Grid-connected Induction Generator Research Articles

Steady-State Modeling and Analysis of Grid-Connected Six-Phase Induction Generator for Renewable Energy Generation

This article presents the steady-state modeling and analysis of a grid-connected six-phase induction generator for renewable energy generation powered by hydro turbine. The basis of the analysis is nodal admittance method as applied to the equivalent circuit, and used to analyze the behavior of the machine for the operating mode such as (i) when only one three-phase winding set is connected to grid, (ii) when one three-phase winding set is connected to grid and other three-phase winding set is subjected to load, and (iii) when both the three-phase winding sets are connected to grid through an interconnecting Y-/Y six-phase to three-phase transformer. Nodal admittance based matrix equations are easier to modify in order to account for mutual leakage coupling between two three-phase winding sets, core loss component, and make the analysis very easy, fast and accurate. Through analytical and practical studies, it is shown that machine can feed direct, reliable, and low cost power to grid without interface network. The analytical results are found to be in good agreement with experimental results.

Grid-Connected Induction Generators Using Delta-Star Switching of the Stator Winding with a Permanently Connected Capacitor

The delta-star switching proposed earlier for grid-connected induction generators has been modified to include a permanently connected capacitor across each phase winding of the stator of the generator. It has been shown that, with this capacitor addition, operating the generator in delta and star connections respectively at higher and lower wind speeds further reduces the reactive power taken by the generator from the grid. Moreover, such switchings do not cause any current harmonics in the supply lines. Test results on a 3-phase, 4-pole, 415 V, 50 Hz, 3.3 kW generator validate such advantages of the proposed scheme, apart from the simplicity of the switching controller involving only five sets of anti-parallel thyristors. The method of choosing a suitable value for the capacitor has also been illustrated. The superiority of proposed scheme, compared to the other single speed schemes of wind driven induction generators, has also been demonstrated through a case study carried out on a 400 V, 250 kW generator, using a typical annual wind data. Any further improvement in grid side power factor can be achieved by employing a STATCOM of reduced VAR rating.

Active Voltage and Frequency Regulator Design for a Wind-Driven Induction Generator to Alleviate Transient Impacts on Power Grid

Grid-connected induction generators (IGs) suffer from voltage and frequency fluctuations when stator circuits are disconnected from the power grid. A typical strategy to prevent this problem is to remove the shunt capacitor to cease self-excitation. However, when paralleling the IG with the power grid, another problem of severe voltage drop caused by excitation current inrush occurs. This paper presents an active voltage and frequency regulator (AVFR) to alleviate the undesired transients for the IG in grid-connection and grid-disconnection cases. The proposed AVFR is realized using a voltage-source inverter (VSI) with a controllable resistor at the dc link. The active power at the ac side is rectified to maintain a constant dc-link voltage using a VSI that also supplies excitation current to the machine. To achieve quick ac current response for the VSI, a stationary α-β-axis-based direct current controller is proposed. A new algorithm based on the normalized voltage vector, which features a linear relation between the α-axis voltage and phase angle, leads directly to a rule that will detect the frequency deviation and provide effective frequency control. The experimental results from a 2.5-kVA digital-signal-processor-based laboratory prototype verify the effectiveness of the proposed AVFR.

Effect of Machine Parameters on Starting Transients of Grid Connected Induction Generators used in Wind Farms

Effect of Machine Parameters on Starting Transients of Grid Connected Induction Generators used in Wind Farms

Application of Facts Controllers for Wind Power Smoothening in Grid-Connected Doubly-Fed Induction Generator Systems

The increasing power demand has led to the growth of new technologies that play an integral role in shaping the future energy market. Keeping in view of the environmental constraints, grid-connected wind turbines are promising increasing system reliability. This paper presents the impact of FACTS controllers on the long-term dynamic behaviour of power systems connected with wind farms based on doubly-fed induction generator systems. Wind speed variations over a period of 1200 s are monitored from the regional meteorological website, and the voltage oscillations/real power injections of the wind farm are observed with and without FACTS devices. EUROSTAG is used for carrying out time domain simulations.

Stability Analysis of a Wave-Energy Conversion System Containing a Grid-Connected Induction Generator Driven by a Wells Turbine

This paper presents the dynamic-stability analyzed results of both dynamic simulations and steady-state performance of a wave-energy power generation system containing a grid-connected induction generator (IG) driven by a Wells turbine. The stator windings of the IG are connected directly to a power grid through a step-up transformer and a transmission line. A d-q axis equivalent-circuit model is employed to establish the IG, the transmission line, and the grid to derive the complete dynamic equations of the studied system under three-phase balanced loading conditions. A frequency-domain approach based on eigenvalue analysis and a time-domain scheme based on nonlinear-model simulations are both carried out to systematically determine the dynamic stability of the studied system under various operating conditions. It can be concluded from the simulation results that the studied wave-energy power generation system subject to different disturbance conditions can maintain stable operation.

