Wound Rotor Induction Machine Research Articles

Design and Control of Microgrid Fed by Renewable Energy Generating Sources

This paper presents a control of a microgrid at an isolated location fed from wind and solar based hybrid energy sources. The machine used for wind energy conversion is doubly fed induction generator (DFIG) and a battery bank is connected to a common dc bus of them. A solar photovoltaic (PV) array is used to convert solar power, which is evacuated at the common dc bus of DFIG using a dc–dc boost converter in a cost effective way. The voltage and frequency are controlled through an indirect vector control of the line side converter, which is incorporated with droop characteristics. It alters the frequency set point based on the energy level of the battery, which slows down over charging or discharging of the battery. The system is also able to work when wind power source is unavailable. Both wind and solar energy blocks have maximum power point tracking (MPPT) in their control algorithm. The system is designed for complete automatic operation taking consideration of all the practical conditions. The system is also provided with a provision of external power support for the battery charging without any additional requirement. A simulation model of system is developed in MATLAB environment and simulation results are presented for various conditions, e.g., unavailability of wind or solar energies, unbalanced and nonlinear loads, and low state of charge of the battery. Finally, a prototype of the system is implemented using a 5-kW solar PV array simulator and a 3.7-kW wound rotor induction machine and experimental results are produced to reaffirm the theoretical model and design.

Detection of mixed eccentricity fault in doubly‐fed induction generator based on reactive power spectrum

Wide application of wind energy in the world towards reduction of global warming, have made the low cost maintenance of these turbines very important. Basically, mechanical faults such as gear-box faults in these turbines lead to a long down time and consequently heavy financial loss. Generally, these faults cause eccentricity in the generator air gap, so by its early detection the fault can be largely prevented. In this paper, eccentricity fault detection is studied in doubly-fed induction generator (DFIG) which is the most established generator in wind turbines and also is more prone to mechanical faults compared to other technologies. One of frequency components of reactive power is introduced as eccentricity fault index. As the reactive power is always evaluated in the control system, there is no need to include a new hardware for fault detection. Finite elements method is used to model the wound rotor induction machine (WRIM) which is coupled with closed-loop control system. Following the validation of the model in the healthy case experimentally, eccentricity under different operating conditions is included in the modeling. Finally, a simple procedure based on simple signal processing methods is introduced for eccentricity fault diagnosis considering the operating conditions of the wind turbine.

Rotor/Stator Inter-Turn Short Circuit Fault Detection for Saturable Wound-Rotor Induction Machine by Modified Magnetic Equivalent Circuit Approach

This paper presents a new method to model healthy and faulty wound-rotor induction machines (WRIMs) with and without inter-turn short circuit fault by a single set of equations. The model is presented for a machine with arbitrary pole, and rotor and stator slot numbers. A single set of equations, including both electric and magnetic parts, is solved to obtain full dynamic behavior of the machine. Therefore, the modified magnetic equivalent circuit with non-linear elements is used for modeling and a numerical technique is employed to solve the non-linear equations. The proposed modified method has a simple calculation procedure in comparison with the conventional magnetic equivalent circuit method. Solving the electric and magnetic equations by a single set of equations is the main advantage of the presented method, which is proposed for a WRIM machine, for the first time. In addition, to increase the accuracy, the saturation phenomenon is also considered in the model. Due to the saturation effect, there are some non-linear algebraic magnetic equations, which should be simultaneously solved with differential equations. A trapezoidal technique is used to convert differential equations to algebraic ones. To solve the non-linear equations, non-linear equations, Newton Newton–Raphson method is employed. Current signature analysis is used for rotor and stator inter-turn short circuit fault detection. Finally, the finite-element method is used to verify the effectiveness of the proposed modeling method. It is worth noting that the proposed method can model a healthy and faulty machine with different faults by a single model.

