Online broadband insulation spectroscopy of induction machines using signal injection

The breakdown of the stator insulation is one of the main causes of failure in medium and high voltage AC machines. Even though there are several techniques available to detect the failure of the ground-wall insulation and the turn insulation there are only few online techniques to detect the deterioration of the insulation prior to its breakdown. Established methods like the partial discharge analysis or the capacitance and dissipation factor test that can be applied to monitor the insulation health online, are limited to assessing the condition of the ground-wall insulation. An entirely different concept that can be used to monitor the ground-wall insulation as well as the turn insulation condition is based on the broadband impedance of the motor. Insulation aging will change the resistance and capacitance of the ground-wall insulation as well as the turn insulation. These changes in the insulation parameters are reflected in changes of the phase-to-ground impedance of the motor. This can be used to monitor the condition of the insulation. To exploit the relationship between the phase-to-ground impedance and the insulation parameters, a signal injection based technique is proposed to measure the impedance of stator insulation on an operating machine and trend the response over time. The basic concept for the signal injection is introduced and measurements on different machine classes (480 V and 4160 V) are presented to show the feasibility of the method.