Stator Side Research Articles

Parameters estimation for multiphase induction machine with concentrated windings through finite element method

The numerical technique for parameters estimation is applicable for the induction machine (IM) with arbitrary number of stator phases and arbitrary number of rotor bars. The self-inductance and mutual inductance matrices for the stator side and rotor side are constructed through the magnetic vector potential method. The Clarke transformation is implemented to separate the magnetising inductance and slot leakage inductance of each sequence plane, where the orthogonality theory of trigonometric function is used. The harmonic leakage inductance of each plane is calculated based on the ratios of the harmonic components among air-gap flux density. The 2D eddy current field solver is conducted to obtain the rotor resistance based on the IM equivalent circuit. A classical mathematical model is established using the acquired parameters under Simulink environment to accomplish the comparisons between the simulated and experimental performance. The non-sinusoidal power supply based on the rotor field-orientated control, which is highly dependent on the precision of parameters, is carried out under no-load and heavy load conditions to indirectly verify the accuracy of parameters.

Pulsation and Vibration Measurement on Stator Side for Turbocharger Turbine Blade Vibration Monitoring

Mechanically robust turbine design with respect to blade vibration is challenging when dealing with nozzle-ring fouling and wear. Especially for engines operating with heavy fuel oil (HFO), the nozzle rings of the turbocharger turbines are prone to severe degradation in terms of contamination with unburned fuel deposits. This contamination will lead to an increased excitation of blade resonances in comparison to the nominal design. Due to the statistical character of contamination, long-term monitoring of blade vibration amplitudes would be beneficial. In the harsh environment of HFO operation, however, conventional blade vibration measurement techniques, such as those using strain gauges or blade tip timing, cannot work reliably for a long period. Thus, the objective of this research is to develop a method that enables the monitoring of turbine blades using pulsation or vibration sensors installed on the stator side. Almost a dozen turbines, both radial and axial, have been examined in order to determine a proper measurement chain/position and analytical method. Even though the challenges specific to the turbocharger turbine application—that high-frequency (up to 50 kHz) acoustic radiation from turbine blades has to be detected by a sensor on the stator side—were demanding, in the course of the investigations several clear examples of turbine blades engine-order resonance detection were gathered. Finally, the proposed method has been tested successfully in a power plant for over one year.

An Intelligent Control Scheme for DFIG Wind Farm to Control the Voltage Fluctuations

<p>In this project we investigates the control of doubly-fed induction generator (DFIG) based wind farms for compensating voltage fluctuations in weak networks. Under an unbalanced power grid voltage conditions, the DFIG’s stator side active power, reactive power, and electromagnetic torque will generate twice-multiplied frequency pulse quantities. The proposed control methodology exploits the potential of the series-DFIG scheme to avoid that grid voltage unbalances compromise the machine operation, and to compensate voltage unbalances at the point of common coupling (PCC), preventing adverse effects on loads connected next to the PCC. The modeling of the system has been discussed and also the system parameters are plotted. MATLAB Simulink has been used as the tool to evaluate the system. The grid parameter variations are also discussed.</p>

Asymmetrical LVRT of DFIG incorporating feed‐forward transient current control and controllable resistive‐type fault current limiter

AbstractDuring the low‐voltage ride‐through (LVRT) of the doubly fed induction generator (DFIG), the change rate of the stator flux is introduced as the feed‐forward compensation (FFC) to prevent the stator flux oscillation from affecting the rotor current control. The dual‐sequence FFC is proposed for the asymmetrical LVRT of the DFIG. To reduce the rotor voltage increase due to the FFC, the controllable resistive‐type fault current limiter (CRTFCL) is implemented at the stator side to improve damping to the stator flux oscillation. The resistance of the CRTFCL is controlled based on the ramp function to relieve oscillations resulted from the switch‐out of the CRTFCL. With the dual‐sequence FFC and CRTFCL applied to the asymmetrical LVRT, analytical expressions of the stator current, rotor current, and rotor voltage are derived. The accuracy of the analytical expressions and the improved LVRT performance with the dual‐sequence FFC and the CRTFCL are verified based on the time‐domain simulations. © 2020 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Small signal stability of three-phase and six-phase synchronous motors: A comparative analysis

Small signal stability analysis is conducted for an asymmetrical six-phase synchronous motor in comparison with its equivalent three-phase counterpart. For this purpose, a linearized model of the six-phase synchronous motor is developed using the dq0 approach, which is used in eigenvalue criteria to determine absolute stability in comparison with its equivalent three-phase counterpart. The analysis includes a comparison of the variation in evaluated eigenvalues associated with the stator and rotor sides according to changes in both the three and six-phase machine parameters and working conditions. Key analytical results are experimentally investigated and validated on a test rig.

