Abstract:
Protection of a cable, its termination and other apparatus from lightning strikes on an overhead line is provided by housing a surge arrestor and a cable termination, which are connected in parallel in a common housing, but as separate components not sharing the same axial line. With this configuration, minimal lead length is provided without sacrificing serviceability.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an electrical protective, high voltage dissipation device for an electrical energy distribution system and, more particularly, to a lightning arrestor which also serves as a riser pole terminator for an Underground Residential Distribution (URD) cable or the like. 
     2. Discussion of Related Art 
     Underground Residential Distribution (URD) cables are presently used for medium- and high-voltage (e.g., 15 kV, 25 kV and 35 kV class) distribution systems which are normally connected to overhead lines at some point by pot-head insulator terminations or connected to transformers or the like by suitable connector insulators. 
     The URD cable is terminated, i.e., the electrical path is changed from an insulated and shielded cable, suitable for underground use, to a bare or insulated overhead line. This termination typically takes the form of an insulated conductor passing through a heavy porcelain connector insulator with arc-over protection, or a polymer connector insulator. The URD cable is also protected from overvoltage surges due to lightning strikes, for example, by a surge arrestor. 
     Typically, these two different functions of terminating a URD cable and protecting the URD cable from overvoltage surges are performed by two separate devices 13, 14 located at the riser pole 10, such as shown in FIG. 1. One device is a riser pole URD overhead cable terminator 14. The second device is a riser pole arrestor 13 mounted in parallel to the terminator 14. The riser pole URD cable terminator 14 may be connected in series to a cut-out fuse device 12, which includes a fuse connected across an insulator mounted to the riser pole 10. The cut-out fuse/terminator combination is mounted in parallel with the surge arrestor 13 as shown. 
     Electric utility companies typically mount the surge arrestor 13 and the cable terminator 14 near to or on the cross arm of a utility pole 10. A central conductor of the URD cable 16 is electrically connected to the overhead high voltage line 11 through the riser pole cable terminator 14, whereas its outer conductor is connected to ground through a ground lead 15, as shown in FIG. 1. Mounting devices of any kind to utility poles is generally a costly proposition and mounting surge arrestors in particular requires care in keeping the leads as short as possible to provide optimal protection. 
     Recurrent failures of URD insulation systems due to lightning strikes has placed a renewed emphasis on the manner in which surge arrestors are connected. Studies have shown that long lead lengths create problems resulting in a lack of protection, such as illustrated in the IEEE guide for unjacketed concentric or semiconducting jacketed direct buried cable used as a pole ground and insulated jacketed cable with separate pole ground conductors, according to C62.22.1-1996. 
     A long lead length can minimize the percent of margin of protection (MOP) when a fast rise time occurs even on a 15 kV system. Fast rise times are prevalent in lightning surges. This is because of the large inductive voltage drop in the connection leads during fast surges. This voltage is equal to Ldi/dt, with the inductive element L being due to the length of connected leads which conduct a substantial and rapidly changing amount of current during a lightning surge. 
     It is an industry recommendation that a 20% margin of protection be provided. A margin of protection is the margin of insulation withstand above the arrestor protection level and it is calculated by using an 8 microsecond rise time. However, even at this margin of protection level, the protective levels are not adequate because lightning surges often do not follow classical guidelines. In accordance with C62.22-1991, lightning surges are often faster rising than 8 microseconds creating higher Ldi/dt voltages and thus lower MOP levels. The percent MOP is calculated with the following equation: ##EQU1## wherein B.I.L. is the Basic Lightning Impulse insulation level, and V max  is the maximum overvoltage found in a URD system. 
     The twenty percent MOP was chosen to allow for aging of transformers, cables, and cable accessories and still adequately protect for overvoltage surges. 
     The 8/20 μsec discharge voltage characteristics of the arrestor plus the Ldi/dt of the leads is used to calculate the MOP. The series connected leads of the arrestor have an inductance approximately equal to 0.4 μh/ft. This voltage drop (kV/ft) of the leads for various surge current magnitudes and rise times of the surge front are as follows. 
