Patent Publication Number: US-7916437-B2

Title: Fault interrupter and operating method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to and benefit from U.S. Provisional Patent Application Ser. No. 60/942,023, filed Jun. 5, 2007, the disclosure of which is hereby incorporated herein by reference for all purposes. 
    
    
     TECHNICAL FIELD 
     This patent relates to a fault interrupting and reclosing device, and more particularly, to a fault interrupting device and associated operating method. 
     BACKGROUND 
     Fault interrupting devices function to isolate a fault condition in a power distribution system. Upon clearing of the fault condition some fault interrupting devices are also operable to reclose the circuit. Faults in a power distribution system can occur for any number of reasons and are often transient. Detection and isolation of the fault mitigates damage to the system as a result of the fault. An ability to reclose the circuit following a fault without replacement of hardware components allows the power distribution system to be returned to normal operation quickly, and in some instances, without operator intervention. 
     Combined fault interrupting and recloser devices may be designed to operate or be operated after a fault interruption to reclose the faulted line or lines. Following reclosing, if the fault is not cleared the device will detect the fault and again operate to open the circuit to isolate the fault. When a fault is determined to be permanent, the fault interrupting device should act to isolate the circuit and prevent further reclosing attempts. 
     Several types of fault interrupting and reclosing devices incorporate vacuum interrupters to perform the circuit interrupting and subsequent reclosing functions. During current interrupting operation, as the vacuum interrupter contacts open, there is redistribution of material from the contacts to the other surfaces within the interrupter. Contact material redistribution occurs with each operation, and therefore, the vacuum interrupter is capable only of a finite number of fault current interrupting operations. The number of fault interrupting operations may be specified for a particular fault protection device based upon design information and intended application. The fault interrupting and reclosing device may include a counter to track the number of operations. 
     The vacuum interrupters in fault interrupting and reclosing devices are capable of operating very quickly under the action of a drive mechanism, such as a drive solenoid. Operation in the presence of an asymmetric current can expose the contacts to large arcing time, for example, arcing times in excess of 10 ms. Such long arcing times have the potential to seriously degrade the life of the fault interrupter and reclosing device. 
     In practice, therefore, the actual number of interrupting cycles a vacuum interrupter is capable of, and hence the fault interrupting and reclosing device incorporating the interrupter, depends on a number of operating characteristics including characteristics of the interrupted fault current and the operating characteristics of the vacuum interrupter. For example, material erosion and corresponding contact degradation become significantly more pronounced with the magnitude and asymmetry of the interrupted current. The number of cycles defining the life of the fault interrupting device is conservatively set to ensure the proper operation of the device throughout its specified life and over its rated current interrupting capacity. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a graphic illustration of a fault interrupting reclosing device in a set or connected position wherein it is operable for connecting a source and load of a power distribution system. 
         FIG. 2  is a bottom view of the fault interrupting device illustrated in  FIG. 1 . 
         FIG. 3  is a graphic illustration of the operative elements disposed within the housing of the fault interrupting reclosing device of  FIG. 1 . 
         FIG. 4  is a block diagram illustrating the operational and control elements for a fault interrupting reclosing device such as the device of  FIG. 1 . 
         FIG. 5  is a flowchart illustrating a method of operating a vacuum fault interrupter. 
         FIGS. 6 and 7  are charts illustrating operation of a vacuum fault interrupter relative to current characteristics. 
     
    
    
     DETAILED DESCRIPTION 
     A fault interrupting and reclosing device includes a circuit interrupting device such as a vacuum fault interrupter, an arc spinner interrupter or the like, coupled to an actuator. The actuator includes at least one force generating element for generating an operating force for operating the circuit interrupter to open the circuit, for example, to generate a linear force to open the contacts of the circuit interrupter, and for generating a restoring force to close circuit interrupter to close the circuit. The actuator may include an electro-magnetic actuator such as a solenoid to drive the contacts open and a spring to close the contacts. The device may further include a latch, such as an electro-mechanical latch, to engage the actuator to retain the state of the circuit interrupter. For example, to hold the vacuum interrupter contacts closed when the circuit is closed and to hold the contacts open when the circuit is opened. Control electronics, which may include one or more of a dedicated processor, a general purpose processor, an application specific integrated circuit, or the like, may be employed to monitor current characteristics, to monitor the position of the vacuum fault interrupter mechanism, and to affect operation of the circuit interrupter responsive thereto. 
