Testing Metal-Oxide Varistors for HVDC Breaker Application

There is growing interest in changing the power grid infrastructure. It is increasingly important to transport a large amount of renewable energy over long distances. To minimize losses, meshed dc-grids are needed but require the development of dc circuit breakers. Without it, a meshed grid cannot be operated. Actually different concepts of dc breakers exist. The majority of them include an energy absorbing element. In ac grids, metal-oxide varistors (MOVs) absorb high energies. However, clarification is needed whether commercial available MOVs are able to handle the energy stresses that arise during dc switching operations. To investigate the energy handling capabilities of conventional metal-oxide material, an endurance test setup has been developed. In this setup, MOV specimens are stressed by repetitive impulse current injections while being permanently connected to dc voltage. This study reports on the endurance behavior of MOV submitted to >10 000 impulse energy injections with 60-100 J/cm³. Large differences are observed for MOVs from different manufacturers. Temperature and superimposing the impulses by a constant dc voltage are two parameters of less impact. Two types of MOV showed moderate changes of their electrical properties after >10 000 impulses and would be suitable for dc breaker application.