A Diagnostic Space Vector-Based Index for Rotor Electrical Fault Detection in Wound-Rotor Induction Machines Under Speed Transient

This paper investigates a new approach for the detection of rotor electrical faults in three-phase wound-rotor induction machines. The method is assessed in a wind energy conversion system, where the rotor side of the wound-rotor induction machine is supplied by a static converter for the control of active and reactive power flows from the generator to the electrical grid. Here, a diagnostic space vector is presented that sensibly improves the rotor fault detection in wound-rotor induction machine by combining the spectral signatures of both rotor current and voltage space vectors. The dc component of this diagnostic space vector, that is equal to zero in healthy conditions and different from zero under rotor asymmetry, is adopted as rotor fault index. The main advantages of the proposed technique are the implementation simplicity and the capability to operate even during speed transients. The technique is also validated by means of simulation and laboratory scale experimental tests.

The Detection and Suppression of Unbalanced Magnetic Pull in Wound Rotor Induction Motors Using Pole-Specific Search Coils and Auxiliary Windings

This paper puts forward a concept for measuring rotor eccentricity in a wound rotor induction machine. The analysis is fully developed in the theory section, then an experimental rig is described and measurements are taken. These are compared with the calculated values. Pole-specific search windings are necessary in a wound rotor machine, because standard sideband current measurement techniques are only valid for cage induction machines. The search coils can also be used to suppress unbalanced magnetic pull.

Scalar controlled induction motor drive speed estimation by adaptive sliding window search of the power signal

This work reports an improved estimation reliability wide range rotor speed estimation algorithm for scalar controlled wound rotor induction machines using spectral search of the stator power signal. An adaptive sliding window speed estimation method is proposed that monitors the value of supply frequency to identify the optimum boundaries of the power signal narrowband maximised by a tracked speed dependent power harmonic, while adjusting to nominal slip variation with supply frequency change characteristic of scalar controlled machines. Analytical expressions are derived that allow direct correlation of the surveyed spectral window boundaries to supply frequency. Once identified, the optimal power spectral narrowband enables supply frequency independent speed evaluation. The proposed method is underpinned by a dichotomous search based frequency tracking technique enabling improvement in the attainable estimation rate. The presented technique’s performance and limitations are assessed in steady-state and transient operation real-time experiments on two different scalar controlled 7.5kW machines. The scheme is shown to provide competent real-time speed estimation in a wide operating speed range for lower dynamics transients and steady-state operation.

Phasor model simulation of a grid integrated variable speed pumped storage system

This study presents a phasor model simulation of a grid integrated variable speed pumped storage (VSPS) system focusing on generating mode. The VSPS is configured by the voltage sourced converter doubly-fed induction machine (DFIM) scheme. A wound rotor induction machine dynamic mathematical model and generalised averaging theory are adopted to derive the phasor model technique of the system. Based on the dq -axis synchronous reference frame concept, the direct power control strategy of the system is designed. The steady-state and transient stability responses of the proposed system are analysed in MATLAB/Simulink platform by considering a 300 MW/18 kV/50 Hz DFIM-based VSPS system and a grid consisting of two hydropower plants with synchronous generator, a diesel power plant with synchronous generator, a solar photovoltaic farm and fixed loads. A comparison of phasor model and detailed model simulations of the system is also conducted. The results show that the grid integrated VSPS system is stable without significant deviation in grid voltages, DC voltage and rotor speed of the DFIM unit under the designed control strategy. Moreover, the phasor model simulation achieves results with good accuracy as the detailed model, but with faster speed. Hence, the phasor model simulation is very effective for analysis and control design of large grid integrated VSPS systems.

Study on Rotary Loop Flow Controller for the Next‐Generation Distribution System

SUMMARYThis paper describes two types of rotary loop flow controller, one is series type and the other is unified power flow controller (UPFC) type. Both of them can maintain the suitable voltage level of distribution system even with a large and uneven penetration of distributed generation. The configurations of proposed controllers are essentially same as wound rotor induction machine with no slip rings. The proposed controllers are less costly, more efficient, and more tolerant to overvoltages/currents as compared to power electronics‐type controllers. Further, UPFC‐type rotary loop flow controller has additional advantages like relatively smaller capability and ability to control reactive power than the series type of rotary loop flow controller. The proper functioning of the proposed controllers is confirmed through simulations and experiments.

