D-axis Stator Research Articles

Coupling DFIG-Based Wind Turbines with the Grid under Voltage Imbalance Conditions

A smooth coupling is implemented between the grid and doubly fed induction generator-based wind turbines (DFIG-WTs) during grid voltage imbalance. The nonlinear characteristics of a grid-connected DFIG-WT system may increase stresses on the mechanical and electrical components of wind turbines. Such difficulties are greatly increased during periods of voltage imbalance. Consequently, in this paper, a new control scheme is proposed to regulate DFIGs in order to support a smooth connection to the grid during voltage imbalance. In synchronization mode, the positive sequence of the rotor dq-axes currents regulates the stator q-axis EMF that is to be synchronized with the q-axis voltage of the grid-side voltage. The phase difference between the grid and stator voltages is compensated by adjusting the stator d-axis EMF to zero. Under normal conditions, a PR controller is used to dampen the negative sequence of the rotor dq-axes currents. PI current controllers are tuned to control the positive sequence of the DFIG rotor currents, while PR current controllers are used to regulate the negative sequence of the rotor currents during synchronization and under normal operation conditions. Experiments are performed to verify the smooth synchronization of the DFIG and the robustness of the proposed control scheme during grid voltage imbalance.

Combined RBFN based MPPT and d-axis stator current control for permanent magnet synchronous generators

This paper proposes a new radial basis function neural network maximum power point tracking controller based on a differential evolution algorithm for machine side converter of permanent magnet synchronous generator wind turbine under variable wind speed. Direct axis stator current control methods of permanent magnet synchronous machine are reviewed shortly. A combined radial basis function neural network-based network maximum power point tracking method and d axis stator current control techniques including zero d axis stator current, unity power factor, and constant stator flux-linkage have been implemented to control the machine side converter of permanent magnet synchronous generator wind turbine. The dynamic performance of the proposed approach is assessed under different operating conditions through a simulation model based on MATLAB. It has been seen that the radial basis function neural network controller can not only track well the maximum power point but also can be reduced costly.

Field Current Control Strategy for Wound-Rotor Synchronous Motors Considering Coupled Stator Flux Linkage

The direct axis (d-axis) stator winding and field winding of a wound-rotor synchronous motor (WRSM) are mutually coupled much like the primary and secondary windings in a transformer are. As the field winding has a large number of turns, small variations of the stator d-axis current can induce a great electromotive force (EMF) in the field windings. This induced EMF voltage disturbs field current regulation, invokes an unwanted field current response, and distorts the stator d-axis flux. In this work, a novel field current control strategy for the WRSM is proposed for compensating the disturbance by d-axis current in field winding. The coupling effect between the stator d-axis winding and field winding is analyzed by using a polynomial regression of the stator d-axis flux linkage. Based on an analysis of the coupling effect, the proposed strategy combines two control methods. First, feedforward control is applied to decouple the field current response from the coupled change in the stator d-axis current flux. Second, a method is used to adjust the d-axis current to ensure that the field inverter voltage is not saturated. Simulations and experiments are presented to verify the proposed control method, proving that the proposed field current control strategy successfully compensates for the field current ripples caused by variations in the coupled stator d-axis current flux.

Dynamic Control of a DFIG Wind Power Generation System to Mitigate Unbalanced Grid Voltage

This paper presents an improved control strategy for a doubly-fed induction generator (DFIG) during unbalanced grid voltage conditions. The proposed strategy was applied in both synchronization and grid-connected conditions. The synchronization process is carried by controlling the extracted positive and negative sequence components of the stator q-axis voltage to follow the grid q-axis voltage. This strategy can be accomplished by controlling the positive and negative sequence components of the rotor d-axis current. By perturbing the rotor d-axis current, the stator EMF builds up and follows the grid voltage accurately. The stator frequency and the phase difference between the stator and grid voltage are compensated by adjusting the stator d-axis positive and negative voltage components to zero. After synchronization, the proposed control strategy focuses on regulating the average stator active and reactive power control by controlling the positive components of q and d-axis currents, respectively. The second target is to minimize the generator torque ripple by controlling the rotor negative sequence components. At the same time, the grid side converter is controlled to minimize the grid power pulsations to reduce the impact of the unbalanced grid voltage. This study focuses on enhancing the dynamics of DFIG during the unbalanced grid voltage by using Multivariable State Feedback (MSF) current controllers. Experiments are carried out to validate the performance improvement by using the proposed method. The simulation and experimental results showed superior performance of the proposed control strategy.

