Enclosed lightning arrester

#This invention relates to a sealed gap-less lightning arrester device enclosed in a grounded metal enclosure filled with a pressurized insulating gas and utilizing nonlinear resistors having excellent nonlinear characteristics and a high electrostatic capacity, and more particularly to means for controlling the potential distribution across such a device.

#In miniature substations installed in small sites, it is required to utilize small-sized lightning arresters and it has been a common practice to insulate the arresters by utilizing sulfur hexafluoride (SF.sub.6). A plurality of silicon carbide (SiC) elements utilized as nonlinear characteristic elements which are the primary elements of lightning arresters, have been connected in series with a discharge gap therein between an associated high voltage conductor and a grounded metal enclosure filled with pressurized gaseous sulfur hexafluoride, the number of the series connections being determined by the associated system voltage. As a result, for a high system voltage there is the disadvantage that it is difficult to make the resulting lightning small-sized.

#Recently, nonlinear resistors of the zinc oxide (ZnO) type have been developed which resistors have the ability to interrupt the power frequency follow current which is produced upon the occurrence of high surge currents at high voltages resulting from lightning strokes etc. and which are not required to be operatively associated with discharge gaps. Those nonlinear resistors have been substituted for silicon carbide elements as above described so as to make it possible to manufacture small-sized lightning arresters more suitable for use in miniature substantions. Such lightning arresters are required only to include a plurality of zinc oxide resistors connected between an associated high voltage conductor and the grounded metal enclosure thereof. The nonlinear resistor formed of the zinc oxide material presents low magnitudes of resistance to high surge currents at high voltages resulting from lightning strokes etc. while presenting very high magnitudes of resistance to currents caused from voltages normally applied thereto so that the nonlinear resistor effectively protects an electric power device from damage due to high surge currents due to lightning strokes etc. However, that nonlinear resistor presents very high magnitudes of resistance to low currents flowing through the system operated in the normal mode so that it functions as an electrostactic capacity rather than a resistance. Accordingly, when operatively coupled to any AC machine, the serially connected nonlinear resistors have different voltage shares in accordance with positions occupied within the associated grounded metal enclosure because of the influence of stray capacities developed between the same and the grounded metal enclosure. As a result, the nonlinear resistors generate unequal amounts of heat and are unevenly deteriorated until they are successively broken. This has resulted in the lightning arresters with a decreased lifetime.

#The present invention will become more readily apparent from the following detailed description taken in conjunction with the accompanying drawings in which:

#FIG. 1 is a diagramatic view of a conventional enclosed lightning arrester device including nonlinear resistors;

#FIG. 2 is a graph illustrating the potential distribution across the nonlinear resistors disposed in the arrangement shown in FIG. 1 and having a normal AC voltage applied thereacross;

#Throughout the Figures like reference numerals and characters designate identical or corresponding components.

#Referring now to FIG. 1 of the drawings, there is illustrated an enclosed lightning arrester device having the conventional construction. The arrangement illustrated comprises a grounded metal enclosure 10 in the form of a hollow cylinder including a lower end closed with a flat metallic plate and an upper end portion having a reduced diameter, and an amount of a pressurized electrically insulating gas 12 having a high dielectric strength, for example, gaseous sulfur hexafluoride (SF.sub.6) filling the enclosure 10. A plurality of nonlinear resistors 14, in this case six resistors, are disposed in superposed relationship on the flat bottom plate of the grounded metal enclosure 10 along the longitudinal axis thereof and are serially interconnected. Each nonlinear resistor 14 has an excellent nonlinear resistance characteristic, has a high electrostatic capacity and is in the shape of a circular pellet, for example of sintered zinc oxide (ZnO). The nonlinear resistors 14 thus disposed have stray capacities 16 developed between the same and the grounded metal enclosure 10. The uppermost resistor 14 as viewed in FIG. 1 is connected to a high voltage conductor 18 in the form of a circular rod extended through and sealed to an electrically insulating spacer 20 rigidly fitted into the upper end portion of the grounded metal endosure 10. The high voltage conductor 18 is connected to a high voltage terminal of an electric power device to be protected although the high voltage terminal and the electric power device are not illustrated only for purposes of keeping the drawings simple. In this way a stack including the superposed nonlinear resistors has been disposed between the high voltage conductor 18 and the grounded metal enclosure 10.

#The operation of the arrangement shown in FIG. 1 will now be described. Each of the nonlinear resistors 14 presents a very low magnitude of resistance to high surge currents at high voltages entering the electric power device (not shown) due to lightning strokes because of their excellent nonlinear resistance characteristic. This prevents the electric power device from being subjected to a high voltage. However, when the electric power device is operated in the normal mode, each of the nonlinear resistors 14 presents a very high magnitude of resistance to a current due to a voltage normally applied across the electric power device with the result that the current is suppressed to a low magnitude sufficient to permit the device to be operated for a long time. Thus each of the nonlinear resistors having a 14 functions as a resistor low magnitude of resistance with respect to high surge currents but it presents very high magnitudes of resistance to low currents flowing through the electric power device operated in the normal mode so that the nonlinear resistors function as electrostatic capacities rather than resistors. Accordingly, this necessarily considered when the electric power device is an AC type.