Nonlinear transient and steady state analysis for self-excited single-phase synchronous reluctance generator

With today\u27s trend for distributed generation and the need for alternative and renewable energy sources, self-excited induction and synchronous reluctance generators have attracted more attention for wind, tidal and hydro power generation applications. Compared to synchronous and DC generators, they have the advantages: they are brushless, they are robust, they do not need DC excitation and they are relatively low cost.;Compared with SEIG, the self-excited reluctance generator (SERG) not only has the advantages of simplicity and ruggedness, but can also have enhanced steady-state characteristics and high efficiency over a wide range of operation. Moreover, its output frequency is determined only by the prime mover speed, rather than by both the load and the prime mover speed as in an induction generator, so SERG can be easily integrated with power electronic devices to implement a control scheme.;Most of the current analyses deal with three-phase reluctance generators, but insufficient attention has been paid to single-phase self-excited reluctance generators (SPSERG). Their unbalanced loads make their analysis more difficult. This research is motivated by the fact that SPSERG provides a good alternative to single-phase induction generators used in stand-alone generation applications. A general methodology is suggested for transient response prediction and steady state performance analysis for the SPSERG type of electric machine.;To establish a design environment, finite element method is an effective tool, which can be integrated in machine modeling to obtain good performance prediction. In this work, an off-line FEM approach is proposed to obtain the saturation characteristics for state space simulation. During the process, transformation between instantaneous inductance and average inductance is investigated. Off-line FEM + SS approach is proved to be a simple and economic method and can fit the experimental results in good accuracy.;Moreover, a steady state model has to be built to reveal the parametric dependence and provide good design guidance. However, because of the unbalanced load and nonlinear feature of the machine, existing models are not suitable for analysis. In this dissertation, a novel inductance-oriented steady state model based on the harmonic balance technique is introduced. The idea is that starting from the inductance determination under certain load, the fluxes can be attained by a nonlinear relationship, after that, the machine variables can be solved according to the fluxes. Comparison between simulation and experiment validates this approach

Selection of Capacitors for the Self Excited Slip ring Induction Generator with External Rotor Capacitance

The self regulating feature of a Self Excited Induction Generator (SEIG) by connecting additional capacitors is examined with the slip ring induction generator. The system consisting of external rotor capacitors at rotor has been analyzed. A methodology has been explained to choose appropriate set of values of these rotor capacitors for desired voltage regulation. Based on the steady-state equivalent circuit model, consideration of the circuit conductances yields a 7th-degree polynomial in the frequency. The polynomial can be solved for real roots, which enables the value of C, 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 are derived for different loads. Using the Same numerical approach, an iterative procedure is also developed for predicting the capacitance required for maintaining the terminal voltage at a preset value when the generator is supplying load. Results of a detailed investigation on a conventional 3.5 kW induction motor operated as a SEIG are presented to illustrate the effectiveness of the proposed method. Close agreement between predicted and test results has been observed thereby establishing the validity of the analysis carried Keywords: capacitance requirements, self-excitation, slip ring induction generator, external rotor capacitanc

Thermal analysis of aluminum and copper rotor self excited induction generators

Wind energy is the one of the most abundantly available forms of renewable energy, and has emerged as a viable alternative to conventional non-renewable energy sources. The wind electrical generation system is the most competitive of all the environmentally clean and safe renewable energy sources. Electricity derived from wind power provides an alternative to conventional generation that could be used to achieve substantial reductions in fossil fuel use and industrial effluents like carbon dioxide. Current utilization of renewable energy systems in the form of wind, small hydro and bio-gas has led to the massive use of grid-connected and self-excited induction generators (SEIG). Besides being commonly used as drives in the industry, three-phase induction machines have earned much attention as wind generators because of the qualities such as ruggedness, fault tolerance and constructional simplicity, and constitute the biggest sector in the present wind power industry. This thesis consists of theory and background of self-excited induction generators (SEIGs), their dynamic and mathematical modeling, development of a laboratory experimental set-up, development of a thermal model and computational and experimental results of thermal modeling and aluminum-rotor and copper-rotor SEIGs and comparative analysis between the two kinds of SEIGs

Study of self excited induction generators with aluminium and copper rotors taking skin effect into account

