  may begin a timer. In the exemplary embodiment, the movement of the movable portion of contacts  104  and  106  is detected using electrical parameters associated with contacts  104  and  106 . After a predetermined distance of travel of the movable portions of contacts  104  and  106 , the movable portion of pre-insertion resistor contact  104  engages a respective non-movable portion. After a predetermined time delay, the movable portion of contact  106  engages a respective non-movable portion of main contact  106 . During testing, the timing of circuit breaker contacts  104  and  106  may be determined. In an embodiment wherein there is no pre-insertion resistor  102  only the timing of main contact  106  and auxiliary contacts (not shown) are determined. 
     FIG. 2  is a schematic illustration of an exemplary equivalent circuit  200  of a contact pair that may be used in circuit breaker phase  100  (shown in FIG.  1 ). Equivalent circuit  200  includes a capacitor  202  that represents contact surfaces of circuit breaker contacts  104  and  106 . During testing, capacitance parameters associated with the circuit breaker contacts, such as, a surface area of each contact surface and a dielectric media surrounding the contact pair each have a constant value. A distance between the contact surfaces of the contact pair is variable based on the contact state, opened or closed, and a amount of travel between fully opened and fully closed. The distance between the contact surfaces is the only capacitance parameter associated with the circuit breaker contacts that substantially varies during operation of the circuit breaker. 
   A first lead  204  of capacitor  202  is electrically coupled to a first lead  206  of a resistor (R a )  208  and a second lead  210  of resistor (R a )  208  is electrically coupled to a first lead  212  of an inductor (L a )  214 . A second lead  216  of inductor (L a )  214  is coupled to a forcing function (not shown) that represents a test signal used to measure the circuit breaker contact timing. Resistor (R a )  208  and inductor (L a )  214  represent the inductance and resistance of the circuit breaker input components. A second lead  218  of capacitor  202  is electrically coupled to a first lead  220  of a resistor (R b )  222  and a second lead  224  of resistor (R b )  222  is electrically coupled to a first lead  226  of an inductor (L b )  228 . A second lead of inductor (L b )  228  is coupled to the forcing function return. Resistor (R b )  222  and inductor (L b )  228  represent the inductance and resistance of the circuit breaker output components. Resistors (R a )  208  and (R b )  222 , and inductors (L a )  214  and (L b )  228  are represented as constant resistance and inductance values, respectively. Circuit parameters affecting these model components, such as, cable length, diameter and material are substantially constant during testing. The absolute values of resistors (R a )  208  and (R b )  222 , and inductors (L a )  214  and (L b )  228  and the steady state value of the contact capacitance are determined by the physical characteristics of each circuit breaker and may vary depending on location and environmental conditions. 
   During testing, equivalent circuit  200  models the high voltage circuit breaker as a dynamic system with one static and one moveable contact, represented by capacitor  202  that changes capacitance value by the motion of the moveable electrode. By measuring the capacitance dynamically, a minimum distance between circuit breaker contacts before the first contact touch, which corresponds to maximum capacitance in the system may be determined. Additionally, the occurrence of the maximum capacitance value may be used to start and/or stop one or more timers measuring a total operating time (timing) and a recorded capacitance waveform enables analyzing other circuit breaker parameters such as, but not limited to contact motion and interrupting media. 
     FIG. 3  is a schematic illustration of an exemplary testing circuit  300  that may be used to test a circuit breaker that is represented by equivalent circuit  200  (shown in FIG.  2 ). Circuit  300  includes a test source that is used to generate a high frequency sine wave signal (Vg(t)) through the circuit breaker contact being measured as represented by circuit  200 . In the exemplary embodiment, a frequency is considered to be a high frequency if the frequency is greater than about ten kilohertz. In an alternative embodiment, a frequency is considered to be a high frequency if the frequency is greater than about one kilohertz. A filter  304  is coupled in electrical series to the output of circuit  200 . Filter  304  includes a resistor  306 , a capacitor  308 , and an inductor  310  electrically coupled in parallel to filter noise. In the exemplary embodiment, values of resistance, capacitance and inductance for each respective component in filter  304  is pre-selected to make filter  304  resonant at a frequency that is equal to the frequency of source  302 . In an alternative embodiment, the frequency of source  302  is adjusted to a resonant frequency of filter  304 . An output voltage (V out (t))  312  of circuit  300  is taken across filter  304 . In the exemplary embodiment, output voltage  312  is electrically coupled to a microprocessor  314 , which is programmed to receive output voltage  312 , analyze data contained within output voltage  312 , control voltage source  302 , receive commands from an operator, execute scripts that include automatic testing procedures, and generate testing data output. Microprocessor  314  is programmed to analyze output voltage  312  to derive other circuit breaker characteristics indirectly, such as, but not limited to pressure in a contact chamber of the circuit breaker, changes in dielectric constant of gas within the chamber, circuit breaker actuating spring elasticity constant, acceleration of circuit breaker components during operation, vibration of circuit breaker component parts, and an operating time of the circuit breaker. 