     
                       TABLE 1______________________________________kV/ft of LeadRise Time   5 kA     10 kA    20 kA  40 kA  65 kA(μsecs)   (kV/ft)  (kV/ft)  (kV/ft)                            (kV/ft)                                   (kV/ft)______________________________________0.1     20       40       80     160    2600.5     4        8        16     32     521.0     2        4        8      16     262.0     1        2        4      8      134.0     0.5      1        2      4      6.58.0     0.25     0.5      1      2      3.2510.0    0.2      0.4      0.8    1.6    26______________________________________ 
    
     The voltage drop significantly increases as the rise time becomes faster for a given current magnitude. Thus, lead length has a direct bearing on the sufficiency of the MOP. For instance, the percent MOP at a 1 μsec rise time for most lead lengths falls below the 20% requirement level according to the following table. 
     
         ______________________________________TYPICAL PERCENT MOP AT 1 μSEC RISE TIMEFOR l5 kV URD SYSTEM  Length of Lead in FeetCurrent(kA)    3      4      %    6    7    8    9    10______________________________________5        64.1   52.2   42.0 33.1 25.2 18.3 120  6.610       25.8   12.3   1.3  -7.7 -15.1                                 -21.4                                      -26.8                                           -31.420       -12.6  -25.3  -34.8                       -42.1                            -47.8                                 -52.5                                      -56.4                                           -59.7______________________________________ 
    
     This table shows that lead lengths are detrimental when fast rising surges occur in a 15 kV URD systems. 
     In recognition of the importance of lead line length, Matthew S. Mashikian poses in U.S Pat. No. 5,210,672 the complete elimination of lead lines for the parallel connection of cable terminator connection to a lightning arrestor using a configuration such as shown in FIG. 2. In the Mashikian patent, an electrical protective device 20 for high voltage cable termination from an overhead transmission line is proposed wherein a tubular insulator 22 1  is provided. An insulated conductor 22 3  passes through the center of the tubular insulator 22 1  of the cable 22. The central conductor 22 3 , when in service, will be exposed to a voltage of the overhead conductor at one of its ends and to a ground voltage at the other end. A surge arrestor 21 3  is concentrically disposed about the tubular insulator 22 1 , and the conductor 22 3  and the surge arrestor 21 3  have common terminals at their ground and high voltage ends. The surge arrestor 22 3  includes voltage-dependent resistors of a toroidal configuration disposed concentrically about the tubular insulator 22 1 , with an outer annular insulator 21 2  disposed about the surge arrestor 21 3 . 
     With this configuration, the surge arrestor 21 3  is said to contribute to the electrical stress grating of the termination and is desirably the sole stress grating means. The Mashikian patent identifies the preferred embodiment of the voltage arrestor 21 3  as being a stack of rings of semiconductive voltage-dependent resistant material with conductive elements 22 5  and 22 6  extending across the ends of the surge arrestor 21 3  and the tubular insulator 22 1  to provide a common terminal therefor. The concentric rings may be in the form of toroidal rings of conventional metal oxide voltage-dependent resistant semi-conductor material but may also take the form of semicircular blocks or otherwise segmented rings to be used to impart a degree of resilience. 
     Thus, the Mashikian patent expressly discloses the desirability of absolutely minimizing the amount of conductive material for interconnecting in parallel the surge arrestor components to the cable termination. 
     However, with this configuration, a number of problems occur. First, for instance, the surge arrestor must be specially formed of semiconductor material in the form of toroidal or segmented toroidal rings. This requires a unique configuration and shape of surge arrestor components not previously found in the prior art, with tight tolerances and little adaptability. This product is difficult to manufacture because the insulator 22 is hard to design. The insulator 22 must be able to accept a wide range of cable diameters, which is hard to accomplish. If the cable diameter is small there will not be enough interference fit and the insulator may flash over. If the cable diameter is large, it will be difficult to install the cable through the insulator 22. 
