Abstract:
A surge arrester having electrical connections to a source of power and to electrical ground is retrofitted with a spark gap assembly to improve performance of the surge arrester. This is accomplished by a providing a spark gap module including at least one spark gap assembly sealed within a housing, disconnecting an electrical connection of the surge arrester, and connecting the spark gap module between the electrical connection and the surge arrester.

Description:
TECHNICAL FIELD 
     The invention relates to surge arresters. 
     BACKGROUND 
     Electrical transmission and distribution equipment is subject to operating voltages within a fairly narrow range under normal conditions. However, system disturbances, such as lightning strikes, poor regulation, unbalanced loads, and switching surges, may produce momentary or extended voltage levels, especially on one or more phases of a multi-phase system, that greatly exceed the levels experienced by the equipment during normal operating conditions. These system variations often are referred to as overvoltage conditions and may lead to the damage of surge arresters installed to protect against transient overvoltages. 
     If not protected from transient current surges, critical and expensive equipment, such as transformers, switching devices, computer equipment, and electrical machinery, may be damaged or destroyed. Accordingly, system designers routinely use surge arresters to protect system components from dangerous overvoltage conditions. 
     A surge arrester is a protective device that is commonly connected in parallel with a comparatively expensive piece of electrical equipment so as to shunt or divert overvoltage-induced current surges safely around the equipment, thereby protecting the equipment and its internal circuitry from damage. The surge arrester normally operates in a high impedance mode that provides a relatively high impedance current path to ground. When exposed to a transient overvoltage condition, the surge arrester operates in a low impedance mode that provides a relatively low impedance current path to electrical ground (or earth). The impedance of the current path is substantially lower than the impedance of the equipment being protected by the surge arrester when the surge arrester is operating in the low impedance mode, and is otherwise substantially higher than the impedance of the protected equipment when in the high impedance mode. 
     Upon discharge of the transient overvoltage condition, the surge arrester returns to operation in the high impedance mode. This prevents normal current at the system frequency from following the surge current to ground (or earth) through the surge arrester. 
     Gapless surge arresters typically include an outer enclosure or housing made of an electrically insulating material, a pair of electrical terminals for connecting the arrester between a line-potential conductor and electrical ground (or earth), and an array of other electrical components that form a series electrical path between the terminals. These components typically include a series assembly of voltage-dependent, nonlinear resistive elements, referred to as varistors. A varistor is characterized by having a relatively high resistance when exposed to a normal operating voltage, and a much lower resistance when exposed to a higher voltage, such as is associated with a transient overvoltage condition. A metal-oxide varistor (“MOV”) is one type of varistor. In addition to varistors, a surge arrester may include one or more spark gap assemblies housed within or outside the insulating enclosure and electrically connected in series with the varistors. 
     SUMMARY 
     In one general aspect, the invention features retrofitting a surge arrester having electrical connections to a source of power and to electrical ground with a series connected spark gap assembly to improve performance of the surge arrester. A spark gap module including at least one spark gap assembly sealed within a housing is provided. An electrical connection of the surge arrester is disconnected, and the spark gap module is connected between the electrical connection and the surge arrester. 
     Embodiments may include one or more of the following features. For example, the spark gap module may be connected between the surge arrester and the source of power, or between the surge arrester and electrical ground. 
     The surge arrester may be a gapless surge arrester. For example, the surge arrester may be a gapless distribution arrester having a 3-36 kV rating, and rated for normal duty (5 kA) or heavy duty (10 kA) operation, although not limited to these ratings. 
     The spark gap module may consist of one or more gap assemblies positioned between a pair of terminals, with the one or more gap assemblies and the terminals sealed within the housing. Each terminal may include a threaded bolt hole. The housing may be a porcelain or polymer housing that may or may not have defined weathersheds. 
     The spark gap assembly may include a resistive or capacitive graded gap structure. The gap structure may include electrodes separated by silicon carbide grading resistors, ceramic capacitors, or other impedance elements. 
