Goran Stojčić

Increasing sensitivity of stator winding short circuit fault indicator in inverter fed induction machines

The inverter fed induction machine is becoming one of the most important drive application due to speed variability, dynamics, and robustness. These advantages are possible by the development of high-power transistors. On the other side these fast switching components lead to steep voltage rises and high stress on the winding insulation of the machine. Faults in the winding system of a machine can cause a dramatic damage due to their fast developing behaviour. An early detection of winding faults can reduce the consequences. In this paper a method is investigated to detect stator winding faults in their early stage. The method is based on voltage pulse excitation done by inverter switching. Current response measurements and subsequent signal processing lead to a fault indicator with high precision and low dependence on the machine point of operation. Measurements on a specially designed machine are carried out to determine the sensitivity of the fault indicator to early stage faults.

Detecting Faults in Doubly Fed Induction Generator by Rotor Side Transient Current Measurement

The doubly fed induction generator (DFIG) is one of the main technologies at variable speed power generation systems. Reliability and efficiency are key factors to realize the maximum energy output of the renewable resources. Detecting generator faults enables the reduction of risk for unexpected outages and thus high economic losses. Stator winding insulation faults count to one of the most frequent failures in electric machines. Common fault detection methods are based on several additional sensors and hardware what makes the system complex, expensive and also fault-prone. In this paper, a method is proposed and investigated to detect stator winding faults based only on measured signals available from inverter build-in sensors. By rotor-side inverter switching the generator is excited by transient voltage pulses and the current response provides the possibility to extract a fault indicator through a specific signal processing. Measurements on a DFIG test stand prove the methods applicability and accuracy.

Detection of Partially Fallen-Out Magnetic Slot Wedges in Inverter-Fed AC Machines at Lower Load Conditions

Electrical machines in the high-voltage class are usually designed with open stator slots. This wide open slots cause an increase of higher order harmonics, vibrations, noise, and temperature; thus, the machine efficiency is decreased. To counteract this disadvantage, magnetic slot wedges are applied. Due to the impact of high magnetic and mechanical forces, these wedges can fall out and may cause further serious damages. Up to now, reliable detection methods for single missing slot wedges are coupled with a disassembling of parts of the machine. In this paper, a method is investigated which provides the possibility of detection, based on the measurement of electrical terminal quantities only.

Detecting partially fallen-out magnetic slot wedges in AC machines based on electrical quantities only

The winding system of high voltage machines is usually composed of pre-formed coils. To facilitate the winding fitting process stator slots are usually wide opened. These wide opened slots are known to cause disturbances of the magnetic field distribution. Thus losses are increased and machines efficiency is reduced. A common way to counteract this drawback is given by placing magnetic slot wedges in the slots. During operation the wedges are exposed to high magnetic and mechanical forces. As a consequence wedges can get loose and finally fall out into the air-gap. State-of-the-art missing slot wedge detection techniques deal with the drawback that the machine must be disassembled, what is usually very time consuming. In this paper a method is investigated which provides the possibility of detecting missing magnetic slot wedges based only on measurement of electrical quantities and without machine disassembling. The method is based on exploitation of machine reaction on transient voltage excitation. The resulting current response contains information on machines magnetic state. This information is composed of several machine asymmetries including the fault (missing wedge) induced asymmetry. A specific signal processing chain provides a distinct separation of all asymmetry components and delivers a high sensitive fault indicator. Measurements for several fault cases are presented and discussed. A sensitivity analysis shows the high accuracy of the method and the ability to detect even partially missing slot wedges.

Detection of defects in stator winding of inverter fed induction machines

Inverter fed drive has become to one of the most important components of modern industrial facilities and propulsion systems. Growing demands on efficiency lead to higher wear of the drive systems and the risk of breakdowns. Unexpected outages of the drive system lead to high economic and/or safety losses. Detecting faults in an early stage helps increase the reliability and reduce the risk of breakdown. Beside short-circuit faults, open-circuit faults count to the most frequent reasons for the breakdown of the winding system. Starting with a crack or cross section deformation of the stator winding conductors the open-circuit fault induces an increased resistance in the corresponding phase. Detecting such small resistance changes provides the possibility to react timely on the developing fault. In this paper a method is presented to detect incipient open-circuit faults in the stator windings of an inverter fed drive. The method gets along with the hardware already present in modern drives, no additional sensors are required. By Applying voltage steps to the machine terminals and measuring the resulting current the phasor resistance can be estimated. By spatially combining the phase measurements of the resistance a phasor of the resistance symmetry can be introduced. With this phasor a high sensitive fault indicator is generated able to detect fault severity and position. Measurements on a laboratory test stand for different fault configurations prove the applicability and accuracy of the proposed method.

