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

Doubly-fed induction generator used in wind energy

Wound-rotor induction generator has numerous advantages in wind power generation over other generators. One scheme for wound-rotor induction generator is realized when a converter cascade is used between the slip-ring terminals and the utility grid to control the rotor power. This configuration is called the doubly-fed induction generator (DFIG). In this work, a novel induction machine model is developed. This model includes the saturation in the main and leakage flux paths. It shows that the model which considers the saturation effects gives more realistic results. A new technique, which was developed for synchronous machines, was applied to experimentally measure the stator and rotor leakage inductance saturation characteristics on the induction machine. A vector control scheme is developed to control the rotor side voltage-source converter. Vector control allows decoupled or independent control of both active and reactive power of DFIG. These techniques are based on the theory of controlling the B- and q- axes components of voltage or current in different reference frames. In this work, the stator flux oriented rotor current control, with decoupled control of active and reactive power, is adopted. This scheme allows the independent control of the generated active and reactive power as well as the rotor speed to track the maximum wind power point. Conventionally, the controller type used in vector controllers is of the PI type with a fixed proportional and integral gain. In this work, different intelligent schemes by which the controller can change its behavior are proposed. The first scheme is an adaptive gain scheduler which utilizes different characteristics to generate the variation in the proportional and the integral gains. The second scheme is a fuzzy logic gain scheduler and the third is a neuro-fuzzy controller. The transient responses using the above mentioned schemes are compared analytically and experimentally. It has been found that although the fuzzy logic and neuro-fuzzy schemes are more complicated and have many parameters; this complication provides a higher degree of freedom in tuning the controller which is evident in giving much better system performance. Finally, the simulation results were experimentally verified by building the experimental setup and implementing the developed control schemes

Advances in wind power generation, transmission, and simulation technology

Wind is an increasingly important piece of electricity generation portfolios worldwide. This dissertation describes advances related to the electromechanical energy conversion system of wind turbines, and the electric transmission system for offshore wind power plants. The contributions of this work are the following: (i) We propose that the power electronics topology commonly called the Vienna rectifier can be used for improved variable-speed wind energy conversion. Theoretical analysis is conducted to show how a Vienna rectifier could drive either a squirrel-cage induction generator or a permanent-magnet synchronous generator-based wind turbine. Computer simulations and experimental results demonstrate the feasibility of the proposed topology and potential improvements in energy conversion efficiency. (ii) We propose a novel low-frequency ac (LFAC) transmission system for offshore wind power plants. A system design and control method is set forth, and key system operational characteristics are illustrated via computer simulations. The LFAC system constitutes a promising option for medium- or long-distance transmission, and could be an alternative to high-voltage dc (HVDC) transmission. (iii) We develop a technique that utilizes a field programmable gate array (FPGA) as a dynamic simulation platform for wind turbines. A doubly fed induction generator-based wind turbine simulation is implemented on an FPGA board, in order to verify the effectiveness and performance advantage of this approach