     In another general aspect, the invention features a retrofit module for adding a spark gap assembly to a surge arrester to improve performance of the surge arrester. The module includes a housing, at least one spark gap assembly sealed within the housing, and structure for electrically connecting the spark gap assembly to a surge arrester, the structure being accessible from outside the housing. 
     Other features and advantages will be apparent from the following description, including the drawings and the claims. 
    
    
     DESCRIPTION OF DRAWINGS 
     FIG. 1 is a partial cross-sectional view of a spark gap retrofit module for a surge arrester. 
     FIG. 2 is a partial cross-sectional view of a surge arrester. 
     FIG. 3 is side view of the module of FIG. 1 mounted on the arrester of FIG.  2 . 
     FIG. 4 is a cross-sectional side view of a spark gap assembly of the module of FIG.  1 . 
     FIGS. 5-7 are graphs showing operating characteristics of gapless and retrofitted surge arresters. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIGS. 1-3, a spark gap retrofit module  100  for a surge arrester  200  may be used to increase the temporary power frequency overvoltage capability of the surge arrester  200 . The module  100  is particularly useful for retrofitting gapless surge arresters, such as the arrester  200 , to reduce their failure rate. For example, one implementation of the module is for use with gapless distribution arresters having 3-36 kV ratings and rated for normal duty (5 kA) or heavy duty (10 kA) operation. However, the module also may be applied to gapless surge arresters having higher ratings, to gapped surge arresters, and to other types of protective equipment. For example, the module may be employed in conjunction with a fuse. 
     The module  100  also may be used to protect surge arresters from ferroresonance related power frequency voltages. Ferroresonance is an overvoltage condition that may be caused by single phase switching of circuit inductance such as in the primary of a transformer when the secondary is lightly loaded or unloaded. These overvoltages may be as high as three times the normal operating voltage of the arresters connected to the primary. The module  100  would increase the power frequency overvoltage capability of the surge arresters connected to the primary isolating the arresters from attempting a discharge of the power system. 
     Referring particularly to FIG. 1, the module  100  includes three gap assemblies  105  positioned between a pair of terminals  110 . The gap assemblies  105  and the terminals  110  are housed within an insulating housing  115  that may define a set of weathersheds  120 . The housing  115  provides a moisture-impervious seal between the gap assemblies  105  and the external environment. Techniques for applying such a housing to a set of electrical components are described in U.S. application Ser. No. 09/142,079, filed Nov. 20, 1998, and titled “SELF-COMPRESSIVE SURGE ARRESTER MODULE AND METHOD OF MAKING SAME,” which is incorporated by reference. 
     While three gap assemblies  105  are shown in FIG. 1, the number of gap assemblies in a module  100  may vary depending on the rating of the arrester  200  to which the module  100  is to be applied. For example, different implementations include from 1 to 12 gap assemblies, but are not restricted to this number of gap assemblies. Depending on the planned application, the gap assemblies may be resistively or capacitively graded, or both. The gap assemblies may include varistors, insulators, other impedance elements or open spark gaps. 
     The addition of the module  100  to the arrester  200  does not detrimentally affect the protective characteristics of the arrester  200 . For example, the discharge voltage and the protective margin of the arrester  200  are unchanged or improved by adding the module  100 . In one implementation, a retrofit module  100  is used to convert a gapless arrester rated at 10 kV to a gapped arrester rated at 12 kV. 
     Each terminal  110  includes a threaded hole  125  sized to receive a connecting bolt  130 . Bolt sizes used in some implementations include, for example, those having 10 mm, 12 mm, or 0.375 inch diameters. The module  100  is connected to a bolt  130 A (FIG. 2) extending from the line side  205  of the arrester  200 . A second bolt  130 B (FIG. 3) is inserted into the module  100  for use in connecting to the power line. 
     Other mechanisms may be used to connect the module  100  to the arrester  200  and the power line. For example, the module  100  may include bolts, with an intermediate bolt connecting the bolt of the module to a bolt of the arrester. Similarly, the module  100  may include a bolt extending from one terminal and a threaded hole in the other terminal. Other attachment techniques include eyebolts, clamps, press fit connections, and conductive adhesives. 