Detecting faults in Doubly fed induction generator by rotor side transient current measurement

The doubly fed induction generator (DFIG) is one of the main technologies at variable speed power generation systems. Reliability and efficiency are key factors to realize the maximum energy output of the renewable resources. Detecting generator faults enables the reduction of risk for unexpected outages and thus high economic losses. Stator winding insulation faults count to one of the most frequent failures in electric machines. Common fault detection methods are based on several additional sensors and hardware what makes the system complex, expansive and also fault-prone. In this work a method is proposed and investigated to detect stator winding faults based only on measured signals available from inverter build-in sensors. By rotor-side inverter switching the generator is excited by transient voltage pulses and the current response provides the possibility to extract a fault indicator through a specific signal processing. Measurements on DFIG test stand prove the methods applicability and accuracy.

Detecting incipient stator winding conductor faults in inverter fed machines

In variable speed drive applications the inverter fed induction machine is one of the most reliable systems. However, increasing demands on efficiency leads to higher wear of all system components. Unexpected machine breakdowns cause economic losses and/or undesired scenarios in safety-critical areas. Thus condition monitoring of all components can reduce the risk of outages due to faults. High electrical and magnetic forces together with increased environmental stresses in rough application fields lead to a higher probability of stator winding faults. Beside inter-turn short-circuit faults open-circuit winding faults are under the most frequent types of faults. Open-circuit winding faults usually start with a crack or conductor deformation. Thus the resistance in the corresponding stator winding phase is increased. Detecting such faults already in an early stage provides the possibility to react timely before a more severe fault occurs. This work presents a method to detect open-circuit winding-faults in an early stage without additional hardware. The method is based on the identification of the phase resistance by combination of measured current reaction to different voltage steps. A specific signal processing chain supplies a fault indictor able to detect the fault by magnitude and position.

Interactive simulation for teaching the influence of power management on hybrid electric cars overall energy consumption

Usage of sustainable energy is a key factor for solving future environmental issues. The purpose of the project SustEner is to modernize sustainable electrical energy vocational training by enhancing existing or establishing new training methods in enterprises and education. This paper presents the SustEner module power management techniques in hybrid electrical cars. Increasing market demand for hybrid electric vehicle (HEV) leads to a higher effort on training and education of high qualified engineers in this field. Beside the development and design of different HEV components also the interaction and efficiency of the entire system is a key factor. Understanding the power flow from one component to the other improves the ability of developing high efficiency HEV. Online teaching tools provide the possibility of teaching complex technical problem in an easy way and with high approval. Thus an interactive simulation tool was developed teaching power management techniques for HEV. In the following an interactive online tool is presented based on a MATLAB/Simulink HEV model enabling configuration of a HEV and efficiency analysis for a specific drive cycle.

Teaching power management techniques for hybrid electric cars by a interactive simulation

Usage of sustainable energy is a key factor for solving future climate issues. The purpose of the project SustEner is to modernize Sustainable Electrical Energy vocational training by enhancing existing or establishing new training methods in enterprises and education. This paper presents the SustEner module power management techniques in hybrid electrical cars.

Control of induction machine based on mathematical model with included anisotropy

The paper concerns with improving the field-oriented controlled induction machine torque and speed accuracy and performance from a control theory viewpoint. A simple model is derived that includes first harmonics of real machine phenomena such as inherent asymmetries, saliencies and anisotropy. We design adaptive proportional-integral controllers based on that model to achieve best possible torque performance. The control algorithm is conceived as an extension of conventional induction machine rotor field-oriented control. The usual Kalman filter approach for state estimation required by field-oriented control is extended to identify the contribution and location of asymmetries in a dual filter approach with unscented Kalman filter. Simulation results are obtained for a 5.5 kW machine with modeled stator and rotor anisotropy. Undesired effects like torque and speed pulsations are greatly reduced with the proposed controller.