Wound Rotor Induction Machine Research Articles

Implementation of variable frequency transformer for integration of stand alone wind energy conversion system with battery energy storage

The conventional power electronics interface used for integration of wind energy conversion system (WECS) with load produces harmonics which deteriorates the quality of power supplied. To overcome this drawback, a variable frequency transformer (VFT) has been used as a link between WECS and load. For this, first a detailed steady-state modeling of VFT has been carried out. The derived model is then used for the analysis of the steady-state performance of VFT which acts as a link between WECS with battery energy storage and load for standalone operation. For this purpose, an equivalent circuit of VFT for standalone operation is developed. A set of equations for the output voltage and power delivered by VFT is derived from its equivalent circuit. The analytical solution of these equations helps in assessing the behavior of VFT. To validate the analytical results obtained from equivalent circuit, an experimental study is also carried out. The experimental study is conducted on a 3 phase, 400 V (L-L), 50 Hz, 2.2 kW, 1440 rpm, 4 pole, wound rotor induction machine. On comparison between analytical and experimental results, it is evident that both results are in good agreement. Further, the analysis is extended for variable load under various values of wind turbine generator speed and load power factor.

Experimental Identification of a Coupled-Circuit Model for the Digital Twin of a Wound-Rotor Induction Machine

Experimental Identification of a Coupled-Circuit Model for the Digital Twin of a Wound-Rotor Induction Machine

Dual Volts-per-Hertz Control of Double-Inverter-Fed Wound Rotor Induction Machine

Double-inverter-fed wound rotor induction machine (DI-WRIM) is a two-port machine capable of maintaining rated torque even at twice the rated speed. Such machines are gaining popularity in high-speed and high-power applications for their versatility and degrees of freedom in control. The existing control methods for DI-WRIM are all based on the dynamic model of the machine and the associated controller structures are complex with multiple controllers. This article proposes a novel control scheme for DI-WRIM based on the steady-state model which requires only one controller. The proposed control scheme has the capability to operate the machine in both sub and super-synchronous modes in all the four quadrants in torque-speed plane. The simplicity of the proposed dual volts-per-hertz control method can be compared to the very popular scalar control used in cage-rotor induction machine drives. Mathematical derivation of the control law with appropriate approximations is discussed. Experimental results are presented from a 5-hp, 8-pole wound rotor induction machine drive.

Improved Sensorless Operation of Double Inverter Fed Wound Rotor Induction Machine With Seamless Mode Transitions

The performance of a sensorless induction motor drive depends on the accuracy in estimating the flux position from the supply voltage and machine parameters. Accurate estimation of flux position at low speed operation is difficult due to the low supply frequency. In a double inverter fed induction motor drive, the low speed operation can be obtained by maintaining the relative frequency between stator and rotor supply <bold xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">voltage</b> without reducing the individual supply frequency to a very low value. In this paper, a novel frequency profile for the stator and rotor supplies of a DFIM drive is proposed, in which the minimum supply frequency of any side inverter is around 0.5 p.u. for the whole speed range of operation. With this proposed scheme, the zero speed operation of the drive is obtained by maintaining both the stator and rotor supply frequencies as 1.0 p.u.. To achieve this, the DFIM is required to be operated as a squirrel cage induction motor for certain speed range. The control scheme for both the stator and rotor side inverters along with seamless mode transitions are also presented. In this DFIM drive, the required motor power is only 50 % of the rated load power and the total <bold xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">volt-ampere (VA)</b> rating of the two inverters is same as that of a conventional squirrel cage induction motor drive. Experimental studies are carried out on a 45 <bold xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">kW</b> wound rotor induction machine to validate the control scheme with the proposed frequency profile and results are presented.