Enhanced Control Algorithm in PMSMs with Machine Current Sensor FDI and Current Management for Hardware Protection

This paper proposes an enhanced control algorithm in permanent magnet synchronous machines (PMSMs) with machine current sensor fault detection and isolation (FDI) and current management for hardware protection. Different from existing work on machine current sensor FDI, the proposed method addresses the sensor FDI problem under co-existence of PMSM current sensor and non-sensor imbalance faults, and thereby help prevent the controller performance degradation caused by a faulty machine current sensor signal. Meanwhile, hardware protection is achieved through the proposed algorithm which has a unique feature of providing online overcurrent regulation and source power management at the same time to enhance protection of power inverter and power source. Benchmark comparison of the proposed sensor FDI method to state-of-the-art techniques is carried out to validate the good performance of the proposed FDI method in terms of fault detection time, false detection rate and missed detection rate. Sensitivity analysis of the proposed current management algorithm with overcurrent regulation and source power management is performed under changing rotor flux linkage and machine stator d-axis self-inductance, as an example to demonstrate the robustness of the proposed algorithm for hardware protection against system parameter variations. The effectiveness of the proposed algorithm is confirmed by simulation results in MATLAB for a PMSM.

Standstill Frequency Response Test and Validation of a Large Hydrogenerator

This paper intends to contribute to the revision process of the IEEE Standard 115 by demonstrating the applicability of the standstill frequency response (SSFR) test on large salient pole hydrogenerators. The presented SSFR tests are carried out on a 55.6-MVA salient pole machine with laminated rotor, non-continuous damper windings, and a nonintegral slot number. The IEEE-115 SSFR test procedure is applied with special care to rotor positioning as well as accurate data acquisition in the low-frequency range. The maximum likelihood estimation method is utilized for machine parameter identification from the SSFR tests. Obtained parameters are compared with design values in addition to the ones obtained using traditional “sudden no-load three-phase short-circuit,” Dalton–Cameron and “open stator d-axis transient time constant” methods. The accuracy of parameters is also confirmed by comparing the measured three-phase short-circuit current waveforms with the ones obtained by simulating the SSFR-based machine models in an electromagnetic transient -type software. Unlike previous SSFR test cases on large salient pole hydrogenerators, accurate results are obtained.

Efficiency enhancement for indirect vector-controlled induction motor drive

ABSTRACTIn this paper, efficiency enhancement algorithms are developed and implemented on an indirect vector-controlled three-phase induction motor (IM) drive, and its performance under different operating conditions is analysed. The controllable electrical losses in the IM are minimised through the optimal control of direct axis (d-axis) stator current, and improvement in motor efficiency is achieved by weakening the rotor flux. The optimal d-axis stator current is also estimated using particle swarm optimisation (PSO) to validate the results obtained through analytical control method. The developed algorithms are tested under various operating conditions and the dynamic performance of the IM drive is analysed. The effectiveness of analytical and PSO-based efficiency optimisation control over conventional constant flux control, especially during light load at rated speed operation, is summarised. The effectiveness of the developed algorithm is validated experimentally through development of laboratory prototype set-up. The effect of parametric variation on efficiency, stator current, torque and speed of IM drive is studied through sensitivity analysis. The effect of variation in stator and rotor resistance due to change in operating temperature of the IM is also analysed and the robustness of the developed algorithm against parametric variations is demonstrated through simulation and experimental studies.

