Patent Document

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. 
   At least some known circuit breakers, use a pre-insertion resistor to facilitate protecting circuits during closing operations of the circuit breaker. Specifically, as the circuit breaker is closed, the pre-insertion resistor is connected in parallel with a gap defined between the open circuit breaker contacts. More specifically, when the pre-insertion resistor is placed in parallel with the gap, the circuit voltage measured to ground (generally line-to-ground voltage) is dropped across the resistor. Accordingly, the current flowing through the resistor is determined by V/Z, wherein V represents the line-to-ground voltage of the circuit and Z represents the vector sum of the resistance of the pre-insertion resistor and the surge impedance of inductive and capacitive elements, such as capacitor banks, reactors, and bus work coupled to and within the circuit. The current determined by this calculation is often referred to as the inrush current and may momentarily achieve a substantially high level when the circuit breaker is used in conjunction with a capacitor bank. 
   During operation, inrush currents with relatively large magnitudes may cause damage to the circuit. For example, without pre-insertion resistors, the inrush current may reach values of about 10 to 30 thousand amperes. However, with a pre-insertion resistor installed, the initial inrush current may reach values of only about 2 to 4 thousand amperes. Following the initial inrush current, the current through the pre-insertion resistor may be limited by the steady state impedance of capacitor banks, and other circuit components, such as, but not limited to loads, reactors, and bus work coupled to the circuit. Consequently, following the initial inrush current, the current flow through the pre-insertion resistor is generally within the range of 100 to 400 amperes. Additionally, after the initial inrush current has subsided, and the current through the pre-insertion resistor has been reduced dropped to a substantially lower level of 100–400 amperes, contacts of the circuit breaker quickly re-engage. If the circuit breaker is switching capacitor banks, the banks discharge directly through the contacts, such that the current is limited by the surge impedance of the banks and the bus work. 
   The engagement of the main contacts shunts the majority of the circuit breaker current through the main contacts so that the pre-insertion resistor does not carry a substantial part of the continuous or normal current through the circuit breaker. Therefore, the timing of the closure of circuit breaker contacts and the resistive integrity of the pre-insertion resistor are factors that may facilitate enhanced operation of a high voltage circuit breaker. Accordingly, verifying such parameters by periodic testing may facilitate proper circuit breaker operation. However, such testing is typically performed in place, for example, in a switchyard or substation where the circuit breaker is normally located, and may subject testing equipment to power line frequency interference from voltages induced into test equipment components and/or cabling from power lines located proximate the circuit breaker and testing equipment. Additionally, when timing a circuit breaker in a substation with high voltage lines surrounding the circuit breaker being tested, a power line frequency current flow may be undesirably induced into the measurement circuits due to capacitive coupling between the test object and adjacent high voltage lines. The interference current may be of substantially the same frequency as the desired measurement result, therefore adversely affecting the measurement result. 
   BRIEF DESCRIPTION OF THE INVENTION 
   In one aspect, a method for measuring a resistance of an electrical contact pair in the presence of a low frequency electromagnetic interference is provided. The contact pair includes at least one movable contact, the movable contact is movable to break electrical contact with the other contact during an open operation, and the movable contact is movable to make electrical contact with the other contact during a close operation. The method includes injecting a high frequency signal across a contact pair in an open state, measuring a voltage across the contact pair, measuring a current flowing through the contact pair, and calculating a resistance of the contact pair using the measured voltage and the measured current. 
   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 determine a circuit breaker contact closing time, determine a circuit breaker contact opening time, and determine a circuit breaker pre-insertion resistor resistance value using three voltage samples and three current samples that facilitate minimizing an induced current measurement error. 
   In yet another aspect, a test device for analyzing circuit breakers is provided. The test device includes a testing circuit electrically coupled to at least one output terminal, an overvoltage protection circuit electrically coupled between the at least one output terminal and electrical ground, and a processor programmed to determine a circuit breaker contact timing measurement and a circuit breaker contact resistance measurement using a square wave test signal. 

   
     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 circuit breaker testing device that may be used to test the circuit breaker phase shown in  FIG. 1 ; 
       FIG. 3  is a graph illustrating an exemplary trace of contact resistance plotted against time as measured by the testing device shown in  FIG. 2 ; 
       FIG. 4  are a series of graphs that illustrate exemplary current and voltage signals used in measuring contact resistance and circuit breaker timing by the testing device shown in  FIG. 2 ; and 
       FIG. 5  is a flow diagram illustrating an exemplary method of measuring the timing of a circuit breaker while suppressing induced current measurement errors. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Periodic testing of circuit breakers may include performing a contact timing test and a pre-insertion resistor resistance test. The timing test measures an elapsed time from the initiation of movement of circuit breaker contacts until the main circuit current stops flowing during an opening test, and also an elapsed time from an initiation of movement of the contacts until the main circuit current starts flowing during a closing test. The pre-insertion resistor resistance test measures the value of the pre-insertion resistor. 
