Abstract:
An over-voltage protection system including a phase bus connected to a phase conductor of an electrical system and one or more transient-suppressing lines connected to the phase bus. Each of the transient-suppressing lines includes a contactor and a transient-suppressing element. The contactor of each transient-suppressing line is selectively opened and closed by a processor, thereby protecting the transient suppressing element from excessive currents. Also a method of protecting at least one transient-suppressing element from over-voltage conditions including providing a transient-suppressing line including the at least one transient-suppressing element in parallel with a load, measuring at least one of a voltage signal representative of a voltage on the transient-suppressing line and a current signal representative of a current on the transient suppressing line, and selectively placing the transient-suppressin line in either an open condition or a closed condition based on at least one of the voltage signal and the current signal.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to transient voltage surge suppression systems, and in particular to a system and method for protecting transient-suppressing elements utilized in a transient voltage surge suppression system against over-voltage conditions.  
         [0003]     2. Background Information  
         [0004]     Electrical systems, such as an electrical power distribution system, periodically experience over-voltage conditions. such as transient over-voltage conditions, also called “surges.” Over-voltage conditions are problematic to electrical systems because they may cause damage to the loads, such as an electronic device or other hardware, that are coupled thereto. As a result, transient voltage surge suppression (TVSS) systems have been developed to protect the loads from over-voltages that would otherwise damage the loads. TVSS systems typically provide such protection by coupling various types of known transient-suppressing elements between the phase, neutral and/or ground conductors of an electrical power distribution system.  
         [0005]     As is known in the art, transient-suppressing elements, such as metal-oxide varistors (MOVs), silicon avalanche diodes (SADs) and gas tubes, typically assume a high impedance state under normal operating voltages. When the voltage across a transient-suppressing element exceeds a pre-determined threshold rating, however, the impedance of the element drops dramatically, essentially short-circuiting the electrical conductors and “shunting” the current associated with the over-voltage through the transient-suppressing element and away from the load.  
         [0006]     MOVs are probably the most commonly used transient-suppressing elements. An MOV consists of two plates separated by an insulator, such as a metal oxide, that has a known voltage breakdown characteristic. When the voltage between the two plates reaches a certain level (the voltage breakdown level), the insulator breaks down and conducts current. MOVs, however, have operational limitations that must be taken into account when designing a TVSS system. Specifically, all MOVs have a maximum transient current rating that, if exceeded, may cause the MOV to fail. An MOV may also fail if subjected to repeated operation, even if the maximum transient current rating is never exceeded. The number of repeated operations necessary to cause failure is a function of the magnitude of transient current conducted by the MOV during each operation: the lower the magnitude, the greater the number of operations necessary to cause failure.  
         [0007]     In light of these limitations, prior art TVSS systems have been developed that use multiple MOVs in parallel combination such that the MOVs share the total transient current. Each individual MOV in such a configuration only conducts a portion of the total transient current, making it less likely that any individual MOV will exceed its maximum transient current capacity. In addition, a TVSS system that uses a plurality of parallel MOVs can withstand a greater number of operations because of the lower magnitude of transient current conducted by each individual MOV. Moreover, a parallel combination of MOVs is advantageous because the failure of any individual MOV will not cause a complete loss of TVSS system functionality.  
         [0008]     When an MOV fails, due to exceeding its maximum current rating or due to frequent operation, it initially falls into a low impedance state in which it draws a large steady-state current from the electrical system. This current, if not interrupted, will drive the MOV into a thermal runaway condition, typically resulting in an explosive failure of the MOV and damage to or destruction of the TVSS system as a whole. To avoid the explosive failure of MOVs in a TVSS system, appropriately-rated current-limiting elements, such as a fuse, are typically employed in series with MOVs, preferably with one such current-limiting element being in series with each MOV. Prior art TVSS systems employing multiple MOVs and one or more fuses are described in, for example, U.S. Pat. No. 5,153,806 to Corey, U.S. Pat. No. 4,271,466 to Comstock, U.S. Pat. No. 6,636,409 to Kladar et al., and U.S. Pat. No. 6,678,140 to Jakwani et al.  
         [0009]     The problem with using fuses to protect against MOV failure is that fuses, while effective in many conditions, are not reliable over the full range of fault currents that may occur. In particular, a fuse may open in response to certain over-current conditions (resulting from an over-voltage) that would not be a problem for (i.e., cause the failure of) the associated MOV. Such fuses are commonly referred to as “nuisance fuses” (having been opened under a condition that was not necessary to protect the MOV) and must be replaced, which is both expensive and inconvenient. Thus there is a need for a system for protecting transient-suppressing elements, such as MOVs, employed in a TVSS system from over-voltage conditions (and the over-currents that result therefrom) that is reliable over the full range of over-currents that may occur.  