Harmonic Order Tracking Analysis: A Speed-Sensorless Method for Condition Monitoring of Wound Rotor Induction Generators

This paper introduces a speed-sensorless method for detecting rotor asymmetries in wound rotor induction machines working under nonstationary conditions. The method is based on the time–frequency analysis of rotor currents and on a subsequent transformation, which leads to the following goals: unlike conventional spectrograms, it enables to show the diagnostic results as a simple graph, similar to a Fourier spectrum, but where the fault components are placed always at the same positions, regardless the working conditions of the machine; moreover, it enables to assess the machine condition through a very small set of parameters. These characteristics facilitate the understanding and processing of the diagnostic results, and thus, help to design improved monitoring and predictive maintenance systems. Also these features make the proposed method very suitable for condition monitoring of wind power generators, because it fits well with the usual nonstationary working conditions of wind turbines, and makes feasible the transmission of significant diagnostic information to the remote monitoring center using standard data transmission systems. Simulation results and experimental tests, carried out on a 5-kW laboratory rig, show the validity of the proposed method and illustrate its advantages regarding previously developed diagnostic methods under nonstationary conditions.

Modelling of a New Configuration for DFIGs Using T-type Converters and a Predictive Control Strategy in Wind Energy Conversion Systems

In this paper, a new topology is introduced for Doubly-Fed Induction Generators (DFIGs), applied in wind energy conversion systems. In the proposed topology T-type converters are used in back-to-back form instead of conventional converters. T-type converters are high-efficient three-level converters with high power quality. Furthermore, a hysteresis-based predictive control method is suggested for rotor current control of DFIG. Therefore, discrete models of the DFIG and T-type converters are extracted. A new equation is also defined for rotor current reference calculation which determines predictive current error. The suggested equation can be applied either for pre-connection to grid for synchronizing the DFIG or after connection to grid for regular control of DFIG. In addition to simulation results, the proposed topology is verified using a laboratory setup. The constructed setup is based on a 16-bit DSPIC and a wound-rotor induction machine. Simulation and practical results confirm effective operation of the proposed topology.

Analysis and Performance of Saturated Three-Phase Wound Rotor Induction Machine

This paper deals with special problem occurs with Induction Machine (IM) which is the saturation problem. Magnetic saturation is accounted by the nonlinear magnetizing curve of the iron core. Two machines of wound-rotor three-phase Induction Machine were used to make a comparison for different situations. The first machine running without saturation condition whiles the other running under saturation condition. Most of the students and researchers were considering the ideal case of IM in their researches. However it is better to study the impact of saturation on the machine when using the IM in different applications. This gives the research a stronger analysis and conclusion, and at the end of the day the results will be fruitful for the researcher. It is also necessary for the researcher to be familiar with the practical problems that occurs in the factories. This paper describes the analysis and characteristics of the induction machines working under the impact of saturation; this can be achieved by initiating a comparison between two machines working under different situations. Based on this basis we can use the facilities of Matlab/Simulink to build the proposed system to study the simulation results obtained in this paper. However there are a lot of problems and faults that occurs in factories when using the IM without finding a logical reason. One of these important problems could be the IM saturation problem which is occurs in the machines during work. Here comes the role of analysing and studying of the situation of saturation of the machine and then we can find out the real reasons and to identify appropriate solutions. So the ideal case study of the machine is not enough during research, especially when we talk about the practical side of the projects then there will be a lot of problems that are difficult to be detected and analysed. Therefore the researchers would have a comprehensive nature by including the saturation problem in there researches. There are many basic concepts that must be confirmed before starting the analysis of electrical machines. The principle of electromagnetic energy

Magnetic equivalent circuit modelling of doubly‐fed induction generator with assessment of rotor inter‐turn short‐circuit fault indices