   During testing, with the breaker contacts in an open state, source  302  injects a high frequency sine wave signal into the circuit breaker. Output  312  receives a signal that corresponds to circuit  200  with a minimum capacitance value for capacitor  202 . The minimum capacitance value occurs when circuit breaker contacts represented by capacitor  202  are open. The circuit breaker is commanded to close and the movable contact begins moving toward the non-movable contact. As the movable contact travels closer to the non-movable contact, the capacitance of capacitor  202  increases proportionally to the distance traveled. The maximum capacitance value occurs just prior to the time when the movable contact electrically touches the non-movable contact. The maximum capacitance value of circuit  200  corresponds to a maximum value of output voltage  312 . The maximum value of output voltage  312  may be obtained by differentiating the output voltage function V out (t) with respect to time and setting the equation to be equal to zero. Mathematically the equation is: 
         Vo   ⁡     (   t   )       =       Vg   ⁡     (   t   )       -     {         (       L   a     +     L   b       )     ⁢         di   g     ⁡     (   t   )       dt       +       (       R   a     +     R   b       )     ⁢     i   g       +       1   C     ⁢       ∫   0   t0     ⁢       i   g     ⁢           ⁢     ⅆ   t             }           
 
   Than, by equating result to zero, capacitance C is given by: 
       C   =       i   g       Vg   -     L   ⁢         d   2     ⁢     i   g         dt   2         -     R   ⁢       di   g     dt               
         Where L=L a +L b  and R=R a +R b          

   The output voltage  312  is electrically coupled to a circuit breaker test device (not shown) that includes a microprocessor for controlling test scripts, computing results from input data, analyzing data received, and generating output displays and printed reports. The term microprocessor, as used herein, refers to microprocessors, microcontrollers, reduced instruction set circuits (RISC), application specific integrated circuits (ASIC), logic circuits, and any other circuit or processor capable of executing the functions described herein. In the exemplary embodiment, testing circuit  300  is a subcircuit of the testing device positioned within testing device  300 . In an alternative embodiment, testing circuit  300  is a separate component electrically coupleable to testing device  300 , and also configurable to electrically couple to an alternate testing device (not shown). 
   By using a high frequency test voltage with resonant filtering of the output voltage the circuit breaker contact position may be measured dynamically by measuring the capacitance between the fixed and the moving contacts of the circuit breaker. By measuring the maximum capacitance value, the minimum distance between electrodes may be determined. 
     FIG. 4  is a schematic illustration of an equivalent circuit  400  of testing circuit  300  (shown in  FIG. 3 ) illustrated at the time of first contact touch. At the time when the movable contact first comes into electrical contact with the non-movable contact, capacitor  202  may be represented as a short circuit  402  and output  312  V out (t) is no longer a monotone function, but becomes a step function with V out  as the step value. The time occurrence of the voltage step corresponds to the time the circuit breaker contacts close. This time value may be recorded for use in calculations and may be used to start and stop circuit breaker operational timers. Similarly, during an opening testing sequence, when the circuit breaker contacts first open V out  will be a negative step function 
   The capacitance based contact timing circuit may facilitate measuring parameters of high voltage circuit breakers contact systems, such as, but not limited to a start event of the contact geometrical position from the circuit breaker electrically isolated open position, the movement of linear travel of the movable contact, the first electrical touch of each contact, and the penetration as a function of dynamic resistance from the first contact touch to a geometrical end position. 
   The circuit also facilitates evaluating data from a synchronized time base to comply with standards and calculating circuit breaker parameters and enabling analysis of circuit breaker operation, such as, but not limited to, measuring actual linear movement of each movable contact, measuring a time elapsed from a synchronized start to first touch or last separation of each movable contact, measure a time elapsed from a start event, such as, the contact geometrical position from the circuit breaker electrically isolated open position, to a contact first touch, or a last separation of each contact to an isolated open position, determine a contact velocity in a function of time and movement within above positions, determine overlap, wherein a movement and time elapsed from each contact separation to arcing contact separation at open an operation is measured, and determine a quality of each contact interrupting medium. 
     FIG. 5  is a schematic illustration of an exemplary testing circuit  500  that may be used to time the contacts of circuit breaker phase  100  (shown in FIG.  1 ). During testing, source  302  injects a test signal into circuit breaker contact  106  within break  108 . If contact  106  is in an open state, the test signal is transmitting capacitively through contact  106  to the input of resonant filter  304 . The filtered output of filter  304  is transmitted to rectifier  502  and low-pass filter  504 . The combination of rectifier  502  and low-pass filter  504  envelopes the output of filter  304  to facilitate reducing high frequency noise and facilitate reducing unwanted non-peak related signal information. In the exemplary embodiment, a corner frequency  1 / 5  of the resonant frequency of capacitor  308  and inductor  310  is used. The signal value from filter  504  is then compared with the voltage value that is greater than RoVg(t)peak/(Ra+Rb+Ro) by comparator  506 . The output signal from comparator  506  is equal to V out  and is transmitted to a digital input of microprocessor  314  as the “make” or “break” timing result. The output of filter  504  is also transmitted to an input of amplifier  508  to provide an analog output signal to microprocessor  314  for further processing. Inductors  510  and  512  are electrically coupled in series with grounding cables  514  and  516 , respectively to drain any currents induced into the circuit breaker circuit. Grounding cables  514  and  516  are applied to the circuit breaker to ensure the personnel safety of operating personnel during testing of the circuit breaker. By introducing an inductance: 
       Lg   &gt;&gt;     1       ω   2     ⁢     C   min               
into the circuit breaker grounding cables  514  and  516 , the circuit breaker timing measurement may be conducted without disconnecting grounding cables  514  and  516  from the circuit breaker thereby providing greater safety protection to operating personnel.
