The effects of multi-current impulses on the low current bulk distribution in varistor materials, under continuous AC operation
The first part of this work gives a broad introduction on the origins of ZnO bismuthbased v aristor materials. The electrical properties of ZnO varistors follow, giving a detailed and up to date review of all the models published with an attempt to explain their complex conduction mechanisms. Stresses found in HV networks are also reviewed in detail, their knowledge being necessary to link the de gradation and fail ure mechanisms of ZnO varistors which are explained at depth. The manufacture of ZnO varistors on a laboratory scale is also presented: disc type as well as thick-film type varistors were manufactured and tested during this inv estigation. The parameters influencing the microstructure and electrical characteristics of varistors during sintering were reviewed. Testing sequences used by manufacturers as well as surge arrester testing standards were also assessed to derive tests for the laboratory produced samples.