Abstract:
A current interrupter ( 4 ) includes a current interrupting device ( 4 ) having at least one movable contact ( 71 ); an actuator ( 8 ) coupled to the movable contact ( 71 ) of the current interrupter ( 4 ); a feedback sensor ( 14 ) for monitoring movement of the actuator ( 8 ); and a control system ( 12 ) coupled to the feedback sensor ( 14 ) so as to receive information from the feedback sensor ( 14 ) concerning the movement of the actuator ( 8 ) and for controlling movement of the actuator ( 8 ) based on the information. The interrupter ( 4 ) further includes a memory ( 202 ) for storing a desired motion profile of the actuator ( 8 ); and a microprocessor ( 202 ) for comparing the movement of the actuator ( 8 ) with the desired motion profile and controlling movement of the actuator ( 8 ) based also on a comparison of the movement of the actuator ( 8 ) with the desired motion profile. The interrupter ( 4 ) further includes a sensor ( 204 ) for sensing a waveform of a voltage in a line to be interrupted and providing information concerning the voltage waveform to the control system ( 12 ); wherein the control system ( 12 ) controls the movement of the actuator ( 8 ) based also on the information concerning the voltage waveform.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation-in-part of U.S. patent application, Ser. No. 08/440,783, filed on May 15, 1995, now abondoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method and device for controlling electrical switchgear. More particularly, the invention relates to a method and device for controlling a switchgear utilizing a voice coil actuator to rapidly and positively open and close a current interrupter. 
     2. Description of Related Art 
     In a power distribution system, switchgear may be incorporated into the system for a number of reasons, such as to provide automatic protection in response to abnormal load conditions or to permit opening and closing of sections of the system. Various types of switchgear include a switch for deliberately opening and closing a power transmission line, such as a line to a capacitor bank; a fault interrupter for automatically opening a line upon the detection of a fault; and a recloser which, upon the detection of a fault, opens and closes rapidly a predetermined number of times until either the fault clears or the recloser locks in an open position. 
     Vacuum interrupters have been widely employed in the art because they provide fast, low energy arc interruption with long contact life, low mechanical stress and a high degree of operating safety. In a vacuum interrupter the contacts are sealed in a vacuum enclosure. One of the contacts is a moveable contact having an operating member extending through a vacuum seal in the enclosure. 
     SUMMARY AND OBJECTS 
     One of the objects of the present invention is to provide a switchgear actuator mechanism and control therefore that minimizes arcing and generated transients during opening and closing. 
     Another object of the present invention is to provide a switchgear actuator mechanism and control therefore that provides accurate monitoring of the system. 
     Another object of the present invention is to provide a switchgear actuator mechanism capable of a range of motion profiles, thereby eliminating the need for many types of mechanical systems. 
     Another object of the present invention is to provide a switchgear actuator mechanism capable of being controlled by any commercially available motor control circuitry or dedicated motion control circuitry. 
     Still another object of the present invention is to provide a switchgear actuator mechanism capable of procuring speeds and forces not readily achievable with prior art mechanical systems. 
     Still another object of the present invention is to provide an improved synchronously operating switchgear that results in a significant reduction in transients generated during the switching operation. 
     Generally, switchgear incorporating vacuum interrupters have utilized various spring loaded mechanisms which are connected to an operating member to positively open or close the interrupter contacts. One such device which is commonly used is the simple toggle linkage. The primary function of these mechanisms is to minimize arcing by very rapidly driving the contacts into their open or closed positions. Various applications may require the use of a number of spring loaded mechanisms with associated latches and linkages. 
     In order to prime these mechanical systems, either by compression or extension of the drive spring, an actuator is normally provided. These actuators can include, but are not limited to, solenoids, motors or hydraulic devices. In comparison to the inherent speed requirements of the interrupter to effectively interrupt current, these actuators are relatively slow with poor response times. For this reason they are not normally used to directly drive the interrupter contacts but are utilized to prime the fast acting spring mechanisms. The prime disadvantage of this system is that the spring driven operation does not lend itself to being easily controllable and it requires considerable engineering effort to finely adjust the mechanism&#39;s performance. 