Zero Direct-axis Current (ZDC) Control for Variable Speed Wind Energy Conversion System using PMSG

This paper presents a control scheme for variable speed wind energy conversion system (WECS) using permanent magnet synchronous generator (PMSG). The control scheme extracts maximum power from wind using maximum power point tracking (MPPT) technique. In this paper, the proposed control scheme is achieved using two back-to-back voltage source converters (VSC) with the DC-link whose voltage is to be maintained. The control scheme discussed in this paper is zero direct-axis current (ZDC) control where the d-axis stator current is kept zero. Performance evaluation of the system is carried out and detail simulation results are demonstrated using MATLAB/SIMULINK environment. The simulation results proves the effectiveness of application of the ZDC control for variable speed WECS.

Particle swarm optimisation in efficiency improvement of vector controlled surface mounted permanent magnet synchronous motor drive

Application of particle swarm optimisation (PSO) in efficiency optimisation of vector controlled surface mounted permanent magnet synchronous motor (PMSM) drive is presented in this study. Analytical expressions for the controllable electrical loss in PMSM are derived and optimised through control of direct axis (d-axis) stator current, according to the desired speed of operation and load torque. The optimal value of d-axis current has been obtained through analytical method as well as through PSO. The motor efficiency is maximised by using the optimal d-axis current to minimise the air gap flux, resulting in reduction of the core losses, especially at light load conditions. The robustness of the efficiency optimisation algorithms are demonstrated through parametric sensitivity analysis. Implementations of efficiency optimisation algorithms are carried out in Matlab and the performance of PMSM drive is described under steady state as well as transient operating conditions. The effect of efficiency optimisation algorithms in different region of operation of PMSM such as constant speed-variable load torque and constant torque–variable speed modes are described.

Analysis and Compensation of Current Measurement Errors in a Doubly Fed Induction Generator

It is necessary to measure the current of rotor for controlling the active and reactive power generated by the stator side of the doubly fed induction generator (DFIG) system. There are offset and scaling errors in the current measurement. The offset and scaling errors cause one and two times current ripples of slip frequency in the synchronous reference frame of vector control, respectively. This paper proposes a compensation method to reduce their ripples. The stator current is variable according to the wind force but the rotor current is almost constant. Therefore input of the rotor current is more useful for a compensation method. The proposed method adopts the synchronous d-axis current of the rotor as the input signal for compensation. The ripples of the measurement errors can be calculated by integrating the synchronous d-axis stator current. The calculated errors are added to the reference current of rotor as input of the current regulator, then the ripples are reduced. Experimental results show the effectiveness of the proposed method.

Performance Enhancement of PMSG Systems with Control of Generator-side Converter Using d-axis Stator Current Controller

This paper presents a performance enhancement of a permanent magnet synchronous generator (PMSG) system with control of generator-side converter for a wind turbine application. This method uses zero d- axis stator current control to minimize winding losses of the generator. The electromagnetic torque of the generator is correlated with the magnitude of the qaxis stator current, while the daxis stator current is regulated at zero, the control scheme decouples the dq-axis stator current control through a vector con- trol for the generator-side converter. This paper also presents mathematical analysis of the active power and stator power factor, and the maximum power point tracking (MPPT) operation. Simulation re- sults are provided to guarantee the proposed control scheme, in which the performance enhancement and eciency are evaluated.

A Radial Position Control Method of Bearingless Motor Based on d - Q -Axis Current Control

This paper presents a radial position control method of bearingless motor (BELM) based on a d-q-axis current control and an integrated winding arrangement. The BELM performs the functions of a motor as well as magnetic bearing. Rotor is suspended with noncontact by suspension force. In a conventional BELM, there are two kinds of stator windings such as motor and suspension windings. The motor winding outputs the rotational torque, and the suspension winding outputs the suspension force. To simplify the structure of conventional BELM, integration of two windings is proposed in this paper. By integrating the windings, the structure of BELM becomes simple, and also, the torque and force productions are increased. The d-axis stator current controls the suspension force, and the q-axis current controls the developed torque of the motor. In the case when there is less number of slots in the machine, the actual suspension force cannot follow the command. In order to make the actual suspension force follow the command, a compensation method is also proposed in this paper. A simulation study is done for the proposed radial position control based integrated winding BELM using finite-element analysis software. Furthermore, a prototype machine of the proposed BELM based on d-q-axis current control is built to confirm the performance of the motor and the magnetic suspension in real time. Rotor stabilization with magnetic levitation is confirmed using the prototype machine.

