A generalized technique for modeling switch-controlled inductionmachine circuits

The main objective is to develop a generalized switch-controlled induction machine circuit modeling technique which is suitable for any switching circuit configuration. The authors present a generalized algorithm which provides a systematic procedure in connecting the formulated induction machine and the switching circuit models through the tensor approach. The formulations of the machine systems for both the switching circuits connected in the stator and rotor sides of the induction machines are derived. By proper selection of the dq reference frame, which depends on the side of the switching circuit connected to the machine, the proposed algorithm will supply an efficient and compact model suitable for arbitrary switching circuit configurations for the induction machine system. Numerical examples on the dynamic simulation of a stator-phase-controlled induction machine circuit are provided to demonstrate the validity of the algorithm     

Doubly-fed induction motor differential cascade. I. Configurationand analysis in the steady state

In this paper, the configuration and the analysis of a doubly-fed differential cascade (DFDC) in the steady state, is presented. The doubly-fed differential cascade consists of two similar doubly-fed induction machines having their corresponding rotor phases connected. The stators are supplied by voltage phasors of variable frequencies. The rotor recovered slip power of the one machine is used to supply the other machine, through the common rotor connection, maintaining the sum of the two machine speeds constant and equal to the difference of the two stator voltage phasor frequencies. The speed of this differential cascade can be controlled by varying the frequency difference of the two stator voltage phasors. The direction of rotation can be changed by changing the sign of the frequency difference of the stators supply voltages. The set of two doubly-fed induction machines connected in a differential cascade may be used in electrical automobile drive systems     

Advanced Backstepping controller for induction generator using multi-scalar machine model for wind power purposes

This paper presents a new non-linear control Algorithm based on the Backstepping approach for an isolated induction generator (IG) driven by a wind turbine. For this purpose and in order to reduce the complexity of the real induction machine mathematical model, the multi-scalar machine model is exploited. The machine delivers an active power to the load via a converter connected to a single capacitor on the dc-side. So, during the voltage build-up process, the necessary stator currents references to be injected by the converter are calculated from the desired active power to be sent to the load and the rotor flux magnitude. Simulation results show that the proposed control provides perfect tracking performances of the DC-bus voltage and the rotor flux magnitude to their reference trajectories.     

Experimental characterization procedure for use with an advanced induction machine model

An advanced induction motor model that includes stator leakage saturation, rotor leakage saturation, magnetizing saturation, and distributed system effects in the rotor circuits has been set forth. This model is considerably more accurate than traditional models, particularly in terms of predicting switching-frequency dynamics. The model proposed is very general in terms of the range of magnetic properties that can be incorporated. This paper provides suggestions for specific forms for the leakage and magnetizing characteristics and derives the resulting small-signal impedance and large-signal steady-state equivalent circuit. Based on these results, a test procedure for experimentally characterizing the machine is developed. The application of the procedure to a 50-hp test machine is included as an example.     

A Synchronous Machine Model With Saturation and Arbitrary Rotor Network Representation

This paper addresses equivalent circuit and magnetic saturation issues associated with synchronous machine modeling. In the proposed synchronous machine model, the rotor equivalent circuits are replaced by arbitrary linear networks. This allows for elimination of the equivalent circuit parameter identification procedure since the measured frequency response may be directly embedded into the model. Magnetic saturation is also represented in both the$q$- and$d$-axis. The model is computationally efficient and suitable for dynamic time-domain power system studies.     

A Voltage-Behind-Reactance Synchronous Machine Model With Saturation and Arbitrary Rotor Network Representation

A voltage-behind-reactance formulation of a synchronous machine model is set forth, which incorporates saturation and cross-saturation, and is general enough to encapsulate a variety of rotor structures by use of arbitrary linear networks instead of equivalent circuits. Specifically, the model's equations - originally expressed in the qd-axes framework - are rewritten in such a way as to allow the actual abc stator windings to be represented by inductive branches, which can then be naturally connected in the desired circuit topology, e.g., to a rectifier. The model's predictions are validated against experimental results.     

An induction machine model for predicting inverter-machineinteraction

The conventional qd induction motor model typically used in drive simulations is very inaccurate in predicting machine performance, except perhaps for the fundamental component of the current and the average torque near rated operating conditions. Predictions of current and torque ripple are often in error by a factor of two to five. This work sets forth an induction machine model specifically designed for use with inverter models to study machine-inverter interaction. Key features include stator and rotor leakage saturation as a function of current and magnetizing flux, distributed effects in the rotor circuits, and a highly computationally efficient implementation. The model is considerably more accurate than the traditional qd model, particularly in its ability to predict switching frequency phenomena. The predictions of the proposed model are compared with those of the standard qd model and to experimental measurements on a 37 W induction motor drive     

Analysis and Detection of Short Circuits in Fractional Horsepower Commutator Machines

An improved mathematical model for transient simulations of commutator machines is presented in this paper. This model can simulate and predict the line current harmonics during healthy and faulty operation conditions. A lumped parameter model that considers every single coil in the stator and rotor as a separate electrical and magnetic circuit is proposed. Self and mutual inductances of the coils are estimated by means of the winding function approach (WFA), accounting for the most relevant space harmonics in the machine air gap. The commutation is modeled by using the real brush width and assuming that the brush-commutator contact resistance is a function uniquely of their contact area, which depends on the armature position. Short circuits between adjacent commutator bars are investigated. It is shown that the model achieves good accuracy, reproducing fairly well the armature current ripple for healthy and faulty operation of the machine.     