This thesis covers the dynamic modeling and analysis of self-excited induction generators (SEIG) used in wind turbine applications. The process of self excitation or build-up of terminal voltage of an induction generator is explained as a physical process and also mathematically using higher order differential equations. A complete system model in d-q axis stationary reference frame has been formulated that consists of several non-linear differential equations. The non-linear variation of the magnetizing inductance with stator current of the induction machine has been taken into account in this model. Moreover this mathematical model takes skin effect into consideration. The rotor parameters determined from standard induction machine tests are modified by taking the rotor bar geometry, the material of the rotor bars and the frequency of the induced emf into account. The developed model has been used to analyze the performance of two industrial type 7.5 hp induction machines, one with an aluminium-rotor, the other with copper-rotor. A comparative performance analysis of these aluminum-rotor and copper-rotor SEIGs, considering saturation and skin effect has been carried out both theoretically and experimentally, and presented in the thesis

Stator current signal crossing for fault diagnosis of self-excited induction generators

This paper presents a novel method for modelling and diagnosis of electrical and mechanical faults in fixed-Speed Self-Excited Induction Generators (SEIGs) operating in autonomous mode in a small-scale wind energy system. The proposed method is validated using the finite element method. After the selection of the magnetising capacitors, the self-excitation process is performed under no-load conditions. Once the stator voltage is established, a symmetrical three-phase load is connected. The fault detection method introduced here is called Stator Current Signal Crossing (SCSC). The SCSC extracts a new signal from the stator currents, that enables the detection of stator inter turn shortcircuits, broken rotor bars, and dynamic eccentricity faults in SEIGs. A spectral analysis of SCSC using the Fast Fourier Transform (FFT) algorithm is used to precisely locate the induced fault components. What sets this fault-tracking method apart from its predecessors is its exceptional ability to detect faults of any magnitude by analysing the modulation of the SCSC signal. These faults are directly identified by the presence of distinct harmonics, each indicative of a specific type of fault. This study also focuses on the SEIG in a wind energy system, whereas previous works have mainly addressed the induction machine in motor mode. In contrast, previous methods involved analysing a single current signal and isolating specific harmonics from a wide frequency range. The effectiveness of the proposed fault detection method and the self-excitation process are illustrated by simulation results and spectral analysis

Optimal energy efficiency of isolated PAT systems by SEIG excitation tuning

[EN] The use of pump working as turbine (PAT) was identified by many researchers as a way to improve the energy efficiency in the water systems. However, the majority of the researches consider the hydraulic machine connected to the electrical grid, which may not fit best when these recovery systems are located in rural or remote areas. To improve the efficiency in these recovery systems for rural areas, this research contributes for a further study and optimization of the off-grid PAT systems with induction generators. The current manuscript proposes a methodology to obtain the best efficiency of the PAT-SEIG (Self-Excited Induction Generator) system when operating under different speeds and loads. For these systems, the selection of capacitors for the SEIG is critical to maximizing the energy efficiency. A methodology is proposed to estimate and select the correct SEIG model parameters and, thus, compute the best capacitor values to improve the PAT-SEIG energy efficiency. Special attention is given to the impact the SEIG parameters have in the efficiency of the recovery system. The accuracy of the analytical model improved, reducing the error between analytical and experimental results from 50.8% (for a model with constant parameters) to 13.2% (with parameters changing according to the operating point of the system). These results showed an increase of the overall PAT system efficiency from 26% to 40% for the analyzed case study.This work was supported by FCT, through IDMEC, under LAETA, project UID/EMS/50022/2019 and the project REDAWN (Reducing Energy Dependency in Atlantic Area Water Networks) EAPA_198/2016 from INTERREG ATLANTIC AREA PROGRAMME 2014-2020 and CERIS (CEHIDRO-IST), the Hydraulic Laboratory, for experiments on PATs.Fernandes, JF.; Pérez-Sánchez, M.; Ferreira, F.; López Jiménez, PA.; Ramos, HM.; Costa Branco, P. (2019). Optimal energy efficiency of isolated PAT systems by SEIG excitation tuning. Energy Conversion and Management. 183:391-405. https://doi.org/10.1016/j.enconman.2019.01.016S39140518

Small-Signal Modelling and Analysis of Doubly-Fed Induction Generators in Wind Power Applications

