Capacitance requirements of self-excited induction generators

A simple method for computing the minimum value of capacitance, C min. required for initiating voltage build-up in a three-phase self-excited induction generator (SEIG) is presented. Based on the steady-state equivalent circuit model, a consideration of the circuit conductances yields a sixth-degree polynomial in the per-unit frequency. The polynomial can be solved for real roots, which enables the value of C_min to be calculated. Critical values of load impedance and speed, below which the machine fails to self-excite irrespective of the capacitance used, are found to exist. Closed form solutions for C_min are derived for no-load and inductive loads. Using the same numerical approach, an interative procedure is developed for predicting the capacitance required for maintaining the terminal voltage at a preset value when the generator is supplying load. Experimental results obtained on a 2 kW induction machine confirm the feasibility and accuracy of the proposed methods     

Performance analysis of a three-phase induction generatorself-excited with a single capacitance

This paper presents a unified method of analysis for a three-phase induction generator self-excited with a single capacitance and supplying a single-phase load. Symmetrical components analysis is used to establish the input impedance of the generator, while the pattern search method of Hooke and Jeeves enables the per-unit frequency and magnetizing reactance of the machine to be determined, a crucial step in computing the generator performance. Best machine performance is obtained using the Steinmetz connection, with the excitation capacitance connected across the lagging phase. Experiments carried out using a 2.2 kW induction machine confirm the accuracy of the theoretical analysis and the solution method     

New approach to determine the critical capacitance for self-excitedinduction generators

This paper presents a new simple approach for computing the minimum value of capacitance necessary to initiate the self-excitation process in three-phase isolated induction generators. The method proposed in this paper is based on the analysis of the complex impedance matrix of the induction generator when loaded with a general inductive load. The advantage of this method is its simplicity since it involves simple algebraic equations and only one equation is solved iteratively to get the value of minimum capacitance. A simple computer algorithm has been developed to predict the minimum value of capacitance necessary for the onset of self-excitation. Using the same approach, the algorithm is modified to predict the minimum value of capacitance necessary to maintain the generator terminal voltage at a preset value under specific load and speed conditions. Computer simulations obtained using the proposed method are compared with those obtained experimentally to confirm the validity and accuracy of the proposed method     

Steady state analysis and performance characteristics of athree-phase induction generator self excited with a singlecapacitor

Steady-state analysis and performance characteristics of a three-phase isolated star or delta connected induction generator self-excited with a single capacitor are discussed. Analytical expressions are derived to determine the no-load capacitance required to maintain self-excitation. The performance characteristics of a self-excited induction generator are affected by the terminal capacitance, C, machine speed, ν, and the load parameters. Generally, the value of C has a stronger influence on the performance characteristics and should be selected such that the terminal voltage, V_t, is near its rated value while keeping C close to its lower limiting value. The accuracy of the method was experimentally verified, and good agreement is obtained between the two sets of results. The performance characteristics of star and delta connected induction generators are compared. In the mode of operation discussed, the voltages and currents are unbalanced with relatively high losses and rather low efficiency. Among the two configurations discussed, the delta connected generator offers a higher current and lower terminal voltage and a wider range of excitation capacitance     

Steady State Analysis and Performance of an Isolated Self-Excited Induction Generator

A general analysis to predict the steady state performance of an isolated self-excited induction generator feeding a balanced R-L load is presented. In the analysis the effect of machine core losses has also been considered. It is shown that besides voltage and frequency the analysis can be used to predict the minimum value of terminal capacitance required for excitation as well as for maintaining a constant terminal voltage. Comparison of the predicted and the experimental results shows a good agreement.     