Vector control of autonomous induction generator taking saturation effect into account

In this paper, we present the voltage build up process and the terminal voltage control of an isolated wind powered induction generator driven by a variable speed wind turbine using rotor flux oriented vector control. A description of the studied system is provided, and a simulation study is presented. The model used for the autonomous induction generator is a diphase one obtained by application of the Park transform. This model permits, when adopting some simplifying hypotheses, taking account the saturation effect. Wind powered isolated induction generators have an input, wind, that is not controllable, but they can be set to operate within a given variation of speed. Unlike a grid connected induction generator, in an isolated case, there should be a control system that keeps the DC bus voltage at a constant value when the speed of the rotor varies. The paper presents the control system to maintain the DC bus voltage at a constant value. The obtained results are presented and discussed.

Reactive power requirements of grid connected induction generator in a weak grid

Induction generators are gaining the popularity due to its simplicity and no synchronization problem. However the major drawback of this machine is its additional reactive burden on the system, where it is connected. In this paper an attempt is made to explore the performance of a grid connected induction generator (GCIG) due to weak grid conditions i.e. voltage and frequency fluctuations. Iterative model developed by the authors is used to predict the reactive burden of the generator due to voltage and frequency fluctuations. The comparison between experimental and simulated results proves the validity of technique proposed. Further proposed modeling is also used to include the effects of saturation.

Fault Ride-through Testing of Wind Turbines

Grid codes concerning low-voltage fault ride-through capabilities of wind turbines are not (yet) harmonized, varying from country to country. Full-scale field testing of wind turbines with respect to all such codes may therefore not be practical. IEC 61400-21 is currently under revision, and the last committee draft (CD 2006) presents a standardized physical test for characterizing a wind turbine's response to a voltage-dip. The results of such physical tests may be used to validate numerical models of the turbine, which in turn can be used for modelling grid code compliance assessment of particular country codes. Success with such modelled validation will provide confidence in numerical simulations. The hypothesis of this paper is that the use of numerical models, that have been validated against the standardized tests, can reduce the number of physical full-scale tests needed. This paper assesses to what degree a validated simulation model is capable of predicting the fault ride-through capabilities of a fixed speed induction generator. This is done by comparing measurements on a wind-turbine emulator in a laboratory with predictions from a validated simulation model of the laboratory emulator. Simulations and laboratory measurements show excellent agreement. The validated simulation model accurately predicts the fault ride-through capability of the direct grid connected induction generator. It is concluded that the proposed methodology is promising and thus has the potential to increase the efficiency of grid code assessments. Full-scale field tests will still be required to validate the modelling completely, but using numerical models reduces the number of field tests needed.

Design of Grid-Connected Induction Generators for Wind Power Generation

The design of an induction machine for grid connected wind energy applications should be different from isolated wind electricity generation and motor applications. Commonly, standard induction machines are used as grid connected induction generators. However, such machines are designed for motor operation and have more reactive power consumption and greater no-load losses than necessary. This paper presents a design for an induction generator exclusively for grid connected applications by reducing both reactive power consumption and no-load losses. The theoretical performance of the proposed generator is compared with the conventional induction motor used as a generator. Evolutionary programming is used as a tool for design optimization of both generator and motor.

Performance of Grid-Connected Induction Generator under Naturally Commutated AC Voltage Controller

This article presents a complete analysis of an induction generator connected to a grid through ac voltage controller utilizing two anti-parallel thyristors. The performance characteristics regarding the harmonic contents, active power, reactive power, power factor, and efficiency have been computed. These characteristics have been determined with the help of a novel abc-dq circuit model. The model posses the advantages of both the dq and direct phase model. The validity of the model and results have been confirmed experimentally.

Finite element analysis of a single-phase grid-connected induction generator with the Steinmetz connection

The Steinmetz connection enables a three-phase induction generator (IG) to operate satisfactorily on a single-phase grid by using only a capacitance phase converter. This paper presents a finite element analysis of this mode of IG operation. A time-stepping, two-dimensional (2-D) finite element method (FEM) is employed in the solver, with the external circuit equations coupled to the set of simultaneous equations formulated with the nodal magnetic vector potentials as the variables. A detailed rotor circuit model that accounts for the current density distribution as well as the end-ring parameters is also presented. Generator performance computed using FEM is compared with that computed using the method of symmetrical components and that obtained experimentally on a small induction machine. The results indicate that FEM gives an accurate prediction of the line current and the output power of the IG.

Innovative Reactive Power Saving in Wind-driven Grid-connected Induction Generators Using a Delta-Star Stator Winding: Part II, Estimation of Annual Wh and VARh of the Delta-Star Generator and Comparison with Alternative Schemes

This paper considers constant-speed electricity generation using induction generators. Taking typical wind-farm site data, a case study allows performance evaluation of the previously proposed delta-star induction generator feeding power to the grid. By making an annual estimation of energy fed to the grid and reactive power drawn from the grid, advantages of this multi-stage generator are shown. A method for studying the effect of variation in wind speed pattern, on the annual reactive power drawn from the grid is indicated. A performance comparison with some other wind-driven constant-speed induction generator systems is also made.

