Recently, unusual transformer failures have been reported in 25kV power systems connected with a VCB and transformer. In this study, performance of the protection system, dielectric strength of the transformers, characteristics of switching surges and resonance phenomena were analysed and assessed. All investigations regarding the insulation breakdown of the transformers were carried out by simulations, laboratory tests, factory tests and on-site measurements in order to thoroughly investigate the root causes. Furthermore, experimental methods for investigating the resonance phenomenon were derived and tests were conducted applying the derived method to recreate the phenomenon in the transformer. It is found that resonance between the transformer and switching surge generated by closing a VCB can initiate the breakdown of internal windings of a transformer.
The AMF(Axial Magnetic Field) electrodes of VI(vacuum interrupters) can be largely classified into two groups; coil type and cup type. While TMF(Transverse Magnetic Field) electrodes do not need structures to support the electrodes, the AMF electrodes should have them inside. In case of separating welded contacts, it is recommended that the structures be brazed with electrodes and contacts. In this case although these structures are generally made of metal of which conductivity is low e.g. stainless steel, the current flows partly through them, which prevents AMF from generating. In this paper, a new structure was proposed to support the AMF electrode and help generate AMF. The structure was made of stainless steel and a form of hollow cylinder with slits like a cup-type AMF electrode. The slits were made on the diagonal to generate AMF in the same direction as the AMF electrodes. The structure can be applied to both coil type and cup type electrodes and preferably to VIs with high breaking capacity, where contact welding occurs sometimes. The AMF distribution and arc appearance of electrodes with the structures were compared with the ones without them. Even though the amplitude of AMF between the electrodes with the structures was lower than the one without them, the former could be increased almost up to the latter depending on the length and direction of the slits. The structures were applied to a 50kA VI and helped pass the ANSI tests.I.
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