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 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     

Single-phase operation of a three-phase induction generator using a novel line current injection method

By using two capacitances and a current injection transformer, a three-phase induction generator can operate with good phase balance and line power factor while delivering power to a single-phase power grid. This paper presents a systematic analysis on this novel induction generator configuration. The solution of the system's inspection equations using the method of symmetrical components enables the steady-state generator performance at any speed to be computed. The conditions for achieving perfect phase balance are deduced from the phasor diagram. It is shown that the capacitances that result in perfect phase balance depend on the generator admittance, power factor angle, as well as the turns-ratio of the current injection transformer. Where possible, the computed results are verified by experiments conducted on a 2-kW induction machine. An experimental investigation on the system waveforms and harmonics is also carried out.     

Single-phase operation of a three-phase induction generator withthe Smith connection

Single-phase operation of a three-phase induction generator with the Smith connection (SMIG) is analyzed using the method of symmetrical components. It is shown that, despite the asymmetrical nature of the winding connection, balanced currents can be made to flow in the three-phase stator winding. The conditions for achieving perfect phase balance are carefully deduced. With the aid of a phasor diagram, expressions for the line power factor and line current under perfect phase balance are also obtained. The effect of the phase-balancing capacitances on the generator performance is investigated. A simple dual-mode control scheme is also proposed with a view of minimizing the phase imbalance over the practical operating speed range. Experiments conducted on a 2.2 kW induction machine confirm the validity of the theoretical analysis and feasibility of the control method     

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     

Performance analysis of a three-phase induction generator connectedto a single-phase power system

This paper analyzes the performance of a three-phase induction generator which is connected to a single-phase power system. Significant improvement in machine performance can be obtained by using a single static phase-converter, provided that the machine is driven in the reverse direction. If two phase-converters, are employed, perfect phase balance can be obtained at any desired value of slip. The theoretical analysis is validated by experiments on a 2-kW test machine     

Capacitors Required for Maximum Power of a Self-Excited Single-Phase Induction Generator Using a Three-Phase Machine

This paper aims to determine the optimal capacitors required for maximum output power of a single-phase self-excited induction generator (SEIG), using a three-phase machine feeding inductive as well as capacitive loads. The generator consists of a three-phase star-connected induction machine with three capacitors and a single-phase load. The algorithm, which gives directly the values of the optimal capacitors for the maximum power output and the maximum power available, has been developed using the steady-state model of the SEIG and sequential unconstrained minimization technique (SUMT). The variations of the maximum power output with power factor (pf) of loads (both inductive and capacitive) and speed of the SEIG have been presented. The voltage regulation of the generator is small due to the effect of the two series capacitors. Experimental results have shown the effectiveness and accuracy of the developed algorithm.     

Effect of excitation capacitors on transient performance ofreluctance generators

The transient responses of a reluctance generator connected to an infinite power system excited by a bank of terminal capacitances are compared to those when load excitation is used. A mathematical model is developed to simulate the machine with its terminal capacitor. With the aid of a least-square-error method, this model is used to optimize the machine parameters. The capacitance excitation requirements for different load conditions are then computed using a steady-state model. The comparison of the transient responses shows that the terminal-capacitor excitation method has several advantages over the load excitation method. It reduces the first rotor swing and gives more damping to the subsequent rotor oscillations. It also increases the critical fault-clearing time and hence the transient stability limits. In addition, it suppresses all power frequency torque oscillations, which are quite pronounced when load excitation is used     

Capacitance requirements of a three-phase induction generatorself-excited with a single capacitance and supplying a single-phase load

This paper presents a practical method for computing the minimum capacitance required to initiate voltage build-up in a three-phase induction generator self-excited with a single capacitance and supplying a single-phase load. Attention is focused on the Steinmetz connection which gives superior performance over the plain single-phasing mode of operation. From a consideration of the input impedance of the induction generator and the self-excitation conditions, two nonlinear equations are obtained. Solution of one equation using the Secant method enables the excitation frequency to be determined, and the minimum excitation capacitance can be calculated from the second equation. This solution technique is subsequently employed in an iterative procedure for computing the capacitance required to maintain the terminal voltage at a preset value when the generator is supplying load. Experimental results obtained on a 2-kW induction machine are presented to verify the theoretical analysis where possible     

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.     

