Mitigation of Harmonics and Inter-Harmonics with LVRT and HVRT Enhancement in Grid-Connected Wind Energy Systems Using Genetic Algorithm-Optimized PWM and Fuzzy Adaptive PID Control

© 2021 Author(s). This is the accepted manuscript version of an article which has been published in final form at https://doi.org/10.1063/5.0015579The growing installed wind capacity over the last decade has led many energy regulators to define specific grid codes for wind energy generation systems connecting to the electricity grid. These requirements impose strict laws regarding the Low Voltage Ride Though (LVRT) and High Voltage Ride Though (HVRT) capabilities of wind turbines during voltage disturbances. The main aim of this paper is to propose LVRT and HVRT strategies that allow wind systems to remain connected during severe grid voltage disturbances. Power quality issues associated with harmonics and inter-harmonics are also discussed and a control scheme for the grid-side converter is proposed to make the Wind Energy Conversion System insensitive to external disturbances and parametric variations. The Selective Harmonic Elimination Pulse Width Modulation technique based on Genetic Algorithm optimization is employed to overcome over-modulation problems, reduce the amplitudes of harmonics, and thus reduce the Total Harmonic Distortion in the current and voltage waveforms. Furthermore, to compensate for the fluctuations of the wind speed due to turbulence at the blades of the turbine, a fuzzy Proportional-Integral-Derivative controller with adaptive gains is proposed to control the converter on the generator side.Peer reviewedFinal Accepted Versio

Detection of faults in a scaled down doubly-fed induction generator using advanced signal processing techniques.

The study ventures into the development of a micro-based doubly fed induction generator (DFIG) test rig for fault studies. The 5kW wound rotor induction machine (WRIM) that was used in the test rig was based on a scaled-down version of a 2.5MW doubly fed induction generator (DFIG). The micromachine has been customized to make provision for implementing stator inter-turn short-circuit faults (ITSCF), rotor ITSCF and static eccentricity (SE) faults in the laboratory environment. The micromachine has been assessed under the healthy and faulty states, both before and after incorporating a converter into the rotor circuit of the machine. In each scenario, the fault signatures have been characterised by analyzing the stator current, rotor current, and the DFIG controller signals using the motor current signature analysis (MCSA) and discrete wavelet transform (DWT) analysis techniques to detect the dominant frequency components which are indicative of these faults. The purpose of the study is to evaluate and identify the most suitable combination of signals and techniques for the detection of each fault under steady-state and transient operating conditions. The analyses of the results presented in this study have indicated that characterizing the fault indicators independent of the converter system ensured clarity in the fault diagnosis process and enabled the development of a systematic fault diagnosis approach that can be applied to a controlled DFIG. It has been demonstrated that the occurrence of the ITSCFs and the SE fault in the micro-WRIM intensifies specific frequency components in the spectral plots of the stator current, rotor current, and the DFIG controller signals, which may then serve as the dominant fault indicators. These dominant components may be used as fault markers for classification and have been used for pattern recognition under the transient condition. In this case, the DWT and spectrogram plots effectively illustrated characteristic patterns of the dominant fault indicators, which were observed to evolve uniquely and more distinguishable in the rotor current signal compared to the stator current signal, before incorporating the converter in the rotor circuit. Therefore, by observing the trends portrayed in the decomposition bands and the spectrogram plots, it is deemed a reliable method of diagnosing and possibly quantifying the intensity of the faults in the machine. Once the power electronic converter was incorporated into the rotor circuit, the DFIG controller signals have been observed to be best suited for diagnosing faults in the micro-DFIG under the steady-state operating condition, as opposed to using the terminal stator or rotor current signals. The study also assessed the impact of undervoltage conditions at the point of common coupling (PCC) on the behaviour of the micro-DFIG. In this investigation, a significant rise in the faulted currents was observed for the undervoltage condition in comparison to the faulty cases under the rated grid voltage conditions. In this regard, it could be detrimental to the operation of the micro-DFIG, particularly the faulted phase windings, and the power electronic converter, should the currents exceed the rated values for extended periods

Doubly fed induction generator with integrated energy storage system for smoothening of output power

Wind energy is one of the fastest growing renewable energies in the world today. However, integration of wind power into the power system network is still confronting many challenges. One of the main challenges is the suppression of wind power fluctuations. This thesis focuses on integration of wind power with energy storage to overcome the integration challenges. The first part of this thesis investigates the suitability of energy storage systems for transmission, distribution and wind farm applications. A review on the available energy storage systems is performed considering several criteria. Efforts are made to investigate solutions that meet all the power system requirements. In the second part of the thesis, a wind turbine generator with integrated energy storage system is modeled and studied for smoothening of the output power fluctuations due to changes in wind velocity. An ultra-capacitor is used as an energy storage system which is integrated into the doubly-fed induction generator through a bidirectional buck-boost dc-dc converter. Different modes of operation for the integrated system are studied and the simulation results verify the effectiveness of the designed model using the software package MATLAB/Simulink. The last part of the thesis focuses on application of the doubly-fed induction machine. The grid-side converter of the machine is used to supply harmonics for nearby non-linear loads. A multiple reference frame synchronous estimator and controller are used to track and eliminate the dynamically changing 6k ±1 harmonics on the power system network. This complete system is developed and tested using the software package PSCAD/EMTDC. The simulation results and the harmonic analysis verify the correct operation of the system --Abstract, page iii