A Novel Mode Un-Mixing Approach in Variational Mode Decomposition for Fault Detection in Wound Rotor Induction Machines

Condition monitoring of induction machines (IMs) with the aim of increasing the machine’s lifetime, improving the efficiency and reducing the maintenance cost is necessary and inevitable. Among different types of methods presented for mechanical and electrical fault tracing in induction machines, stator current signature analysis has attracted great attention in recent decades. This popularity is mainly due to the non-invasive nature of this technique. A non-recursive method named variational mode decomposition (VMD) is used for the decomposition of any signal into several intrinsic mode functions (IMFs). This technique can be employed for detection of faulty components in a current signature. However, mode mixing of extracted IMFs makes the mechanical and electrical fault detection of IMs complicated, especially in the case where fault indices emerge close to the supply frequency. To achieve this, we rectify the signal of stator current prior to applying VMD. The main advantage of the presented approach is allowing the fault indices to be properly demodulated from the main frequency to avoid mode mixing phenomenon. The method shows that the dominant frequencies of the current signal can be isolated in each IMFs, appropriately. The proposed strategy is validated to detect the rotor asymmetric fault (RAF) in a wound rotor induction machine (WRIM), in both transient and steady-state conditions.

A Case for Micromachines in Laboratory-Based DFIG Wind Turbine Systems for Fault Studies

For laboratory-based systems, the selection of components is critical to ensure proper representation of the actual system. In past investigations, researchers made use of small-scale off-the-shelf wound rotor induction machines (WRIMs) to perform laboratory-based studies. However, there are no reports validating their applicability in comparison to using the actual machines. In this regard, this paper investigates the correlation between the dimensional parameters and the electrical asymmetry indicators for a large-scale wind generator and laboratory-based generators. For the latter, a comparison is made between using a standard small-scale off-the-shelf WRIM and a micromachine. It is shown that the harmonic frequencies in a healthy machine and those due to inherent asymmetries appear at the same positions on the stator current spectrum for the three machines. However, there is a difference in the amplitudes of these components, given their dependence on the winding factors for a specific machine. The dimensional analysis techniques used to scale down the large-scale generator into a micromachine ensure that these parameters are related. However, this is not the case for the standard small-scale WRIM. Further work demonstrates the correlation of the fault-related harmonics between the micromachine and the large-scale machine for a rotor inter-turn fault condition. Micromachines are designed to ensure that the time constants are proportional representations of the larger machine, thus providing an accurate reflection of what is expected under transient operating conditions of wind turbines. This would assist in the development of condition monitoring strategies that are suited for the actual systems in the industry.

Manipulation of stator current signature for rotor asymmetries fault diagnosis of wound rotor induction machine

In this paper, a new technique based on the manipulation of stator current signature for induction machines fault diagnosis is introduced. The goal of the proposed method is to demodulate the characteristic frequencies from supply frequency and preserve the information of the supply frequency and its harmonics. The proposed method can be easily implemented and used in experimental systems due to its low computational complexity. The validity of the proposed method is proved through theoretical analysis and experimental results in steady-state and transient conditions. In this regard, the 270-W wound rotor induction machine (WRIM) is tested under different fault severities and load levels.

Design and implementation of optimal controller for DFIG-WT using autonomous groups particle swarm optimization

&lt;span&gt;There are many types of generators used within wind energy such as doubly fed induction generator (DFIG). Particle swarm optimization (PSO) algorithm is simple, robust and easy to implement. In addition to the privilege of PSO, autonomous groups particle swarm optimization (AGPSO) has the advantages of using diverse autonomous groups which result in more randomized and directed search. Applying AGPSO to tune PI controller to control DFIG is proposed in this paper. An implemented laboratory prototype consists of brushless DC motor (BLDC) for simulating the various wind speeds. Wound rotor induction machine, working as DFIG. This system is a stand-alone system. System identification strategy was introduced in this work. In this study, AGPSO is suggested for tuning the PI controller. Different case studies are performed, such as step changes in both speed and electrical load for showing the effectiveness of the proposed algorithm. For comparison PSO is used to tune the PI controller. Results from experiments clarify the feasibility of the proposed methodology. It is approved that AGPSO achieves the prevalent control execution (quicker transient response and more modest steady state error (ess)) contrasted with the PSO in tuning PI controller when applied to be used with off-grid systems. &lt;/span&gt;

