Abstract:
Methods, apparatus, and systems are provided for a selectivity device for a circuit breaker. The invention includes a modular assembly adapted to be coupled to a load conductor assembly. The modular assembly includes a tripping plunger including a cam surface adapted to interact with a breaker tripping mechanism; an armature coupled to the tripping plunger; a yoke adapted to generate a magnetic field in response to an electric current in the load conductor assembly and disposed to apply a magnetic force proportionate to the electric current on the armature; and a spring assembly adapted to counter-balance the magnetic force up to a predefined tripping current in the load conductor assembly. Numerous additional aspects are disclosed.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to circuit breakers, and more particularly is directed to methods and apparatus for an improved magnetic armature selective tripping device of a circuit breaker. 
     BACKGROUND OF THE INVENTION 
     Modern electrical distribution circuits are fed from a step down transformer which takes in higher transmission voltages (e.g., 1000s of volts) used for transmission and converts the voltages to lower, more usable voltages. As a general rule, distribution networks are designed with higher amperage and voltage rated circuit protection devices closer to the transformer and lower amperage and voltage rated protection devices further away from the transformer. An example of a simple power distribution network  100  is illustrated in  FIG. 1 . In the example, a step down transformer  102  is coupled to a “downstream” 2000 amp circuit breaker  104  which protects three downstream 800 amp circuit breakers  106 ,  108 ,  110  in parallel on three branches. One of the 800 amp circuit breakers  106  protects three downstream 250 amp circuit breakers  112 ,  114 , and  116  in parallel on three sub-branches. Finally, one of the 250 amp circuit breakers  116  protects three downstream 160 amp circuit breakers  118 ,  120 , and  122  in parallel on three sub-sub-branches. With each downstream step, the amount of amperage protection required is reduced. When an electrical fault event occurs, it is desirable to maintain operation of as much service as possible to the remaining parts of the network  100 . When the network  100  is able to isolate the fault event and maintain service to the rest of the network  100 , the application is known as “selective.” When power is lost to an unaffected part of the network  100  (i.e., the fault event only occurs on a sub-branch but causes all the upstream breakers to trip) the application is “non-selective.” 
       FIGS. 2A and 2B  more clearly illustrate the difference between a non-selective and a selective application, respectively.  FIG. 2A  depicts a non-selective network  200 A in which a fault event  202  has caused circuit breakers  122 ,  116 , and  106  to all trip which results in an outage  204  that leaves most of the network  200 A without electrical service. In contrast,  FIG. 2B  depicts a selective network  200 B in which a fault event  202  has caused only circuit breaker  122 ′ to trip which results in an outage  204 ′ that only leaves a relatively small portion of the network  200 B without electrical service. 
     In order to be selective, the circuit protection devices (e.g., circuit breakers) must identify where the electrical fault has occurred and act accordingly as fast as possible. This means that an upstream breaker must be able to distinguish between a fault that occurs nearby, and one that occurs downstream of another breaker. A selective power distribution system means lower downtime costs for the electrical service customer and a more stable distribution network even when problems occur. In the past, this has been achieved using tripping characteristic curves. 
     An example of a tripping characteristic curve  300  used for selective coordination of circuit protection devices is illustrated  FIG. 3 . A first curve  302  represents the behavior of a downstream circuit breaker and a second curve  304  represents the behavior of a circuit breaker located immediately upstream from the downstream circuit breaker. Using tripping characteristic curves  300  to implement selectivity has many limitations which are mainly governed by the physical attributes of the contact system within the circuit breakers. 
     Some manufacturers have attempted to add extra devices inside the circuit breakers to increase the ability of the breaker to distinguish where an electrical fault has occurred in the dynamic breaker behavior region  306  of the tripping characteristic curve. These extra devices are typically designed as integrated components of the breaker and for the specific physical attributes of the contact system within a particular circuit breaker. These devices are not applicable or re-useable for different circuit breakers. For example, a prior art circuit breaker that includes an integrally formed, non-removable, and non-modular selectivity device is the Model Tmax T6 circuit breaker manufactured by ABB Asea Brown Boveri Ltd of Zurich, Switzerland. This example breaker includes a custom designed, integral selectivity device developed based on tripping characteristic curves. Thus, what is needed are methods and apparatus for an improved magnetic armature selective tripping device that is modular and can be easily configured for use in different circuit breakers. 
     SUMMARY OF THE INVENTION 
     Inventive methods and apparatus are provided for a selectivity device for a circuit breaker. The apparatus includes a modular assembly adapted to be coupled to a load conductor assembly. The modular assembly includes a tripping plunger including a cam surface adapted to interact with a breaker tripping mechanism; an armature coupled to the tripping plunger; a yoke adapted to generate a magnetic field in response to an electric current in the load conductor assembly and disposed to apply a magnetic force proportionate to the electric current on the armature; and a spring assembly adapted to counter-balance the magnetic force up to a predefined tripping current in the load conductor assembly. 