     While the present invention has application to virtually any fault interrupting device, the following discussion of a particular type of fault interrupting device provides an environment for describing and understanding the various embodiments and aspects of the invention. Referring to  FIG. 1 , a fault interrupting and reclosing device  100  includes a housing  102  including a first tap  104  and a second tap  106 . The housing  102 , first tap  104  and second tap  106  are configured to allow the device  100  to couple to mounting  110 , such as a mounting commonly referred to as a cut out mounting or other suitable mounting. The mounting  110  may include a support  112  permitting the mounting  110  to be secured to a pole or other structure (not depicted) for supporting the mounting  110  relative to the lines of the power distribution system. The first tap  104  may be secured to a supply coupling  114  of the mounting  110  and the second tap may secure to a load coupling  116  of the mounting  110 . The supply coupling  114  may include an alignment member  118  that engages an alignment member  120  of the device  100  for aligning the tap  104  relative to a contact  122  that electrically couples the tap  104  to the supply of the power distribution system. 
     The load mounting  116  may include a trunnion  124  secured to the mounting  110 . The trunnion  124  is formed to include a channel  125  within which a sliding contact/pivot member  126  is disposed. The member  126  is coupled as part of a release mechanism  128  that provides for releasing the device  100  from the mounting  110 , for example, after a predetermined number of failed reclose attempts. 
       FIG. 1  depicts the device in a connected position wherein the device is electrically coupled to both the supply side  114  and the load side  116  of the power distribution system via the cut out mounting  110 . The device may also be disposed in a disconnected position. The device  100  includes a hook ring  132 . Using a “hot stick” or other suitable insulated tool, a technician can grasp the hook ring, and pulling away from the cut out mounting  110 , cause the tap  104  to separate from the strap  122 . The strap  122  normally bears against the tap  104 , the force of which is sufficient in normal operation to retain the device  100  in the connected state and ensure electrical conductivity. However, by applying a force to the hook ring  132 , the tap  104  may be separated from the strap  122 . Once separated, the device  100  is free to rotate about the pivot  130  away from the cut out mounting  110 . If mounted vertically, as depicted in  FIG. 1 , gravity will act to cause the device  100  to rotate about the pivot  130  to a disconnect position. The hook ring  132  also allows the device  100  to be moved to the connected position depicted in  FIG. 1 . 
     The device  100  may be operated, as will be explained, in an automatic mode. In the automatic mode, upon fault detection, the device  100  operates to open, without disconnecting from the power distribution system, to isolate the fault. The device  100  may then attempt to reclose one or more times. If after reclosure the fault is no longer detected, the device  100  remains closed. If, however, the fault is persistent, the device  100  will again open. After a predetermined number of reclose attempts, the release mechanism acts to release the device  100  from the mounting  110  allowing the device to drop out of the connected state shown in  FIG. 1  and into the disconnected state. 
     In certain applications it may be desirable to disable the reclose function. In that case, upon a first fault detection the device will release or “drop out” of the mounting to the disconnected position. A selector  136  ( FIG. 2 ) is provided to allow a technician to set the operating mode, automatic (AUTO) or non-reclosing (NR). For example, the selector  136  may include a ring  136  so that the selector  136  may be actuated using a hot stick or other suitable tool from the ground or a bucket truck. A cycle counter  138  may also be provided. The cycle counter  138  provides an indication of the total interrupt cycles, and hence provides an indication of when the device may require service or replacement, a record of fault activity and data for statistical analysis of device and/or system performance. 
       FIG. 3  depicts a circuit interrupting device  140  of the device  100 . The circuit interrupting device  140  may be any suitable device examples of which include vacuum interrupters and arc spinner interrupters. The circuit interrupter  140  may be coupled by an insulating coupling  142  to a solenoid  144 . The solenoid  144  may be configured with a first, primary coil  146  conducting the line-to-load current that is used to generate, as a result of a fault current, an opening force on the coupling  142  for actuating the circuit interrupting device  140 , for example, exerting an opening force on the contacts of the vacuum interrupter. If the circuit interrupting device is a vacuum interrupter, as depicted in the exemplary embodiment illustrated in  FIG. 3 , it may include an axial magnetic field coil  141  allowing the vacuum interrupter  140  to interrupt a fault current in excess of that for which it is rated. 
