Paul S. Barendse

The detection of interturn stator faults in doubly-fed induction generators

Presently, many condition monitoring techniques that are based on steady-state analysis are being applied to wind generators. However, the operation of wind generators is predominantly transient, therefore prompting the development of non-stationary techniques for fault detection. In this paper we apply steady-state techniques, e.g. motor current signatures analysis (MCSA) and the extended Parks vector approach (EPVA), as well as a new transient technique that is a combination of the EPVA, the discrete wavelet transform and statistics, to the detection of turn faults in a doubly-fed induction generators (DFIG). It is shown that steady-state techniques are not effective when applied to DFIGs operating under transient conditions. The new technique shows that stator turn faults can be unambiguously detected under transient conditions.

Design and Analysis of an Electromechanical Battery for Rural Electrification in Sub-Saharan Africa

This paper presents the design and analysis of an electromechanical flywheel energy storage system to enhance rural electrification in sub-Saharan Africa. The system consists of a flywheel rotor, an electrical machine, control system, bearings, and a containment structure. With the exception of the power electronics and magnets, local materials were used for the manufacture of the flywheel system. The flywheel rotor is made from glass fiber-epoxy composite, designed using novel shape profiles and utilizes a stress based solution by introducing a central hole for shaft inclusion. The system was accelerated to 6000 r/min storing up to 227 kJ. Numerical stress analyses were performed during the design stage to ensure that the maximum tensile strength is not exceeded. A lumped parameter thermal model is used to estimate the temperature distribution to ensure safe operating conditions of the flywheel system and environment. A life cycle cost analysis performed found that by integrating a flywheel system into a Solar Home System implies a cost savings of 35% per kilowatthour when compared with lead-acid batteries.

Fuel Cell Condition Monitoring Using Optimized Broadband Impedance Spectroscopy

Determining the state of health of fuel cell systems is essential to improving its performance and life expectancy. This paper presents the development of a new rapid online condition monitoring system using optimized broadband impedance spectroscopy (OBIS). The hardware was specifically designed to be low cost and scalable to meet the needs of single cell and stack level testing. It is shown how classic electrochemical impedance spectroscopy (EIS) is limited when testing under extreme nonlinear conditions. The design process of the broadband signal is tailored for polymer electrolyte membrane fuel cell diagnostics in order to minimize measurement time and system disturbance while maximizing accuracy. The long measurement time of standard EIS makes it impractical to use for rapid fault diagnosis, and it is shown how the OBIS system is able to deliver impedance measurements under conditions where EIS cannot be applied. The proposed system is tested for an array of possible operational phenomena and faults and the results compared with that obtained from a commercial frequency response analyzer to demonstrate performance.

Development of an HT PEM Fuel Cell Emulator Using a Multiphase Interleaved DC–DC Converter Topology

This paper presents a new emulator topology for a high-temperature (HT) proton-exchange membrane (PEM) fuel cell (FC). Emulators are used to predict FC behavior and facilitate development of the power-conditioning subsystems. In this paper, the high-temperature system is modeled and emulated both in the steady state and transient domains. The model is tailored to operate effectively in real time on the emulator hardware and to deliver acceptable performance during steady-state and dynamic conditions. In particular, a two-stage approach is applied to the design of the emulator hardware. The first stage is based on a multiphase interleaved converter, capable of maximizing ripple cancellation, while ensuring rapid dynamic performance through the use of reduced filter components. These benefits are only apparent by operating the converter at its critical duty ratio. This is achieved through the introduction of a power-stage converter, which tracks the steady-state behavior of the FC, allowing the multiphase converter to account for the rapid transient behavior. This operating principle improves the quality of the output dc voltage and dynamic performance beyond that achieved by conventional emulator topologies. The experimental results of the FC stack, HT PEM FC model and emulator are presented to confirm the performance of the proposed system.

Influence of Slot Openings and Tooth Profile on Cogging Torque in Axial-Flux PM Machines

Slotted axial-flux machines have excellent power and torque densities. However, it is difficult to reduce their cogging torque due to the complexity associated with implementing classical techniques. In this paper, slot-opening widths and tooth profiles will be shown to be significant in mitigating cogging torque in these machines. In particular, varying the slot opening reduced it by 52%, whereas a parallel-tooth (rectangular) profile lowered it by 24%, when compared with a conventional trapezoidal-tooth profile. An analytical quasi-3-D analysis was formulated and used to analyze and determine cogging torque. It was validated numerically and experimentally. Its versatility is in its ability to analyze different shapes of poles and slot openings, which can be extended to model air-gap nonuniformity. This paper also presents cogging torque minimization techniques that maintain the ease of manufacture of the parallel-tooth stator. Experimental results showed 73% and 48% reduction in cogging torque, which are achieved by the use of alternating pole arcs and skewed poles.