Energy Model Based Direct Torque Control of Induction Motor Using IP Controllers

This paper deals with direct torque control of an induction motor (IM) with constant switching frequency. The desired torque is obtained from the speed controller which is designed using the IP controller. Decoupling control of torque and flux is developed based on the energy model of IM using the IP controller strategies. The desired d-axis and q-axis stator voltage components are obtained from the designed controller, which decouples torque and flux. The constant switching frequency can be applied using space-vector pulse width modulation, since the desired stator voltage can be known from the decoupling torque and flux controllers. In order to achieve stable operation of the proposed IP controllers, the gains of the controllers are chosen by setting the poles in negative (left) half of s-plane and by choosing the rising time for the response of the step function. The proposed controller was verified in simulations using Matlab/Simulink and results have proven excellent performance. It was found that the proposed IP controllers can provide excellent performance to track the desired torque and speed and to reject the disturbance of load.

High-Efficiency Control for a Wind Energy Conversion System With Induction Generator

In this paper, an improved efficiency control scheme for wind energy conversion systems (WECSs) with squirrel cage induction generators is proposed. Thus, the power harvesting from the WECS is increased and additionally expansion of the exploitable wind speed region toward the lower speed range is accomplished. The generator is connected to the power grid by means of two space-vector-controlled back-to-back converters. A minimum ohmic loss (MOL) controller is introduced in order to minimize the generator resistive loss that is accomplished by adjusting the d-axis stator current according to torque conditions. The implementation of the proposed controller is easy and cost effective because neither additional control signals nor the knowledge of the generator loss model is required. The effectiveness of MOL controller and its successful cooperation with two types of maximum power point tracking (MPPT) controllers, which are employed to maximize the wind turbine output power, are experimentally verified. The MPPT controller is implemented by using an adaptive search control and a fuzzy-logic-based control technique, since both are independent of wind turbine characteristics and widely used. Selective experimental results are presented to demonstrate the resulting improvements of the suggested control scheme.

Left-inversion soft-sensor of synchronous generator

SUMMARY The online measurement of some crucial variables in synchronous generator (such as the power angle, the d-axis and q-axis transient electric and magnetic fields, and the d-axis and q-axis stator circuit currents) is very important in many applications, but they are difficult to be measured by ordinary sensors. In the paper, these variables are estimated (soft-sensed) on the basis of the online directly measurable variables of generator (such as the active and reactive powers and the amplitude of current). First, a new model of synchronous generator is proposed, which fully includes the online measurable variables of synchronous generator and is suitable for designing soft-sensors. Second, aiming at the specialty of the proposed model (a typical kind of nonlinear differential-algebraic subsystem), the original left-inversion soft-sensing algorithm proposed in a previous paper is expanded. Then a left-inversion soft-sensor is designed while the fourth-order transient practical model of synchronous generator is considered, and all of the to-be-estimated variables are soft-sensed. Finally, the simulation is performed on the basis of MATLAB/SimPowerSystems (MathWorks, Natick, MA, USA), in which the sixth-order subtransient practical model of generator is used to simulate the real generator, and the results demonstrate the validity of the proposed method. Copyright © 2011 John Wiley & Sons, Ltd.