     The solenoid  144  may further include a secondary coil winding  148  that may be used as a transformer source for providing electrical energy to storage devices  190  such as capacitors for operating the solenoid  144  a release latch assembly  160  and a controller and/or control electronics  192  ( FIG. 4 ). The solenoid  144  may also include a spring  149 . The spring  149  provides a closing force on the coupling  142  for returning the circuit interrupter to the closed or connected state, for example, by urging the contacts closed. More than one spring may be provided. For example, a first spring may be used to provide a closing force while a second spring is used to provide a biasing force to maintain the contacts in contact. Therefore, the device  100  includes a solenoid  144  operable to provide an opening force (energized coil) and a closing force (spring). 
     A pin or other suitable coupling  152  couples the solenoid plunger  150  to a lever  154 . The lever  154  is mounted within the bracket (not depicted) to pivot about a pivot point  156 . The coupling of solenoid plunger  150  to the lever  154  causes pivoting motion of the lever  154  upon extension and retraction of the solenoid plunger  150  relative to the solenoid  144 . 
     Still referring to  FIG. 3 , the device  100  may further include a latch assembly  160 . The latch assembly  160  is secured within the housing  102  and has a generally “C” or claw shape structure including a first latching portion  162  and a second latching portion  163 . The latch assembly  160  essentially consists of a pair of electrically controllable “horseshoe” magnets  164  and  165  (magnetic stator pieces); the respective end positions of which define the first latching portion  162  and the second latching portion  163 . The magnets  164  and  165  are spaced apart so as to define a slot  167  within which an armature  168  of the lever  154  is disposed. The armature  168  itself may be magnetic or made of magnetic material, or, as depicted, the end may include a magnet insert  169 . 
     The magnet stator  164  and  165  is formed by combining “C” or “horseshoe” shaped permeable members  170  and  172  having magnetic material  174  disposed between them at a specific location. Combined with the magnetic material  174  is a coil  176 . The coil  176  is coupled to the control electronics to receive an electric current the affect of which is to neutralize the magnetic field of the magnetic material  174 . Absent current in the coil, the magnetic material  174  acts to create a magnetic field shared by the members  170  and  172  within the first and second latching portions  162  and  164  to retain the lever  154  at either of the first or second latching portions  162  and  164 , depending on the state of the actuator and the circuit interrupter. The magnetic material may be disposed closer to one end of the “C” shape than the other, such that by its relative position, the magnetic force applied to the magnet insert (armature)  169  may be greater at one latching portion, for example  162 , than the other, for example  164 . Application of current within the coil acts to neutralize the magnetic field in the first and second latching portions  162  and  164  such that under action of the solenoid  144  the circuit interrupting device may be driven from the closed or connected state to the open or disconnected state, or, under action of the return spring  149 , the circuit interrupting device may be driven from the open or disconnected state to the closed or connected state. This is explained in more detail below. 
     With the solenoid  144  in the circuit closed position or connected state, the end  168  is disposed adjacent the first latching portion  162 . Absent current in the coil  176 , a magnetic field is present in the first latching portion  162  that exerts a retaining force on the end  168  and/or the magnetic insert  169 , as the case may be. The retaining force resists movement of the end  168 , and hence the lever  154 , latching it and the solenoid  144 , in the circuit closed position. Upon detection of a fault current, the solenoid  144  generates a force on the solenoid plunger  150  to open the circuit interrupting device  140 . Concomitantly, the control electronics  192  applies a current to the coil  176  neutralizing the magnetic field releasing the lever  154 . Axial movement of the solenoid plunger  150  in conjunction with the opening of the circuit interrupter  140  causes the lever  154  to rotate such that the end  168  is disposed adjacent the second latching portion  164 . The current is removed from the coil  176  restoring the magnetic field such that the second latching portion  164  exerts a force on the end  168 , which resists movement of the end  168  and latches the lever  154 , and hence the solenoid  144 , in the circuit open position or disconnected state. Current may be removed from the coil  176  at any point in the travel of the lever  154 , to minimize the energy drawn from the energy storage means. The force of the magnet, in combination with the mechanical advantage provided by having the magnet act on the end  168  relative to the pivot  156 , provides sufficient force to resist the closing force exerted by the spring  149 . Of course, it should be understood that in other embodiments, various combinations of linkages, gears or other force-multiplying arrangements may be employed. 