       SUMMARY OF THE INVENTION  
       [0010]     These needs, and others, are addressed by the present invention which provides a system for protecting a load connected to an electrical system from over-voltage conditions. The system includes a phase bus connected to a phase conductor of the electrical system and one or more transient-suppressing lines connected to the phase bus. Each of the transient-suppressing lines includes a contactor and a transient-suppressing element, such as an MOV, connected in series with the contactor. The system further includes a processing unit and a memory storing one or more routines executable by the processing unit. The contactor of each transient-suppressing line is in electronic communication with the processing unit, and the routines are adapted to selectively open and close each contactor, thereby protecting the associated transient suppressing element from excessive currents.  
         [0011]     In the preferred embodiment, each of the transient-suppressing lines has operatively coupled thereto at least one of: (i) a voltage transducer in electronic communication with the processing unit, the voltage transducer generating a voltage signal, and (ii) a current transducer in electronic communication with the processing unit, the current transducer generating a current signal. In this embodiment, the routines are further adapted to selectively open and close each contactor based on at least one of the associated voltage signal and the associated current signal. The memory may store one or both of a normal voltage signature and a normal current signature for each of the transient-suppressing lines. The routines may then be further adapted to open and close each contactor based on at least one of: (i) a first comparison between the associated voltage signal and the normal voltage signature, and (ii) a second comparison between the associated current signal and the normal current signature. In particular, the contactor will be opened is abnormal voltage and/or current conditions are detected.  
         [0012]     Moreover, the phase bus may include a phase bus contactor in electronic communication with the processing unit, wherein the routines are further adapted to selectively open and close the phase bus contactor. In particular, the phase bus may have operatively coupled thereto at least one of: (i) a phase bus voltage transducer in electronic communication with the processing unit, the phase bus voltage transducer generating a phase bus voltage signal, and (ii) a phase bus current transducer in electronic communication with the processing unit, the phase bus current transducer generating a phase bus current signal. In this configuration, the routines are further adapted to open and close the phase bus contactor based on at least one of the phase bus voltage signal and the phase bus current signal, such as by comparing those signals normal signatures to detect abnormalities.  
         [0013]     According to another aspect of the invention, a phase conductor voltage transducer generating a phase conductor voltage signal is operatively coupled to the phase conductor and in electronic communication with said processing unit. The phase bus contactor is in a normally open condition, and the routines are adapted to close the phase bus contactor only if the phase conductor voltage signal is determined to be at or below a predetermined level.  
         [0014]     According to yet another aspect of the invention, at least one of the transient-suppressing lines includes a parallel combination of a series contactor and a series transient-suppressing element which is connected in series with the transient-suppressing element of the transient-suppressing lines. The routines are adapted to selectively open and close said series contactor in order to provide greater voltage handling capability.  
         [0015]     The invention also relates to a method of protecting at least one transient-suppressing element from over-voltage conditions, wherein the at least one transient suppressing element is part of a system for protecting a load connected to an electrical system. The method includes providing a transient-suppressing line including the at least one transient-suppressing element, the transient-suppressing line being connected to the electrical system and being in parallel with the load, measuring at least one of a voltage signal representative of a voltage on the transient-suppressing line and a current signal representative of a current on the transient suppressing line, and selectively placing the transient-suppressing line in either an open, non-conducting condition or a closed, conducting condition based on at least one of the voltage signal and the current signal. As a result, the transient-suppressing element is protected form damaging over-voltage conditions.  
         [0016]     In one particular embodiment, the method includes storing one or both of a normal voltage signature and a normal current signature for the transient-suppressing line, and performing at least one of: (i) a first comparison between the voltage signal and the normal voltage signature, and (ii) a second comparison between the current signal and the normal current signature. In this embodiment, the step of selectively placing the transient-suppressing line in either an open, non-conducting condition or a closed, conducting condition is based on at least one of the first comparison and the second comparison.  