The Role of Doubly-fed Motor with Low Voltage Ride Through by Controlling Converter in Wind Power Generation

To improve the performance of the doubly-fed wind power converter system, according to the doubly-fed wind turbine system and its mathematical model, a vector control based on doubly-fed wind power converter control strategy and the rotor side stator flux linkage attenuation in the grid voltage drop of the doubly-fed wind power system are proposed. MATLAB/Simulink software is used to simulate different voltage sags and to compare and analyze the changes of stator flux and zero-sequence current between original control and stator flux attenuation control based on rotor side. It is found that when the grid voltage drops by 20%, the impact on the system is small, and when it falls to 80%, it would cause huge damage to the converter. And compared with the original control, the low voltage crossing control based on stator flux attenuation on the rotor side can restore the stable running state of the system more quickly. The results show that the stator flux decay control based on rotor side can make the doubly-fed wind machine transit the process quickly and reach a stable state, which lays a foundation for better control of power grid and reactive power support. The results of this study are intended to provide a basis for the control strategy of the converter in the doubly-fed wind power generation system and the stable operation of the system.

Design of Novel Intelligent Controller for Doubly-Fed Induction Generator-Driven Wind Turbine to Improve Transient Control Performance

This paper presents a design for the novel intelligent control (NIC) of the stabilization of a doubly fed induction generator (DFIG)-based wind turbine. The intelligent control scheme was developed to control the rotor side voltage source converter that allowed independent control of the generated active and reactive power as well as the rotor speed to track the maximum power point (MPP). Moreover, the NIC was used to reduce the interaction between the real and reactive power flow and improve the performance. The proposed NIC consisted of an improved recurrent fuzzy neural network (IRFNN) and ant colony optimization with genetic algorithms (GACO). GACO was adopted to adjust the learning rates to improve the online learning capability of the IRFNN. Furthermore, an NIC controller was proposed for both the rotor and stator side converters to improve the steady-state and transient voltage stability of the DFIG system for different operating conditions. In addition, the method is highly simple to implement. Finally, the proposed approach stabilized the internal dynamics through rotor voltage control and improved the dynamic behavior of the DFIG after clearing a fault.

Improved Voltage Harmonics Compensation Strategy for a Stand-Alone DFIG Operating under Unbalanced and Nonlinear Load Conditions

The study presents an improved control and operation for a stand-alone Doubly-Fed Induction Generator (DFIG) energy system under unbalanced and nonlinear load conditions. This type of loads results in unbalance and distorted stator voltage and current at the Point of Common Coupling (PCC), with subsequent power quality degradation and electromagnetic torque pulsations of DFIG. A new and simple control strategy based Proportional-Resonant Controllers (PRCs) in the stationary reference frame is applied in the Load-Side Converter (LSC) for fully compensate the unbalance and harmonic distortion in the PCC and avoid the torque pulsations of DFIG. As a result, symmetrical and sinusoidal AC voltage is applied to the PCC, which indicates that the Rotor-Side Converter (RSC) can be still controlled with the traditional vector control strategy, to adjust the generator torque and to balance active power. This strategy is conceptually simple, it does not require for rotating coordinate transformation and the sequence decomposition process, in addition no current sensors need to be installed at load or stator side, so that the number of current sensors employed is reduced. Simulation results using MatLab/Simulink are given and show the effectiveness and validity of the proposed control scheme to eliminate the harmonics in the PCC and torque pulsations of DFIG.