     In practice, this means that many different mechanisms must be designed to accommodate the different operating requirements for switches, fault interrupters and reclosers and within each one of these switchgear classes, there are different mechanisms required depending on the application, including voltage and current requirements. 
     Furthermore, in view of the high voltages that are typically used in power applications, rapid and accurate movement of the interrupter contacts is desired to minimize arcing between the contacts and the generation of transients. Depending upon the application, whether it is capacitor bank switching or fault interruption, it can be determined by those skilled in the art when the most advantageous time to open or close the interrupter contact occurs. This optimum time correlates to a precise point on the voltage or current wave where current interruption or contact make would produce minimal arcing and transients. Since conventional spring driven mechanisms do not lend themselves to this degree of fine control, this invention offers a viable means to achieve point-on-wave or synchronous switching. Such synchronous operation of the interrupter is beneficial both in terms of the reduced wear on the interrupter contacts and the significant reduction in general transients experienced by the power system downstream of the switchgear unit. 
     A further feature of a controlled, synchronously operating switchgear unit is that the velocity at which the contacts close can be controlled. In conventional systems, the contacts are driven together in an uncontrolled fashion at very high velocity and it is possible that the contracts will bounce open a number of times before coming to rest. This bounce phenomenon is undesirable because the ensuing arcing can soften the contacts and create strong welds when the contacts finally mate. 
     In accordance with the present invention, a current interrupter includes a current interrupting device having at least one movable contact; an actuator coupled to the movable contact of the current interrupter; a feedback sensor for monitoring movement of the actuator; and a control system coupled to the feedback sensor so as to receive information from the feedback sensor concerning the movement of the actuator and for controlling movement of the actuator based on the information. The interrupter further includes a memory for storing a desired motion profile of the actuator; and a microprocessor for comparing the movement of the actuator with the desired motion profile and controlling movement of the actuator based also on a comparison of the movement of the actuator with the desired motion profile. The interrupter further includes a sensor for sensing a waveform of a voltage or current in a line to be switched and providing information concerning the waveform to the control system; wherein the control system controls the movement of the actuator based also on the information concerning the waveform. 
     The foregoing features and advantages of the present invention will be apparent from the following more particular description of the invention. The accompany drawings, listed hereinbelow, are useful in explaining the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the text which follows, the invention is explained with reference to illustrative embodiments, in which: 
     FIG. 1 shows a schematic diagram of switchgear employing a voice coil actuator; 
     FIG. 2 shows a cross-sectional view of one embodiment of a switchgear; 
     FIG. 3 is a cross-sectional view of the vacuum module shown in FIG. 2; 
     FIG. 4 shows an enlarged view of the operating mechanism of the embodiment displayed in FIG. 2; 
     FIG. 5 shows an exploded view of the primary components of the operating mechanism; 
     FIG. 6 shows a graph illustrating the system voltage vs. time and the dielectric descent of the interrupter; 
     FIG. 7 is a schematic view of a circuit that may be used with the present invention; 
     FIG. 8 is a graph illustrating a motion profile that may ,be used with the present invention; 
     FIG. 9 is an illustration of a voice coil actuator that may be used with the present invention; 
     FIG. 10 is a view of a latching mechanism that may be used with the present invention; 
     FIG. 11 is a view of a contact pressure spring mechanism that may be used with the present invention; 
     FIG. 12 is a graph illustrating the synchronous timing of an opening operation of a capacitor switch. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     For a better understanding of the invention, reference may be made to the following detailed description taken in conjunction with the accompanying drawings, wherein preferred exemplary embodiments of the present invention are illustrated and described. Each reference number is consistent throughout all of the drawings. 
     In FIG. 1, an incoming power line  2  is coupled in series with a current interrupter  4 , thereby allowing the current interrupter  4  to open the line. The line  2  may be opened upon a predetermined command or, in the case of a fault interrupter, if a fault exceeds a predetermined threshold level. One of the contacts of the current interrupter  4  is connected to one end of an operating rod  6 . The other end of the operating rod  6  is operatively coupled to an actuator, such as a voice coil actuator  8 . The voice coil actuator  8  directly acts upon the operating rod  6  in order to open or close the contacts of the current interrupter  4 . 