         [0017]     Where the transient-suppressing line is connected to a phase bus connected to a phase conductor of the electrical system, the method may include measuring at least one of a phase bus voltage signal representative of a voltage on the phase bus and a phase bus current signal representative of a current on the phase bus, and selectively placing the phase bus in either an open, non-conducting condition or a closed, conducting condition based on at least one of said phase bus voltage signal and said phase bus current signal. The decision to place the phase bus in either an open, non-conducting condition or a closed, conducting condition may be based on a comparison to normal voltage and current signatures to detect abnormal conditions. In addition, the method, according to another aspect of the invention, may include connecting the phase bus to a phase conductor of the electrical system only if a phase voltage on the phase conductor is determined to be at or below a predetermined level. Finally, the method may include selectively connecting at least one additional transient-suppressing element in series with the at least one transient-suppressing element to increase the voltage handling capacity.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]     A full understanding of the invention can be gained from the following description of the preferred embodiments when read in conjunction with the accompanying drawings in which:  
         [0019]      FIG. 1  is a schematic diagram of a TVSS system according to one embodiment of the present invention;  
         [0020]      FIGS. 2, 3  and  4  are graphical representations of sample normal and abnormal voltage and current signatures that may be measured using the TVSS system of  FIG. 1 ;  
         [0021]      FIG. 5  is a schematic diagram that illustrates application of TVSS system of  FIG. 1  to a three-phase electrical distribution system; and  
         [0022]      FIG. 6  is a schematic diagram of a TVSS system according to an alternate embodiment the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]      FIG. 1  is a schematic diagram of a TVSS system  5  according to one embodiment of the present invention. TVSS system  5  is coupled to a phase conductor  10 , designated as Phase A, which is a phase conductor of an electrical power distribution system. Phase conductor  10  may be the sole phase conductor of a single phase electrical power distribution system, or one phase conductor of a multi-phase electrical power distribution system, such as, for example, a three-phase electrical power distribution system. TVSS system  5  includes TVSS circuit  15  in electronic communication with processing unit  20 , which may be, for instance, and without limitation, a microprocessor (μP). As seen in  FIG. 1 , TVSS circuit  15  includes phase bus  25  that is selectively connectable to phase conductor  10  by way of contactor  30 . Contactor  30  may be any type of known electronically controlled switch or relay, such as a TRIAC or an SCR (silicon controlled rectifier). Contactor  30 , identified as K A  in  FIG. 1 , is in electronic communication with and under the control of processing unit  20 , which is able to selectively open and close contactor  30  using appropriate electronic signals represented by arrow  35  in  FIG. 1 .  
         [0024]     In addition, a voltage transducer  40 , such as, for example, a potential transformer or similar voltage measuring device, is operatively coupled to phase conductor  10  and is in electronic communication with processing unit  20 . Voltage transducer  40  generates a signal V A  which represents the voltage carried by phase conductor  10  (the source voltage). The V A  signal is transmitted to processing unit  20  as indicated by the arrow  45  in  FIG. 1 . A voltage transducer  50  is operatively coupled to phase bus  25  and is in electronic communication with processing unit  20 . Voltage transducer  50  generates a signal V a  which represents the voltage carried by phase bus  25 . A current transducer  55 , such as, for example, a current transformer, a Hall Effect device or any other suitable current measuring device, is also operatively coupled to phase bus  25  and is in electronic communication with processing unit  20 . Current transducer  55  generates a signal I aT  which represents the total current carried by phase bus  25 . The V a  and I aT  signals are transmitted to processing unit  20  as indicated by arrows  45  and  60 , respectively, in  FIG. 1 .  
         [0025]     TVSS circuit  15  also includes a plurality of transient-suppressing lines  65  arranged in a parallel configuration as seen in  FIG. 1 . Each transient-suppressing line  65  is connected to phase bus  25  at a first end thereof and to ground at a second end thereof. Transient-suppressing lines  65  are also connected in parallel with a load that is to be protected. Furthermore, each transient-suppressing line  65  includes a contactor  70  as described above (identified as K 1 , K 2  . . . K n ) and a transient-suppressing element  80 , which preferably is an MOV (identified as M 1 , M 2  . . . M n ), but may be another type of known transient-suppressing element. In addition, a voltage transducer  75  as described above and a current transducer  85  as described above are operatively coupled to each transient-suppressing line  65 . Each contactor  70 , voltage transducer  75 , and current transducer  85  is in electronic communication with processing unit  20 . Processing unit  20 , through appropriate electronic signals represented by arrow  35 , selectively controls the operation of (opening and closing) each contactor  70 . Each voltage transducer  75  generates a signal (V a1 , V a2  . . . V an ) which represents the voltage carried by the associated transient-suppressing line  65  (arrow  45 ), and each current transducer  85  generates a signal (I a1 , I a2  . . . I an ) which represents the current carried by the associated transient-suppressing line  65  (arrow  60 ). Those signals are communicated to processing unit  20  for processing thereby.  