Exploitation of the Power Capability in a Five-Phase Doubly Fed Induction Motor Drive for Contact-Less Energy Transfer

Rotary assembly stations are commonly used in automated manufacturing processes. These production platforms require the speed control of the plate and need power for the actuators and sensors located on the rotating frame. These requirements are usually met through a gearmotor and sliding electrical contacts. Among the solutions that avoid the slip-rings, this paper focuses on an integrated electric drive consisting of a gearless five-phase wound-rotor induction machine fed by two inverters, one placed on the stator reference frame and one placed on the rotating one. The main contribution is a novel control scheme that maximizes the power transferred to the rotating loads in compliance with the thermal constraint of the rotor and the voltage constraint of the rotor inverter. The theoretical analysis allows deriving an analytical solution to the control problem, and the experimental results confirm the effectiveness of the proposed algorithm.

Control mechanism of unbalanced magnetic pull in doubly fed induction generator using extra pole‐specific stator windings

The doubly fed induction generator is commonly used in commercial wind turbines because of its simplicity and robustness; it is a wound rotor induction machine. It has a relatively small airgap which makes it more vulnerable to unbalanced magnetic pull. This paper presents a simulation model to investigate the ability of extra stator windings with pm ± 1 pole-pairs to control the unbalanced magnetic pull (UMP). These auxiliary windings are inserted in the stator slots and they are used to produce counteracting flux that reduces the UMP. In the theory section, an UMP control matrix is fully developed that involves additional winding currents for obtaining active UMP control. The algorithm for controlling the UMP is then introduced. This uses vector control with stator flux orientation. Several simulations are carried out using MATLAB and Altair Flux 2D finite element analysis. These show a good reduction of UMP under open circuit and locked rotor tests. These models allow the study of the combined effects of radial forces with changes in the machine design. It was found that the average power absorbed by the auxiliary windings equals 13% of the total power losses in normal operation conditions.

An Improved Dual DTC of Double-Inverter-Fed WRIM Drive With Reduced Torque Ripple by Emulating Equivalent 3L NPC VSC

Wound rotor induction machine (WRIM) connected to stator and rotor voltage source converters (VSCs), known as the double-inverter-fed WRIM drive, can be viewed as two virtual squirrel cage induction machine drives, when seen from air-gap. In this article, this feature of WRIM is explored to develop an improved dual direct torque vontrol (DDTC) technique such that, two individual and independent conventional DTC controllers can be directly employed to control the stator and rotor VSCs. The improved technique is inculcated with all the benefits of conventional DTC controller, especially the benefit of zero dependence on rotor position. Furthermore, the increased torque ripple with the conventional DTC controllers is mitigated by modifying the switching tables (STs) such that, the voltage vectors emulated in the air-gap are equivalent to a 3-level neutral point clamped VSC. The STs are developed by analyzing the WRIM as an open-end winding induction machine (OEWIM). To demonstrate the performance of the proposed DDTC technique, the experimental results obtained from 3 kW WRIM are presented, under various transients and steady-state conditions. The reduction in the torque ripple is demonstrated by estimated torque ripple waveforms, at different speeds.