Modeling and Emulation of Induction Machines for Renewable Energy Systems

Electric motors with their drive systems utilize most of the generated power. They account for about two-thirds of industrial energy utilization and about 45% of the global energy utilization. Therefore, optimizing a motor and its corresponding drive system with their control can save energy and improve system efficiency. It can be risky and difficult however to test large prototype electrical machines and study their dynamic characteristics; or to test electric drives at high power levels with different machine ratings and operating conditions. One way to effectively evaluate these systems is to emulate the electric machine using a power electronic converter with the help of a real-time simulation system. Power electronic converters and their control systems are increasingly being used in industries with different power ratings at high switching frequencies in the kHz range. In this thesis, an induction motor emulator based on a power electronic converter is developed to allow detailed testing the converter and controller. A proportional-resonant current controller in the abc-frame and pulse-width modulation are employed. The conventional model of the induction machine (IM) with constant parameters does not represent accurately the machine’s performance for severe transients specifically during starting and loading conditions. Magnetic saturation effects should be considered. Hence, experimental procedures to determine the flux saturation characteristics in the main and both stator and rotor leakage flux paths are achieved. Machine models that consider or neglect the main and leakage flux saturation are compared with experimental results. The model which considers the magnetic saturation effect in both flux paths results in more accurate transient responses. Likewise, the dynamic response of the induction motor emulator during startup and loading transients show the effectiveness of using the developed emulator to resemble closely a real motor. The relationship between the stator and rotor leakage reactance of the induction machine according to IEEE Std. 112™ is assumed to be constant under all operating conditions. However, this is not accurate during severe transients such as the direct online startup and loading conditions of a three-phase induction motor. The leakage reactance of the machine can vary widely during severe conditions. Hence, using constant parameters in the machine model will result in an inaccurate dynamic performance prediction. Moreover, considering a constant ratio between the stator and rotor leakage reactance is no longer valid for all current levels. In this thesis, a direct and precise method is proposed to estimate and separate the stator and rotor leakage reactance parameters under normal operating conditions and when the core is deeply saturated. The method exploits the 2D time-stepping finite element method (FEM) with a coupled circuit. The obtained current-dependent reactance functions in both leakage flux paths are included in the dq-model of the IM. Other machine parameters are determined by implementing the standard tests in FEM. To verify the effectiveness of the proposed method, the predicted results are compared to the dynamic responses obtained experimentally from a three-phase, 5-hp squirrel-cage IM. A power electronic converter-based self-excited induction generator (SEIG) emulator is developed. The testbed replaces a wind- or microhydro-turbine driven squirrel-cage induction generator that works within an isolated power system to feed power in remote areas. It supports testing and analyzing the dynamic performance of islanded generation systems which comprise numerous kinds of parallel-operated renewable energy sources. The risk and cost associated with the testing, analysis and development of novel control topologies and electrical machine prototypes are reduced considerably. The dq-model of the SEIG in the rotor reference frame is implemented in a real-time controller. Saturation in the main and leakage flux paths are included in the machine model. The generator model with modified parameters is verified and used in the emulator. The cascaded voltage and current loops utilizing the proportional-integral controllers in dq-frame are employed. A voltage-type ideal transformer model is used as a power interface for the emulator whereas an excitation capacitance is added to the power-hardware-in-the-loop block diagram. Likewise, the dynamic response of the induction generator emulator during voltage buildup and loading conditions validates the effectiveness of using the developed emulator to resemble closely a real generator

Studies in Electrical Machines & Wind Turbines associated with developing Reliable Power Generation

The publications listed in date order in this document are offered for the Degree of Doctor of Science in Durham University and have been selected from the author’s full publication list. The papers in this thesis constitute a continuum of original work in fundamental and applied electrical science, spanning 30 years, deployed on real industrial problems, making a significant contribution to conventional and renewable energy power generation. This is the basis of a claim of high distinction, constituting an original and substantial contribution to engineering science

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

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

Wind Turbine Generator Condition Monitoring via the Generator Control Loop

This thesis focuses on the development of condition monitoring techniques for application in wind turbines, particularly for offshore wind turbine driven doubly fed induction generators. The work describes the significant development of a physical condition monitoring Test Rig and its MATLAB Simulink model to represent modern variable speed wind turbine and the innovation and application of the rotor side control signals for the generator fault detection. Work has been carried out to develop a physical condition monitoring Test Rig from open loop control, with a wound rotor induction generator, into closed loop control with a doubly fed induction generator. This included designing and building the rotor side converter, installing the back-to-back converter and other new instrumentation. Moreover, the MATLAB Simulink model of the Test Rig has been developed to represent the closed loop control, with more detailed information on the Rig components and instrumentation and has been validated against the physical system in the time and frequency domains. A fault detection technique has been proposed by the author based on frequency analysis of the rotor-side control signals, namely; d-rotor current error, q-rotor current error and q-rotor current, for wind turbine generator fault detection. This technique has been investigated for rotor electrical asymmetry on the physical Test Rig and its MATLAB Simulink model at different fixed and variable speed conditions. The sensitivity of the each proposed signal has been studied under different operating conditions. Measured and simulated results are presented, a comparison with the results from using stator current and total power has been addressed and the improvement in condition monitoring detection performance has been demonstrated in comparison with previous methods, looking at current, power and vibration analysis