The worldwide demand for more diverse and greener energy supply has had a significant impact on the development of wind energy in the last decades. From 2 GW in 1990, the global installed capacity has now reached about 100 GW and is estimated to grow to 1000 GW by 2025. As wind power penetration increases, it is important to investigate its effect on the power system. Among the various technologies available for wind energy conversion, the doubly-fed induction generator (DFIG) is one of the preferred solutions because it offers the advantages of reduced mechanical stress and optimised power capture thanks to variable speed operation. This work presents the small-signal modelling and analysis of the DFIG for power system stability studies. This thesis starts by reviewing the mathematical models of wind turbines with DFIG convenient for power system studies. Different approaches proposed in the literature for the modelling of the turbine, drive-train, generator, rotor converter and external power system are discussed. It is shown that the flexibility of the drive train should be represented by a two-mass model in the presence of a gearbox. In the analysis part, the steady-state behaviour of the DFIG is examined. Comparison is made with the conventional synchronous generators (SG) and squirrel-cage induction generators to highlight the differences between the machines. The initialisation of the DFIG dynamic variables and other operating quantities is then discussed. Various methods are briefly reviewed and a step-by-step procedure is suggested to avoid the iterative computations in initial condition mentioned in the literature. The dynamical behaviour of the DFIG is studied with eigenvalue analysis. Modal analysis is performed for both open-loop and closed-loop situations. The effect of parameters and operating point variations on small signal stability is observed. For the open-loop DFIG, conditions on machine parameters are obtained to ensure stability of the system. For the closed-loop DFIG, it is shown that the generator electrical transients may be neglected once the converter controls are properly tuned. A tuning procedure is proposed and conditions on proportional gains are obtained for stable electrical dynamics. Finally, small-signal analysis of a multi-machine system with both SG and DFIG is performed. It is shown that there is no common mode to the two types of generators. The result confirms that the DFIG does not introduce negative damping to the system, however it is also shown that the overall effect of the DFIG on the power system stability depends on several structural factors and a general statement as to whether it improves or detriorates the oscillatory stability of a system can not be made

Excitation and control of a high-speed induction generator

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2005.Includes bibliographical references (leaves 97-99).This project investigates the use of a high speed, squirrel cage induction generator and power converter for producing DC electrical power onboard ships and submarines. Potential advantages of high speed induction generators include smaller size and weight, increased durability, and decreased cost and maintenance. Unfortunately, induction generators require a "supply of reactive power" to run and suffer from variation in output voltage and frequency with any changes to the input reactive power excitation, mechanical drive speed, and load. A power converter can resolve some of these issues by circulating the changing reactive power demanded by the generator while simultaneously controlling the stator frequency to adjust the machine slip and manage the real output power. This combination of real and reactive power control will ensure a constant voltage DC bus over the full load range. Tests were performed on a three horsepower motor to help validate models and simulations at both the two kilowatt and 5 megawatt level. After determining the equivalent circuit of the demonstration motor, it was tested as a generator under grid connected and capacitor excited conditions. A stand-alone five megawatt, 12,000 RPM generator designed specifically to operate at high efficiency and power factor over the full load rang was used to design converter parameters. A variety of reactive power excitation strategies were briefly examined before the flow of reactive currents through a converter was explained using a six step inverter with two different switching schemes.(cont.) Steady state and transient simulations matched the measured machine performance and illustrated the performance of the control strategy as the load changes. Keywords: induction generator, self-excitation, reactive power, power converter, rectifier.by Steven Carl Englebretson.S.M

IDENTIFYING VOLTAGE AND FREQUENCY REGULATION CURVES OF SELF-EXCITED INDUCTION GENERATOR

A simple method for computation of voltage and frequency regulation curves of a self-excited induction generator is presented in this paper. The method has been successfully exploited for identifying several different types of regulation curves of a 1.5 kW cage rotor self-excited induction generator. The proposed approach is universal and can be used for identification of any generator’s regulation curves, if the parameters of the equivalent circuit are known. Information contained in these computed curves can be very useful while designing systems for automatic control of self-excited induction generators

MODELING AND ANALYSIS OF AC CONDUCTION EFFECTS IN ALUMINUM & COPPER-ROTOR INDUCTION MACHINES AND DEVELOPMENT OF A NOVEL VOLTAGE REGULATION SCHEME FOR DISTRIBUTED WIND POWER GENERATION

Centralized generation is being supplemented or replaced fast by distributed generation, a new way of thinking about electricity generation, transmission and distribution. Understanding the significance and prospects of self-excited induction generators (SEIGs) in autonomous distributed wind power generation (ADWG), this thesis exclusively presents the following : 1) A developed dynamic model of SEIG developed using the conventional two-axis transformation technique, commonly known as Park\u27s transformation. 2) A developed electromagnetic model of the AC conduction effects and the significance of incorporating them into the conventional two-axis model of the SEIG (improved mathematical model). 3) A comprehensive study of commercially available niche copper-rotor induction motor (CRIM) and conventional aluminum-rotor induction motor (ARIM) to be used as induction generators in the above application. 4) An experimental three phase short-circuit fault analysis in SEIGs for ADWG. 5) A novel low-cost embedded system based on Daubechies wavelet transforms and swarm intelligence technique for voltage regulation and fault detction in the above application