C. Zoeller

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

Transient distribution of voltages in induction machine stator windings resulting from switching of power electronics

{\rm{(tan\,\delta)}}

Separation of fundamental wave and transient components of the current signal for machine insulation state monitoring

, 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.

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

The fields of application of adjustable speed drives fed by voltage source inverters is constantly increasing. For better exploitation of the systems capabilities all drive components are more and more operated near and even above their rated values. This leads to more strains for the drive. However, at the same time the demands for reliability are also increasing. To reach this requirement different strategies like fault tolerant design, fault detection and condition monitoring can be implemented. However, to implement such strategies a deep understanding of the effects that lead to problems or faults in the drive system is necessary. The fast switching of modern voltage source inverters leads to transient overvoltage stressing the machines insulation system. A short literature study presenting the characteristic of and the parameters influencing the non-linear voltage distribution in the stator winding of inverter-fed machines will be presented. Furthermore the findings will be compared with experimental results on an induction machine with tapped windings.

Detection of AC machines insulation health state based on evaluation of switching transients using two current sensors and eigenanalysis-based parameter estimation

In this work a method to detect insulation degradation of an adjustable speed drive (ASD) based on analyses of the transient current reaction caused by inverter switching is proposed. The drive system consists of an inverter fed three phase induction machine with 1.4MW. The standard current sensors are used to analyze the transient signal ringing to extract an insulation state indicator of the winding system and for the control of the machine as well. Using the inverter as a source of excitation, it is possible to perform an insulation test by evaluation of the resulting transient current sensor signals. The influences of the fundamental wave portion and transient component of the current signal on the insulation state indicator are analyzed. An effective separation of these components is investigated with an algorithm applicable for real time evaluation of the insulation state.

Influence of parasitic capacitances of IGBT inverter on insulation condition monitoring of traction machines based on current signal transients analysis

In order to investigate insulation degradation of high voltage traction drives, series of measurements on form wound coil specimens have been done. These stator bars have been aged by different accelerated aging cycles until the failure of the insulation is determined by voltage exposure tests. After every aging cycle the dissipation factor (tan δ) and capacitance (C) of the insulation system is recorded. With these non destructive diagnostic measurements, the correlation between the indicator values (tan δ, PI, C) and the degradation of the insulation strength until a breakdown can be investigated. Furthermore, an online insulation condition monitoring method for traction drives is proposed. In this method the inverter is used as a source of excitation with a simple switching pattern. The current transducers transient response at step excitation is used to evaluate a change of the insulation system. The drive system containing machine, cabling and inverter can be represented with a complex network of electrical, also parasitic components like resistances, inductances and capacitances. Insulation degradation within the machine also changes these components as investigated at the accelerated aging tests, and thus the tendency can be used for the monitoring method.

Inverter-fed drive stator insulation monitoring based on reflection phenomena stimulated by voltage step excitation

Application of monitoring systems is growing in modern drive applications to ensure continuous operability. Insulation health state can be examined offline 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 available in modern drives. The trace of the machines transient current reaction is influenced by parameters like the parasitic winding capacitances. Due to degradation of the winding insulation these parasitic parameters change and as a consequence also the shape of the transient current signal is changed. As the degradation induced changes in the signal are no very prominent, measurement procedure and signal processing play an important role in the estimation of the insulation health state. In the following, a method is presented to derive an insulation health state indicator based on the availability of two phase current sensors only and using eigenanalysis of the transient signal frequencies. The performance of the health state indicator derived is verified on a 1.4MW induction machine with preformed stator coils equipped with additional tapings.

Insulation condition monitoring of traction drives based on transient current signal resulting from differential and common mode excitation

At inverter-fed traction drives stray currents due to parasitic capacitances in frequency converters and machine cause high frequency voltage components spreading into all conductive components of the drive system. These hf voltages are harmful and origin from the inverter side through fast switching power semiconductors with high levels of output voltage steepness. Additionally overvoltage at the machine occurs by inverter operation of the machine and is thus stressing the machine insulation system. These effects can lead to a reduction of the machine insulation lifetime due to loss of insulation strength. Thus, insulation condition monitoring systems are necessary to enable a safe and reliable operation of a drive. Insulation condition monitoring of inverter-fed traction drives is implemented in this work by evaluating the transient current response after voltage step excitation. The current measurement is performed using the built-in current transducers of the inverter, normally used for the control of the drive, to save cost and space. Insulation degradation usually is a slowly developing process, at which the parasitic capacitances of the winding system change and thus also the transient current response. However, influences from the inverter capacitances cannot be neglected and thus the influence of the parasitic capacitances located in the inverter on the proposed insulation condition monitoring is investigated.

Investigation of sampling frequency and jitter effects on transient current signal evaluation for insulation condition monitoring

Reflected wave transient phenomena in voltages and currents of inverter-fed drives caused by voltage source inverters (VSI) operating with pulse width modulation (PWM) with high dv/dt rates are well known and analyzed in many studies. The reflection phenomena occurring at the machine terminals, resulting in high overvoltages are one reason for the stress and aging mechanism of AC machines insulation system. The motivation of this work arises out of the need for a continuous monitoring of the insulation health state for traction drives systems in order to ensure high reliability of the drive over many years of operation. With a voltage step initiated by a switching transition, the inverter elicits a response in the drive system. By using the information of the resulting transient effects, conclusions can be drawn on basis of the oscillation behavior which are indicative for a change in the insulation system. Investigations in this work show that an alteration in the shape of the system transients, in peak values as well as frequency components, correlates with a dielectric insulation capability of the machine winding insulation. Measurements on a small, low voltage, random wound, induction machine (5.5kW) and a medium voltage induction machine (1.4MW) with form-wound coil based stator system are performed. Artificially induced insulation aging is realized by accelerated thermal aging to demonstrate the effect. Additionally, two different inverter types are used, with standard IGBT modules (dv/dt ∼2–4kV/μs) and new SiC semiconductor inverter technology (dv/dt up to 20kV/μs), to analyze the influence of the shape of the excitation voltage step.

Detection and localization of insulation deterioration in traction drives based on specific high frequency current response evaluation

In this work a method to detect stator insulation degradation of traction drives based on analyses of the transient current reaction caused by inverter switching is proposed. Two simple excitation methods are proposed to analyze the state of the inter-turn insulation as well as the groundwall insulation. With the standard hall-effect based closed loop current transducers used for the control of the machine as well, the transient signal ringing resulting from a switching transition of the inverter is analyzed. Based on a comparative monitoring method the transient part of the current response is analyzed to extract an insulation state indicator of the winding system. The approach is tested with a SiC-MOSFET inverter prototype as a source of excitation. Due to fast switching of the voltage source inverter and high output voltage dv/dt rates, the motor windings suffer under increased stress that leads to insulation degradation. With the ability to change the effective gate resistance small dv/dt variations and their influence on the proposed method is analyzed. The method is tested with 1.4MW and 5.5kW induction machines.