Arrestor unit

Abstract: An arrestor unit is disclosed for protecting a power transmission (5) line against lightening surges. The arrestor unit includes a discharge electrode (9) that is electrically separated from the power transmission line (5) by a series air gap (G). A plurality of non-linear resistors (13) are provided between the discharge electrode (9) and an electrical ground. A container (20) is provided to house the non-linear resistors (13). The container (20) is arranged to discharge residual electric charges that accumulate in the non-linear resistors (13). The discharge constant of the container (20) and non-linear resistors (13) is set to a value less than the time interval between voltage surges of a normal multiple surge lightning current.

Description: The present invention relates to arrestor units for power transmission lines. More particularly, the invention relates to an arrestor unit container structure that is arranged to discharge residual electrostatic charges that accumulate in the non-linear resistors. The container structure is capable of discharging the resistors during the surge intervals that occur in multiple surge lightning currents. JP-A-2/170385 discloses an arrestor unit having a series gap as illustrated in Figs. 4 to 6. Fig. 5 shows the equivalent circuit diagram of the arrestor unit illustrated in Fig. 4. Fig. 6 shows an arrestor unit embodied in accordance with the block diagram illustrated in Fig. 5. As shown in these figures, the arrestor unit of the prior art has a series gap 51 of an electrostatic capacity C 1 and a non-linear resistor 52(R) in an insulating housing 50. A resistor 53(r) for discharging residual electric charge is electrically connected in parallel with the non-linear resistor 52 respectively through seals 54. The discharge time constant of the arrestor unit depends on the electrostatic capacitance C2 of the resistor 53 and the non-linear resistor 52, and is set to be less than the time interval between voltage surges of a normal multiple surge lightning current. With this arrangement, the residual voltage of a first surge can be discharged before the appearance of the next surge even when the arrestor unit interrupts the follow current during the low voltage period between surges. Additionally, the flashover voltage of the series gap 51 can be maintained at a constant level. The arrestor unit includes an internal series gap 51, inside its insulating housing 50. Thus, the operational line voltage is applied to the arrestor unit; in particular it is inevitable that the insulator housing 50 will be subject to the operational line voltage. For this reason, the insulating housing 50 needs sufficient insulating strength to withstand the continuous use of the arrestor unit, without current leakage. With the resistor 53 housed in parallel with the non-linear resistor 52 inside the insulating housing 50, the resultant arrestor unit is too large in diameter and weight to be easily installed on power transmission lines. It is therefore a main object of the present invention to provide a new and useful arrestor unit construction that includes a series gap, enabling a smaller and lighter unit and also good insulating characteristics during multiple surge lightning currents, even when the exterior of the insulator is soiled by dirt and/or pollution. By the invention, an arrestor arrangement according to claim 1 is provided for protecting a power transmission line against lightening surges. The invention, together with the objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which: Fig. 1 is a cross sectional view of an arresting insulator which is part of the present invention. Fig. 2 is a front view of the arresting insulator shown in Fig. 1. Fig. 3 is a diagram of the equivalent circuit of the arresting insulator shown in Fig. 1. Fig. 4 is a schematic diagram of an arresting unit of the prior art having a serial gap. Fig. 5 is a diagram of the equivalent circuit of the arrestor unit illustrated in Fig. 4. Fig. 6 is a cross sectional view of an arrestor unit of the prior art having a series gap as shown in Fig. 4. In the following paragraphs, a detailed description is given of one embodiment of the present invention. As seen in Fig. 2, a suspension insulator 9 is suspended from the end of a support arm 2 that extends from a transmission tower 1. A power transmission line 5 is supported by the bottom part of the suspension insulator 3 via a support 4. Arc horns 6 and 7 are attached to the top and bottom ends of the suspension insulator 3 respectively to protect the suspension insulator 3 in the event of a flashover. A discharge electrode 9 is supported by the support 4. An arresting insulator 8 is supported by the intermediate part of the support arm 2 via an adapter. A discharge electrode 10 is mounted to the bottom part of the arresting insulator 8, such that it is opposite to the discharge electrode 9 with a series air gap G existing therebetween. Arc horns 6A and 7A are mounted to the top and bottom end parts of the arresting insulator 8 respectively to protect the arresting insulator 8 from damage in the event of flashover. In Fig. 1, the arresting insulator 8 includes a plurality of non-linear resistors 13 housed in a container 20. The container 20 has an inner wall 11 and an outer wall 21. The inner wall 11 is a cylindrical glass cloth made of fiberglass reinforced plastics impregnated with, for example, epoxy