     The voice coil actuator  8  is a direct drive, limited motion device that uses a magnetic field and a coil winding  10 , to produce a force proportional to the current applied to the coil. The electromechanical conversion of the voice coil actuator  8  is governed by the Lorentz Force Principle, which states that if a current-carrying conductor is placed in a magnetic field, a force will act upon it. The magnitude of the force is determined by the equation: 
     
       
         F=kBLIN 
       
     
     where F equals force, k is a constant, B is the magnetic flux density, L is the length of the conductor, I is the current in the conductor, and N is the number of turns of the conductor. 
     The current passing through the voice coil winding  10  is controlled by a control mechanism  12 . Any commercially available control mechanism  12  could be utilized. For example, suitable control mechanisms  12  include: single loop controllers, programmable logic controllers, or distributed control systems. The control mechanism  12  may be coupled to a feedback device  14 , which provides input regarding the position of the operating rod  6 . 
     The control mechanism  12  may also be coupled to a latching device  16 . When instructed to secure the operating rod  6  by the control mechanism  12 , the latching device  16  fastens the operating rod  6  in its current position. In an alternative device, the latching mechanism  16  may be a permanent magnet or mechanical latch that is not coupled to the control device  12 . 
     In FIG. 2, a cross-sectional view of one of the embodiments of the invention is shown. A one piece, elongated, solidly insulated encapsulation  18  encloses the operating rod  6  and the current interrupter  4 . The encapsulation  18  may be formed out of ceramic, porcelain, any suitable epoxy, or any other appropriate solid insulating material. A line side high voltage electrical terminal  22  and a load side high voltage electrical terminal  20  protrude through the solidly insulated enclosure  18 , and are coupled to the current interrupter  4 . The high voltage electrical terminals  20  and  22  are diametrically disposed,  180  degrees apart, and are parallel with respect to one another. The encapsulation  18  provides both the solid insulation between the high voltage electrical terminals  20  and  22  and the solid insulation between each high voltage electrical terminal  20  and  22  and electrical ground (not shown). 
     The current interrupter  4  includes a vacuum module or bottle  24 , shown in cross section in FIG. 3, with a pair of switch contacts  71 ,  72  disposed within the vacuum module  24 . The vacuum module  24  provides a housing and an evacuated environment for the operation of the pair of switch contacts. The module  24  is usually constructed from an elongated, generally tubular, evacuated, ceramic casing  73 , preferably formed from alumina. One of the switch contacts  71  is movable, and the other switch contact  72  is stationary or fixed. 
     A special fitting  76  is attached to the stem of the stationary contact  72 , permitting the associated high voltage electrical terminal  22  to exit at a 90° angle. 
     The movable switch contact  71  is fastened to the uppermost, longitudinal end of the operating rod  6 . One method of fastening is to use a stud  32  threaded into a tapped connection  74  in the moving stem  75  of the movable contact  71 . When the switch contacts are in the closed position as shown, a low resistance or short circuit electrical path is created between the high voltage electrical terminals  20  and  22 . The current interrupter  4  further includes a current exchange assembly and an interface  26  between the vacuum module  24  and the current exchange assembly. The current exchange assembly contains a moving piston  28  and a fixed outer housing  30 . In this embodiment, the operating rod  6  is made from an electrically insulated material. 
     The other end of the operating rod  6  is secured to a flange  34  on the voice coil actuator  8  by a rigid pin  36 . The pin  36  which retains the foregoing components in position, can be secured by any suitable means, such as a pair of retaining rings. A recirculating linear ball bearing  38  and split rings  40 , which hold the ball bearing, provide smooth movement of the operating rod  6 . The voice coil winding  10  is disposed between the outer body of the voice coil actuator  8  and the flange  34 . Side flanges  42  are attached to the outer body of the voice coil actuator  8 , and connect to side brackets  44 , thereby securely fastening the voice coil actuator  8  to a protective case  46 . The protective case  46  is attached to a lid  50  for the protective case  46  via housing flanges  48 , and the protective case lid  50  is connected to the solid insulation enclosure  18  via lid flanges  52 . Just as the solid insulated encapsulation  18 , the protective case  46  is also formed out of ceramic, porcelain, any suitable epoxy, or any other appropriate solid insulating material. 