     Referring also to FIG. 4, one implementation employs a resistance graded gap structure  400  as a gap assembly  105 . The structure  400  employs an upper electrode  405 , a center electrode  410 , and a lower electrode  415 . Silicon carbide grading ring resistors  420 ,  425  separate corresponding pairs of electrodes. The electrodes also are positioned so that a spark gap  430  is formed between the upper electrode  405  and the center electrode  410 , and a spark gap  435  is formed between the center electrode  410  and the lower electrode  415 . 
     A retrofitted arrester  200  including one or more gap structures  400  in the retrofit module  100  has three different modes of operation: the steady state mode, the temporary overvoltage (TOV) withstand mode, and the impulse mode. The particular mode in which the arrester operates depends upon the applied voltage and the frequency of that voltage. In explaining operation of the retrofitted arrester  200 , the module  100  is treated as including three gap structures  400 , and the arrester  200  is treated as including three 3 kV MOV disks to form a 9 kV retrofitted arrester. For purposes of explanation only, operation of this retrofitted arrester is discussed relative to a 9 kV gapless arrester including three 3 kV MOV disks. It is important to note that comparable results could be obtained using a hybrid arrester that includes MOVs and gap assemblies within the housing. However, such an approach requires the considerable expense of replacing an existing gapless arrester with a hybrid arrester, rather than just retrofitting the gapless arrester with a retrofit module. 
     Steady state is the most common mode (mode  1 ) of operation, and occurs when the arrester has normal line-to-ground voltage applied to it at a normal operating frequency (e.g., 50 or 60 Hz). In this mode, the voltage is distributed across the arrester  200  and the retrofit module  100 , with the voltage distribution being a function of the individual component impedances. 
     The temporary overvoltage (TOV ) mode (mode  2 ) of operation occurs when an increased voltage occurs at the normal operating frequency. The retrofitted surge arrester is better able to endure such abnormal but commonly occurring conditions than is the surge arrester  200  standing alone. Referring to FIG. 5, as the applied voltage (V a )  500  increases, the voltage  505  across the gap assemblies  400  may be optimally designed to (V g −V d ) increase at a faster rate than does the voltage  510  across the MOV disks (V d ). This is due to the lower non-linearity of the impedance of the grading structure  420 ,  425  relative to the impedance of the MOV disks. This relative change in impedance results in a larger portion of the applied voltage being shifted from the MOV disks to the grading structure. This shift occurs without substantial current conduction, since the resistance-to-voltage curve of the grading structure is relatively flat due to the non-linear characteristic of the device. 
     This partial isolation from system voltage delays the onset of conduction through the MOV disks, which results in the higher TOV capability of the retrofitted arrester. This continues until the sparkover voltage  515  is reached, at which point a spark is generated between the electrodes and the impedances  420 ,  425  are shunted. At this point, the resistance of the spark gap assemblies becomes essentially zero and the applied voltage  500  shifts to the MOV disks. FIGS. 6 and 7 illustrate the difference in temporary overvoltage capabilities between a gapless arrester design including MOV disks (FIG. 6) and a retrofitted arrester design (FIG.  7 ). 
     The impulse mode (mode  3 ) of operation occurs when a high single polarity voltage, such as associated with a lightning stroke, is applied to the arrester. The discharge voltage of such a retrofitted arrester is a function of the resistance of the MOV disks. Accordingly, the discharge voltage of a properly retrofitted arrester will be essentially the same as that of a gapless arrester. 
     However, the retrofitted arrester does achieve improved performance with respect to its ability in modes  1  and  2  to discharge current during the impulse mode of operation. During an impulse, once the sparkover voltage  515  is reached, the grading elements are shunted and begin to cool. At that time, the impulse voltage is applied to the MOV disks, which heat up and become less resistive. After the impulse is completely discharged, the cooled grading elements of the spark gap retrofit module assume a larger portion of the total arrester voltage. The corresponding reduction in the voltage to the MOV disks reduces the power dissipated by them, allowing them to cool faster. This enhances arrester durability and recovery during impulse events. 
     Other embodiments are within the scope of the following claims.