     In some other embodiments, a circuit breaker including a selectivity device is provided. The circuit breaker includes a housing enclosing the circuit breaker and the selectivity device. The selectivity device includes a modular assembly adapted to be coupled to a load conductor assembly of the circuit breaker. The modular assembly includes a tripping plunger including a cam surface adapted to interact with a breaker tripping mechanism; an armature coupled to the tripping plunger; a yoke adapted to generate a magnetic field in response to an electric current in the load conductor assembly and disposed to apply a magnetic force proportionate to the electric current on the armature; and a spring assembly adapted to counter-balance the magnetic force up to a predefined tripping current in the load conductor assembly. 
     In yet other embodiments, a method of selectively tripping a circuit breaker is provided. The method includes selecting a cam surface having a shape selected to configure tripping characteristics of a selectivity device for the circuit breaker; installing the selectivity device with the selected cam surface on a load conductor assembly of the circuit breaker; and monitoring current flowing through the load conductor assembly so that the selectivity device reacts to a current flow at or over a predefined amount of current. 
     Numerous other aspects are provided. Other features and aspects of the present invention will become more fully apparent from the following detailed description, the appended claims, and the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram depicting an example of a simplified power distribution network. 
         FIG. 2A  is a block diagram depicting an example of a non-selective power distribution network. 
         FIG. 2B  is a block diagram depicting an example of a selective power distribution network according to embodiments of the present invention. 
         FIG. 3  is a graph depicting an example of tripping characteristics curves. 
         FIG. 4  is a perspective drawing of an example of a selectivity device of a circuit breaker according to embodiments of the present invention. 
         FIG. 5  is a perspective drawing of an example of a load conductor assembly of a circuit breaker according to embodiments of the present invention. 
         FIG. 6  is a perspective drawing of an alternative example of a selectivity device of a circuit breaker according to embodiments of the present invention. 
         FIGS. 7A and 7B  are side view drawings illustrating operation of an example selectivity device of a circuit breaker according to embodiments of the present invention. 
         FIGS. 8A and 8B  are perspective view and side view drawings respectively of a tripping plunger of an example selectivity device of a circuit breaker according to embodiments of the present invention. 
         FIG. 9  is a graph depicting how the force available for tripping a breaker using the selectivity device of the present invention can be calculated according to embodiments of the present invention. 
         FIG. 10  is a flow chart depicting an example method of tripping a circuit breaker using the using the selectivity device of the present invention according to embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention provides improved methods and apparatus for a modular, configurable, magnetic armature selective tripping device (i.e., a selectivity device) inside a molded case circuit breaker (MCCB) that may be used to improve the selective coordination between circuit protection devices in a power distribution network. The inventive selectivity device includes a tunable cam profile, a “U” shaped yoke, and springs with a relatively low spring rate that provide an increased amount of energy available for tripping the MCCB. The invention facilitates improved selectivity particularly in devices with higher levels of mechanism energy. Further, the inventive selectivity device is modular and can be configured or tuned to be attached to various different load conductor/current transformer assemblies. 
     The present invention helps to provide selectivity to distribution networks by enabling the MCCB to distinguish more accurately when an electrical fault event occurs proximate to the breaker and when the issue is further downstream in the network. A selectivity device embodied as a tripping device, which is magnetically activated when a specific amount of current passes through the MCCB, is used in the present invention. The specific amount of current that triggers the tripping device corresponds to a level of current which could only exist when the electrical fault is directly downstream of the MCCB where the selectivity device is installed. Instead of being based solely on specific physical attributes of the contact system of the breaker, the tripping characteristics of the selectivity device of the present invention are governed by the selected shape of a cam on the tripping plunger of the selectivity device. 
     In other words, a novel feature of the tripping device of the present invention is that the cam-tripping surface is tunable. This means some of the tripping characteristics can be changed by simply changing the shape of one surface. Another advantage of the present invention is its open nature. This allows selection of larger diameter springs which have a flatter spring rate. Flatter spring rates ensure that the maximum amount of energy possible is available to be used to trip the MCCB. This feature facilitates increased selectivity performance on larger breakers which have higher required tripping energy while being space constrained. Finally, the device is mounted in such a manner as to be as modular (e.g., removable as a unit, configurable for different applications/breakers) as possible, using a current carrying structure that is unique to electronic trip unit (ETU) style breakers. 
