SYSTEM AND METHOD FOR POINT ON WAVE CLOSING OF A VACUUM INTERRUPTER

Abstract

A system and method for point on wave closing of a switching device, where the close initiation point is selected so that a minor current loop is generated that minimizes the accumulated current during the closing operation. The method includes determining bounce characteristic of contacts in the switching device during the closing operation, identifying available close initiation points on a voltage wave for a voltage across the contacts, selecting one or two of the close initiation points on the voltage wave that minimizes the accumulation of current during the closing operation, and closing the switching device using the selected close initiation point.

Claims

1. A system for closing contacts in a switching device during a closing operation, comprising: a sensor configured to detect bounce characteristics of the contacts during the closing operation; a waveform analyzer configured to identify available close initiation points on a voltage waveform across the contacts; a processor configured to select a close initiation point from the identified initiation points that minimizes accumulated current during the closing operation by generating a minor current loop; and a controller configured to initiate closure of the switching device at the selected close initiation point, wherein the processor selects the close initiation point based on pre-characterized contact bounce dynamics and actuator response time, and wherein the selected close initiation point is not constrained to a zero-crossing of the voltage waveform.

2. The system of claim 1, wherein the switching device comprises a vacuum interrupter having a movable contact and a fixed contact enclosed within a vacuum chamber.

3. The system of claim 2, wherein the vacuum interrupter is operated by a magnetic actuator comprising: a stator with an electrical coil; a plunger movable within the stator; and a spring assembly coupled to the plunger and the movable contact.

4. The system of claim 3, wherein the magnetic actuator includes a compliance spring configured to absorb contact bounce energy during closure and reduce contact welding.

5. The system of claim 3, wherein the magnetic actuator includes permanent magnets configured to hold the plunger in a latched position after closure.

6. The system of claim 3, wherein the processor selects the close initiation point based on a modeled response time of the magnetic actuator, including the time delay between coil energization and contact engagement.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] FIG. 1 is a cross-sectional type view of a switching device including a vacuum interrupter and a magnetic actuator;

[0012] FIG. 2 is a graph with time on the horizontal axis and voltage/current on the vertical axis showing a closing operation of the switching device that generates a symmetrical current wave;

[0013] FIG. 3 is a graph with time on the horizontal axis and voltage/current on the vertical axis showing a closing operation of the switching device that generates an asymmetrical current wave;

[0014] FIG. 4 is a graph with time on the horizontal axis and voltage/current on the vertical axis showing a closing operation of the switching device that generates a minor loop current wave that provides low accumulated current during the closing operation; and

[0015] FIG. 5 is a block diagram showing a process for selecting the point on wave closing of the switching device to provide low accumulated current during the closing operation to reduce contact arcing damage.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0016] The following discussion of the embodiments of the disclosure directed to a system and method for adjusting the point on wave closing of a vacuum interrupter to reduce contact arcing damage, where the close initiation point is selected so that a minor current loop is generated that minimizes the accumulated current during the closing operation, is merely exemplary in nature, and is in no way intended to limit the disclosure or its applications or uses. For example, the system and method have particular application for use in a switching device associated with transformers in a residential loop circuit. However, the system and method may have other applications.

[0017] FIG. 1 is a cross-sectional type view of a switching device 10 intended to represent any switching device suitable for the purposes discussed herein and has application as a switching device associated with a transformer in a residential loop circuit. The switching device 10 includes a vacuum interrupter 12 having a vacuum enclosure 14 defining a vacuum chamber 16, an upper fixed terminal 18 extending through the enclosure 14 and into the chamber 16 and having a contact 22 and a lower movable terminal 24 extending through the enclosure 14 and into the chamber 16 and having a contact 26, where a gap 28 is provided between the contacts 22 and 26 when the vacuum interrupter 12 is open. A bellows 30 allows the movable terminal 24 to move without affecting the vacuum integrity of the chamber 16. The movable terminal 24 is coupled to a drive rod 32.

[0018] The switching device 10 also includes an actuator 40 that controls the drive rod 32 through a coupling rod 60 to open and close the vacuum interrupter 12. The actuator 40 includes an annular latching plate 42 having a central opening 44 through which the coupling rod 60 extends. The actuator 40 also includes a stator 46 defining a central opening 48, where a magnetic plunger 50 having a top shoulder 52 is slidably positioned within the opening 48. A coil 56 is positioned against the stator 46 in the opening 48 and a series of permanent magnets 58 are positioned between the plate 42 and the stator 46. A cup member 62 is rigidly secured to the plunger 50 and an opening spring 64 is provided within the cup member 62 and is positioned against the stator 46. A stop member 66 including an annular flange 68 is provided within the plunger 50 and is rigidly attached to the coupling rod 42 through the opening 44 in the plunger 50. A compliance spring 70 is provided within the cup member 62 and is positioned against the flange 68, which pushes the flange 68 against the shoulder 52.