Minimum airgap flux linkage requirement for self-excitation instand-alone induction generators

A minimum airgap flux linkage is required for the self-excitation and stable operation of an isolated induction generator feeding an impedance load. With the aid of bifurcation theory, it is shown that the minimum airgap flux linkage requirement is the value at which the derivative of the magnetizing inductance with respect to the airgap flux linkage is zero. This minimum airgap flux linkage determines the minimum or maximum load impedance and minimum excitation capacitance requirements. This result is demonstrated using single-phase and three-phase induction generators     

Apparatus for supplying an isolated DC load from a variable-speedself-excited induction generator

This paper describes the design and laboratory testing of novel generation apparatus for supplying an isolated DC load from a self-excited induction generator operable at variable speed. The variable-speed generating apparatus consists of a self-excited induction machine, a controlled Graetz bridge rectifier, a voltage-boost power converter, and a control system. The induction generator supplies the rectifier. The voltage-boost power converter interfaces the variable output voltage of the rectifier to the fixed DC voltage required for the load. The rectifier is operated at levels of average DC current and voltage which control machine voltage to the rated AC voltage and which also draw the necessary power to supply the DC load. Performance is enhanced with respect to earlier apparatus in that both the DC voltage supplied to the load and the AC voltage on the machine are simultaneously controlled to fixed reference levels over broad operating ranges of load and speed     

Voltage and frequency control of self-excited slip-ring induction generators

By varying the effective rotor resistance of a self-excited slip-ring induction generator (SESRIG), the magnitude and frequency of the output voltage can be controlled over a wide speed range. A steady-state analysis based on a normalized equivalent circuit enables the control characteristics to be deduced. For a given stator load impedance, both the frequency and the voltage can be maintained constant as the speed is varied, without changing the excitation capacitance. When the stator load is variable, simultaneous voltage and frequency control requires the excitation capacitance to be changed as the rotor resistance is varied. Experiments performed on a 1.8-kW laboratory machine confirm the feasibility of the method of control. Practical implementation of a closed-loop control scheme for an SESRIG using chopper-controlled rotor resistance is also discussed. With a properly tuned proportional-plus-integral (PI) controller, satisfactory dynamic performance of the SESRIG is obtained. The proposed scheme may be used in a low-cost variable-speed wind energy system for providing good-quality electric power to remote regions.     

Optimisation of voltage and frequency regulation in an isolated wind-driven six-phase self-excited induction generator

This paper presents a constant voltage operation of a Six-Phase Self-Excited Induction Generator (SPSEIG) driven by a fixed speed wind turbine using an Ant colony optimisation (ACO) technique to predict the behaviour of a the machine. In this paper, an attempt has been made to estimate the excitation capacitance requirements of a SPSEIG for maintaining rated terminal voltage and frequency. The range of capacitance variation required for maintaining constant terminal voltage while supplying a load of variable magnitude is evaluated. Analytical approaches, suitable for all the configurations of shunt capacitances such as variable excitation capacitance connected across (i) single three-phase winding set only and (ii) both the three-phase winding sets of an SPSEIG for operation as a simple shunt on no load and pure resistive load, are presented. The mathematical model developed is based on loop impedance method using graph theory. It is shown that the proposed technique is very effective and useful for making the SPSEIG feasible for remote areas with wind potential. The proposed approach is tested and compared with Genetic Algorithm (GA) and Fmincon technique.     

A novel analysis on the performance of an isolated self-excitedinduction generator

This paper presents a novel approach based on eigenvalue and eigenvalue sensitivity analyses to predict both minimum and maximum values of capacitance required for the self-excitation of a three-phase induction generator. Numerous numerical methods based on steady-state equivalent circuit models have been proposed to find the minimum capacitance of self-excited induction generators by solving simultaneous nonlinear equations. Steady-state and sensitivity analyses of different capacitance values with respect to various system parameters are performed. Transient analyses of the studied induction generator under different loading conditions are also carried out. Experimental results obtained on a 1.1 kW induction machine confirm the feasibility and effectiveness of the proposed approach     

Performance characteristics and optimum utilization of a cagemachine as capacitance excited induction generator

