Enhancing Performance of Filament Cage Metal Oxide Surge Arresters

Arresters

Metal-oxide surge arresters are increasingly used in AC as well as in DC applications to protect equipment by limiting overvoltage stresses and absorbing high levels of energy due to lightning or faults in the grid. Apart from this basic requirement, it is also essential to deliver these arresters in a way that facilitates customizing electrical and mechanical requirements while also meeting market levels for price and competence in insulation coordination.

Mechanical load and electrical stresses that affect an arrester’s housing, including dielectric strength of the equipment, are such that failure can often be expected. For reasons of cost, insulation strength of electrical equipment as well as of the active part of an MO arrester cannot always be designed to be as high as desired. Rather, economic and reliable operation of MV and HV grids is only feasible through installation of MO arresters with an adequate protection level against unwanted and unacceptably high voltages or high mechanical loads from the grid. The challenge for MO arresters is therefore to safely limit repetitive short-term overvoltages, to handle the charge transfer and manage the thermal stresses that arise during bypassing surge currents. They must also function reliably in failure free operation as well as handle all mechanical forces that can arise.

Uc is one of the most important design criteria for dimensioning an MO arrester since a high level of protection can only be achieved if the arrester is not overloaded electrically, thermally or mechanically during normal operation, thereby creating an unstable situation (e.g. thermal runaway of varistors or cracking of the housing). Given this, work is needed when dimensioning and selecting filament cage type MO arresters in applications, where for example operation is characterized by DC voltage with high fluctuations.

Required mechanical considerations result from application of surge arresters in regions due to a broad range of diverse mounting configurations, climatic conditions, varying risk of earthquakes, differing weight and vibration of cable connections as well as electromagnetic forces. It can prove challenging to cover all these factors with only one housing variant that, while light and compact, also offers high resistance to possible different mechanical forces.

Fig. 1: Schematic diagram of bending moment test bench for SSL and SLL type test on MO arresters, according to IEC.
Fig. 2: Result of bending moment test – SSL on MO arrester to check one design configuration with 525 Nm.

Attend the 2022 INMR WORLD CONGRESS this October, where arrester technical expert, Christoph Hippler, of Tridelta-Meidensha in Germany will report on an experimental evaluation of the design of filament cage type MO arresters. The aim of this research was to demonstrate that it is possible using an exact approach to classify all mechanical and electrical stresses that can occur and take these into account to design an arrester that will last more than 30 years. In addition, his contribution will explain creation of a protection concept.