Torsional Vibration Analysis of Shaft in an Induced Draft Fan Due to Variable Frequency Drive

The torsional vibration problem of shaft in an induced draft fan (IDF) caused by frequency conversion driving technology is a result of dynamic electro-mechanical coupling behavior during the energy conversion. Therefore, it is difficult to understand the mechanism of torsional vibration by only analyzing the mechanical structure or the electrical structure. In this paper, eigenvalue structural analysis (ESA) and time-domain transient analysis, which are typically used to examine sub-synchronous resonance (SSR), are employed to resolve this issue. It is difficult to develop a linear model for complex controllers with ESA as ESA can cause certain errors during the linearization and estimation of initial values of equilibrium points. On the contrary, time-domain analysis not only offers a small-step integration technique to simulate a realistic condition but also facilitates transient analysis of complex controllers. Transient analysis has been used to study the influence of elastic parameters of shaft, self-damping coefficients, resistance of the stator side, smooth reactance of the stator side, and voltage-frequency ratio on the control of target frequency converters and cascade multilevel high-voltage frequency converters. In this paper,It is also advised to avoid torsional resonance region with torsional-vibration monitoring during the operation of the system, and stator resistance can be added and the voltage-frequency rate can be adjusted to obtain an integrated control of an active torsional-vibration system.

Matrix structure of unified mathematical model of electric AC machines at control

The matrix structure of the equations of a generalized electric alternating current machine is proposed, which, based on the Parke equations, is written in the coordinate axes of the machines rotating with the rotor speed. In the matrix structure, the column matrices of the derivatives of the stator, excitation and rotor windings are equal to the product of diagonal matrices consisting of the machine parameters and the column matrices of the flux links themselves and the sum of the matrix columns of the control parameters which are the matrix columns of the stator voltage, excitation voltage, and rotor voltage. It is shown that the matrix structure of a generalized controlled AC machine is transformed into mathematical models of almost all encountered AC electric machines, namely, into a synchronous machine with two excitation windings - a longitudinal and a transverse one; in a synchronous machine with a longitudinal field winding (classic); in an asynchronous machine with a squirrel-cage rotor; into an asynchronous machine with a phase rotor. It has been shown that the matrix structure includes the controls of these machines both from the stator and from the rotor. On the stator side for synchronous machines, it is a frequency control that regulates both the amplitude and frequency of the applied voltage, and on the rotor side, a constant voltage control is supplied to the longitudinal and transverse windings. For asynchronous machines, the stator and rotor are frequency-controlled. The following are examples of frequency control of an asynchronous machine both from the stator and from the rotor.

Sensitivity Analysis and Design Optimization of a New Hybrid-Excited Dual-PM Generator With Relieving-DC-Saturation Structure for Stand-Alone Wind Power Generation

This article proposes a new hybrid-excited dual-permanent-magnet generator with the relieving-dc-saturation (RDCS) structure for stand-alone wind power generation. The proposed generator integrates the stator dc source and rotor permanent-magnet (PM) source in a single brushless structure, which enables coordinated power generation and flexible power adjustment. Moreover, considering the parasitic saturation effect in the stator core caused by the dc field source, a constant PM bias is introduced at the stator side to construct an RDCS structure, so that the core utilization ratio is improved as well as the generator power density. In this article, the generator structure and the operation principle are introduced. Some leading design parameters are determined based on the sensitivity analysis. A finite element and genetic algorithm combined method is further adopted to realize a multi-objective design optimization, and the optimal design of the proposed topology is verified to be a potential generator candidate for stand-alone wind power generation.

2-D Analytical Model for Predicting Magnetic Flux Distribution in Slotless Single-sided Axial Flux Permanent-Magnet Synchronous Machines

This paper estimates the magnetic flux density components in the slotless single-sided axial flux permanent-magnet synchronous machines (SAFPMSMs). For this purpose, a 2-D analytical model based on the sub-domain method is utilized in which the cross-section of the presented machine is divided into the seven sub-regions such as stator side exterior, stator, winding, air-gap, permanent-magnets (PMs), mover and mover side exterior. Based on the Maxwell equations, the related partial differential equations (PDEs) of magnetic flux density components are formed in each sub-region which are identified as the essential step for obtaining the machines quantities. According to the superposition theorem, two separate steps are implemented for calculating the magnetic flux density components. In the first step, open circuit analysis includes various type of magnetization patterns, i.e. parallel, ideal Halbach, 2-segment Halbach and bar magnet in shifting direction is investigated and armature currents are zero and in the second step PMs are inactive and the magnetic flux density components are originated due to only armature reaction. Eventually, 2-D finite element method (FEM) is determined to confirm the accuracy of the presented analytical approach and an acceptable agreement between the analytical and FEM models can be observed.