     Turing now to  FIG. 4 , an example embodiment of the selectivity device  400  of the present invention is shown. In some embodiments, the selectivity device  400  includes a selectivity mounting base  402  that is adapted to be easily attached to existing circuit breakers. The base  402  supports an assembly that includes a tripping plunger  404 , a tripping plunger base  406 , two armature plates  408 , two extension springs  410  and a yoke  412 . The selectivity device  400  is mounted to a load conductor assembly  414  which is not part of the selectivity device  400 . 
     Note that the existing structure of the load conductor assembly  414  need not be modified to support the selectivity device  400  of the present invention. Thus, the selectivity device  400  is a modular assembly that can be added to or removed from an existing circuit breaker. A detailed perspective view of the load conductor assembly  414 , without a selectivity device  400  mounted, is depicted in  FIG. 5 . The load conductor assembly  414  includes a braid terminal  502 , a load side conductor  504  and a current transformer  506  disposed between. The entire selectivity device  400  may be mounted securely and reliably to the load conductor assembly  414  with only two fasteners (e.g., screws, bolts, etc.) to integrate with the current transformer  506 . This arrangement allows for maximum flexibility and modularity of the design. The selectivity device  400  may be installed as an optional enhancement to an electronic trip unit (ETU) based circuit breaker or the selectivity device  400  may be easily omitted for a cost-reduced breaker when selectivity is not required. Known prior-art selectivity devices are integrally connected to and form part of the conducting path making modularity impracticable. 
     Turning now to  FIG. 6 , an alternative embodiment of a selectivity device  600  according to the present invention is depicted. The alternative embodiment of the device  600  is adapted to be used in a center pole position of a circuit breaker. The structure and function of the alternative device  600  is similar to the device  400  depicted in  FIG. 4  except the tripping plunger  602  includes a lateral offset member  604  that locates the cam surface off center of the load conductor assembly  414 . This arrangement facilitates easy alignment of the cam surface with the tripping shaft (e.g., see  FIG. 7A, 706 ) of the circuit breaker. Otherwise, as with the device  400  depicted in  FIG. 4 , the alternative device  600  includes a selectivity mounting base  402  that is adapted to be easily attached to existing load conductor assembly  414  of existing circuit breakers. Also as with the device  400  depicted in  FIG. 4 , the base  402  supports an assembly that includes the modified tripping plunger  602 , a tripping plunger base  406 , two armature plates  408 , two extension springs  410  and a yoke  412 . 
     Operation of the selectivity device  400  (or  600 ) is illustrated in  FIGS. 7A and 7B . As current (indicated by small arrows  702 ) passes through the load conductor assembly  414 , a magnetic field (indicated by the large arrow  704 ) is generated in yoke  412 . The magnetic field  704  forces (i.e., pulls) the metal armature plates  408  downward against the counter-force of the springs  410  toward the yoke  412 . Since the armature plates  408  are attached to the tripping plunger base  406  and tripping plunger  404  (or  602 ), a downward motion results. The downward motion forces an interaction of the cam surface of the tripping plunger  404  (or  602 ) on the tripping shaft  706  of the breaker mechanism  710 . The tripping shaft  706  is rotated (as indicated by arced arrow  712 ) by the interaction which trips the breaker as shown in  FIG. 7B . 
     Turning now to  FIGS. 8A and 8B , the tripping plunger  404  is illustrated in more detail in a perspective drawing and a side view drawing, respectively. The cam head  802  includes a cam surface  804  that is adapted to interact with the tripping shaft  706  ( FIGS. 7A &amp; 7B ) of the breaker mechanism  710  ( FIGS. 7A &amp; 7B ). The cam head  802  is mounted on the tripping plunger shaft  806  which includes one or more guide rails  808  to keep the tripping plunger  404  aligned as it moves up and down in matting grooves of the tripping plunger base  406  ( FIG. 4 ). The tripping plunger  404  also includes a flange  810  which facilitates coupling to the armature plates  408  ( FIG. 4 ). 
     The cam surface  804  of the tripping plunger  404  allows the selectivity device  400  to be tuned or configured for different circuit breakers and different triggering currents. By changing the shape of the cam surface  806 , the amount of force required to trip the breaker can be adjusted. In some embodiments, a variety of cam heads  802  may be manufactured with different cam surfaces  804  and the desired cam surface  804  may be selected simply by choosing which cam head  802  is attached to the tripping plunger  404 . Thus, dramatic variations in performance can be achieved with minimal impact to the parts. 