     In this embodiment the feedback device  14  is a position sensor, such as a linear potentiometer  14 . The linear potentiometer  14  can be made from a three-terminal rheostat or a resistor with one or more adjustable sliding contacts, thereby functioning as an adjustable voltage divider. The linear potentiometer  14  provides information regarding the position of the operating rod  6  to the control mechanism  12 , which controls the voice coil. actuator  8 . Alternatively, the feedback device  14  may be an optical encoder. 
     The latching device  16  is intended to secure the operating rod  6 . The latching device may be a controllable device, such as an electromagnet, or a simple mechanical or permanent magnet latch including: a latching magnet  54 , a spacer  56  made from nonferrous material, a bolt  58  securing the latching magnet  54  to the protective case lid  50 , a latch plate  60  made from steel or iron, and a latch plate pin  62  securing the latch plate  60  to the operating rod  6 . 
     In order to more fully understand the invention, reference may be had to FIGS. 4 and 5. FIG. 4 shows an enlarged view of the operating mechanism of the preferred embodiment displayed in FIG. 2, and FIG. 5 shows an exploded view of the primary components of the operating mechanism. 
     Details concerning the control mechanism of the present invention will now be described. 
     FIG. 6 illustrates a voltage signal  100  plotted on a graph comparing the voltage level v(t) versus time t. In a 60 Hz application, each half cycle is ideally 8.33 ms. However, actual cycles may vary due to harmonics or assymetric conditions so that a given half cycle may be greater than or less than 8.33 ms. 
     In order to minimize arcing and the generation of transients in a capacitor switch application, the contacts of the interrupter are ideally closed instantaneously at the null points when v(t) equals zero. See point A in FIG.  6 . However, since the contacts cannot close instantaneously, the timing of the initiation of the opening and closing sequences should be carefully controlled in order to minimize transients and arcing. 
     A preferred embodiment of a control circuit  200  for use with the present invention is illustrated in FIG.  7 . At the heart of the control circuit  200  is a microprocessor  202  that is suitable for use in a broad temperature range. 
     The voltage waveform of the power line being controlled by the interrupter  4  is analyzed with a voltage waveform analyzer  204 , a phase lock loop circuit  206 , and a V zero  crossing detection circuit  208 . Information concerning the voltage waveform of the line to be interrupted, including the timing of null points A wherein the voltage v(t) is zero, is input to the microprocessor  202 . Alternatively, a voltage waveform analyzer  204  could be used that measures the voltage waveform directly off the line without the phase lock loop circuit  206 . 
     Open and close commands are input to the microprocessor  202  via inputs  210  and  212 , respectively. The open and close commands may be created manually, may be initiated at preset times by a clock, may be initiated by an external control, or may be triggered by the detection of a fault, depending on the particular application of the interrupter  4 . 
     A reset signal  214  may be input to the microprocessor  202  to manually reset the microprocessor  202  when necessary. For example, if the interrupter  4  is manually manipulated, the microprocessor  202  may not be set to the current status of the interrupter  4 . In such a situation, the microprocessor  202  should be reset. 
     Status indicators may be provided to indicate various conditions of the circuit  200  or the interrupter  4 . Such indicators may include a maintenance light  216  to indicate when maintenance is required, a power on light  218 , a switch open indicator  220 , a switch closed indicator  222 , and a counter  224  that may be used to count cycles or operations of the system. 
     A preferred embodiment of the present invention may include two control systems. A first control system is conventional, and thus not disclosed herein in detail, and determines when the line controlled by the interrupter  4  is to be opened or closed. The first control system may include a fault detector or a timer for interrupting the line upon the detection of a fault, or at a predetermined time. 
     Alternatively, an open or close command may be input directly to the system. The open and close commands, whether originating from the first control system or manually, are input to the microprocessor  202  at inputs  210  and  212 , respectively. 
     The second control system  200 , illustrated in FIG. 7, analyzes the voltage waveform of the line and determines the best time for initiating opening and closing the interrupter  4  in order to minimize transients and arcing. 