     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 coupled in parallel with a moving main contact  106 . In the exemplary embodiment, phase  100  includes two breaks  108  that each include a pre-insertion resistor (only one is shown in  FIG. 1 ). 
   In operation, when the circuit breaker receives a command to close from an open position, linkages within the circuit breaker cause movable portions of contacts  104  and  106  to shift towards engaging respective non-movable portions of contacts  104  and  106 . During a testing sequence, movement of the movable portion of contacts  104  and  106  may initiate a timer. After a predetermined distance of travel of the movable portions of contacts  104  and  106  has lapsed, the movable portion of pre-insertion resistor contact  104  engages the non-movable portion of contact  104 , causing current to flow through contact  104  and pre-insertion resistor  102 . A current surge through contact  104  may be limited by pre-insertion resistor  102 . After a predetermined time delay, the movable portion of contact  106  engages the non-movable portion of main contact  106 . Since the resistance of main contact  106  may be substantially less than the resistance of pre-insertion resistor  102 , substantially all current flowing through the circuit breaker flows through main contact  106 . During testing, the resistance values of contacts  104  and  106  may be determined, in addition to the resistance value of pre-insertion resistor  102  and the timing of circuit breaker contacts  104  and  106 . More specifically, the resistances are measured dynamically and the value of pre-insertion resistor  102  is measured in a time period elapsed between the closing of resistor contact  104  and the closing of main contact  106 . Based on the measured resistance values, known threshold values are used to determine when main contact  106  and resistor contact  104  are each considered to be open and/or closed, such that the contact timing may be calculated. In one embodiment no pre-insertion resistor  102  is included in the circuit breaker, and only the timing of main contact is determined. 
     FIG. 2  is a schematic illustration of an exemplary circuit breaker testing device  200  that may be used to test circuit breaker phase  100  (shown in  FIG. 1 ). Testing devise  200  includes a testing circuit  201  that includes a four-quadrant voltage source  202  for supplying measurement signals to the object being tested. In the exemplary embodiment, only a single phase of a substation circuit breaker is illustrated. A current limiting resistor  204  and an ammeter  206  are electrically coupled in series with voltage source  202 . A voltmeter  207  is electrically coupled in parallel to the series combination of resistor  204 , source  202 , and ammeter  206  at a first node  208  and a second node  210 . Nodes  208  and  210  are electrically coupled to nodes  212  and  214 , respectively, in an over-voltage protection circuit  216 . Nodes  212  and  214  are also electrically coupled to output terminals  218  and  220 , respectively. Over-voltage protection circuit  216  includes a first transient voltage surge suppressor (TVSS) that is electrically coupled to a grounded case  234  of testing device  200  and node  212 . A second TVSS  236  is electrically coupled between nodes  212  and  214 , and a third TVSS  238  is electrically coupled between node  214  and grounded case  234 . Terminal  218  is electrically coupled to a line side  222  of one phase of circuit breaker  224 , and terminal  220  is electrically coupled to a load side  226  of circuit breaker  224 . Disconnect switches  228  and  230  are electrically coupled to line side  222  and load side  224  of circuit breaker  224 , respectively, to isolate circuit breaker  224  from other components during testing and/or maintenance activities. 
   A microprocessor  240  is communicatively coupled to testing circuit  201  for controlling voltage source  202 , ammeter  206 , voltmeter  207  and the operation of circuit breaker  224  during testing. Microprocessor  240  may also receive signals from ammeter  206 , voltmeter  207  and components (not shown) of circuit breaker  224  for monitoring circuit breaker  224  operation, initiating testing sequences, and collecting, storing, and manipulating data to facilitate determining a circuit breaker  224  timing, and contact and resistor resistances. 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, microprocessor  240  executes instructions stored in software. In another embodiment, microprocessor  240  executes instructions stored in firmware (not shown). In yet another embodiment, microprocessor  240  is a circuit defined to perform the functions described herein. Additionally, although the herein described methods and apparatus are described in an industrial setting, it is contemplated that the benefits of the invention accrue to non-industrial systems such as those systems typically employed in a commercial setting, such as, for example, but not limited to, electronics and communications. 