Coordinate Control of Grid Power, Battery SoC and LVRT Protection in Single VSC Tied DFIG

This paper proposes a coordinate control scheme for the single VSC tieddoubly-fed induction generator (DFIG). In this control scheme, both the gridpower and battery SoC (State of Charge) are maintained to provide an un-interrupted power supply. During the continuous operation of DFIG in thesub synchronous region, there is scope for complete battery discharge. Henceto overcome this drawback, the coordinated control scheme maintains thebattery SoC level within the limits. If the SoC falls below the specified lowerlimit, then the proposed scheme curtails the grid power. Instead of dischargingthe battery, the control shifts the battery to charging mode until the safelimit of SoC is attained. During the continuous operation of DFIG in thesuper synchronous region, if the SoC reaches its upper limit, the proposedscheme discharges the extra power to the dump load. Further, this controlscheme also introduces the low voltage ride through (LVRT) aspect accordingto IEGC (Indian electricity grid code) of 15% of nominal voltage and also an enhanced rotor position computation is implemented for the effectiveestimation of rotor position for single VSC tied DFIG. This control makesthe topology more robust and improves the reliability of the system. Theproposed scheme is validated for a test system of 3.7 kW Wound RotorInduction Machine based DG unit and investigations are done in MATLABsimulation.

Dual-Mode Brushless Wound Rotor Synchronous Machine for High Starting Torque

This paper proposes a dual-mode brushless wound rotor synchronous machine (DBL-WRSM) for high starting torque. The proposed DBL-WRSM consists of two modes: induction and brushless (BL)-synchronous modes. First, the induction mode operates as a wound rotor induction machine (WRIM) to achieve a high starting torque. Second, the BL-synchronous mode operates as a brushless wound rotor synchronous machine (BL-WRSM) using the subharmonic of the stator magnetomotive force (MMF) in the steady state. To realize the dual-mode topology of the proposed machine, the stator is composed of two divided armature windings and two inverters that can produce subharmonics. The rotor has five switches that convert the rotor winding configuration between the two modes. To verify the proposed topology of DBL-WRSM, finite element analysis (FEA) was performed. Through the FEA results of the transient electromagnetic torque for the voltage source, it was confirmed that the proposed DBL-WRSM not only operates as a BL-WRSM in a steady state but also has a sufficiently high starting torque even under a full-load condition.

Design and Control of Micro-Grid fed by Renewable Energy Generating Sources

This paper presents a control of a micro-grid 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. unviability of wind or solar energies, unbalanced and nonlinear loads, 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.

Magnetic Flux Analysis for the Condition Monitoring of Electric Machines: A Review

Magnetic flux analysis is a condition monitoring technique that is drawing the interest of many researchers and motor manufacturers. The great enhancements and reduction in the costs and dimensions of the required sensors, the development of advanced signal processing techniques that are suitable for flux data analysis, along with other inherent advantages provided by this technology, are relevant aspects that have allowed the proliferation of flux-based techniques. This article reviews the most recent scientific contributions related to the development and application of flux-based methods for the monitoring of rotating electric machines. Particularly, aspects related to the main sensors used to acquire magnetic flux signals as well as the leading signal processing and classification techniques are commented on. The discussion is focused on the diagnosis of different types of faults in the most common rotating electric machines used in industry, namely: squirrel cage induction machines, wound rotor induction machines, permanent magnet machines, and wound field synchronous machines. A critical insight of the techniques developed in the area is provided and several open challenges are also discussed.

Stator and rotor turn‐to‐turn fault detection in wound rotor induction machines based on the air‐gap magnetic field distortion

An efficient online flux-based diagnostic approach is proposed to detect turn-to-turn faults in the stator and rotor windings of wound rotor induction machines, at an early stage of development, which can be applied in the online mode. Some flux sensors are installed in the stator slots to detect the rotational magnetic field at various positions along the air-gap circumference. These sensors measure the flux linked to all winding coil groups, in the three phases. Any fault occurring either in the stator or rotor windings will disturb the air-gap flux and create an asymmetry in the rotational magnetic field, which is detectable through the difference of the induced voltages in the corresponding flux sensors. The proposed diagnostic technique is evaluated by finite element analysis as well as by multiple experimental tests. The obtained results show that such invasive technique is able to detect and discriminate various faults in the stator and rotor windings with high accuracy, low calculation burden, proper sensitivity and appropriate robustness in the face of machine load level variations.

