Online detection of insulation degradation in inverter fed drive systems based on high frequency current sampling

The electrical part of modern drive systems usually consists of different components like power electronics, cabling, windings. Each of these components is usually operated at or near its rated values. In addition, high dynamic operation, overload cycles, as well as the fast switching of the power devices puts additional stress on all drive components. As a consequence overall reliability is reduced. Especially in security critical applications a continuous monitoring of the different components is thus necessary. In order to detect possible defects in an early stage or to enable prediction of a developing fault very sensitive measurement and evaluation methods are essential. In this paper a new method is proposed to monitor the state of different drive components like windings insulation, cabling and power electronics very accurately. The method is based on the current sensors available in the inverter in combination with a high frequency sampling. Each change in the switching state of the inverter leads to a transient voltage excitation of the electrical system consisting of power electronics, cabling, machine windings, leading to a step response of the machine current. Assuming a sufficient high sampling rate of the current, the characteristic features of the step response like eigenfrequency, damping rate and steady state value can be calculated and compared. Each of these characteristic values is influenced by the inductances, capacitances and resistances of the system. If for example the inter-turn insulation deteriorates the capacitance changes before the actual breakdown leading to a change in the mentioned characteristic values. Measurement results are presented to verify the applicability to detect changes in the phase winding capacity.