         [0026]     Signals V A , V a , V a1 , V a2  . . . V an  and I aT , I a1 , I a2  . . . I an  not only provide information about a particular voltage or current level at a particular time, but also provide information and representations of the AC voltage and current waveforms that are present at each particular location in TVSS circuit  15 . In addition, as is known in the art, under normal, safe operating conditions, TVSS circuit  15  will have a consistent “normal” voltage signature (AC waveform) and a consistent “normal” current signature (AC waveform) at each location where signals V a , V a1 , V a2  . . . V an  and I aT , I a1 , I a2  . . . I an  are measured.  FIG. 2  shows an example of a normal voltage signature  90  and a normal current signature  95  measured at, for example, the locations where V a1  and I a1  are measured. These signatures may be measured, recorded and stored in, for example, a memory  100  associated with processing unit  100  for use by processing unit  20  as described herein. Memory  100  may be one or more of any type of known storage element such as RAM, ROM, PROM and the like, alone or in combination. For example, memory  100  could be a combination of a RAM component and a ROM component. Under abnormal operating conditions, such as an over-voltage condition. the voltage signature and/or current signature at one or more of the locations where signals V a , V a1 , V a2  . . . V an  and I aT , I a1 , I a2  . . . I an , are measured will differ from the normal current and/or voltage signature for that location.  FIGS. 3 and 4  show two examples of a voltage signature ( 105  and  115 ) and a current signature ( 110  and  120 ) measured at the locations where V a1  and I a1  are measured under an abnormal operating condition, such as an over-voltage condition. Current signature  110  presents a phase shift as compared to normal current signature  95  due to microstructure damage of the associated transient-suppressing element  80  (e.g., MOV M 1 ) resulting from excessive joule heating. Similarly, current signature  120  presents a phase shift as compared to normal current signature  95  due to an over-voltage condition in the associated transient-suppressing line  65 .  
         [0027]     According to an aspect of the present invention, memory  100  is provided with one or more software routines executable by processing unit  20  for receiving the voltage and/or current signatures represented by signals V a , V a1 , V a2  . . . V an  and I aT , I a1 , I a2  . . . I an  and comparing them to the pre-stored normal voltage signature and normal current signature for the appropriate location within TVSS circuit  15 . Based on these comparisons, if one or more abnormal signatures are detected, processing unit  20  will generate and transmit an appropriate signal for opening the associated contact  70  to thereby protect the associated transient-suppressing element  80  from further exposure to a harmful voltage and/or current condition. As a result, damage, possibly catastrophic, to the TVSS circuit  15  as a whole will likely be prevented, with only the associated transient-suppressing element or elements  80  possibly needing to be replaced. As will be appreciated, under some circumstances, it will be advantageous to open contactor  30  to isolate TVSS circuit  15  (and protect all elements thereof) in its entirety until normal operating conditions are restored.  
         [0028]     According to a further aspect of the invention, when operation of TVSS circuit  15  is first initiated in a particular application, contactor  30  begins in a normally open condition, thereby isolating TVSS circuit  15 . Processing unit  20  will then monitor signal V A  to determine whether it is within a predetermined normal operating range for TVSS circuit  15 , and will only generate a signal to close contactor  30  if it is determined that the signal V A  is within the normal operating range.  
         [0029]      FIG. 5  illustrates the application of the present invention to a three-phase electrical distribution system having Phases A, B, and C. As seen in  FIG. 5 , three TVSS circuits  15  as described above are provided, one for each of the Phases A, B and C. Processing unit  20  in this configuration is adapted to independently monitor each TVSS circuit  15  and take appropriate action as described above in connection with  FIGS. 1-4 .  
         [0030]      FIG. 6  is a schematic diagram of a TVSS system  5 ′ according to an alternate embodiment of the present invention. As described below, TVSS system  5 ′ includes one or more additional series transient-suppressing elements that may be selectively switched in and out to provide increased operating voltage capability. TVSS system  5 ′ includes all of the elements of TVSS system  5  described above, and such elements are designated with like reference numerals in  FIG. 6 .  
         [0031]     As seen in  FIG. 6 , one or more of the transient-suppressing lines  65  in TVSS system  5 ′ include, in series with the other elements thereof, a parallel combination of a series transient-suppressing element  125  (identified as M s1  . . . M sn ) and a series contactor  130  (identified as K s1  . . . K sn ). In addition, a series current transducer  135  is operatively coupled to the line containing each series transient-suppressing element  125  to provide a signal (I sa1  . . . I san ) representative of the current flowing through the series transient-suppressing element  125 . Each series contactor  130  is in electronic communication with and under the selective control of processing unit  20  (represented by arrow  35 ′). Similarly, each series current transducer  135  is in electronic communication with processing unit  20  (represented by arrow  60 ′). If a series contactor  130  is in an open condition, then the associated series transient-suppressing element  125  will be by-passed. If, however, a series contactor  130  is in a closed condition, then the associated series transient-suppressing element  125  will be placed in series with the associated transient-suppressing element  80 , thereby increasing the voltage level that the transient-suppressing line  65  can handle. In this embodiment, memory  100  is provided with one or more routines for selectively opening and closing series contactors  130  when it would be desirable to increase the voltage that a particular transient-suppressing line  65  can handle.  
         [0032]     Thus, the present invention provides a system and method of providing transient voltage surge suppression in which the transient-suppressing elements that are utilized are protected over a full range of over-current conditions yet remain functional for all over-voltage conditions that they can appropriately handle (that may otherwise have caused a fuse to open).  
         [0033]     While specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art of various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of the invention which is to be given the full breadth of the claims appended and any and all equivalents thereof.