Evaluation and Current-Response-Based Identification of Insulation Degradation for High Utilized Electrical Machines in Railway Application

The demand for condition monitoring systems to prevent a breakdown of electrical machines is continuously increasing. A failure or collapse of the insulation is one of the main reasons for a machine outage, both in the field of high-voltage machines as well as low voltage. Especially at inverter-fed motors, several kinds of stresses due to parasitic phenomena exist and cause decrease of the insulation lifetime. With the method proposed, the evaluation of the stator insulation health condition of inverter-fed motors is possible by analyzing the current sensors transient response after inverter voltage step excitation. The analysis of the transient signal portion gives evidence of a possible change in the insulation system by comparison of the characteristics with the known pattern of the healthy machine transient signal portion. Tests are performed on a 1.4-MW induction machine designed for traction applications. By the availability of taps accessible on the machine terminal, different scenarios to emulate winding insulation degradation are conducted. Additionally, in order to investigate insulation degradation characteristics of the used insulation materials, accelerated aging procedures are applied on specially manufactured stator slot models with form-wound coils. These stator test segments have been aged by different accelerated aging cycles until the failure of the insulation is determined by voltage exposure tests. With additional nondestructive diagnostic measurements, e.g., dissipation factor (tan δ), capacitance, the correlation between these indicator values, the results of the proposed method, and the degradation of the insulation strength until a breakdown are investigated.