Ljubiša Čičkarić

A high voltage rotating machinery is subjected to high electrical, mechanical, thermal and environmental stresses over the course of their service life. All that cause aging and degradation of the stator insulation system. A damaged insulation can cause the formation of larger eddy currents and local hot spots, which graduallly spread out during time. Therefore, it is essential to regulary check the insulation integrity to avoid possible consequential damage such as core burning or damage of the bar insulation, which can result in a stator ground fault leading to an unscheduled downtime. It is hard to detect local hot spots of a stator cores by monitoring of working parameters. The flux test is used to check the condition of the stator core insulation and thus to detect possible local insulation damage, which can result in local hot spots between several sheets of the core during magnetization. For assessing the condition of stator cores, three measurement methods are used: – Electromagnetic core imperfection detection (EL-CID) – 50 (60) Hz rated flux test – 500 Hz ring flux test These testing methods became an integral part of generator investigations during the building, overhauls and interventions with partially or completely removed rotor.

Keywords: Magnetic Core, Stator laminations, Eddy currents, Inter-laminar voltage, Hot spot