     Each interrupter  4  has a dielectric strength that defines the likelihood of an arc jumping from one contact to another. The dielectric strength depends upon a number of factors including the medium inside the interrupter  4  and the distance between the contacts  71 ,  72 . FIG. 6 illustrates the changing or descent of the dielectric strength between the contacts  71 ,  72  versus time as the distance between the contacts closes. See line C in FIG.  6 . Ideally, the dielectric strength between the contacts would be infinite until the exact moment of closing of the contacts  71 ,  72 . See line B in FIG.  6 . In reality, the dielectric slopes downward, reducing quickly as the contacts approach each other. See line C in FIG.  6 . If the slope of the dielectric descent is sufficiently high, and the dielectric strength remains greater than the voltage of the waveform, the generation of arcing and transients is eliminated or significantly reduced. 
     Another factor to be considered during the operation of an interrupter is the relative velocity between the contacts upon opening and closing. If the contacts are moving slowly, the slope of the dielectric descent will be low, and arcing will likely occur. Conversely, if the contacts are moving too quickly, especially upon closing, the contacts will likely bounce off of each other, causing unnecessary arcing and transients. Accordingly, a unique ideal motion profile may exist for each application of an interrupter. FIG. 8 illustrates an example of a motion profile, wherein the abscissa represents the location of the moving contact  71  and the ordinate represents the velocity at which the contact  71  is moving. Point  0  on the abscissa represents the starting or maximum open position of the contact  71 , and point x represents the closed position, wherein the contact  71  is touching the stationary contact  72 . At point  0 , when the close command is initiated, the velocity is zero. The velocity is increased as quickly a possible to a maximum velocity V max . The velocity remains at V max  for as long as possible, but is then reduced as the point of contact x approaches in order to minimize bounce. 
     During an opening sequence, the motion profile is also important to prevent the occurrence of restrikes or re-ignitions shortly after opening. If the contacts separate at too slow a speed, or at a time when the voltage level is too high, excessive arcing may occur. Desired motion profiles for opening and closing sequences can be determined by those of skill in the art and preprogrammed into the circuit  200 . 
     Turning attention to FIG. 12, the timing of the opening operation in a capacitor switching application may be better understood. FIG. 12 relates to the opening sequence of a system that includes a capacitor bank. Line  4  indicates the voltage level of the fully charged capacitors. The switch begins to open at point  2 , and an arc forms. However, at this point, the current is decaying and the arc is extinguished at current zero, point  3 . The system voltage is now at its peak, but the voltage across the contacts is small because of the charge on the capacitor bank, which approximates the peak system voltage. As the system voltage begins to drop, the voltage on the capacitor bank stays high, resulting in an increase in the voltage across the contacts. The contacts should part with enough acceleration so that the dielectric rises faster than the escalating voltage between the contacts in order to avoid restrikes and re-ignitions. 
     The motion control function can be achieved by means of software loaded into the microprocessor/microcontroller or by the addition of dedicated motion control chips which interface with the microprocessor. A particular motion profile is programmed into a memory, which may be a separate EEPROM chip in an external motion control circuit  226 , or onboard memory on the microprocessor or microcontroller. The motion control circuit  226  is connected to the feedback device (encoder)  14  and to a pulse width modulation (PWM) circuit  228 . The PWM  228  controls the current that is applied to the voice coil actuator  8 . Since the force driving the voice coil actuator  8  is proportional to the current supplied to the voice coil actuator  8 , the velocity of the actuator  6  (and the moving contact  71 ) is controlled by the PWM  228 . As a result, the voice coil actuator  8  is controlled by a closed loop feedback system that includes the position encoder  14  that sends a position signal of the actuator  8  to the motion control circuit  226 . The motion control circuit  226  compares the actual position of the actuator  8  to the ideal motion profile preprogrammed into the motion control circuit  226 . Based on the comparison of the actual position to the ideal motion profile, the voice coil actuator  8  is controlled by the PWM so that its motion closely approximates the ideal intended motion. 