     Referring back to  FIGS. 7A and 7B , it is apparent that the slope/shape of the cam surface  804  affects how quickly the selectivity device  400  will trip the breaker as the magnetic force  704  pulls down the armature plates  408  and consequently, the tripping plunger  404 . In other words, if the slope/shape of the cam surface  804  is changed to be more vertical, less force is required to pull the tripping plunger  404  down but the time it takes to trip the breaker is increased. If the slope/shape of the cam surface  804  is changed to be more horizontal, more force is required to pull the tripping plunger  404  down but the time it takes to trip the breaker is decreased. In addition, the slope of the cam surface  804  can have a changing slope from the bottom to the top so that, for example, the initial force required to pull the tripping plunger  404  down is relatively low (e.g., a more vertical slope at the lower end of the cam surface) and the subsequent required force is higher but the breaker is tripped sooner (e.g., a more horizontal slope towards the upper end of the cam surface). Other shapes and performance tuning concerns may be used. In summary, the ability to tune or select the cam surface  804  afforded by the present invention changes the amount of energy required from the magnetic structure to trip the breaker and therefore, controllably and configurably allows changes to be made to the performance characteristics of the selectivity device  400 . 
     As with any mechanical system, there is a critical amount of energy that is required to achieve a desired result. In this case, the energy of concern is that which is required to trip the breaker at the tripping shaft  706 . Since the magnetic affect on the selectivity device  400  has limited margin in the amount of energy that can be provided to trip the breaker, it is desirable that the springs  410  used in the selectivity device  400  have as flat a spring rate as possible. The present invention allows for this flat rate by having an overall larger volume for the springs to occupy within the circuit breaker than prior art selectivity devices. This increased availability of volume allows for selection of springs  410  with larger diameters. Larger diameter springs  410  mean flatter spring rates. For example, in a selectivity device for a 1000 amp circuit breaker that is designed to trip at 35000 amps, springs with a diameter of approximately 5.5 mm to 8 mm, and a spring rate of approximately 2.5 N/mm to 0.7 N/mm may be used. In prior art selectivity devices, the springs are typically internal to the load bus and are thus much more constricted by available space. Therefore, the prior art devices typically result in less energy available to trip the breaker. 
     Another advantage of the increased available volume of the present invention, allows the use multiple springs rather than a single spring. When multiple springs are used, larger magnetic attraction forces between the armature plates  408  and the yoke  412  can be accommodated and therefore, the activation point of the selectivity device  410  can be higher. 
     Turning to  FIG. 9 , the relationship between the force available for tripping the breaker, the magnetic force of the yoke  412 , the spring rate of the springs  410 , the opening distance between the yoke  412  and armature plates  408 , and the friction loss from lateral magnetic forces is illustrated in a graph  900 . The relationship can be represented by the equation:
 
 F=F   A   −F   spr   −F   f  
 
where F represents the force available for tripping the breaker, F A  represents the vertical magnetic force at the desired “must trip” amperage, F spr  represents the calibrated spring force to be exerted at the desired “no trip” amperage, and F f  represents the friction loss from lateral magnetic forces. To clarify, F spr  is calibrated to be an amount of upward force exerted by the springs sufficient to prevent downward motion of the armature plates  408  at the desired “no trip” amperage. Referring the example graph of  FIG. 9 , curve  902  represents a plot of F A  at an example “must trip” amperage of 35 kA. Curve  904  represents a plot of F spr  at an example “no trip” amperage of 30 kA. Curve  906  represents a plot of F f . Line  908  represents a plot of the spring force calibrated to resist 30 kA at an opening of 12 mm between the yoke  412  and armature plates  408 . Finally, curve  910  represents the force available for tripping the circuit breaker calculated based on the above equation.
 
     Turning now to  FIG. 10 , an example method  1000  of using the selectivity device of the present invention to selectively trip a circuit breaker is depicted as a flow chart. In step  1002 , a cam surface  804  of a cam head  802  of a tripping plunger  404  of the selectivity device  400  is selected with the desired shape to adjust or configure the tripping characteristics of the selectivity device  400  for the circuit breaker. In step  1004 , the selectivity device including the selected cam surface  804  is installed on the load conductor assembly  414  of the circuit breaker and disposed to interact with the tripping shaft  706  of the breaker mechanism  710 . In step  1006 , the selectivity device  400 , which is monitoring the current flowing through the load conductor assembly  414 , reacts to current at or over the desired breaker trip amperage. By energizing the yoke  412  in response to the specific current to generate sufficient downward magnetic force to overcome the spring force on the armature plates, the device  400  pulls the tripping plunger  404  to cause the cam surface to interact with the tripping shaft  706  of the breaker mechanism  710  and thereby trip the breaker. 
     Accordingly, while the present invention has been disclosed in connection with the preferred embodiments thereof, it should be understood that other embodiments may fall within the spirit and scope of the invention, as defined by the following claims.