Low voltage ride through of doubly-fed induction generator connected to the grid using sliding mode control strategy

Wind Energy Conversion System (WECS) based on Doubly Fed Induction Generator (DFIG) connected to the grid is subjected to high transient currents at rotor side and rise in DC-link voltage during voltage sag at stator/grid side. To secure power system operation wind turbines have to meet grid requirements through the Low voltage ride through (LVRT) capability and contribute to grid voltage control during severe situations. This paper presents the modeling and control designs for WECS based on a real model of DFIG taking into account the effect of stator resistance. The non-linear control technique using sliding mode control (SMC) strategy is used to alter the dynamics of 1.5 MW wind turbine system connected to the grid under severe faults of grid voltage. The paper, also discusses the transient behavior and points out the performance limit for LVRT by using two protection circuits of an AC-crowbar and a DC-Chopper which follow a developed flowchart of system protection modes under fault which achieved LVRT requirements through results. The model has been implemented in MATLAB/SIMULINK for both rotor and grid side converters.     

Harmonic compensation using a synchronous machine with resonantfield circuits

This paper deals with a harmonic compensation method using a synchronous machine with resonant field circuits. Harmonics have become one of the major problems in power systems and a lot of methods of harmonic suppression have been studied. The authors have proposed a new method of harmonic suppression using a synchronous machine excited with the 6th harmonic current. This paper shows that armature inductances for the 5th or 7th harmonics decrease when the field circuits are connected to capacitors resonating at the 6th harmonic. The variations of armature impedances are explained theoretically by Park's equations. The optimal capacitances are selected so as to resonate with field inductances for the short-circuited armature. The experiments to suppress the 5th and 7th harmonics using the synchronous machine are successful     

Initialization of DFIG wind turbines with a phasor‐based approach

The objective of this paper is to propose a simple approach to solve the steady state of a wind turbine (WT) equipped with a doubly fed induction generator (DFIG), which can be used to initialize dynamic studies of the machine. The idea is to model the rotor‐side converter (RSC) as a constant current source connected to the rotor of the DFIG. The resulting equivalent circuit consists of a voltage source in series with a reactance, which makes it possible to obtain simple phasor expressions that can be used to obtain the Park components of the variables. The proposed method is compared with the traditional Newton–Raphson algorithm, showing that it is easier and faster to implement, as it makes use of the phasor expressions and it does not require an iterative process to obtain the final solution. Finally, the results of the proposed method are used to simulate a 2‐MW DFIG‐based WT under three‐phase faults, considering three different WT‐operating points. In these simulations, the idea of constant rotor current is extrapolated to the entire event. The simulated results show that both current at torque peaks are reduced. The analytical study and the simulations have been carried out in Matlab ?.     

A novel finite-element transient computation of two-axis parametersof solid-rotor generators for use in power systems

An original finite element method for calculating the two-axis transient parameters of solid rotor turbine-generators is presented. Eddy current effects are lumped into equivalent damper circuits, representing layers of massive conductors within the rotor. The time variation of machine parameters of a two-axis model is determined and the equivalent circuits obtained are tested by comparing the two-axis and finite element simulations. The two-axis equivalent circuits can simulate a wide range of operating conditions, with appropriate modification of the magnetizing inductances     

Power quality analysis of wind farm connected to Alaçatı substation in Turkey

This paper presents simulation results and power quality measurements of a wind farm. The wound rotor induction generator at 600 kW is employed for power conversion in the wind energy conversion system (WECS). This induction machine is connected to the drive circuit via rotor terminals and speed control is carried out by means of chopper circuit. The model used in the package program is experimentally tested on the single machine drive system at 3.5 kW in the laboratory, after which the power quality issues of the wind farm are investigated by using the same model for 12 wind turbines in PSCAD.     