     Control of the actuator is further modified by the circuits  204 ,  206 ,  208  that monitor that actual voltage waveform of the line to be interrupted. For example, for a particular application, it may be determined that the contacts  71 ,  72  should open or close within 1 ms of the zero crossing A (FIG. 6) of the voltage signal v(t). The ideal motion profile preprogrammed into the motion control circuit  226  includes the total reaction and travel time of the actuator  8  from the time an initiating signal is sent to the time the contacts  71 ,  72  close. If the ideal motion profile indicates that the reaction and travel time for the contacts to close after the initiating signal is 7 ms, the microprocessor analyzes the actual voltage waveform of the line to be interrupted and determines a specific time between null points at which the initiating signal should be sent. The circuits  204 ,  206 ,  208  first establish the actual cycle period and the resulting length of time between zero crossings. The control circuit  200  then initiates operation of the voice coil actuator  8  at a time after a zero crossing that is equal to the actual time between null crossings minus the reaction and travel time of the actuator  8 . Accordingly, if the actual voltage waveform indicates that there are 8.3 ms between zero crossings and the reaction and travel time is 7 ms, the opening sequence is initiated at 1.3 ms after a zero crossing. In an alternative embodiment, the system may assume that the actual time between zero crossings is 8.33 ms, and the initiation is calculated based on that assumption. 
     In some embodiments of the present invention, a plurality of motion profiles can be preprogrammed into the circuit  200 , and the appropriate motion profile can be selected by an input from the operator. 
     Once the sequence is initiated, the actual motion of the actuator  8  is monitored by the encoder  14  and compared against the ideal motion profile. The current applied to the actuator  8  is adjusted by the PWM  228  based on the comparison of the actual movement of the actuator  8  to the ideal motion profile. 
     FIG. 9 illustrates another embodiment of a voice coil actuator  308  that may be used with any of the embodiments of the present invention. The voice coil actuator  308  includes a ring shaped magnet  310 , which is preferably a 4 MGO ceramic magnet. The magnet  310  is housed with a bottom pole piece  312  and a top pole piece  314 . These pole pieces are formed from ferromagnetic materials, such as iron or steel. The pole pieces  312 ,  314  include a central aperture  316  through which an operating rod  318  extends. The operating rod  318  is supported in the pole pieces  312 ,  314  with self-lubricating polymer bearings  320 , such as IGUS™ bearings  320 . 
     An aluminum plate  328  is fixed to the rod  318 . At a peripheral edge of the plate  328 , a coil  330  extends from the plate  328  into an air groove  332  formed between the bottom pole piece  312  and the magnet  310 . The coil  330  may be formed from flattened wire so as to maximize the number of turns that will fit within the air groove  332 . 
     The actuator  308  may be driven by a 24 volt battery, or any other suitable power source, including an autoranging AC to DC converter. 
     In order to latch the device in a particular position, the operating rod  318  may include a groove  320  within which is located a ball  322 . See FIG. 10. A spring  324  and cap  326  urge the ball  322  into the groove  320  to retain the rod  318  in a fixed position. The rod  318  may be freed from the ball  322  upon the application of a force, the level of which depends on the strength of the spring  324 . 
     In order to ensure a good connection between the contacts  71 ,  72 , a spring  340 , or other force, may be applied to the rod  6  (or  318 ) to urge the contact  71  against the contact  72  with a predetermined force, such as 60-100 pounds. The spring may be compressed by the action of the actuator. Turning attention to FIG. 11, the operating rod  6 ,  318  may include a flange  342  that provides a surface against which the spring  340  presses. Another abutment surface  344  may be provided to support the opposite end of the spring  340 . 
     The spring  340  provides the additional benefit of maintaining an adequate force between the two contacts  71 ,  72 . For example, after repeated operations, arcing may cause the contacts to wear. Because of the spring force, the two contacts are urged against each other, even if they have become worn. In addition, the application of the force causes a reduction in the electrical resistance between the contacts in the closed position, thereby reducing heat losses. 
     If the contacts become worn, the operating rod  6 ,  318  will move a greater distance in order to accommodate the wear. Since the position sensor  14  senses the distance moved by the operating rod  6 ,  318 , the system can be programmed to illuminate the maintenance signal  216 , or some other indicator, to indicate that excessive wear has occurred on the contacts  71 ,  72 . The system can also modify its motion profile to allow for such incremental increases in stroke. 
     Although only preferred embodiments are specifically illustrated and described herein, it will be appreciated that many modifications and variations of the present invention are possible in light of the above teachings and within the purview of the appended claims without departing from the spirit and intended scope of the invention.