     SUMMARY OF THE INVENTION 
     In light of these and other problems of the prior art, the present invention has for an object the provision of an electrical protective device which provides surge arresting properties for high voltage connections to overhead lines in which the length of the connecting lead is substantially reduced, but not at an absolute minimum, while providing an adaptability and a serviceability, the combination of which is not found in the prior art. 
     This object and other objects are provided by a device in which the surge arrestor and the cable termination are provided in a common housing but as discrete elements within that housing and with highly desirable insulating characteristics. 
     The present inventor has now found the foregoing and related objects may be attained by an electric protective device or high voltage cable termination from an overhead transmission line comprising a cable including an outer conductor adaptedly connected to ground, an intermediate insulating layer, and an inner conductor adapted to carry current during normal operation; a surge arrestor including at least one voltage dependent resistor; an arrestor/cable electrical connector electrically connecting at least one voltage dependent resistor of the surge arrestor to the inner conductor of the cable, a ground electrical connector for electrically connecting another end of the voltage dependent resistor of the surge arrestor to the outer conductor of the cable; and a housing in which the surge arrestor and a terminal portion of the cable are commonly housed, wherein an axial line of the at least one voltage dependent resistor of the surge arrestor is spaced from the axial line of the terminal portion of the cable. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will now be described by way of exemplary embodiments, to which it is not limited, as shown in the accompanying figures in which: 
     FIG. 1 illustrates a conventional parallel connection of a lightning arrestor and a riser pole terminator; 
     FIG. 2 illustrates a combination of a toroidal lightning arrestor surrounding a cable in accordance with U.S. Pat. No. 5,210,676; 
     FIG. 3 illustrates a first embodiment in accordance with the present invention; and 
     FIG. 4 illustrates a second embodiment in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION 
     The present invention is illustrated using two embodiments shown in FIGS. 3 and 4. As shown in FIG. 3, the electrical protective device 30 includes a housing 31 formed of materials such as an outdoor polymer. This polymer could be silicone, EPDM, or other suitable outdoor rubber. The electrical protective device 30 could also be porcelain. The exterior surface of the housing 31 can include sheds or undulations 31 1  which increase the electrical leakage path. The increased surface leakage paths assists in avoiding breakdown through surface flashover. The housing 31 can have an upper rubber seal or boot 31 2  for receiving a pin connector 32 1  of a cable 32. Inside the housing 31, a tube 31 3  of fiberglass or a molded epoxy resin can be to hold insulating fluid. On the lower portion of the housing 31 and the tube 31 3  are openings through which the cable 32 is inserted via a lower rubber seal 31 4  or the like. In the context of this application, &#34;upper&#34; means closer to the overhead power lines and &#34;lower&#34; means further from the overhead power line. The tube 31 3  can be filled with an insulating fluid such as polybutylene in the first embodiment shown in FIG. 3. As shown in FIG. 4, the insulating fluid 31 5  is replaced with a housing filler 41 3  having an interference fit for the cable 32. Other insulating mechanisms may be employed such as silicone oil. 
     The cable 32 includes the pin connector 32 1  projecting out of the top of the housing 31. The pin connector 32 1  is attached to a crimp connector 32 2 . The crimp connector 32 2  is connected to the inner conductor 32 3  of the cable 32. Around the inner conductor 32 3  of the cable 32 is an insulating layer 32 4 , which may include insulating material and, perhaps, a semiconducting layer. A neutral wire 32 5  forms the outer sheathing of the cable 32. The insulating layer 32 4  and the neutral wire 32 5  are maintained at ground potential while the central conductor is energized to an elevated potential level, e.g., connected to an overhead conductor 11 (FIG. 1). 
