Electricity: electrical systems and devices – Safety and protection of systems and devices – High voltage dissipation
Patent
1995-09-11
1996-10-29
Williams, Howard L.
Electricity: electrical systems and devices
Safety and protection of systems and devices
High voltage dissipation
361117, 361118, H02H 100
Patent
active
055702643
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The present invention relates to a surge arrester with an elongated outer housing which consists of an insulator of polymeric material comprising at least one stack of a plurality of cylindrical arrestor elements, preferably made of zinc oxide varistor material, which are arranged one after the other in the axial direction of the arrestor elements between two metal electrodes which are each arranged at a respective end of the polymer insulator.
In connection with pollution, surge arresters are subjected to heavy radial electrical stresses. This is due to the differences between the internal voltage distribution which is controlled by the ZnO blocks and the external voltage distribution which is caused by the pollution. In arresters with polymer insulators, the high electrical stress is applied to the polymer material, with an ensuing risk of degradation of the polymer material. The stress in connection with pollution increases, in principle, with the length of the arrester. One way of reducing the risk of degradation of the polymer is to limit the electrical stress to a certain level, independently of the length of the arrester. This can be done, as is common in arresters with porcelain housings, by constructing the arrester in several mechanically separated parts (multi-part arrester), in which each arrester part comprises an insulating housing provided with metal flanges and comprising a stack of series-connected ZnO blocks. The metal flanges at the joint between adjacent arrester parts thereby form a galvanic connection between the stacks of blocks and the outer surface of the housing. This is, however, a uneconomical solution and, in addition, it makes the arrester unnecessarily long.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a surge arrester with a polymer housing which is intended for high system voltages, in which the possible voltage stress within an arrester part can be limited in an economical way to an acceptable level taking into account the polymer material, while at the same time making possible a minimization of the length of the arrester in relation to the system voltage. This is achieved according to the invention by a surge attester with the field-limiting control electrodes along the length of the polymer insulator outer housing, the control electrodes being electrically connected with the stack of arrestor elements within the housing.
By providing the insulator on the outside with bands or rings of metal at regular mutual distances along the arrester, and electrically connecting these bands to the ZnO stack on the inside, the voltage difference between the bands, and hence the maximum stress on the polymer, will be determined by the number of ZnO blocks between the bands. In this way, an arrester can be constructed as a single-part arrester, even for high voltages.
If, in addition, the field-equalizing bands or rings are made mechanically strong, an additional advantage is obtained, namely, in connection with an internal short circuit in the arrester. An arrester with a polymer insulator may be provided with an inner glass-fibre reinforced tube or a winding to obtain mechanical strength. If, for example, in connection with a fault in an arrester element, a short-circuit arc is formed inside the arrester with an ensuing overpressure, such a tube will normally crack up. The cracking may, however, become too great, especially if the arrester is long, which may result in parts of the ZnO blocks being thrown out. By providing the arrester with strong bands or rings according to the above, the advantage can be achieved that the tube cracks in a more controlled manner.
The connection between the control electrode and the stack of arrester elements is suitably achieved with the aid of a contact pin which extends through a radial hole in the polymer insulator and which is pressed, by a spring arranged in a recess in the control electrode, against a cylindrical spacer of metal arranged in the element stack.
REFERENCES:
patent: 4340924 (1982-07-01), Kresge et al.
patent: 4467387 (1984-08-01), Bergh et al.
Lundquist Jan
Stenstrom Lennart
Asea Brown Boveri AB
Medley Sally C.
Williams Howard L.
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