resin with carbon black. The resistance of the container 20 can be controlled by adjusting the content of the carbon black in the epoxy resin of which the inner wall 11 is made. The outside circumference of the inner wall 11 is provided with a plurality of pressure release holes 12. A plurality of non-linear resistors 13 are housed within the inner wall 11. The top and bottom ends of the inner wall 11 are sealed with end caps 14 and 15. The non-linear resistors are designed to pass a large current at lightening surge voltages, but only small currents at operational voltages. Thus, they can cut off follow currents in order to prevent ground faults in the affected line. That is, they prevent the normal operational line current (referred to as follow currents) from being grounded (i.e. a ground fault) through the arrestor unit after the lightning surge has passed. In a preferred embodiment, the main material of the non-linear resistors 13 is zinc oxide (ZnO). An upper seal 16 and a lower seal 17 are adhesively attached to the upper and lower ends of the inner wall 11 respectively by adhesive 18. A coil spring 19 is set between the end cap 15 and the lower seal 17 at the bottom part of the non-linear resistors 13 in such a way that the end cap 15 and the lower seal 17 are electrically connected with a low impedance. A conductive foil shunt 100 extends between the top and bottom ends of the spring. The outer wall 21 is fit airtight about the inner wall 11 and over the side walls of the upper seal 16 and the lower seal 17 by means of rubber molding. A plurality of sheds 21a are formed on the outer side of the outer wall 21. In order to ensure long-time reliability, the same rubber material as that of the outer wall 21 is filled into the clearance between the non-linear resistors 13 and the inner wall 11 from the pressure release holes 12 at the time of molding. A method suitable for determining the minimum allowable resistance Rmin and maximum allowable resistance Rmax of the container 20 is now described. Fig. 3 shows the equivalent circuit of the arrestor unit in this preferred embodiment. In this equivalent circuit the electrostatic capacity C 1 of the series air gap G, the total resistance r of the non-linear resistors 13, and the total resistance R of the container 20 are indicated. These should be set so that the discharge constant C2·R of the container 20 is smaller than the expected time interval τ between surges of a multiple surge lightning current. That is, R<τ/C2 in mathematical terms. It is generally understood that the interval τ between surges of a multiple lightning surge, (i.e. the available discharge time) is in the range of 1ms to 10ms. Assuming that the interval τ is lms, the maximum allowable resistance Rmax is obtained from an equation expressing the time constant (τ = C2·R). Table 1 shows Rmax for several representative power transmission voltage ratings. Line voltage (kv) 33 66 77 110 154 187 220 275 Line voltage max. (kv) 36 72 84 120 168 204 240 300 Rmin. (kΩ) 11 21 24 35 48 44 52 65 Rmax (mΩ) 16 34 40 50 67 67 100 100 Electrostatic Capacity C2 (PF) 65 30 25 20 15 15 10 10 In Table 1, the minimum allowable resistance Rmin is an experimentally determined resistance that is necessary to insure that the follow current will be interrupted. The experiments were based on an assumed maximum value of 5A for the allowable current (whereas the effective value is 3.5A). Next, the function of the described arrestor unit is explained. A lightning surge (caused by lightning striking the power transmission line 5) flows from the support 4 and the discharge electrode 9 and flashes over to the discharge electrode 10 though the series air gap G. This begins the discharge. The lightning surge then flows through the lower seal 17, the non-linear resistors 13, the upper seal 16, the adaptor and the support arm 2 and is discharged to the tower 1, which is grounded. After the lightning surge has passed the normal line current would typically try to act as a follow current (i.e. following the lightning surge to ground). However, the resistance of the non-linear resistors 13 recovers after the lightning surge passes and the follow current is interrupted by the recovered resistance of the non-linear resistors 13 and the serial air gap G. This prevents a ground fault in the line. After a lightning surge is applied to and discharged through the non-linear resistors 13, a residual electric charge will remain in them. Since the arresting insulator 8 is insulated from the applied voltage side by the series air gap G, the residual charge cannot flow towards the applied voltage side. However, the resistance R of the container 20 is set so that the discharge constant C2·R of the container 20 is smaller than the surge interval τ. Therefore, the residual charge in the non-linear resistors 13 will drain through the container 20 within the specified time and is then discharged to the ground. Note that if the residual charge is not drained, the arrestor unit will have a residual voltage. The voltage at the support 4 would then have to be higher by a corresponding amount in order to reinitiate flashover through the air gap G. In the present arrangement, however, residual voltage of the non-linear resistors 13 is discharged so that when the next lightning surge occurs, the voltage differential necessary to initiate another flashover through the serial air gap G is very similar to the voltage