Patent Publication Number: US-2013250469-A1

Title: Use of fuses to connect parallel columns of metal oxide varistors

Description:
BACKGROUND OF THE INVENTION 
     The subject matter disclosed herein relates generally to surge protection for power systems. More particularly, the subject matter disclosed herein relates to the use of fuses to connect multiple, parallel, columns of metal oxide varistors. 
     Multiple parallel columns of metal oxide varistors (MOVs) are typically used as overvoltage protection of devices that use a high amount of energy but still need surge protection, for example, capacitors. Because the columns do not share current equally, connection of multiple parallel columns of MOVs requires some method of controlling the sharing of current between columns. Conventionally this is accomplished through current-share testing at a factory. For example, using voltage impulses on one or more MOV&#39;s, then measuring and comparing currents through each column tested, then reconfiguring and swapping columns, then testing again. This process can be lengthy and costly. 
     When MOVs fail, they typically fail as a short-circuit, as a low-impedance arc. Depending on the application, short-circuit currents flowing through this failed MOV will dissipate energy in the form of increasing pressure (which can lead to explosions) and/or heat radiated from a high-power arc that is difficult to control. Conventionally this is mitigated by pressure-relief and arc-control devices built-into the units, and these devices must be tested in high-current power laboratories using costly and cumbersome testing methods. 
     BRIEF DESCRIPTION OF THE INVENTION 
     A system including fuses connected to multiple, parallel, columns of metal oxide varistors (MOV) is disclosed. In one embodiment, the system includes a plurality of MOV columns connected in parallel, and at least one fuse connected to one or more MOV columns. The fuse(s) are configured to isolate the MOV column in the case that the fuse is activated. 
     A first aspect of the invention includes a system comprising: a plurality of metal oxide varistors (MOVs) columns connected in parallel; and at least one fuse connected to at least one MOV column, the at least one fuse configured to isolate the MOV column in the case that the at least one fuse is activated. 
     A second aspect of the invention includes a system comprising: a plurality of metal oxide varistors (MOVs) columns connected in parallel; at least one fuse connected to at least one MOV column, the at least one fuse configured to isolate the MOV column in the case that the at least one fuse is activated; and at least one capacitor connected in parallel with the plurality of MOV columns. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features of this invention will be more readily understood from the following detailed description of the various aspects of the invention taken in conjunction with the accompanying drawings that depict various embodiments of the invention: 
         FIG. 1  shows a system using fuses to connect multiple parallel columns of MOVs according to an embodiment of the invention; 
         FIG. 2  shows a system using fuses to connect multiple columns of MOVs in parallel and series, according to an embodiment of the invention; 
         FIG. 3  shows a system using fuses to connect multiple parallel columns of MOVs according to another embodiment of the invention; and 
         FIG. 4  shows a system using fuses to connect multiple parallel columns of MOVs and a plurality of fused capacitors according to an embodiment of the invention. 
     
    
    
