M. Subbiah

Certain control strategies for three-phase semi-converters for the operation of self-excited induction generators

The application of certain firing angle control techniques such as Negative Firing Angle Control, Extinction Angle Control, and Symmetrical Angle Control has been studied for the first time, for the operation of three-phase semi-converters. The input and output voltage and current waveforms and the expressions for the performance quantities of the converter, in terms of the respective control angles /spl alpha/, /spl beta/, and /spl gamma/, have been analytically derived. These results have been confirmed in the simulation of the converter with these control schemes employing MATLAB software tools. The schemes were applied for the control of Self-Excited Induction Generators popularly employed in the renewable energy conversion systems. Illustrative examples of such generator-converter systems, studied both theoretically and experimentally, have been presented for different types of loading patterns that would be required in practical applications and relative merits of the schemes are discussed. To ensure compactness, reliability and flexibility as closed loop schemes, microcontroller based firing circuits have been developed.

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.

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.

A Novel Unified Approach for the Analysis and Design of Wind-Driven SEIGs Using Nested GAs

A unified procedure for the design optimization and performance predetermination of wind-driven three-phase Self-Excited Induction Generators (SEIGs) has been attempted using Genetic Algorithm (GA) twice, in two nested loops. The outer loop is meant for the calculation of the values of turns per coil of the stator winding and the excitation capacitor which would meet the specified performance constraints of the generator. The inner loop applies the GA tool for the first time, for a straightforward predetermination of these performance quantities. Further, additional schemes such as, switching of the stator connection from delta to star, at times of lower wind speed and short shunt arrangement suitable for lagging power loads, have also been included. Method of estimating the ranges for the variables, required to be given as input data into each of these two loops for the application of GA, has been explained. Experimental results obtained on a test machine and performance values predicted employing Genetic Algorithm, have been shown to agree closely, validating the comprehensive methodology proposed for the first time for the economical design of SEIGs for wind-driven applications.

Major methods of steady-state analysis of three-phase SEIGs - A summary

Through an elaborate survey of the papers published so far, a summary of three major methods of performance analysis of Self-Excited Induction Generators (SEIGs) has been presented. Among the various methods, the optimization approach has been shown to be the best in handling the variation in the generator and load parameters. The comparative study presented herein would be useful for any researcher as a starting platform, for further investigations in this field of wind energy electric conversion systems.

Wind-Driven SEIGs for Supplying Isolated Loads Employing DSP Based Power Electronic Controllers

A DSP based closed-loop system has been developed for wind-driven three-phase Self-Excited Induction Generators (SEIGs) using a diode bridge rectifier (DBR) followed by an IGBT inverter. This system is meant for supplying isolated ac loads with a voltage of constant magnitude and frequency, for any operating condition of varying nature of wind speed and consumer load. The configuration and implementation of this proposed system have been fully described. The detailed method of analysing and predetermining the performance characteristics of such stand-alone systems has also been explained. Test results obtained on an SEIG operated with the controller built in the laboratory, demonstrate the successful working of both the hardware and software of the control scheme and the usefulness of the set-up as a whole for supplying isolated ac loads. With the varying nature of wind speed, a method of meeting the power balance between the wind power available and the load power requirement has also been explained with extensive Simulation studies.

Determination of boundary values of excitation capacitance and minimum load impedance for Wind-driven SEIGs

A simple method involving only a quadratic equation has been developed for calculating the minimum and maximum values of excitation capacitance that will sustain self-excitation in an induction generator meant for isolated power supplies. This approach has also been further extended for the determination of maximum load of a given power factor that could be applied to the generator without losing self-excitation, while operating with a specified capacitance and rotor speed. Both ac loads directly connected across the generator and also dc loads supplied through controlled rectifiers have been considered. Details of the various control schemes, which could be adopted for the rectifier and the effect of varying the control angle in each case, are presented. Experimental results obtained on a 400V, 3-phase, 3.7 kW, induction machine run as a generator validates the derived expressions.

Performance predetermination of variable speed wind-driven grid connected SEIGs

A Self Excited Induction Generator (SEIG) system has been proposed for supplying power to the grid through a combination of a three-phase semi-converter (SC) and a line commutated inverter. This system is capable of operating at variable speed to enable maximum power, proportional to the cube of the wind speed, to be extracted from the wind. The method of analysing the system has been explained by deriving the relevant expressions for the various performance quantities. The different control schemes to be adopted for the SC to suit the various speed ranges of the generator has been described with an example along with the predetermined performance characteristics of the system, following cube law power curve. The simulated waveforms of voltage and current at the various stages of the proposed system are also presented.

Analysis and control of three-phase SEIGs supplying battery charging loads through single-phase converters

A method employing Genetic Algorithm (GA) has been applied for the analysis of three-phase self-excited induction generators supplying battery charging load through single-phase semi-converters. The battery circuit is represented as equivalent resistance and reactance in the equivalent circuit of the generators and such equivalent values are functions of the dc charging voltage, current and control angle set in the converter. Apart from the conventional Phase Angle Control, other firing control schemes namely Extinction Angle Control and Symmetrical Angle Control have been considered for the operation of the converter. The suitability of each of these control schemes for specific load conditions is discussed with examples. Experimental results obtained on a three-phase SEIG charging suitable batteries through a single-phase IGBT based semi-converter, controlled with all these schemes, confirm the usefulness of the proposed system and the validity of its analysis.