Effectiveness evaluation of passive resistive element placement on a fault ride‐through enhancement in a DFIG‐based wind energy conversion system

AbstractOver the last few decades, doubly fed induction generators (DFIG) have become one of the most successful and preferred types of wind energy generators (WEG). The DFIG has the advantages of a wide range of speed operations, a high efficiency, and partial rated converters. However, direct coupling of the stator with the grid makes the system more prone to grid disturbances. The consequences of grid disturbances, such as a rotor overcurrent, stator overcurrent, electromagnetic torque oscillations, and direct current (DC) link overvoltage, are the predominant considerations that affect the rotor circuit, stator circuit, mechanical components, and DC‐link capacitor of the DFIG, respectively. These uncertainties affect the operation of the generator and may lead to the generator to be shutdown. In this paper, a novel position for the placement of a passive resistive element (PRE) is illustrated. This position of the PRE placement is compared with all other possible locations for the PRE. The different locations for PRE placement are the stator side, rotor side, across the DC‐link capacitor, and between the rotor side converter (RSC) and grid side converter (GSC). This paper aims to determine a cost‐effective solution among all possible locations of the PRE placement. The novel position of the PRE, ie, between the RSC and GSC, is the best position among the other possible locations of the PRE, when the performance, cost, and loss are taken into consideration. The effectiveness of the PRE is further compared with the resistive‐type superconducting fault current limiter (R‐SFCL). The PRE performs better and has a lower cost than the R‐SFCL.

Performance of Brushless Induction Excited Synchronous Generator

This paper reports a brushless induction excited synchronous generator. An induction machine (IM) and a synchronous machine (SM) are embedded in the same machine frame. To avoid the magnetic interaction between these two machines, they are configured for different numbers of poles. However, rotor windings of both machines are electrically coupled using a rotating rectifier. Therefore, field current of the SM can be controlled smoothly from stator side of the IM. Thus, this machine is expected to be more compact and cost effective compared to traditional pilot exciter-based brushless and permanent magnet less synchronous generators. In this paper, SM and IM are configured for four and six poles, respectively. The IM is operated in self-excited or power-zero mode such that it need not draw any active power from the external source at steady state. This is achieved by operating the IM at negative slip through a closed loop controller. A Vector control-based voltage regulation algorithm is proposed and dynamic performance of this controller is examined using PLECS. A 5.5 kVA laboratory prototype has been designed and fabricated to demonstrate the usefulness of the proposed generator and the controller.

Minimization of Current Ripple and Switching Losses in Double-Inverter Fed Wound Rotor Induction Machine Drive Using PWM Techniques

In the topology of double-inverter fed wound rotor induction machine (DI-WRIM) two voltage source inverters (VSIs) are connected one each on stator and rotor terminals. This leads to ripple in both stator and rotor currents. In this paper, these current ripples are analyzed in terms of the stator- and rotor-flux ripples, which are estimated from their respective terminals. To reduce the current ripples, the VSIs of the stator and rotor terminals are modulated with different pulsewidth modulation (PWM) sequences. The switching loss in the DI-WRIM is also analyzed for the proposed stator–rotor PWM sequence combinations, based on the flexibility of changing the power factor, by changing the flux component of the current fed from either side of a DI-WRIM drive. The proposed PWM combinations with the proposed flux component in currents are found to reduce the stator and rotor current ripples and, the switching losses. The analytically estimated root mean square current ripples for the proposed PWM combinations are validated using the simulation and the experimental results obtained from a 3-kW DI-WRIM drive. The analysis on switching losses is validated by measuring the temperature rise of heat sinks present in the individual stator and rotor side VSIs of the DI-WRIM drive.

Determination of the most effective switching signal and position of braking resistor in DFIG wind turbine under transient conditions

A quick post fault recovery of wind generator system could be achieved with the help of a series dynamic braking resistor (SDBR). This paper investigates the dynamics, size, optimal location of a series dynamic braking resistor in the doubly fed induction generator (DFIG) structure based on different switching strategy of the braking resistor. Three signals were investigated in switching the SDBR during transient conditions of the DFIG wind generator; the DC-link voltage, rotor current and grid voltage, respectively. Also, three locations were considered in connecting the SDBR in the DFIG system; the stator, rotor and grid side converter, respectively. Furthermore, the optimum size and best insertion time of the SDBR were investigated based on the most effective performance of the SDBR scheme considering its switching signal and position in the DFIG system. Simulations were run in power system computer-aided design and electromagnetic transient including DC (PSCAD/EMTDC). The results show that when the SDBR is connected to the DFIG, improved performance was achieved; however, the stator based SDBR topology in conjunction with a grid voltage actuating signal, fast insertion time and short duration of operation quickly recovered the DFIG wind generator variables during transient conditions.