In this study, a wound rotor induction machine, as the core of a doubly-fed induction generator (DFIG) system, is modelled using the magnetic equivalent circuit approach. The entire DFIG system is implemented in Matlab/Simulink environment, including the machine model, power converters, control system and the grid. The model, which includes the ability to simulate rotor inter-turn short-circuits (RITSCs), is subsequently used for the investigation and comparison of the impact of this fault in several DFIG quantities, namely in the stator current and in some control variables. This research also deals in detail with the differences found when comparing RITSC and unbalanced rotor resistance (URR) faults. It is observed that on the contrary to the URR, the stator current spectrum is more appropriate than the control signals for the detection of RITSC. Simulation and experimental results obtained with a DFIG system operating at sub-synchronous and super-synchronous speeds, for different values of the active and reactive powers injected into the grid, corroborate the findings reported in this study.

Sensorless speed estimation in wound rotor induction machines by spectral search of the stator phase power signal

This paper investigates the feasibility of utilising the stator phase power signal to establish sensorless speed estimation schemes in wound rotor induction machines (WRIMs). To this end, the authors derive and validate closed form expressions defining the possible power signal spectral content containing rotor speed information. Based on these, two novel sensorless speed estimation techniques are proposed: the first enables supply frequency independent speed estimation in WRIM extended slip applications, while the second facilitates estimation in a wide operating speed range for scalar controlled WRIM applications based on a sliding-window search of the power spectrum. The proposed techniques are underpinned by a dichotomous search enabled spectral processing algorithm and shown to deliver an increase in attainable real-time estimation rate without compromising estimation accuracy. The performance of reported schemes is assessed and validated in real-time laboratory experiments.

A data-based technique for monitoring of wound rotor induction machines: A simulation study

Detecting faults induction machines is crucial for a safe operation of these machines. The aim of this paper is to present a statistical fault detection methodology for the detection of faults in three-phase wound rotor induction machines (WRIM). The proposed fault detection approach is based on the use of principal components analysis (PCA). However, conventional PCA-based detection indices, such as the T2 and the Q statistics, are not well suited to detect small faults because these indices only use information from the most recent available samples. Detection of small faults is one of the most crucial and challenging tasks in the area of fault detection and diagnosis. In this paper, a new statistical system monitoring strategy is proposed for detecting changes resulting from small shifts in several variables associated with WRIM. The proposed approach combines modeling using PCA modeling with the exponentially weighted moving average (EWMA) control scheme. In the proposed approach, EWMA control scheme is applied on the ignored principal components to detect the presence of faults. The performance of the proposed method is compared with those of the traditional PCA-based fault detection indices. The simulation results clearly show the effectiveness of the proposed method over the conventional ones, especially in the presence of faults with small magnitudes.

Impacts of rotor inter-turn short circuit fault upon performance of wound rotor induction machines

This paper investigates the performance of a wound rotor induction motor (WRIM) under rotor inter-turn short circuit (SC) fault. Magnetic equivalent circuit (MEC) and finite elements method (FEM) are used to model the machine. Impacts of the fault on the signals of the machine including rotor current, stator current, and developed torque of the motor are addressed. Current of SC turns is also the signal that is considered here. A method based on the MEC is used to estimate different inductances of WRIM in which the impact of yokes, stator and rotor tooth, windings distribution and also core saturation are included. The obtained inductances are compared with that of the FEM and effect of the SC turns on the inductances are investigated. Finally, the fault is diagnosed using Fourier transform of the stator current and verified by experimental results.

Assessment of Time Frequency Color Index for Electrical Machines Diagnosis and Fault Severity

Improving availability and reliability of Induction Machine (IM) implies the minimization and the prediction of the maintenance operations. In this paper, we focus on detection and location of the rotor electrical faults in IM. This kind of fault generates non-stationary signatures in the stator current of the machine. Advanced signal processing techniques are required to deal with that type of signals. In this context, several recent studies suggest the use of Time-Frequency Representations (TFR) for the diagnosis of IMs' faults. This work presents a comparative study of some linear TFR. This last relies on good experimental results conducted on a test bench using a wound rotor induction machine operating in no load condition. The purpose is to showcase the TFR performances in faults detection. Static indicator is used to check the fault severity.