The performance characteristics of a cage induction machine operating as a self-excited induction generator (SEIG) in stand-alone mode are presented. A static capacitor bank is considered to self-excite the machine and to maintain its terminal voltage constant. The lagging reactive power requirement of self-excited induction generator is obtained for different load values. The effect of speed on the excitation requirement of the cage machine has also been studied. An algorithm is developed to achieve these characteristics using the Newton-Raphson method and a steady-state equivalent circuit of the machine. The developed analytical technique is extended to evaluate the number of steps of switching capacitors for loading the machine up to its full load rating while maintaining the terminal voltage within desired limits. The selection of an optimum terminal voltage corresponding to the maximum output of the machine for its optimum utilization is also made using single-variable optimization     

Steady-State Analysis of a Self-Excited Single-Phase Reluctance Generator

This paper presents an analytical method for predicting the steady-state performance of a self-excited single-phase reluctance generator (SESPRG), which supplies R-- L load. The proposed analysis is based on the d--q axis model and phasor diagram of such a generator in the steady-state condition. Excitation capacitors are connected across both the main and auxiliary windings. Magnetic saturation is taken into account and is assumed to be confined to the direct axis, and is accounted for a variable direct-axis magnetizing reactance. Conditions of self-excitation and the minimum value of the capacitance required to achieve self-excitation are also given. Special attention is focused on the machine performance when it operates as a pure single-phase reluctance generator (PSPRG). A fixed-capacitor (FC) thyristor-controlled reactor (TCR) scheme is used to regulate the generator terminal voltage by controlling the thyristor conduction angle. Further stability limits are investigated by developing the active-reactive (P-Q) power diagram. Reasonably close agreement between the measured and predicted results is observed confirming the validity of the proposed analysis.     

Autonomous micro hydro power plant with induction generator

Autonomous micro hydro power plants (MHPP) are a reliable solution for supplying small power consumers in areas located far from the distribution grid. When an induction generator (IG) is used in such a power plant, voltage and frequency need to be stabilized. This paper presents a single control structure that ensures both the voltage and frequency regulation of an isolated induction generator (IG). The control structure consists in a voltage source inverter (VSI) with a dump load (DL) circuit on its DC side. The VSI operates at constant frequency, thus stabilizing the IG frequency also. For voltage regulation two cascaded regulators are used, which have as reference the line voltage and the VSI DC voltage, respectively. Simulations and experiments are carried out in order to investigate the reliability of such configuration when supplying static and dynamic loads.     

A novel excitation scheme for a stand-alone three-phase induction generator supplying single-phase loads

This paper presents the operating principle and steady-state analysis of a novel excitation scheme for a stand-alone three-phase induction generator that supplies single-phase loads. The phase windings and excitation capacitances are arranged in the form of the Smith connection and the excitation scheme is referred to as the SMSEIG. In addition to providing the reactive power for self-excitation, the capacitances also act as phase balancers. With this novel excitation scheme, isolated single-phase loads can be supplied with good phase balance in the induction machine, resulting in high efficiency, large power output, and quiet machine operation. Performance analysis is based on the method of symmetrical components, from which the input impedance of the generator can be determined. Numerical solution of a simplified equivalent circuit for the machine variables, namely the excitation frequency and magnetizing reactance, enables the generator performance to be evaluated for any load and speed. With the aid of a phasor diagram, the conditions for achieving perfect phase balance are deduced and a method to compute the capacitances required is developed. Experimental investigations on a 2.2-kW induction machine confirm the feasibility of the SMSEIG.     

Steady-state analysis and performance of a stand-alone three-phaseinduction generator with asymmetrically connected load impedances andexcitation capacitances

This paper presents a steady-state performance analysis of a stand-alone three-phase induction generator self-excited with unbalanced capacitances and supplying unbalanced loads. Using the method of symmetrical components, the complex three-phase generator-load system is reduced to a simple equivalent passive circuit. A function minimization technique is employed to solve this equivalent circuit in order to determine the excitation frequency and magnetizing reactance. The proposed method enables practically all cases of unbalanced operation of the self-excited induction generator to be analyzed. Emphasis is next focused on single-phase loads and a special phase-balancing scheme, namely the modified Steinmetz connection (MSC), is investigated. It is shown that perfect phase balance of the SEIG can be obtained with an appropriate combination of excitation capacitances and loads. The theoretical analysis is validated by experiments on a 2.2 kW induction machine     

