Sinisa Djurovic

Origins of stator current spectra in DFIGs with winding faults and excitation asymmetries

In this paper the authors derive simple expressions for the frequencies of the harmonic components in the steady state stator line current of a DFIG operating under various conditions of supply and/or winding unbalance. The underlying purpose of the work is to identify signature frequencies which may be used in condition monitoring instrumentation to both identify and discriminate between a range of likely faults that may arise in practice. The analytical expressions are verified first by comparison with the spectra calculated using an advanced time-stepped circuit model, and then by comparison with measurements made on an experimental test rig.

Influence of supply harmonic voltages on DFIG stator current and power spectrum

This paper investigates the impact of supply harmonic voltages on DFIG stator current and power spectra. The influence of individual higher order supply harmonics on DFIG stator current and on total instantaneous power spectral content is investigated using a time-stepped circuit model. The observed effects are identified in experimental data obtained from a laboratory test rig. A set of analytical expressions that define the spectral characteristics of DFIG power and current signals is also presented. This work demonstrates that supply harmonic voltages have a significant impact on DFIG harmonic emissions and result in a range of well defined and calculable slip-dependant interharmonic current and power frequency components.

Estimation of Composite Load Model Parameters Using an Improved Particle Swarm Optimization Method

Power system loads are one of the crucial elements of modern power systems and, as such, must be properly modelled in stability studies. However, the static and dynamic characteristics of a load are commonly unknown, extremely nonlinear, and are usually time varying. Consequently, a measurement-based approach for determining the load characteristics would offer a significant advantage since it could update the parameters of load models directly from the available system measurements. For this purpose and in order to accurately determine load model parameters, a suitable parameter estimation method must be applied. The conventional approach to this problem favors the use of standard nonlinear estimators or artificial intelligence (AI)-based methods. In this paper, a new solution for determining the unknown load model parameters is proposed-an improved particle swarm optimization (IPSO) method. The proposed method is an AI-type technique similar to the commonly used genetic algorithms (GAs) and is shown to provide a promising alternative. This paper presents a performance comparison of IPSO and GA using computer simulations and measured data obtained from realistic laboratory experiments.

Supply Induced Interharmonic Effects in Wound Rotor and Doubly-Fed Induction Generators

This paper investigates the spectral effects that stator supply harmonics can induce in wound rotor and doubly-fed induction generators. The purpose of this study is to clarify the nature and the origin of the reported significant inter harmonic effects in these machines. To this end, the authors derive analytical expressions that define the spectral signature frequencies of individual supply harmonics in the current and torque signals. The expressions are first validated through comparison with the spectra calculated using a harmonic time-stepped machine model and then by spectral measurements on a laboratory test rig comprising a 30 kW machine excited by a programmable power supply unit. It is shown that the presence of supply harmonics can result in multiple and variable wide-band interharmonic effects in the current and torque signals. Finally, the interharmonic current emissions for the wound rotor and the doubly-fed mode of operation are experimentally quantified using the relevant IEC standard indices.

Fault frequency tracking during transient operation of wind turbine generators

As remote and offshore wind turbines increase their contribution to renewable energy generation, effective and reliable condition monitoring techniques will be required to reduce failure downtime. This paper examines the possibilities for analysing electrical signals from the stator of wound rotor induction generators, commonly used in wind turbines. Previous work derived analytical expressions for the frequency content of line current and total instantaneous power for healthy and faulty wound rotor induction generators under steady state conditions. This paper builds upon those results to examine fault detection under transient, variable speed conditions, such as encountered in a wind turbine. Through comparison between a time-stepped analytical model and a physical test rig it is concluded that the tracking of speed dependent fault frequencies is possible and could be an effective and reliable way to monitor the health of a wind turbine wound rotor induction generator.

Investigation of induction generator wide band vibration monitoring using fibre Bragg grating accelerometers

This paper investigates the use of fibre Bragg grating (FBG) accelerometers for wide band vibration monitoring in wound rotor induction generators. The sensor performance is assessed in a series of experiments on a laboratory test rig comprising a 30 kW induction machine operating in healthy conditions and with electrical or mechanical fault. Vibration measurements are processed and analyzed in the frequency domain for fault feature extraction. The fibre optic sensor effectiveness in measurement of wide band fault signatures in the vibration signal is compared with that of a commercial piezoelectric based solution. The potential and limitations of the investigated FBG accelerometer design are evaluated for use in condition monitoring applications.

Evaluation of fiber-optic sensing performance for embedded thermal monitoring of electric machinery wound components

This paper investigates the thermal monitoring performance potential of fibre-optic (FO) thermal sensors embedded into the structure of random wound coils. The implications of FO sensor packaging and installation procedure on thermal monitoring performance are assessed on a prototype coil in a series of laboratory experiments in a controlled thermal ambient. It is shown that, while the procedure of embedding the FO sensor into the coil structure alters its operational characteristics to an extent, the resulting in-situ hot spot thermal monitoring provides a reliable output when benchmarked with conventional thermocouple measurements.

Real-time insulation lifetime monitoring for motor windings

This paper investigates the development and implementation of a real-time thermal ageing model for polymer-based electrical wire insulation using the classical Arrhenius relationship for chemical reaction rates. The paper presents the theoretical development and implementation of the method for predicting the insulation lifetime based on real-time temperature measurements using fibre-optic sensors embedded inside copper-wound coils. The performance of the presented lifetime model in delivering consistent results for winding insulation lifetime predictions is then assessed and validated using real-time steady-state and transient thermal experiments on a wound test coil mounted into a purpose built motorette test rig.

FBG sensing for hot spot thermal monitoring in electric machinery random wound components

This paper investigates the application of Fibre Bragg Grating sensors for thermal hot spot temperature monitoring of random wound electric coils. The paper first presents the design, installation and in-situ characterisation details of a thermal sensor used for enabling thermal monitoring in the centre of the wound coil structure, in close proximity to the thermal hot spot location. A number of thermal monitoring experiments are then undertaken on a prototype current carrying test coil under a range of controlled static and dynamic thermal conditions. It is shown that reliable, improved fidelity information on the coils thermal status can be obtained from embedded Fibre Bragg Grating thermal sensors when compared to conventional thermal sensing solutions.

Real-time sensorless speed estimation in wound rotor induction machines using a dichotomous search algorithm

This paper investigates the development and realtime implementation of a dichotomous search algorithm based sensorless speed estimation technique in wound rotor induction machines. To this end the authors present the description of the structure and implementation of the technique proposed to extract the desired slip dependent frequency and hence the rotor speed information from the machine stator current spectrum. The performance of the presented algorithm in delivering estimation rate and accuracy improvements is then assessed and validated in real-time speed estimation tests undertaken on a 7.5 kW induction generator operating in the nominal and extended slip range.