次世代配電系統に向けた回転型潮流制御装置の検討

This paper describes two types of Rotary Loop Flow Controller, one is series type and the other is UPFC type. Both of them can maintain the suitable voltage level of distribution system even with a large and uneven penetration of distributed generation (DG). The configurations of proposed controllers are essentially same as wound rotor induction machine with no slip rings. The proposed controllers are less costly, more efficient, more tolerant to over voltages/currents as compared to power electronics type controllers. Further, UPFC type rotary loop flow controller has additional advantages like relatively smaller capability, ability to control reactive power than the series type of rotary loop flow controller. The proper functioning of the proposed controllers is confirmed through simulations and experiments.

Interconnection of a wind driven induction machine with the power utilities: a simulation study

A three-phase wound rotor induction machine, operating as generator and driven by a wind turbine is interconnected with the power utilities grid. This is done through the enforcement of the machine to run at over synchronous speed conditions. Extraction of a maximum power from a wind turbine is made possible through the installation of a three-phase controlled bridge rectifier as a part from a slip energy recovery system. The controlled bridge converter is operated in the 'inverter mode of operation'. The installation of the controlled bridge converter results in the generation of harmonic currents. To overcome the effects of such harmonic currents, three-phase active filters are shunted with the induction machine. The shunted active filters guaranty the sinusoid quality of the power utilities line currents. The overall performance of the proposed configuration has been simulated in MATLAB/SIMULINK environment. Under different pretended wind speeds, quite satisfactory and encouraging results have been obtained.

Computation of Wound Rotor Induction Machines Based on Coupled Finite Elements and Circuit Equation Under a First Space Harmonic Approximation

This paper presents a fast method to compute wound rotor induction machines in steady state. Coupled time-harmonic finite-element (FE) circuit equation is used under a first space harmonic approximation for the air-gap magnetic field. It is shown that only four magnetostatic FE computations are necessary to determine the machine performances for any slip value. The performance comparison with a conventional complex magnetodynamic and time-stepping FE analyses show the effectiveness of the proposed approach.

Detection of Rotor Eccentricity in Wound Rotor Induction Machines Using Pole-Specific Search Coils

Condition monitoring is a standard method for scheduling maintenance and ensuring that catastrophic failures do not occur. It is used in many applications where there is high capital cost or the location is remote. Condition monitoring is increasingly used in renewable energy as stated in [1]-[4] and these mostly address the rotating equipment and in particular the electrical machinery. In terms of faults in rotating machines then there are several detection methods as addressed in [1]-[4]. The faults were listed in [5] where 41 % are bearing faults, 37 % are stator faults, 10 % are rotor faults and 12 % are others. In this digest the monitoring of the non-centering of the rotor in the stator bore is addressed (called rotor eccentricity); this can indicate bearing faults or miss-fitting. Vibration monitoring is popular and the vibration signature can be complex. For overheating problems then thermal monitoring can be used. Another popular method is to use line current monitoring where signature current sidebands are monitored [6]. However, these are relevant to cage induction machines and in renewable energy, particularly wind turbines, wound rotor induction generators are used and these do not produce the same sideband currents as shown in [7]. A chart showing the relevant airgap flux waves for cage and wound rotor machines are shown in Fig. 1. The vital difference that negates cage monitoring systems being applied to wound rotor machines is the absence of the rotor cage that can also have, for a 4 pole machine, 2 and 6 pole MMF waves induced into the cage. There have been several attempts to use search coils to assess for rotor eccentricity [8][9] though these do not seem to have been developed. In this digest pole-specific search windings are put into the stator slots. Since the example machine is a 4 pole machine then 6 pole and 2 pole search windings are put into a machine at the top of a slot. This is in the area where a wedge is often fitted.