A novel analysis of parallel operated self-excited inductiongenerators

This paper presents a novel approach based on eigenvalue and eigenvalue sensitivity analyses to predict both minimum and maximum values of the capacitance required for the self-excitation of parallel operated three-phase induction generators. Numerous numerical methods based on steady-state equivalent circuit models have been proposed to find the minimum capacitance of a self-excited induction generator by solving simultaneous nonlinear equations. Steady-state and sensitivity analyses of different capacitance values concerning various system limits are performed. Transient analyses of the studied induction generators under different loading conditions are also carried out. The results show those induction generators with different speeds can be operated in parallel properly, but the requirement before they may be connected in parallel is the phase sequence of the running and incoming induction generators must be the same. Experimental results obtained on 1.1 kW induction machines confirm the feasibility and effectiveness of the proposed approach     

Capacitance Requirements for Isolated Self Excited Induction Generators

Capacitance requirements for isolated self excitd induction generators are discussed. It is shown that numerical methods based upon the steady state as well as operational equivalent circuit give similar predicted values. An analytical method is proposed to compute, Cmin, the minimun capacitance value required for self excitation under no load conditons. It is shown that Cmin is inversely proportional to the square of the speed. Furthermore, it is inversely proportional to the maximum saturated magnetizing reactance. The theoretical results are verified experimentally for a number of test machines and a good agreement is observed between the theoretical and the experimental values. The influence of load impedance and its power factor on the terminal capacitance required to maintain self excitation under steady state is also examined and it is shown that when machine is loaded, the terminal capacitance should be several times that required at no load. Furthermore, the influence of terminal capacitor upon the maximum available output power fron isolated induction generators is also examined.     

Performance of self-excited single-phase induction generators withshunt, short-shunt and long-shunt excitation connections

The influence of three capacitor excitation topologies (shunt, short-shunt and long-shunt) on the steady-state and dynamic performance of a single-phase, self-excited induction generator is explored in this paper. Attention is focused on the influence of the different capacitor connections on the generator overloaded and output power capabilities. The generator voltage with shunt excitation connection collapses when overloaded, while with either the long or short-shunt excitation connection the generator is able to sustain the load at a lower operating voltage and larger load current     

A novel single-phase self-regulated self-excited inductiongenerator using a three-phase machine

This paper presents a steady-state analysis of a novel single-phase self-regulated self-excited induction generator which employs a three-phase machine. Performance equations are derived using the method of symmetrical components, while the pattern search method of Hooke and Jeeves is used for the determination of the machine variables. The advantages of the generator include simple circuit configuration, small voltage regulation, good phase balance, and large power output. With an appropriate choice of the series and shunt capacitances, a nearly balanced operating condition can be obtained for a certain load. The theoretical analysis is validated by experiments performed on a small induction machine     

Performance of a three-phase AC generator with inset NdFeB permanent-magnet rotor

This paper presents the analysis and performance of a three-phase AC generator with an inset, neodymium-iron-boron (NdFeB) permanent-magnet (PM) rotor. Such a rotor construction gives rise to an inverse saliency effect (i.e., the direct-axis synchronous reactance is less than the quadrature-axis synchronous reactance). This feature results in an improvement in the voltage regulation characteristics when the generator supplies an isolated, unity-power-factor load. By solving the equations derived from the two-axis theory, it is found that there exists, in general, two values of load current at which zero voltage regulation is obtained. The relationship between armature resistance, inverse saliency ratio, and the operating speed to give zero voltage regulation is investigated. The finite-element method (FEM) is used for computing the pertinent generator parameters for performance evaluation, namely the no-load voltage and the synchronous reactances. Flux plots are presented to confirm the origin of inverse saliency in the inset PM rotor. The theoretical analysis is validated by experiments carried out on a 2.5-kVA prototype generator.