Considerations for improving the non-intrusive efficiency estimation of induction machines using the air gap torque method

The air gap torque (AGT) method is a well established technique for determining the efficiency of an induction motor, yet there are still discrepancies in the literature when applying the method. Additionally, the AGT method is known to be highly accurate, however, it is considered to be too intrusive for industry applications. To resolve this, the efficiency of an induction machine can be estimated non-intrusively by implementing the non-intrusive air gap torque (NAGT) method. The NAGT method combines various estimation techniques to determine the stator resistance, rotor speed and losses non-intrusively. However, due to the various estimation techniques associated with this method, the accuracy of the method is compromised. This paper assesses the various methods for estimating these parameters. It proposes a technique for improving the estimation of SLLs and addresses the discrepancy, as seen in literature, in interpreting the AGT method. Efficiency results from the NAGT method are compared to the IEC Std 34-2-1 and direct methods. The effects of voltage supply unbalances on efficiency estimation using the NAGT method are also examined. Lastly, an error analysis is conducted on experimental data to investigate the effects of instrumental errors on efficiency determination using the IEC Std 34-2-1, direct and NAGT methods.

The application of wavelets for the detection of inter-turn faults in induction machines

The most popular methods of induction machine condition monitoring utilize the steady-state spectral components of the stator quantities. These stator spectral components can include voltage, current and power and are used to detect turn faults, broken rotor bars, bearing failures and air gap eccentricities. Presently, many techniques that are based on steady-state analysis are being applied to induction machines. However, induction motors are not always operating under complete steady state conditions, therefore prompting the development of non-stationary techniques for fault detection. In this paper, it will be shown how the steady-state fault detection technique, Extended Parks Vector Approach (EPVA), can be improved to detect inter-turn faults during transients by incorporating the discrete wavelet transform (DWT) to the detection scheme. The new technique shows that stator turn faults can be unambiguously detected under transient conditions.

A New Algorithm for the Detection of Faults in Permanent Magnet Machines

Most condition monitoring techniques are based on steady state analysis. However, machines typically operate under transient conditions thus prompting the development of non-stationary fault detection techniques. In this paper, the steady state analysis technique (motor current signature analysis) is extended to a new non-stationary fault detection technique to detect several different faults in a low-voltage high current PMSM. It will be shown that the motor current signature analysis (MCSA) technique cannot be applied directly to machines operating under transient conditions. The new technique, which is based on an adaptive algorithm capable of extracting non-stationary sinusoids, is able to extract fault information during transient operation of the machine

Electrical Circuit Analysis of CO Poisoning in High-Temperature PEM Fuel Cells for Fault Diagnostics and Mitigation

High-temperature proton exchange membrane (HTPEM) fuel cells are able to withstand a substantial amount of CO poisoning while still maintaining stable output. High concentrations of CO will however degrade performance and can lead to instability. This paper presents a detailed study on the impact of CO poisoning on the performance of High-temperature PEM fuel cells and proposes a new method for mitigating the long term effects by using natural current profiles. A dedicated test setup was constructed to perform both steady state and dynamic analysis on the fuel cell under a wide range of operating conditions and variations in CO content. The drop in performance captured in the polarization curves is modeled using a simple circuit model with a dedicated fault element. The captured impedance spectra from the electrochemical impedance spectroscopy tests provide insight to the changes in the electrochemical circuit parameters that can be used to diagnose the extent of CO poisoning. Possible load control strategies that can reverse the CO poisoning is explored and the optimal profile is experimentally determined along with the long term effects.

Classification of High-Temperature PEM Fuel Cell Degradation Mechanisms Using Equivalent Circuits

This paper presents the evaluation of a high-temperature proton exchange membrane (HTPEM) fuel cell for different degradation mechanisms using equivalent circuit analysis. Specific consideration is given to the variation of phosphoric acid content in the polybenzimidazole membrane and the effect on the equivalent circuit. The importance of the cell assembly and operating conditions on acid migration are discussed, and it is shown how it affects performance both in the short-term and the long-term operation of the HTPEM cell. A new method is developed, whereby acid leaching can be greatly accelerated in order to quantify performance loss. The change in system parameters as a function of membrane electrode assembly acid content is investigated using electrochemical impedance spectroscopy and compared with the changes that take place for catalyst degradation, CO poisoning, and reactant starvation. It is shown, by using the equivalent circuits, that the drop in performance relating to the individual degradation mechanisms in the cell can be investigated and isolated for fault classification in online diagnostic systems.