Innovative Reactive Power Saving in Wind-driven Grid-connected Induction Generators Using a Delta-star Stator Winding Part I: Performance Analysis of the Delta-Star Generator and Test Results

An innovative delta-star three-phase winding, developed earlier for harmonic reduction in 3-phase induction motors, is applied for constant speed grid-connected induction generators. A method has been developed for the prediction of the performance of the generator with such an unconventional winding. It has been demonstrated that with a suitable switching arrangement, the proposed delta-star generator can result in increased real power delivered to the grid and reduced reactive power drawn from the grid, as compared to a fixed stator configuration. A suitable microcontroller-based switching-scheme has also been designed for changing the stator connection according to the wind speed. The facility for using the star mode of the generator while connecting it to the grid with much reduced inrush current is also illustrated.

Initialisation of Grid-Connected Wind Turbine Models in Power-System Simulations

This paper describes a method of initialising dynamic wind turbine models in power-system simulations. The method uses grid-status information to initialise the grid-connected wind turbine model. The goal is to start a power-system simulation in a stationary state, without unnecessary transient behaviour at the beginning of the simulation. Correct initialisation is necessary to reach quickly the steady state of the grid-connected wind turbine, before a real transient on the grid occurs. Consequently, the dynamic performance of the system can be independently and accurately evaluated. The method presented considers wind turbines with directly grid-connected induction generators, with constant or variable blade angle (passive stall control, blade angle control or active stall control). These aspects apply to both constant and variable speed wind turbines. The investigation is performed in the commercial dedicated power-system simulation-tool DIgSILENT. Attention is especially drawn to the initialisation of the non-electrical parts of the wind turbine. The suitability of this initialisation strategy is illustrated by an example.

Solid-State Control of a Grid Connected Induction Generator

This paper presents complete analysis of induction generators linked to the network through transistorized AC voltage controller. Three different control strategies are investigated, and various performance characteristics have been computed. The generator performance has been determined with the help of a novel abcdq circuit model. The model possesses the advantage of both the dq and direct phase models.

Dynamic analysis of a grid-connected induction generator driven by a wave-energy turbine through hunting networks

The presence of forced oscillations occurring in a Wells turbine driven grid-connected induction generator enables one to seek a solution by considering the analogy between the dynamics of Wells turbine driven systems with those associated with diesel-engine driven generators or electric motors driving reciprocating compressors, where also such forced oscillations occur, although there they occur in synchronism with shaft-position or speed. This difference is taken into account and the hunting network theory, which has hitherto been applied to the dynamic analysis of motors driving reciprocating compressors, is here applied to analyse the dynamics of a Wells turbine driven grid-connected induction generator. The method enables the generator current, power and slip to be predicted from a knowledge of the shaft-torque harmonics. The result is compared with that obtained through d-q analysis.

A wind turbine emulator based on a dual DSP processor system

In this paper the principles and structure of a wind turbine emulator (WTE) are described. The WTE is a versatile system specially designed for the purpose of developing and testing new control strategies for wind energy conversion systems. The WTE is built around a processor module, which controls a separately-excited DC motor. The core of the processor module is a dual DSP system working in a tightly-coupled architecture. The WTE provides a friendly environment to readily modify wind speed conditions, change turbine parameters and supervise system variables. Experimental results from the WTE coupled to a grid-connected induction generator are analyzed.

A Wind Turbine Emulator Based on a Dual DSP Processor System

In this paper the principles and structure of a Wind Turbine Emulator (WTE) are described. The WTE is a versatile system specially designed with the purpose of developing and testing new control strategies for Wind Energy Conversion Systems. The WTE is built around a Processor Module, which controls a separately-excited DC motor. The core of the Processor Module is a dual DSP system working in tightly-coupled architecture. The WTE provides a friendly environment to readily modify wind speed conditions, change turbine parameters and supervise system variables. Experimental results from the WTE coupled to a grid-connected induction generator are analyzed.

Analysis of wind driven grid connected induction generators under unbalanced grid conditions

A wind driven induction generator feeding power to the grid has been analyzed under the abnormal condition of unbalanced grid voltages. Using the symmetrical component and double revolving field theory, appropriate equivalent circuits and model equations have been derived for the generating mode with suitable realistic modifications. It is emphasized that the active and reactive power components and their directions for both positive and negative sequence systems need to be properly identified in order to obtain the cumulative response of the generator under different wind power conditions. In view of the fact that the reactive power is drawn from the grid while the active power is fed into the grid, the extent of variations in power fed to the grid and the reactive VAr due to unbalanced grid voltages for different wind power conditions need to be estimated to provide guidelines in the design and operation of the wind energy conversion system. Both experimental and theoretical results for a 3.7 kW laboratory model have also been presented, to validate the theoretical formulations, extendable to large units. Extensive data have been presented and discussed for a 55 kW unit installed in site. >