Dynamic Simulation of Unbalanced Magnetic Force in Doubly-Fed Induction Machine with Inter-turn Short-Circuited Stator

In the presented work, a dynamic model is provided for the wound-rotor induction machines with short-circuited stator winding. Both inter-turn phase-to-ground and inter-turn phase-to-phase short circuit faults are considered in the provided model. The self- and mutual-inductances of the windings of the faulty machine are the parameters of the provided state-space equations. As well as the machine electromagnetic torque and its stator and rotor currents, the waveform of the unbalanced magnetic force (UMF) in the faulty induction machine is predicted in the provided model. The UMF of the faulty machine is expressed by a static function of the machine currents. The parameters of the UMF function are obtained by using the Maxwell definition. The machine inductances and the air gap flux density distribution are obtained by means of winding function method. Since the exact waveforms of the machine turn functions are considered in computation of the air gap flux density, the special harmonics are taken into account inherently in the provided model. Finally, the developed model is verified by means of the time-stepping finite element model.

Mathematical Modeling and Current-Oriented Control of Double-Inverter-Fed Wound Rotor IM Emulated as Two Virtual Cage Rotor IMs

Stator- and rotor-side voltage source inverters (VSIs) (VSI-S and VSI-R) of a double-inverter-fed wound rotor induction machine (DI-WRIM) drive individually feed 1-p.u. power each, to extract 2-p.u. power at a 2-p.u. speed. This feature of the DI-WRIM drive is explored, in this article, to model this drive as two virtual cage rotor induction machines (CRIMs). The VSI-S connected to the stator is emulated as one VSI-fed CRIM with the rotor short circuited, while the VSI-R connected to the rotor is emulated as another VSI-fed CRIM with the stator short circuited. The mathematical modeling of the DI-WRIM drive as two virtual CRIMs, in the synchronous reference frame, is presented by considering the angle between the stator and rotor currents (γ) as a medium of coupling between the two virtual CRIMs. The proposed mathematical model of the DI-WRIM is used to demonstrate the performance of the drive in a current-oriented control (COC) technique. In the COC technique, the VSI-S is controlled in the stator current-oriented frame and the VSI-R is controlled in the rotor current-oriented frame. The proposed model and technique is tested on a 4-hp WRIM drive and the experimental results are also presented to demonstrate the satisfactory performance of the drive.

Real-Time Digital Twin of a Wound Rotor Induction Machine Based on Finite Element Method

Monitoring and early fault prediction of large electrical machines is important to maintain a sustainable and safe power system. With the ever-increasing computational power of modern processors, real-time simulation based monitoring of electrical machines is becoming a topic of interest. This work describes the development of a real-time digital twin (RTDT) of a wound rotor induction machine (WRIM) using a precomputed finite element model fed with online measurements. It computes accurate outputs in real-time of electromagnetic quantities otherwise difficult to measure such as local magnetic flux, current in bars and torque. In addition, it considers space harmonics, magnetic imbalance and fault conditions. The development process of the RTDT is described thoroughly and outputs are compared in real-time to measurements taken from the actual machine in rotation. Results show that they are accurate with harmonic content respected.

Modeling, simulation and control of a doubly-fed induction generator for wind energy conversion systems

In recent years, wind energy has become one of the most promising renewable energy sources. Various wind turbine concepts with different generator topologies have been developed to convert this abundant energy into electric power. The doubly-fed induction generator (DFIG) is currently the most common type of generator used in wind farms. Usually the DFIG generator is a wound rotor induction machine, where the stator circuit is directly connected to grid while the rotor’s winding is connected to the grid via a three-phase converter. This paper describes an approach for the independent control of the active and reactive powers of the variable-speed DFIG. The simulation model including a 1.5 MW-DFIG driven by a wind turbine, a PWM back-to-back inverter and the proposed control strategy are developed and implemented using MATLAB/Simulink/SimPowerSystems environment.