     To close the circuit interrupting device, current is again applied to the coil  176  to neutralize the magnetic field. With the magnetic field neutralized, the lever  154  is free to move and the spring  146  has sufficient strength to force circuit interrupting device  140  to the closed position or connected state. Once the end  168  is substantially disengaged from the second latching portion  164 , the current within the coil  176  is terminated restoring the magnetic field and the retaining magnetic force. The lever  154  is again latched on contacting the first latching portion  162 . Thus, the latch assembly  160  provides for latching the solenoid  144  in both the circuit open position/disconnected state and the circuit closed position/connected state. The required mechanical advantage and magnet strength is determined for the particular application. For example, the latch assembly  160  in combination with the mechanical advantage may provide a hold force that is greater than the solenoid acting force, e.g. two or more times the solenoid acting force. 
     A flexible conductive strap (not depicted) may couple from a moving contact  172  of the circuit interrupter  140  to the solenoid  144  for providing electrical power to the first coil  146  and the second coil  148 . The flexible strap may also couple fault current to the solenoid  144 . When a fault current exists, the fault current passing through the solenoid coil  146  develops an axial force sufficient to drive the circuit interrupter  140  to an open/disconnected state. Once opened, the circuit interrupter  140  is held open by the latching capability of the latch  160  acting on the lever  154 . 
     The controller  192  is operable upon fault detection to energize the coil  176  to negate the magnetic field of the magnetic material  174  to allow the solenoid  144  to drive the circuit interrupter  140  to the open state. The controller  192  is also operable to energize the coil  176  to negate the magnetic field of the magnetic material  174  to allow the circuit interrupter  140  to close under action of the spring  149 . Once the contacts are closed, the circuit interrupter  140  again conducts, and current is coupled by the strap to the solenoid coil  148 . If the fault current persists, the device  100  again acts to open the circuit. 
     The controller  192  is operable to provide for and manage reclose attempts, and for example, to provide a delay between reclose attempts and to count the number of reclose attempts. Should the number of reclose attempts exceed a threshold value, then the device  100  may be caused to drop out. The controller further may delay energizing the coil  176  thereby restraining the solenoid until its release will result in the minimum arcing time at the contacts of the interrupter while still assuring successful latching in the circuit open position. For example, the block diagram of  FIG. 4  illustrates the solenoid  144  mechanically coupled to the circuit interrupter  140 . The solenoid  144  also couples to an energy storage device  190 , such as a capacitor or series of capacitors. A controller  192  couples to the solenoid  144  to monitor fault current and the number of interrupt operations and to energize the coil  176  to release the latch  160 . The controller  192  also couples to the actuator  182  in order to affect drop out, if necessary. 
     For fault currents above a threshold, which can be user defined and/or dynamically/automatically determined, and in one example 2 kiloAmps (kA), the controller only causes activation of the fault interrupter  140  within a prescribed window of a cycle of the periodic waveform subsequent to the decision having been made to open the fault interrupter. This window of time may be a set period of time following the time of occurrence of the first peak of the preceding cycle of current. Alternatively, the window of time may be dynamically determined. The window may preferably be determined so as to minimize arcing time during opening of the contacts by causing the opening to occur at a favourable point on the current wave for reducing arcing time. 
     With reference to  FIG. 5 , coupled to monitor the current in the conductor of the power distribution system ( 200 ), the controller  192  is able to monitor current magnitude in the time domain and to determine whether the current is above or below a given threshold ( 202 ). This monitoring is in addition to the normal relay-like measurement of the rms current, compensating for any asymmetric components. Once the controller  192  determines that the measured symmetric current is both above a trip threshold and above an algorithm threshold ( 204 ) it initiates a delay algorithm ( 206 ). This algorithm ( 206 ) may call for energizing the solenoid  144  ( 210 ) following a predetermined delay following the first maximum absolute magnitude current measurement. For example, the controller  192  may delay a release signal to the solenoid  144  for a time period ( 208 ), which may be fixed or dynamic and may be related to the operating characteristics of the fault interrupter  140 . In a device that employs sixteen current measurements per cycle the time interval may be set to expire fourteen sample periods after the first maximum absolute magnitude current measurement. The time when the first peak is measured relative to when the first current measurement sample that is taken following processor activation will be different for currents having different degrees of asymmetry. However, the time delay from the time of occurrence of the first peak to the initiation of the opening of the interrupter will be the same for both symmetric and asymmetric currents. Following operation of the fault interrupter to isolate the fault, the controller  192  then initiates a reclose or lockout operation based upon the fault persistence, end-of-life of the fault interrupter, non-reclose setting of the fault interrupter or the like ( 212 ). 