     It is noted that the drawings of the disclosure are not to scale. The drawings are intended to depict only typical aspects of the disclosure, and therefore should not be considered as limiting the scope of the disclosure. In the drawings, like numbering represents like elements between the drawings. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The subject matter disclosed herein relates generally to a surge protection system for a power system. More particularly, the subject matter disclosed herein relates to the use of current-limiting fuses to connect multiple, parallel, columns of metal oxide varistors (MOVs). 
     Turning to  FIG. 1 , a system  100  is shown according to an embodiment of the invention. System  100  includes a plurality of metal oxide varistors (MOVs) columns  102  connected in parallel. As shown in  FIG. 1 , each MOV column  102  includes at least one MOV  104 . MOV  104  can comprise any known or later developed varistor as known in the art, configured to protect circuits against excessive transient voltages. As known in the art, each MOV  104  has a “reference voltage” that once the voltage across MOV column  102  reaches or exceeds that reference voltage, MOV  104  will begin to conduct current to limit the voltage across the MOV to some maximum value (sometimes referred to as its “protective level voltage”) according to its material characteristics. 
     As shown in  FIG. 1 , each MOV column  102  is positioned between buses  106 ,  108 . For example, each MOV column  102  has a first end connected to a first bus  106 , and a second end connected to a second bus  108 . It is also understood that while  FIG. 1  shows two buses  106 ,  108  and MOV columns  102  between those buses  106 ,  108 , any number of buses can be used, and MOV columns  102  can be connected in series as well as in parallel. For example, as shown in  FIG. 2 , a third bus  109  can be included, and MOV columns  102  can be connected in series as well as in parallel. 
     System  100  further includes at least one fuse  110  positioned on an MOV column  102 . For example, at least one fuse  110  can be positioned in series with an MOV  104 , between MOV  104  and bus  106 , and/or between MOV  104  and bus  108 . In one embodiment, each MOV column  102  includes a fuse  110 , but it is understood that any number of fuses  110  can be used. For example, one fuse  110  can be connected to a plurality of MOV columns  102 . Also, fuses  110  shown in  FIGS. 1-4  are external fuses  110 , in that they are not integrated with MOVs  104 , but it is understood that internal fuses can also be used, i.e., fuses that are inside or integrated with MOVs  104 . 
     Fuse  110  can comprise any type of fuse that is configured to isolate MOV column  102  when activated, for example, a current-limiting fuse. Fuse  110  is configured to isolate the MOV column  102  that it is connected to, in the case that fuse  110  is activated. In this way, if a current passing through MOV column  102  exceeds an activation limit of fuse  110 , fuse  110  will activate by creating an open circuit and therefore current will not flow through fuse  110  and MOV column  102 . 
     It is also understood that fuse  110  will also be activated, i.e., break the current, in the case that a MOV  104  connected to fuse  110  takes a current beyond a specified limit for MOV  104 . In other words, a surge through MOV  104  of a current higher than the voltage limit of MOV  104  will also activate fuse  110  and will prevent current from flowing through MOV column  102 . 
     In operation, when a MOV column  102  conducts more than its rated current, and more than its “fair share” of current, this excessive conduction can lead to column failure. MOV columns typically fail as a short-circuit. However, embodiments of this invention include a current-limiting fuse  110  that first limits, and then interrupts, the short-circuit current of a failed MOV column  102 . In some cases fuse  110  can limit, and then interrupt, the current that exceeds the MOV current rating, and in so doing may also prevent column failure. 
     Turning to  FIG. 3 , in another embodiment, system  100  further includes a high voltage MOV column  112 , including a high voltage MOV  114 , connected in parallel with MOV columns  102 . In the embodiment shown in  FIG. 3 , high voltage MOV column  112  is connected at an end of the plurality of MOV columns  102 . High voltage MOV  114  can be any MOV capable of conducting at a higher voltage than any of the MOVs  104  included in MOV columns  102 , and will typically not conduct a significant amount of current at voltage levels that are limited by the operation of MOV columns  102 . As shown in  FIG. 3 , a relay  116  can also be connected in series with the high voltage MOV  114  in high voltage MOV column  112 . Relay  116  can be configured to activate in the case that high voltage MOV  114  conducts current, for example, relay  116  can activate by triggering a switch  118  as discussed herein. 
     Because high voltage MOV  114  will only conduct current if voltage gets above its reference voltage, and MOV  114  will not conduct if a pre-set, minimum, number of MOV columns  102  are connected and effectively limit voltage to a value below the reference voltage of MOV  114 , MOV  114  and relay  116  can act as an indicator when all (or a significant number) of fuses  110  have been activated. Relay  116  can be connected to a switch  118  that could switch off current through system  100 , or alternately to close a switch to bypass system  100  by electrically connecting bus  106  to bus  108  through a bypass switch, and by doing so collapsing the voltage between the two  106 ,  108  buses to zero, in the case that relay  116  is activated. 
     Turning to  FIG. 4 , one embodiment of system  100  in use is shown. As shown in  FIG. 4 , system  100  is connected to at least one capacitor  120 , each capacitor  120  in a column connected in parallel with the plurality of MOV columns. In this way, system  100  acts as a surge protection system for capacitor(s)  120 . As shown in  FIG. 4 , each capacitor  120  can be connected in series with a fuse  110 . In this embodiment, the parallel MOV columns  102  also limit the recovery voltage of an operating fuse  110  to prevent fuse  110  from re-striking Current limiting fuses  110 , when interrupting high-frequency current from a high-energy source such as voltage stored in a parallel capacitance, isolate the current by creating a high-magnitude “back voltage” that can compromise dielectric systems nearby. The MOV columns  102  connected in close proximity to capacitor  120  in this embodiment will limit that voltage and mitigate both the chance of fuse-restrike, and also the negative effects of the back voltage from fuse operation. 
     Embodiments of the invention disclosed herein use current limiting fuses to quickly isolate units that fail due to unequal current sharing, which can produce, over time, an assembly of parallel columns where the lower-voltage units that conduct more than their “fair share” are removed from the circuit by fuse operation, over time leaving an assembly of parallel columns that share current better than at initial assembly. In contrast to prior art methods, embodiments of this invention allow the application of off-the-shelf arresters and fuses, and results in an efficient assembly of parallel columns without requiring conventional current share testing. While external fuses have been used in other applications, e.g., in capacitor applications to isolate failed capacitors, external fuses have not been used in conjunction with MOV columns as discussed herein. Specifically, the fused MOV columns disclosed herein use fuses as a substitute for current sharing, and as a substitute for the post-failure pressure relief and arc control functionality of a conventional MOV design. 
     Embodiments of the invention disclosed herein use current limiting fuses to quickly isolate units that fail due to unequal current sharing or any other cause of MOV failure. The ensuing short-circuit currents caused by this failure can, if not limited and interrupted by the current limiting fuse, lead to a rise in local pressures that can lead to explosions if not relieved and/or a power arc that is difficult to control and can lead to collateral damage of surrounding equipment from either the arc touching the equipment or the radiated heat from the arc dissipating energy into that equipment. In contrast to prior art methods, embodiments of this invention allow the application of current limiting fuses  110  in lieu of pressure relief and/or arc control devices for the MOV columns  102 , and allow for minimized pressure relief and arc control testing, as long as fuses  110  have passed testing to prove they are capable of limiting and quickly interrupting the short-circuit current through the failed MOV columns  102  before pressure buildup occurs and/or the arc gets out of control. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the disclosure. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. It is further understood that the terms “front” and “back” are not intended to be limiting and are intended to be interchangeable where appropriate. 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.