Comparison of stator‐ and rotor‐surface‐mounted PM brushless machines

Flux-reversal permanent magnet (PM) machines can be considered as stator-surface-mounted PM (SSPM) machines, where normally a pair of magnets with reversal magnetization polarities is mounted on the surface of each stator tooth. Hence, the SSPM machines and the conventional rotor surface-mounted PM (RSPM) machines are rather similar since both machines house the PMs in the air-gap, and the only difference is the location of magnets in the stator side and rotor side, respectively. In this study, a comparative study between SSPM and RSPM machines is conducted to investigate the difference including topology, operation principle and performance. First of all, the topology of both machines is compared. Then, the operation principle in both machines is analysed based on the air-gap field modulation theory. Correspondingly, the characteristics of flux density harmonics either due to PMs or armature reaction are obtained. Therefore, the harmonics that make contribution to back-electromotive force and rated torque are quantified. Besides, other electromagnetic performances, e.g. overload capability, flux weakening ability and loss distribution are also evaluated. Finally, a 12/14 prototyped SSPM machine is manufactured and tested to verify analytical and finite element method-based prediction.

Immunizing Variable Frequency Transformer From Dual-Side Asymmetrical Grid Faults via a Single-Converter-Based Novel Control Strategy

Both variable frequency transformer (VFT) and high voltage direct current converter are feasible solutions for asynchronous interconnection of different power grids. The VFT possesses inherent natural damping and high overloading capability, which is important for stability and reliability of power grids. The VFT, however, has not been widely used, since it cannot well solve some grid faults like asymmetrical grid faults. In this paper, a novel control strategy is designed on a single series compensation converter (SCC) of traditional VFT. With only replacement of this proposed strategy, the traditional VFT is immune to dual-side asymmetrical grid faults. In detail, three controllers are designed and implemented in a single SCC, i.e., two negative sequence controllers are designed to deal with asymmetrical grid faults at stator and rotor sides of the VFT, respectively; A positive sequence controller is adopted to control the reactive power flowing through the VFT, which reserves the original function of the SCC. Detailed theoretical analysis and hardware-in-loop experiments are carried out to verify the proposed control strategy.

Improved Direct Resonant Control for Suppressing Torque Ripple and Reducing Harmonic Current Losses of DFIG-DC System

The most prominent problem of the doubly fed induction generator-dc (DFIG-dc) system is the highly distorted stator voltage due to the diode bridge on stator side. The highly distorted stator voltage not only influences the stator voltage orientation control but also causes harmonic currents and torque ripples. In this paper, an enhanced stator fundamental voltage orientation method is proposed to deal with the highly distorted stator voltage. Furthermore, the relationship between torque ripples and harmonic currents is investigated. And then an improved direct resonant control strategy is proposed for suppressing the torque ripple and reducing the harmonic currents simultaneously. In this way, the loss of harmonic currents can be reduced while suppressing the torque ripple. Finally, the experimental results are presented to validate the effectiveness of the proposed control strategy.

Direct Power Control of Grid Connected Double-Fed Induction Generator in Wind Energy Conversion System

Direct Power Control is a rotor side control of a doubly fed induction generator by which decoupled control of stator active and reactive power can be obtained is presented. It is based on the measurement of active and reactive power on the grid side where voltages and currents are alternating at fixed frequency. The active and reactive powers are made to track references using hysteresis controllers. The measurements are taken on stator side and the control is made on rotor side. This control system eliminates the need for rotor position sensing and gives an excellent dynamic performance with simulation results for a variable speed constant frequency induction generator system. The system can be operated below, at and above synchronous speed. The modeling of the Doubly Fed Induction Generator (DFIG) system is discussed in this paper. The power is controlled by rotor current injection. The controlling scheme is explained and the modeling of the complete system is done in MATLAB-SIMULINK. The performance of this control technique is observed in wind power applications. Optimum power control and pitch angle control for wind turbine is proposed and the response of generator speed, pitch angle, optimum power, turbine torque, stator current, stator and rotor powers for different wind speeds are shown by simulation results. The direction of stator and rotor power flow is explained with simulation results.