   Testing device  200  is illustrated testing one phase of a high voltage circuit breaker. With additional inputs and software timers, testing device  200  is capable of testing all phases of a circuit breaker simultaneously, as well as testing all phases of multiple circuit breakers simultaneously. 
   In operation, terminal  218  may be electrically coupled to line side  222  via cable  242  and terminal  220  may be coupled to load side  226  via cable  246 . Processor  240  initiates a testing sequence upon a user&#39;s command. In the exemplary embodiment, processor  240  directs testing device  100  through a diagnostic routine that verifies operation of components internal to testing device  100  and also verifies proper connection of test leads  242  and  246 . Source  202  generates a voltage signal that is transmitted through ammeter  206  to load side  226  through main contact  106  of circuit breaker  224  to line side connection  222  before being returned to voltage source  202 . In the exemplary embodiment, voltage source  202  transmits a ten kilohertz square wave signal that varies between approximately ten volts and fifty-five volts. Microprocessor  240  may control operation of voltage source  202  to other parameters depending on the requirements of a specific test being run. During testing, a current may be induced into testing circuit  201  from high voltage power lines adjacent breaker  224 . The current may be induced into circuit  201  at a power line frequency, such as, for example sixty Hertz or fifty Hertz. Such an induced signal may interfere with an accurate measurement of resistance and timing if the induced signal is not suppressed, eliminated, and/or accounted for in the measurement sequence. 
     FIG. 3  is a graph  300  illustrating an exemplary trace of contact resistance plotted versus time as measured by testing device  200  (shown in  FIG. 2 ). Graph  300  includes an x-axis  302  that represents time (t) from the initiation of a testing sequence at t=0. A y-axis  304  represents a magnitude of resistance (R) measured between terminals  218  and  220  of testing device  200 . A first legend  306  illustrates when main contact  106  is in a closed state and a second legend  308  indicates when resistor contact  104  is in a closed state. A heavy black line on each legend  306  and  304  represents the contacts in the closed state. 
   During testing, circuit breaker  224  may start in the open state at t=0 wherein a resistance  310  of contacts  104  and  106  is relatively large, and wherein substantially zero current flows through contacts  104  and  106 . While monitoring circuit voltage and current, and while calculating contact resistance, processor  240  commands circuit breaker  224  to close, thereby initiating a timing sequence to facilitate determining a time it takes for each contact to close. When resistor contact  104  closes at time t=1, trace  304  illustrates a contact resistance  312  that is approximately equal to a resistance of pre-insertion resistor  102 , for example, approximately one hundred ohms. At time t=2, main contact  106  closes and trace  304  illustrates a contact resistance  314  that is relatively low, for example, on the order of several milli-ohms or less. After time t=2 but before time t=3 the contact  104  open. The opening of contact  104  can not be monitored using the method described here since contact  104  in series with pre-insertion resistor  102  are paralleled by the very low resistance contact  106 . 
   Processor  240  may calculate contact operating time based on measured resistance of contacts  104  and  106  and the command to the breaker to close. Similarly, operation of circuit breaker  224 , during an open operation may also be monitored. As illustrated at time t=3, contact  106  opens when commanded by processor  240  to open. An elapsed time between the initiation of the open command and time t=3 represents a circuit breaker  224  actuation time in the open sequence. 
     FIG. 4  are a series of graphs  400 ,  402 ,  404 , and  406  that illustrate exemplary current and voltage signals used to measure contact resistance and circuit breaker timing by testing device  200  (shown in  FIG. 2 ). Graph  400  illustrates a trace  408  of current that may be induced into the circuitry of testing device  200  based on a location proximate high voltage power lines in a switchyard or a substation, for example, where the testing may be taking place. Trace  408  illustrates one cycle of power line frequency. Trace  408  also illustrates the voltage across the pre-insertion resistor caused by the induced current. The magnitude of the trace is illustrated qualitatively. 
   In the exemplary embodiment, graph  402  illustrates a trace  410  of the output of voltage source  202  (shown in  FIG. 2 ) in the exemplary embodiment, as a ten kilohertz square wave that varies between ten volts and fifty-five volts. Trace  410  also illustrates the current through the pre-insertion resistor caused by the generated voltage. In other embodiments, voltage source  202  may utilize a signal at a different frequency and/or different voltage magnitudes. 
