M. A. Vogelsberger

Induction Machine Insulation Health State Monitoring Based on Online Switching Transient Exploitation

Todays variable-speed drives are usually operated close to their maximum tolerable conditions. The fast switching of modern power electronic devices leads to high stress of the winding insulation. As a result, an insulation breakdown may lead to sudden breakdown and high economic loss. To avoid unpredictable downtimes and enable repair on demand, monitoring of the insulation health state is getting more and more important. This paper proposes a method to monitor changes in the insulation health state by evaluating the machine high-frequency properties. The deterioration of the insulation condition is usually linked with a change of insulation capacity and thus also influences high-frequency properties. Initiating a voltage step excitation of the machine by the switching of the inverter, the high-frequency properties can be identified by measuring the resulting current response. This response is usually seen as current signal ringing and contains the machine high-frequency information. By applying signal processing tools, changes in the high-frequency information are extracted, and an insulation state indicator is derived. The applicability of the method is verified by measurements on two test machines (5.5 kW and 1.4 MW) having different power ratings as well as different insulation systems.

Separation of disturbing influences on induction machine's high-frequency behavior to ensure accurate insulation condition monitoring

The range of industrial applications requiring adjustable speed drives (ASD) is continuously increasing. Although generally highly reliable, a breakdown of an electrical machine has to be prevented under any circumstance for certain applications like safety-critical devices or industrial systems with many drives installed. To increase efficiency and return-on-investment the drives are operated at or near their rated values. High dynamics, overload and the fast switching of the inverter cause additional stress for the different drive components. One of the major causes for drive breakdown is insulation deterioration. Before an actual breakdown of the insulation system occurs, the machines high-frequency behavior is altered. Therefore monitoring of the drives hf-behavior can be employed to evaluate the insulation condition. However, other drive components like cabling, grounding or the inverter may change the drives hf-behavior as well. The possibility to separate the different influences of the machines hf-behavior is thus essential and will be shown by experimental results.

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

Insulation monitoring of three phase inverter-fed AC machines based on two current sensors only

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

, 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.

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

The demand of monitoring systems to prevent a breakdown of electrical machines is continuously increasing. Regarding economic issues, usage of system resources and additional components to establish monitoring is restricted. With the method proposed, the evaluation of the stator insulation condition of all three phases is possible using only the two current sensors already available in modern drive systems. The analyses of the transient part of the current response to voltage step excitation give evidence of a possible change in the insulation system. The degradation of the insulation health state is linked with a change in its capacitance, which in turn influences the electrical high frequency properties of the machine and as a consequence the transient part of the current signal resulting from a voltage step. To enable the monitoring of the phase with the missing sensor, a simple summation of all measured currents does not deliver enough accuracy for insulation monitoring. The non-measured phase however, can still be monitored by a special excitation sequence and signal processing without significant deterioration of sensitivity compared to the results if a sensor is available. Further, the influence of temperature and moisture on the proposed method is analyzed.

Sensitivity analysis of insulation state indicator in dependence of sampling rate and bit resolution to define hardware requirements

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.

Exploitation of induction machine's high-frequency behavior for online insulation monitoring

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.

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

Industrial application of adjustable speed drives (ASD) has been constantly growing in the last decades. In addition to the sheer number of inverter-fed drives the variety of application fields is increasing too. The use of such ASD in safety critical devices or industrial systems with many drives installed leads to the requirement of increased reliability. Furthermore many drives are operated at or near their rated values to increase efficiency and return on investment. In comparison to mains-fed machines, high dynamics and the fast switching of the inverter adds significant stress to different components of inverter-fed drives. The electrical machine is one of the most important components in these drives. A major cause for machine breakdown is insulation related fault. It is important to mention that such faults do not occur suddenly but origin in an alteration of the insulation condition and finally lead to short circuits. This alteration of the insulation leads to changes in the machines high-frequency (hf) behavior. To reduce the number of breakdowns and apply predictive maintenance insulation monitoring can be implemented. Monitoring of the machines hf-behavior can be used to assess the insulation condition by investigation of the transients in the current or voltage. Thus oversampling of the sensor signals is needed. The needed sampling rate and resolution to detect alterations in the machines hf-behavior is investigated by experimental results.

Aspects of electromagnetic-thermal coupled optimization of asynchronous machines for traction drives

The trend to increased energy efficiency and profitable capital expenditure leads to the operation of drive systems at or near their rated values and to increased running time. Thus, down time due to machine breakdown (e.g. in a traction drive) leads to high economic losses for the operator. Furthermore, the breakdown of safety-critical devices may lead to dangerous situations. Due to the above stated reasons the demand for operation reliability is constantly increasing. The fast switching of the voltage source inverter (VSI) in adjustable speed drives (ASD) leads to increased stress for the winding insulation. Thus, it gets even more important to monitor the condition of the insulation in inverter-fed drives. Degradation of the insulation results in an alteration of the machines high-frequency behavior. The proposed method is capable of detecting such changes by evaluation of the transient current reaction on inverter switching.