 
     FIG. 6  is a graph  600  of an exemplary trace  602  of output voltage  312  of testing circuit  300 . Graph  600  includes an x-axis  604  indicative of time, and a y-axis  606  that illustrates a magnitude of output voltage  312  at each corresponding unit of time. At t( 0 )  608 , a circuit breaker operating signal is triggered. Between time, t( 0 )  608  and a time t( 1 )  610  the circuit breaker movable contact is moving towards the circuit breaker non-movable contact. As the contacts move closer together, the capacitance and hence, the voltage across the contacts increases. The impedance of the contacts may be determined from the equation: Z=Ra+Rb+ω(L a +L b )+1/ωC). At time t( 1 )  610 , a first contact is detected by the step jump in V out  at point  612 . At point  612  a “make” signal is generated based on the detected step jump. During jump  614 , a dynamic resistance (Z) between the circuit breaker contacts in motion, termed the penetration process, may be determined based on the equation. Z=R a +R b +ω(L a +L b ). At a point  616 , the dominant impedance on the system becomes the inductance of the circuit breaker cables and may be determined by the equation Z=ω(L a +L b ) 
     FIG. 7  is a flow diagram of an exemplary method  700  for analyzing a circuit breaker. Method  700  includes electrically coupling  701  each circuit breaker contact to ground. A first grounding cable may be coupled to a line side contact of the circuit breaker to a local ground connection. In one embodiment, a inductance of about one microhenry is coupled in series with the grounding cable. In an alternative embodiment, an inductance of less than about one millihenry is coupled in series with the grounding cable. Similarly, a second grounding cable may be electrically coupled to a load side of contact of the circuit breaker to a local ground connection. Inductance may be coupled in series with the grounding cable as described above. A test voltage is applied across the circuit breaker contact pair. In the exemplary embodiment, the test voltage is a high frequency sine wave signal. The frequency of the test signal is selected to match the resonant frequency of the circuit breaker contact and filter circuit, which are electrically coupled in series with the source. Alternatively, impedance values of the components of the filter circuit may be selected such that the resonant frequency of the filter matches the output frequency of the test source. During testing, the output voltage taken across the filter circuit is proportional to a capacitance value of the circuit breaker contact. Accordingly, measuring  704  an output voltage of the testing circuit provides indication of the capacitance of the circuit breaker contact. The gap defined between each contact determines the capacitance of the circuit breaker contact. The state of the circuit breaker contact is changed  706  from an open position to a closed position or the closed position to the open position by automatic action taken by a microprocessor executing within the test device or by a manual command initiated by an operator. As the movable contact of the circuit breaker contact pair moves relative to the non-movable contact, the capacitance between the contacts changes proportionally with respect to the distance separating the contacts. As the contacts engage the testing circuit configuration changes such that the output voltage changes by a step amount. The step change is detected  708  in the output voltage that corresponds to the change of state of the circuit breaker. An output signal at the time of the step change is generated for use in analyzing a condition of the circuit breaker contacts and dielectric medium. 
   While the present invention is described with reference to measuring timing and resistance of contacts of a high voltage circuit breaker, numerous other applications are contemplated. For example, it is contemplated that the present invention may be applied to any system wherein electromagnetic interference may induce currents into measured parameters and measuring devices such that the accuracy of such measurements is reduced without suppression of the induced currents. 
   The above-described high voltage circuit breaker testing system is cost-effective and highly reliable for determining a circuit breaker contact timing and resistance in the presence of induced currents from electromagnetic interference. More specifically, the methods and systems described herein facilitate determining circuit breaker operating times and contact resistances accurately in the presence of electromagnetic induced currents in the circuit breaker circuit and testing circuit. In addition, the above-described methods and systems facilitate providing an accurate and repeatable circuit breaker timing and contact resistance measurement with minimal operator interaction. As a result, the methods and systems described herein facilitate maintaining high voltage circuit breakers in a cost-effective and reliable manner. 
   Exemplary embodiments of circuit breaker testing systems are described above in detail. The systems are not limited to the specific embodiments described herein, but rather, components of each system may be utilized independently and separately from other components described herein. Each system component can also be used in combination with other system components. 
   While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.