   Graph  404  illustrates a trace  412  that is a sum of traces  408  and  410  that shows both the voltage and the current traces that are a result of the voltage produced by the induced current flowing through the contact resistance modulating the square wave output of voltage source  202 . A point  414  represents a measurement point during testing at a local minimum value of trace  412 . Point  416  represents a measurement point at a local maximum value of trace  412  occurring prior to point  414  in time, and point  418  represents a measurement point at a local maximum value of trace  412  occurring subsequent to point  414  in time. The sum of the voltage caused by the induced current passing through the resistance in circuit breaker  224  and the resistance in the measurement circuit and the generated square wave voltage is measured by voltmeter  207  in testing circuit  201 . 
   Graph  406  illustrates an enlarged portion of trace  412 . A magnitude difference  420  represents a difference between the magnitude of trace  412  at point  418  and at point  416 . Difference  420  may be an offset in the voltage/current received during testing by testing device  200  that is attributable to the current induced into the testing device circuitry from the high voltage power lines. Processor  240  may modify a result of testing by difference  420  to facilitate determining a more accurate value of contact resistance, pre-insertion resistor resistance, and circuit breaker timing. 
   If there is substantially zero induced current, the quotient between the voltage and the current, measured by voltmeter  207  and ammeter  206 , may represent the resistance across terminals  218  and  220  of testing device  200 , and the contacts of circuit breaker  224 . When the induced current is not substantially zero, the circuit includes two unknown variables: the induced current and the circuit breaker contact resistance. To determine the circuit breaker contact resistance in the presence of the induced current, two equations are used to determine the two circuit unknowns. Voltage and current samples from points  414  and  416  are used to populate an equation for the circuit breaker contact resistance at a first level of induced current and voltage and current samples from points  414  and  418  are used to populate an equation for the circuit breaker contact resistance at a second level of induced current. Three samples are populated such that there are two equations with two unknown variables: the induced current and the circuit breaker contact resistance. From the equation system, the induced current is eliminated and the circuit breaker contact resistance is solved for. 
   The resulting resistance is then given by the following formula:
 
 R   n =( V   N −( V   N−1   +V   N+1 )/2)/( I   N −( I   N−1   +I   N+1 )/2);
 
   where: 
   R n  is the resistance of at least one of the pre-insertion resistor and the circuit breaker contacts; 
   V N  is a local minimum magnitude of the injected and induced voltage signal; 
   V N−1  is a local maximum magnitude of the injected and induced voltage signal preceding the local minimum; 
   V N+1  is a local maximum magnitude of the injected and induced voltage signal succeeding the local minimum; 
   I N  is a local minimum magnitude of the injected and induced current signal; 
   I N−1  is a local maximum magnitude of the injected and induced current signal preceding the local minimum; and 
   I N+1  is a local maximum magnitude of the injected and induced current signal succeeding the local minimum; 
     FIG. 5  is a flow diagram illustrating an exemplary method  500  for measuring the timing of a circuit breaker while suppressing induced current measurement errors. Method  500  includes determining  502  a circuit breaker contact closing time from a testing sequence circuit breaker operation command and a circuit breaker measurement that indicates the breaker has actuated. The testing device processor initiates a circuit breaker operation command, such as, for example, a circuit breaker close command. Initiation of the circuit breaker operation also initiates a timer. The time elapses until a measured circuit breaker parameter indicates that the circuit breaker has closed. In the exemplary embodiment, a voltage across the circuit breaker contacts, and a current through the contacts are measured to enable a contact resistance to be calculated. If the calculated contact resistance is within a predefined threshold value, the calculation indicates the circuit breaker contacts are closed and the timer is stopped. In the exemplary embodiment, a first timer is used to measure the closing time of a circuit breaker pre-insertion resistor contact and a second timer is used to measure the closing time of the circuit breaker main contact. In an alternative embodiment, wherein a pre-insertion resistor is not used, only one timer is used to monitor the main contact closing time. In another alternative embodiment, additional timers are used to monitor other circuit breaker parameters. 
   Similarly to determining the circuit breaker closing time, a circuit breaker contact opening time is determined  504 . The processor initiates an opening command to the circuit breaker, which also starts a timer. The time elapses from the opening command until circuit breaker parameters indicate the circuit breaker contacts are open. During testing, the circuit breaker pre-insertion resistor resistance value and main contact resistance value may be determined  506  using three voltage samples and three current samples. Such a method facilitates minimizing an induced current measurement error due to currents induced into testing device cabling and circuits, and the circuit breaker components due to their location in close proximity to high voltage power lines in. 
   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 low frequency 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.

Technology Category: 3