[0019] When the vacuum interrupter 12 is to be closed, the coil 56 is energized with current flow in one direction, which draws the plunger 50 and the cup member 62 upward against the bias of the opening spring 64. When the contacts 22 and 26 touch the compliance spring 70 compresses, the cup member 62 continues to move and the flange 68 stops moving so that when the vacuum interrupter 12 is completely closed the compliance spring 70 is more compressed than it was when the contacts 22 and 26 first touched. When fully closed, the plunger 50 is seated against the latching plate 42. The current to the coil 56 is turned off, and the permanent magnets 58 hold the plunger 50 in the closed position. When the vacuum interrupter 12 is to be opened, the coil 56 is energized in the opposite direction, which breaks the magnetic hold of the permanent magnets 58. The opening spring 64 and the compliance spring 70 provide the force to open the contacts 22 and 26 and may be used to help break the welding force on the contacts 22 and 26.

[0020] As discussed above, when the contact 26 engages the contact 22 during a closing operation of the vacuum interrupter 12, the contact 26 bounces off of the contact 22, usually twice, before the contacts 22 and 26 are fully engaged. For one common example of vacuum interrupter bounce, the contacts 22 and 26 touch for about 1 ms, then separate for about 2 ms, then touch for about 1 ms, and then separate for about 1 ms before they are fully engaged. At about a 1 mm gap between the contacts 22 and 26 and a typical fault current of 6.3 kA, arcing occurs at a 20 kV instantaneous voltage. It is be desirable to minimize the accumulation of current amp seconds (I.sup.2T) of the current wave over time caused by the arcing when the vacuum interrupter 12 is being closed to help prevent contact damage and welding. It is known to initiate a close operation of the vacuum interrupter 12 at a certain point on the voltage wave to achieve certain results based on modeling of the switching device 10 that considers many factors, such as contact bounce characteristics, actuator response time, etc. For one typical controller operational speed, the close operation of the vacuum interrupter 12 can be initiated at sixteen different voltage angles along the voltage wave.

[0021] FIG. 2 is a graph with time on the horizontal axis and voltage/current on the vertical axis, where graph line 80 is vacuum interrupter open and close position, where a high signal is the closed position and a low signal is the open position, graph line 82 is the current wave and graph line 84 is the voltage wave. The pulses 86 and 88 show the vacuum interrupter bounce during the closing operation of the vacuum interrupter 12 described above. For this example, the vacuum interrupter 12 is commanded closed at a time so that the first contact between the contacts 22 and 26 at point 90 is at or near a peak, here negative, of the voltage wave and the current wave is near a zero crossing, which provides a symmetrical current wave. For the bounce and fault current of 6.3 kA example given above, the accumulated current I.sup.2T during the time between points 92 and 94 when contact bounce is occurring during the closing operation of the vacuum interrupter 12 is 237 A.sup.2s.

[0022] FIG. 3 is the graph shown in FIG. 2, but where the close initiation point is selected so that the first time the contacts 22 and 26 engage at the point 90, the voltage angle is at or near a zero crossing, which causes an asymmetrical current wave. For the bounce and fault current of 6.3 kA example given above, the accumulated current I.sup.2T during the time between the points 92 and 94 when contact bounce is occurring during the closing operation of the vacuum interrupter 12 is 129 A.sup.2s.

[0023] This disclosure proposes choosing the voltage angle of when to initiate the closing operation of the vacuum interrupter 12 that minimizes the accumulated current I.sup.2T between the points 92 and 94 when contact bounce is occurring during the closing operation of the vacuum interrupter 12 when fault current is present. FIG. 4 is the graph shown in FIG. 2, but where the close initiation point on the voltage wave 84 is selected so that a minor current loop of the current wave 82 is generated between the points 92 and 94 that minimizes the accumulated current I.sup.2T. For the best closing angle and the bounce and fault current of 6.3 kA example given above, the accumulated current I.sup.2T during the time between the points 92 and 94 when contact bounce is occurring during the closing operation of the vacuum interrupter 12 is 24 A.sup.2s. This shows that for a given vacuum interrupter bounce, there is a closing angle that results in a much lower accumulated current I.sup.2T than closing at the peak of the voltage to get a symmetrical wave as shown in FIG. 2 or closing at the zero crossing of the voltage wave to get a fully asymmetrical wave as shown in FIG. 3. This result is idealized and does not account for variations in the closing angle and the bounce, but it shows that there is an optimum point that can greatly reduce the accumulated current I.sup.2T.

[0024] As mentioned, each voltage angle for initiating the closing operation of the vacuum interrupter 12 produces a different accumulated current I.sup.2T during the closing operation. By experimenting using the various parameters of the particular switching device, the voltage angle closing point that produces the lowest accumulated current I.sup.2T can be identified. The voltage angle closing points that are available would depend on processor speed used in the controller of the switching device. FIG. 5 is a block diagram of a system 100 that illustrates this analysis. The system 100 includes a processor 102 that receives the various parameters on lines 104, such as vacuum interrupter bounce characteristics, actuator response time, fault current, etc., and produces a voltage angle that minimizes the accumulated current I.sup.2T, which can then be stored in a controller 106 that controls the switching device.

[0025] The foregoing discussion discloses and describes merely exemplary embodiments of the present disclosure. One skilled in the art will readily recognize from such discussion and from the accompanying drawings and claims that various changes, modifications and variations can be made therein without departing from the spirit and scope of the disclosure as defined in the following claims.