required to initiate the original flashover. Therefore, even if a multiple surge lightning current is encountered, the residual charge in the non-linear resistors 13 will be reduced sufficiently to be substantially negligible. Thus the described arrestor unit has improved discharge characteristics. The container 20, being electrically conductive, distributes the voltage equally between the applied voltage side and the grounded side. Because of this characteristic, the distributed voltage can be improved when the creeping surface of the container 20 has been soiled or otherwise polluted. Thus, the creeping flashover voltage characteristic can be improved. From a different point of view, when pollution withstand voltage is considered, the present arresting insulator 8 can be decreased in size and weight relative to existing arresting insulators by minimizing the increase in the creeping leak distance of the container 20. Furthermore, due to the same function, the creeping flashover voltage against the discharge voltage in the event of lightning surge can be improved, and the series air gap G of the arresting insulator 8 can be short-circuited. In the present arrangement the impedance of the arresting insulator 8 connected in series to the series air gap G is much smaller than that of the existing arresting insulators because the container conducts as a resistor R. As a result, the distributed voltage of the serial air gap is much larger and the lightning flashover voltage at the initial lightning surge application is lower than with existing arrestor units. This means that, in comparison with existing arrestor units, flashover at the arresting insulator side is easier because the flashover voltage of the arresting insulator is lower than that against lightning surge of the suspension insulator 3. In addition to the improvement in the arresting characteristic, the impedance of the arresting insulator falls as describe above. Furthermore, the electrostatic induction voltage caused by the operational line voltage falls, and thereby safety can be improved. The present invention is not limited to the previously described embodiment and may be embodied in many other forms as well. Specific modifications include the following. (1) The inside circumference of the inner wall 11 and/or outer wall 21 can be coated with conductive paint. As a result, existing materials or parts can be used, reducing the cost. (2) The inner wall 11 is used in the afore-mentioned preferred embodiment. By dispensing with the inner wall 11 and reducing the resistance of the outer wall 21, an insulating tube made of a material which has the required mechanical strength, such as epoxy resin, can be used in the arrestor unit of the present invention. Therefore, the present examples and embodiments are to be considered as illustrative and not restrictive.

Claims: An arrestor arrangement for supporting a power transmission line (5) comprising a supporting insulator (3) to support the line (5), the supporting insulator (3) having a first discharge electrode (9) to discharge lightning surge from the line (5); an arresting insulator (8) comprising a plurality of non-linear resistors (13) housed in a container (20) and having a second discharge electrode (10), spaced from the first discharge electrode (9) on the supporting insulator (8) by a predetermined series air gap (G) so as to receive lightning discharge therefrom and discharge it through the non-linear resistors (13); the container 20 housing the non-linear resistors (13) constituting a discharge resistor adapted to discharge residual charges after a lightning surge, the resistance of the container (20) and the electrostatic capacitance formed with the non-linear resistors (13) being predetermined such that the discharge constant dependent on these values is less than the time interval between surges of multiple-surge lightning. An arrestor arrangement according to claim 1 in which the container 20 has an inner wall (11) made of electrically conductive synthetic resin. An arrestor arrangement according to claim 2 in which the synthetic resin of the inner wall (11) contains carbon black, whereby its resistance may be determined by adjusting the carbon black content of the resin. An arrestor unit according to claim 2 or claim 3 in which the container (20) comprises an outer wall (21) having a plurality of sheds (21a), and formed in an airtight fashion around the inner wall (11) by rubber moulding. An arrestor arrangement according to claim 4 in which the outer wall (21) is of a synthetic resin containing carbon black, whereby its resistance can be determined by adjusting the carbon black content of the resin. An arrestor arrangement according to any one of the preceding claims in which the discharge constant is less than 1 ms. An arrestor arrangement according to any one of the preceding claims in which the supporting insulator (3) and the arresting insulator (8) are both suspended vertically from a support arm (2) of a transmission tower (1). An arrestor arrangement according to any one of the preceding claims in which each of the supporting insulator (3) and arresting insulator (8) has a respective pair of upper and lower arc horns (6, 7; 6A, 7A) to protect their exteriors against flashover.

Inventors: NAKAYAMA TETSUYA, NAKAYAMA, TETSUYA
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1992
Main classification: H02H9
IPC codes: H01B17, H01T4, H01T1, H01C7