High performance sensorless speed vector control of SPIM Drives with on-line stator resistance estimation

The aim of this paper is to implement a sensorless Indirect Stator Field Oriented Control (ISFOC) of Single-Phase Induction Motor Drive (SPIMD) with stator resistance tuning. The proposed method for the estimation of speed is based only on measurement of stator currents. A very simple identification algorithm using d-axis stator current error for identifying the stator resistance is proposed. Experimental results for SPIMD are presented and analyzed by using a dSpace system with DS1104 control board based on the Digital Signal Processor (DSP) TMS320F240. Simulated results are compared with experimental results on a test 1.1 kW SPIM setup. The agreement between simulated and experimental results proves the validity of the proposed method.

Semi-analytical method for determining d-axis synchronous generator parameters using the DC step voltage test

The DC step voltage experiment is an alternative and non-destructive time-domain test to extract parameters from electrical machines that follows the standstill frequency response test procedure described in IEEE standard 115A. This work presents a new method for semi-analytically determining the characteristic parameters of the d-axis synchronous machine model. The test is carried out when the machine is at rest and the rotor's position is at d-axis; a DC-controlled power source is applied suddenly at two of the three terminals of the stator machine and the field circuit is short-circuited. Test measurements of d-axis stator voltage excitation and field and d-axis stator current responses were used. Both stator and field current patterns are simultaneously fitted to exponential functions which coefficients are analytically related to the d-axis characteristic parameters of the equivalent circuit. These characteristic parameters come from the conventional two transfer functions: direct-axis operational inductance Ld(s) and the armature to field transfer function sG(s). To validate the proposed methodology, test data is used in the d-axis parameter determination of a 7 kVA, 220 V, 60 Hz synchronous generator.

On-line efficiency optimization of a synchronous reluctance motor

This paper deals with an on-line optimum-efficiency control of a synchronous reluctance motor drive. The input power minimization control is implemented with a search controller using Fibonacci search algorithm. It searches the optimal reference value of the d-axis stator current for which the input power is minimum. The input power is calculated from the measured dc-bus current and dc-bus voltage of the inverter. A rotor-oriented vector control of the synchronous reluctance machine with the optimization efficiency controller is achieved with a DSP board (TMS302C31). Experimental results are presented to validate the proposed control methods. It is shown that stability problems can appear during the search process.

A high-performance sensorless indirect stator flux orientation control of induction motor drive

A new method for the implementation of a sensorless indirect stator-flux-oriented control (ISFOC) of induction motor drives with stator resistance tuning is proposed in this paper. The proposed method for the estimation of speed and stator resistance is based only on measurement of stator currents. The error of the measured q-axis current from its reference value feeds the proportional plus integral (PI) controller, the output of which is the estimated slip frequency. It is subtracted from the synchronous angular frequency, which is obtained from the output integral plus proportional (IP) rotor speed controller, to have the estimated rotor speed. For current regulation, this paper proposes a conventional PI controller with feedforward compensation terms in the synchronous frame. Owing to its advantages, an IP controller is used for rotor speed regulation. Stator resistance updating is based on the measured and reference d-axis stator current of an induction motor on d-q frame synchronously rotating with the stator flux vector. Experimental results for a 3-kW induction motor are presented and analyzed by using a dSpace system with DS1102 controller board based on the digital signal processor (DSP) TMS320C31. Digital simulation and experimental results are presented to show the improvement in performance of the proposed method.

Controllable-Fulx PMSM With Novel Design of Magnetic Circuit in Rotor — Memory Motors

This paper presents a novel magnetic circuite structure of internal composite-rotor controllable-fulx permanent-magnet synchronous machines (PMSMs), or named Memory Motors. According to the given pole pair number, the special placement and dimension of NbFeB and AlNiCo permanent magnets (PMs) brings a ture wide range speed operation to the motors. The magnetization statu of PMs can be changed with d-axis stator current vector pules regulated by the closed loop control system. Further more, once the indensity of magnetization has been changed, the motors will memorize the flux density level. Following the description of the motors' principle, the internal magnetic field has been analyzed by finite element method, and the proposed assumption has been proved feasible. Studies point out that the result of flux weakening can be better, and the influence of armature reaction on air-gap PM-flux is diminished when the larger q-axis magnetic resistance is designed.