A laboratory model for harmonic measurements in windpowergenerators

In this letter we present the principles of an integrated control and measurement system which is linked to a DC servo motor used to drive a single-phase induction machine. The role of the DC servo motor is to model the drive from the turbine of a wind energy conversion system (WECS). This drive is in turn physically connected to a single-phase induction machine that is in turn linked to an external single-phase supply and various loads applied. Measurements of the harmonic components from the system are obtained from a desktop computer with analog to digital interface cards running programs developed under LabVIEW software. The system demonstrates the greatest change in harmonic components just before onset of power generation and thereafter settles to a constant level. Work is planned to expand the system to include more nodes, three phase capabilities, and reactive power electronic control     

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 brushless, exciterless, single-phase, sinusoidal wave synchronousmachine having an auxiliary stator winding

Analytical and experimental studies of a brushless, exciterless, single-phase, sinusoidal-wave synchronous machine operating as a generator or a motor, derived from a three-phase machine, are reported. One phase armature winding of the three-phase machine is used as an auxiliary stator winding of the single-phase machine and is used to supply the exciting power for the other two-phase armature windings acting as the load winding of the single-phase machine. A 1.5 kW, 200 V, 60 Hz, four-pole synchronous machine was used the experiments. It is shown that the waveforms of the armature terminal voltage and the load current are nearly sinusoidal. The advantages of the single-phase machine as a portable generator or small-load motor are discussed     

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.     

Aggregate load-frequency control of a wind-hydro autonomous microgrid

This paper presents an aggregate load-frequency controller for an autonomous microgrid (MG) with wind and hydro renewable energy sources. A micro-hydro power plant with a synchronous generator (SG) and a wind power plant with an induction generator (IG) supply the MG. Both generators directly feed power into the grid without the use of additional power electronics interfaces, thus the solution becoming robust, reliable and cost-effective. An original electronic load controller (ELC) regulates the MG frequency by a centralized load-frequency control method, which is based on a combination of smart load (SL) and battery energy storage system (BESS). SL and BESS provides the active power balance for various events that such systems encounter in real situations, both in cases of energy excess production and energy shortage. Moreover, the proposed ELC includes an ancillary function to compensate the power unbalance produced by the uneven distribution of the single-phase loads on the MG phases, without the use of extra hardware components. A laboratory-scale prototype is used for experimentally assessment of the proposed solutions. The experimental results emphasize the effectiveness of the ELC while also showing its limitations.     

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.     

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     

Effects of forced power transfer on high speed generator-loadsystems

A computer-aided method for investigating disturbances due to the forced paralleling of out of phase high speed salient pole AC generator systems feeding isolated loads is presented. The method is used to predict the system performance including the reverse voltage across the rotating bridge rectifier of the field exciter. This paper presents the fundamentals and the modeling approach used in the development of this method. In addition, the results of using this approach to compute the machine parameters under different load conditions including saturation effects due to magnetic material nonlinearities and space harmonics effects due to machine geometry and winding layouts are presented. The computed parameters are validated by comparison to test data. These parameters form the main data for simulating the forced paralleling of out of phase high speed salient pole AC generator systems feeding isolated loads. Further, the results of using this modeling approach in a case study to predict the system performance due to forced power transfer are summarized and are shown to be in good agreement with test data     

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     

Voltage and Frequency Controller for a Three-Phase Four-Wire Autonomous Wind Energy Conversion System

This paper deals with control of voltage and frequency of an autonomous wind energy conversion system (AWECS) based on capacitor-excited asynchronous generator and feeding three-phase four-wire loads. The proposed controller consists of three single-phase insulated gate bipolar junction transistor (IGBT)-based voltage source converters (VSCs) and a battery at dc link. These three single-phase VSCs are connected to each phase of the generator through three single-phase transformers. The proposed controller is having bidirectional flow capability of active and reactive powers by which it controls the system voltage and frequency with variation of consumer loads and the speed of the wind. VSCs along with transformer function as a voltage regulator, a harmonic eliminator, a load balancer, and a neutral current compensator while the battery is used to control the active power flow which, in turn, maintains the constant system frequency. The complete electromechanical system is modeled and simulated in the MATLAB using the Simulink and the power system blockset (PSB) toolboxes. The simulated results are presented to demonstrate the capability of the proposed controller as a voltage and frequency regulator, harmonic eliminator, load balancer, and neutral current compensator for different electrical (varying consumer loads) and mechanical (varying wind speed) dynamic conditions in an autonomous wind energy conversion system.