     To terminate the cable 32 to a bare electrical conductor 32 3 , the insulating layer 32 4  is cut back. In this configuration, the voltage would rise abruptly from zero at the edge of the cutback insulating layer 32 4  to a significant portion of the total conductor voltage. This abrupt voltage rise would produce a high gradient capable of causing an electrical arc or flashover and damaging the cable termination. To reduce this gradient or electrical stress, stress relief devices such as shown schematically as the high voltage stress control 32 6  may be used adjacent to the cutback edge of the insulating layer 32 4 . 
     A surge arrestor 33 connected in parallel to the cable 32 includes a top conducting block 33 1  and a bottom conducting block 33 4  between which are voltage dependent resistors 33 3  made from such semiconductor materials as metal oxide and zinc oxide, in the form of a metal oxide varistor (MOV) or spark gaps with resistive rings. The top conducting block 33 1 , the voltage dependent resistors 33 3 , and the bottom conducting block 33 4  can be held together by a glass tape and epoxy coating 33 2 . A hold plate 33 5  schematically represents a means for maintaining voltage dependent resistors 33 3  in close electrical contact to the top conducting block 33 1  and the bottom conducting block 33 4 . The hold plate 33 5  may be in the form of a simple metal plate or a metal plate and biasing means (e.g., spring) mechanism to provide bias for holding the various components together. 
     The cable 32 and the surge arrestor 33 are connected together by a ground connector 34 and an arrestor/cable connector 35. The ground connector 34 in the first embodiment shown in FIG. 3 includes a spring loaded connector 34 1  for attaching the neutral wire 32 5  of the cable 32 to the bottom connecting block 33 4  of the surge arrestor 33. Between the spring loaded connector 34 1  and the surge arrestor 33 is a conductive material such as a metal rod threadingly engaging the hold plate 33 5  and a lower portion of the housing 31 by means of a connector bolt 34 2 . 
     As illustrated in FIG. 4, the ground connector 44 includes a threaded connection 44 1  for connecting the surge arrestor 33 to a ground and connection block 44 2 , which in turn is electrically connected to a common ground connection 44 3  electrically connected to the neutral wire 32 5  of the cable 32. 
     At the other end of the surge arrestor 33 is an arrestor/cable connector 35 shown in FIG. 3 or an arrestor/cable connector 45, as shown in FIG. 4. The surge arrestor/cable connector 35 of the first embodiment includes a spring loaded connector 35 1  and a connector for connection to the pin connector 32 1  of the cable 32 and a connector bolt 35 2  for connecting the spring loaded connector 35 1  to the top connecting block 33 1  of the surge arrestor 33 to the housing 31. The arrestor/cable connector 35 of the first embodiment is completely within the housing 31. In contrast, in the second embodiment shown in FIG. 4, the arrestor/cable connector 45 can be external to the housing 31 and includes a connector plate 45 1  such as a stainless steel with a spin welded crimp connector attached thereto for connecting the inner conductor 32 3  to the pin connector 32 1  of the cable 32. A connector bolt 45 2  connects the plate connector 45 1  to the top conductor block 33 1  of the surge arrestor 33 in the exemplary embodiment. Optionally, a silicone rubber boot 41 7  can be placed over the pin connector 32 1 , the crimp connector 32 2  and the connector bolt 45 2  in the second embodiment shown in FIG. 4. 
     The common housing 31 improves aesthetic, provides an adequate margin of protection while reducing assembly and mounting cost without the needed complexity of toroidal voltage dependent resistors and the like. 
     It can be seen from the foregoing detailed description and the attached figures that the protection device of the present invention combines cable termination and surge arrestor functions in a simple and effective way to increase the protection level attainable with reduced installation time while improving serviceability and maintenance. The device may be employed in several forms to effect cable terminations to overhead transmission lines or transformers or the like and may take the form of bushing connectors or elbow connections in various electrical devices. 
     The present invention has been described by way of exemplary embodiments to which it is not limited. Other variations and modifications will occur to those skilled in the art without departing from the spirit and scope of the present invention as defined in the appended claims.