Theory and performance of series connected self-excited synchronousgenerators

The series connected self-excited synchronous generator is a slip ring type induction machine with stator and rotor windings connected in series along with excitation capacitors. The machine, when self-excited, will yield an output voltage with a frequency equal to half the rotor angular speed. This paper presents analytical as well as experimental investigations into machine performance. The analysis is based on a deduced phasor diagram. The suggested method of analysis is simple and makes it possible to study the effect of machine parameters on its performance. Useful conclusions are given showing proper design considerations to be accounted for to allow the machine to develop acceptable output levels     

Application of Floquet's theory to the analysis of series-connectedwound-rotor self-excited synchronous generator

In this paper, Floquet's theory for solving differential equations with periodically varying coefficients has been utilized in evaluating the steady state performance of a three phase wound rotor series-connected self-excited synchronous generator SCSESG. This type of generator is practically realized by the series connection of stator and rotor windings of a conventional wound-rotor induction machine. Self excitation may occur when a suitable capacitor bank is connected across the machine terminals. The analysis gives the same results that are obtained when the d-q transformation model is utilized. Application of Floquet's theory has the advantage of reducing the mathematical manipulation needed. The results are checked experimentally. Saturation effects on each axis inductance as well as iron losses show satisfactory agreement. The generator acts as a hypothetical salient pole machine operating at half the rotor electrical angular frequency and is independent of load conditions provided that the prime mover speed is kept constant     

Direct active and reactive power control of DFIG for wind energy generation

This paper presents a new direct power control (DPC) strategy for a doubly fed induction generator (DFIG)-based wind energy generation system. The strategy is based on the direct control of stator active and reactive power by selecting appropriate voltage vectors on the rotor side. It is found that the initial rotor flux has no impact on the changes of the stator active and reactive power. The proposed method only utilizes the estimated stator flux so as to remove the difficulties associated with rotor flux estimation. The principles of this method are described in detail in this paper. The only machine parameter required by the proposed DPC method is the stator resistance whose impact on the system performance is found to be negligible. Simulation results on a 2 MW DFIG system are provided to demonstrate the effectiveness and robustness of the proposed control strategy during variations of active and reactive power, rotor speed, machine parameters, and converter dc link voltage.     

A comprehensive method for the calculation of inductance coefficients of cage induction machines

In this paper, a new comprehensive method for the calculation of inductance coefficients of squirrel cage induction machine based on combined winding function approach (WFA) and magnetic equivalent circuit (MEC) is presented. By taking into account machine geometry, rotor skewing, stator and rotor slots effects and type of windings connection, this method is able to model most of the important features of an induction machine. The effects of each machine parameter on the inductance coefficients are verified. Also, effects of several rotor asymmetries on these inductances are shown. Simulation results are verified by more elaborate nonlinear finite element model and finally with experimental results.     

An introduction to electric machine modeling by systems ofnoninteger order. Application to double-cage induction machine

The flowing of induced currents in electric machines, especially in the deep-bars of induction machine cages and the dampers of synchronous machines, implies an increase in the order of the equivalent circuits used to model their frequency response. But, physical meanings of the circuit parameters are often lost. Thus, the authors propose to go back over the modeling of skin effect in order to define noninteger order equivalent circuits which would be linked with the consequences of induced currents on the frequency response. In this paper, the proposed equivalent circuit is based on half-order systems. It is used to represent the frequency response of a 30 kW 4 pole double-cage induction machine and its results are compared with measurements     

Integrated power characteristic study of DFIG and its frequency converter in wind power generation

A doubly fed induction generator (DFIG) is a variable speed induction machine. It is a standard, wound rotor induction machine with its stator windings directly connected to the grid and its rotor windings connected to the grid through a back-to-back AC/DC/AC PWM converter. The power generation of a DFIG includes power delivered from two paths, one from the stator to the grid and the other from the rotor, through the frequency converter, to the grid. The power production characteristics, therefore, depend not only on the induction machine but also on the two PWM converters as well as how they are controlled. This paper investigates power generation characteristics of a DFIG system through computer simulation. The specific features of the study are (1) a steady-state model of a DFIG system in d–q reference frame, (2) a simulation mechanism that reflects decoupled d–q control strategies, (3) power characteristic simulation for both generator and converter, and (4) an integrative study combining stator, rotor and converter together. An extensive analysis is conducted to examine integrated power generation characteristics of DFIG and its frequency converter under different wind and d–q control conditions so as to benefit the development of advanced DFIG control technology.     

ALA转子无刷双馈风力发电机的参数计算方法与转子制造工艺探讨

在大型变速恒频风电系统中采用新型无刷双馈发电机，既运行可靠又可降低所需控制系统的容量和成本。该文介绍了该种风电系统的组成和新型无刷双馈发电机结构；提出了各向异性轴向叠片（ALA）转子无刷双馈发电机电感参数的准确计算方法；研究了主要设计尺寸对电机耦合能力的影响；对该种电机转子的制造工艺进行了探讨。研究表明，该文提出的电感参数叠加法较好的解决了该种发电机的参数计算问题；在无刷双馈发电机的设计中，应兼顾考虑主要设计尺寸的综合影响，以得到最佳的设计方案。