Increasing accuracy of winding insulation state indicator of three phase inverter-fed machines using two current sensors only

In modern traction drives the application of monitoring systems is growing to ensure continuous operability. Because of the voltage source inverters (VSI) and the high steep voltage change (dv/dt), increased stress of the winding insulation exist. Stator insulation faults are common reasons for a machine breakdown. Insulation health state can be examined by evaluating the transient reaction to a voltage step excitation. Using the inverter as a source of excitation, it is possible to perform an insulation test by evaluating the resulting transient current sensor signals. The trace of the machines transient current reaction depends on the state of the winding insulation system. If insulation degradation occurs the parameters like the parasitic winding capacitances are changing and influencing the trace of the ringing. Normally, for a three phase AC machine the state of every phase is analyzed with the corresponding current sensor. However, regarding the economic issue, the usage of system resources and additional components is restricted. With the proposed method the evaluation of the stator insulation condition is possible only with two current sensors. The state of the phase without a sensor can be analyzed by a special excitation sequence without significant deterioration of sensitivity compared to the results if a sensor is available. Because the transient reaction of non-excited phases is very small, enhanced signal preprocessing is required to prevent sensitivity losses.