     The frequency of current sampling by the controller  192  needs to be at least eight times that of the system frequency in order to identify, with useful resolution, the occurrence in time of the peak magnitudes of the periodic current. The window of time for activating the opening of the fault interrupter  140  needs to be determined based upon the timing variability of the fault interrupter operating mechanism and also upon the time resolution of the acquisition of the current samples. 
     For example, the device  100  may include a fault interrupter  140  with contacts that are capable of going from being fully closed to being locked open state in no less than 3 ms and no more than 5 ms. The controller  192  for the device  100  may take current samples 16 times per 60 Hz cycle. Upon detecting current above the instantaneous current magnitude threshold, e.g., RMS current exceeds a threshold, the controller  192  records the time, t peak , at which this first current peak is detected. It also initiates a delay counter. The delay may be set to cause activation of the fault interrupter  140  opening mechanism at a time t peak-to-trip , which may be 6.46 milliseconds (ms) past detecting the peak magnitude current. Activation is initiated at a time t trip . In this current example, time t peak-to-trip  is 14.79 ms (t peak-to-trip +t single cycle ; 6.46 ms+8.333 ms for a 60 Hz system) past the time t peak  of occurrence of the first peak current of the preceding cycle where the first peak current exceeds a threshold. In the example, time t peak-to-trip  of 6.46 ms is the 4.167 ms time from the time t peak  of occurrence of the most recent peak magnitude to the next zero-current crossing (¼ of a cycle of the 60 Hz current) plus the 3.333 ms from that zero-current crossing to the point in time that is 5 ms prior to next zero-current crossing minus 1.041 ms, the time period between samples of current taken by the control. Equation 1 illustrates this relationship generally. 
                     t     peak   -   to   -   trip       =       3     4   ⁢           ⁢   f       -     t     trip   -   mech   -   max       -     1     f   sample                 Equation   ⁢           ⁢   1               
t peak-to-trip =time from occurrence of most recent peak magnitude of most recent current cycle to time of activation of fault interrupter trip mechanism; t trip-mech-max =maximum time required by the mechanism for the fault interrupter contacts to go from closed to locked in the open state; (e.g., 5 ms typical for a vacuum fault interrupter but dependent upon the type of fault interrupting device); f=electrical distribution system frequency, typically 50 Hz or 60 Hz. (60 Hz in the example); f sample =frequency of the acquisition of current samples by the control. (e.g., minimum 8 times f sample ; and demonstrated 16 times f sample , or 960 Hz in the example).
 
       FIG. 6  and  FIG. 7  additionally provide graphical illustrations of the timing of the primary current, the detection of the first (positive in this case, but may be negative) current peak, and the initiation of the fault interrupter  140 . A symmetric current is depicted in  FIG. 5  by the trace  200 .  FIG. 6  is similar to  FIG. 5  but illustrates an asymmetric current depicted by the trace  202 . The controller  192  detects the first current peak exceeding the threshold occurs at time t peak . The controller  192  then initiates a delay, time t peak-to-trip . Fault interrupter  140  operation is initiated at the time t trip . The operation of the fault interrupter  140  is delayed to a point on the current wave  200  that reduces arcing time, and hence, enhances fault interrupter useful life. Advantageously, because the device designer has accounted for and has reduced the possibility of fault interruption at a point on the current wave that would result in long arcing times and significant contact degradation, the designed delay may increase the number of fault interrupting cycles before establishing the end-of-life of the device. 
     While the present disclosure is susceptible to various modifications and alternative forms, certain embodiments are shown by way of example in the drawings and the herein described embodiments. It will be understood, however, that this disclosure is not intended to limit the invention to the particular forms described, but to the contrary, the invention is intended to cover all modifications, alternatives, and equivalents defined by the appended claims. 
     It should also be understood that, unless a term is expressly defined in this patent using the sentence “As used herein, the term ‘______’ is hereby defined to mean . . . ” or a similar sentence, there is no intent to limit the meaning of that term, either expressly or by implication, beyond its plain or ordinary meaning, and such term should not be interpreted to be limited in scope based on any statement made in any section of this patent (other than the language of the claims). To the extent that any term recited in the claims at the end of this patent is referred to in this patent in a manner consistent with a single meaning, that is done for sake of clarity only so as to not confuse the reader, and it is not intended that such claim term by limited, by implication or otherwise, to that single meaning. Unless a claim element is defined by reciting the word “means” and a function without the recital of any structure, it is not intended that the scope of any claim element be interpreted based on the application of 35 U.S.C. §112, sixth paragraph.