Grid Connected Doubly Fed Induction Generator Based Wind Turbine under LVRT

This project concentrates on the Low Voltage Ride Through (LVRT) capability of Doubly Fed Induction Generator (DFIG) wind turbine. The main attention in the project is, therefore, drawn to the control of the DFIG wind turbine and of its power converter and to the ability to protect itself without disconnection during grid faults. It provides also an overview on the interaction between variable speed DFIG wind turbines and the power system subjected to disturbances, such as short circuit faults. The dynamic model of DFIG wind turbine includes models for both mechanical components as well as for all electrical components, controllers and for the protection device of DFIG necessary during grid faults. The viewpoint of this project is to carry out different simulations to provide insight and understanding of the grid fault impact on both DFIG wind turbines and on the power system itself. The dynamic behavior of DFIG wind turbines during grid faults is simulated and assessed by using a transmission power system generic model developed and delivered by Transmission System Operator in the power system simulation toolbox Digsilent, Matlab/Simulink and PLECS

Control of a permanent magnet synchronous general-based wind energy conversion system.

Master of Science in Electrical Engineering. University of KwaZulu-Natal, Durban 2016.Wind energy has proven to be a competitive and an environmentally friendly renewable energy resource for generating electricity. Wind farms are usually located far from the load centers; hence the generated power has to be transmitted over long distances to load centers. High voltage direct current (HVDC) transmission system is the preferred means for transmitting bulk power over long distances when compared to high voltage alternating current (HVAC) transmission system. An HVDC transmission system increases the transmission capacity, improves the system stability, and possesses lower transmission losses. In this research investigation, a 690V, 2MW wind turbine-driven permanent magnet synchronous generator is modelled to be integrated into a local 33kV AC grid via a three- level neutral-point-clamped voltage source converter (VSC)-based HVDC transmission system. Three control schemes were implemented, namely: pitch-angle controller, generator-side converter controller, and a grid-side converter controller to optimize the system performance. The stability analysis and controller modeling was carried out in MATLAB using bode plots and step response curves. The proposed subsystems and the control schemes were implemented in PSIM software package to evaluate the overall system's performance. The simulations were carried out on the model and it was concluded that the grid-side converter controller ensured maximum power point tracking when the wind speed was lower than the wind turbine(WT)'s rated wind speed. Conversely, as the wind speed exceeded the WT's rated wind speed, the pitch-angle controller was activated. This increased the angle of attack thereby reducing the power coefficient in order to shed off the aerodynamic power. Furthermore, the DC-link voltage was stabilized within the allowable limits to ensure a continuous flow of active power from the WT to the grid and the reactive power transfer between the grid-side converter and the AC utility grid was maintained to a minimum to ensure a unity power factor. The comparison analysis of the new control approach to the traditional control approach illustrated that for the new control approach, the ability of the DC-link voltage controller to keep the DC-link voltage within the allowable limits does not get impaired during fault conditions. Therefore, the power continues flowing from the WT generator to the grid. Conversely, it was observed that for the traditional control approach, the ability of the DC-link voltage controller to stabilize the DC-link voltage gets impaired and therefore it can no longer effectively transfer as much active power from the WT generator to the grid. Therefore, the new control approach proved to be effective in terms of stabilizing the DC-link voltage during fault conditions thereby enhancing the WT’s fault-ride-through capability

Development of fast multi-system simulation models for permanent magnet synchronous motor and generator drive systems

This research project investigates the development and validation of alternative simulation models for voltage source inverter fed permanent magnet synchronous machine drive systems which can rapidly and accurately analyse and evaluate the performance of PM machine drives and associated control system designs. Traditionally simulations have been conducted using switching models and state space average value methods. The simulation of switching models is time consuming and that of state space averaging involves complex mathematical transformation to d-q axis, with additional circuitry and this limits their application in a time critical design process. Even if the complex calculations of state space are overcome, the proposed model can still achieve better results. This thesis presents the development of fast multi system simulation models for permanent magnet synchronous motor and generator drive systems. The fast simulation model: Average Voltage Estimation Model (AVEM) was developed for two-level, three phase VSI-fed PMSM drive systems and two-level three phase full-scale back-back VSI incorporated in a PMSG wind energy conversion system. The method uses the principle of control strategy and switching function to derive the average phase voltage in one switching period and then uses the average voltages to drive piecewise-linear voltage sources across the terminals of the permanent magnet synchronous machine and three phase system. A voltage source inverter loss model was also developed and incorporated into the AVEM to simulate the drive system power flow and its performance evaluated. The average voltage estimation model is also used to estimate and simulate the energy output of the variable speed PMSG wind energy conversion system. Practical implementation of this technique is achieved using a DSP based controller and validation made through comparison of the DSP AVEM energy estimation method with calculated energy. The study also presents the development of detailed VSI switching models for a variable speed PMSM and a PMSG wind energy conversion system which serve as benchmarks for the proposed AVEM models. A detailed description of both models will be presented. Since models require a control strategy: these control strategies were also developed using the carrier-based sinusoidal (SPWM) and implemented with PI regulators. In the permanent magnet synchronous generator wind energy conversion system application, the SPWM is applied to control the speed of the generator side converter to track maximum power as wind speed varies using the developed passive MPPT control technique and controls the AC load side converter to maintained constant DC link voltage. The sinusoidal PWM control provides a simplified control suitable for the variable speed PMSM drive system and the PMSG wind energy conversion system. Lastly, this thesis presents a detailed development of an experimental test rig. The test rig is developed to provide flexibility for the validation and comparison of the results of both simulation models against real practical implementations for PMSM drive system and PMSG wind energy conversions system. Several simulation case studies were performed using the PORTUNUS simulation package to validate and analyse the steady state accuracy of the proposed average voltage estimation model and control system against the switching model. Experiments were also carried out to validate the results of the simulation models. The simulation models results are presented and compared with experimental results. Suitable steady state performance analysis of two-level, three phase voltage source inverter fed permanent magnet synchronous motor and two-level three phase full scale back-back voltage source inverter with permanent magnet synchronous generator drive simulation and experimental performance are also carried out. The results show good agreement of the proposed average voltage estimation model with the switching model and experimental data, and where necessary the reasons for differences are discussed. The simulation of the AVEM is approximately 50 times faster than the switching model. The limitation of the proposed model is also discussed; mainly it cannot be used for the study and analysis of the internal dynamics of the voltage source inverter. The results from the proposed modelling method utilising the average voltage estimation confirm that this method can be used as an alternative to the detailed switching model for fast simulation and steady state analysis of PM machine drive systems given the advantages of speed, simplicity and ease of implementation. Note that the proposed model is only used for steady state performance analysis; however, in future its application can be extended to transient analysis. In addition, the model is not about maximium power point tracking techniques but it can accommodate maximium power point tracking techniques. It should also be highlighted that exactly the same digital control block is used in both the switching and AVEM models thus allowing a true comparison of controller behaviour. The model developed in this research project has application beyond PMSM drive system and PMSG wind energy conversion system. It can be applied to modelling, simulation and control of other electrical machine drives such as induction machines, switched reluctance machines and three-phase VSI-fed systems

Wind Power

This book is the result of inspirations and contributions from many researchers of different fields. A wide verity of research results are merged together to make this book useful for students and researchers who will take contribution for further development of the existing technology. I hope you will enjoy the book, so that my effort to bringing it together for you will be successful. In my capacity, as the Editor of this book, I would like to thanks and appreciate the chapter authors, who ensured the quality of the material as well as submitting their best works. Most of the results presented in to the book have already been published on international journals and appreciated in many international conferences

Synchronverter-based control for wind power

More and more attention has been paid to the energy crisis due to the increasing energy demand from industrial and commercial applications. The utilisation of wind power, which is considered as one of the most promising renewable energy sources, has grown rapidly in the last three decades. In recent years, many power converter techniques have been developed to integrate wind power with the electrical grid. The use of power electronic converters allows for variable speed operation of wind turbines, and enhanced power extraction. This work, which is supported by EPSRC and Nheolis under the DHPA scheme, focuses on the design and analysis of control systems for wind power. In this work, two of the most popular AC-DC-AC topologies with permanent magnet synchronous generators (PMSG) have been developed. One consists of an uncontrollable rectifier, a boost converter and an inverter and a current control scheme is proposed to achieve the maximum power point tracking (MPPT). In the control strategy, the output current of the uncontrollable rectifier is controlled by a boost converter according to the current reference, which is determined by a climbing algorithm, to achieve MPPT. The synchronverter technology has been applied to control the inverter for the grid-connection. An experimental setup based on DSP has been designed to implement all the above mentioned experiments. In addition, a synchronverter-based parallel control strategy, which consists of a frequency droop loop and a voltage droop loop to achieve accurate sharing of real power and reactive power respectively, has been further studied. Moreover, a control strategy based on the synchronverter has been presented to force the inverter to have capacitive output impedance, so that the quality of the output voltage is improved. Abstract The other topology consists of a full-scale back-to-back converter, of which the rectifier is controllable. Two control strategies have been proposed to operate a three-phase rectifier to mimic a synchronous motor, following the idea of synchronverters to operate inverters to mimic synchronous generators. In the proposed schemes, the real power extracted from the source and the output voltage are the control variables, respectively, hence they can be employed in different applications. Furthermore, improved control strategies are proposed to self-synchronise with the grid. This does not only improve the performance of the system but also considerably reduces the complexity of the overall controller. All experiments have been implemented on a test rig based on dSPACE to demonstrate the excellent performance of the proposed control strategies with unity power factor, sinusoidal currents and good dynamics. Finally, an original control strategy based on the synchronverter technology has been proposed for back-to-back converters in wind power applications to make the whole system behave as a generator-motor-generator system