Development of a system for testing grid-connected permanent magnet wind generators

Renewable energy will be included in the South African Energy Mix over the next two decades. The introduction of renewable energy will reduce South Africa's carbon emissions and also stimulate the economy through job creation as well as creating a local manufacturing sector. South Africa has a large coastal region which is ideal for wind energy deployment. The integration of wind power into the grid needs to be understood as well as the possible problems associated with it. The objective this thesis is to develop a laboratory-based system which can serve as a tool for studying non-ideal conditions associated with the integration of grid-connected Permanent Magnet (PM) wind generators

Study on a hybrid wind turbine system with Intrinsic compressed air energy storage provision

Recent years have witnessed the expansion of the wind power industry, spawned from international legislation that commits countries to increasing their share of renewable energy, compared to their gross energy consumption. However, increased exploitation of wind power poses challenges for power network operation. The variability and uncertainty of wind power may lead to network capacity constraints, system stability issues and potential wind power curtailment in the near future. Energy storage is considered to be one of the most viable options to support the integration of increased wind power to the power network. This project is concerned with developing a hybrid wind turbine with intrinsic compressed air energy storage provision, so that the power output of a wind turbine can be controlled, thus providing flexibility. The proposed hybrid wind turbine makes use of compressed air energy storage on a turbine level. An efficient power split device in the form of a planetary gear box is designed to couple a horizontal axis wind turbine with a scroll air expander/compressor machine, i.e. a single device that can swap its operation, and a permanent magnet synchronous generator/motor. The hybrid wind turbine can operate in conventional standalone fashion. In addition, power can be added and taken away to/from the system through the expansion and compression mode. The hybrid wind turbine also offers standalone energy storage provision, so that power can be extracted from the grid and returned at later times. The whole system mathematical model is derived and successfully validated by means of a small scale experimental test rig. Equipped with feedback control, the hybrid wind turbine can smooth the generated power output under varying wind speeds. Following this, efficiency analysis is carried out in this thesis and a feasibility study on a 1MW hybrid wind turbine is conducted. The results obtained from the project demonstrate that the proposed hybrid wind turbine system is feasible and that it can help improve the wind turbine efficiency in addition to smoothing the power output. Therefore, it can be a valuable asset for wind power integration into the power network

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

Development of a converter for grid-tied and isolated operation of an interior permanent magnet synchronous generator, coupled to a twin-shaft gas turbine

South Africa’s overreliance on coal fired power generation has led to the government’s commitment to diversifying the country’s energy mix. Gas turbine generators are poised to play a larger role in South Africa’s energy mix, due to the country’s abundance in natural gas reserves. Therefore, there is a need to developed gas turbine emulation systems to investigate how this transition is to be implemented and to discover new efficient ways to generate power through gas turbines. This thesis presents the development of a twin-shaft gas turbine emulator. A DC-machine that accepts both torque and speed references is used to emulate the behaviour of the gas turbine according to a modified Rowen gas turbine model. The emulator is coupled to a 1.5kW interior permanent magnet synchronous generator (IPM). The power density of a DC-machine is significantly lower than that of a gas turbine of the same rating. Thus, the DC-machine is rated at double the rating of the IPM to overcome the high inertia it has when compared to a gas turbine of the same rating. This means that the DC-machine can produce large toques to successfully emulated the dynamic behaviour of the gas turbine. A maximum error 2.5% in the emulation of the gas turbine’s speed is reported. A two-level active converter is used to compare control strategies for an IPM. Ninety-degree torque angle (NTA) control, maximum torque per ampere (MTPA) control and unity power factor (UPF) control are compared for performance. The UPF and MTPA control result in the lowest and second lowest DC-link utilisation respectively when compared to NTA control. This is due to a negative d-axis current component as opposed to a zero d-axis current component in the case of NTA control. It is also concluded that to achieve a high power factor and torque development, a negative d-axis current component is required. UPF and MTPA control perform well in both categories, with UPF control and MTPA control resulting in the highest power factor and developed torque respectively. A fourth control strategy that maximises the efficiency of the IPM is developed experimentally. The maximum efficiency (ME) control strategy minimises mechanical, core, windage and conduction losses. It also results in near unity power factor and near maximum developed torque. A nonconventional control structure that involves control of the DC-link from the generatorside converter is presented. This frees the outer-loop control of load-side converter to regulate voltage across the load when the system is supplying power to an isolated load. This control structure also allows the grid-side converter to employ reactive power compensation, without having to regulate the DC-link voltage at the same time. In doing so, large grid currents are avoided. A recursive least squares (RLS) algorithm is used to separate negative and positive sequence current components during grid voltage unbalance. A method to minimise the presence of negative sequence components in the load current is presented and implemented successfully in an experiment

Hardware-In-the-Loop operations with an emulator rig for SOFC hybrid systems

This paper shows the Hardware-In-the-Loop (HIL) technique developed for the complete emulation of Solid Oxide Fuel Cell (SOFC) based hybrid systems. This approach is based on the coupling of an emulator test rig with a real-time software for components which are not included in the plant. The experimental facility is composed of a T100 microturbine (100 kW electrical power size) modified for the connection to an SOFC emulator device. This component is composed of both anodic and cathodic vessels including also the anodic recirculation system which is carried out with a single stage ejector, driven by an air flow in the primary duct. However, no real stack material was installed in the plant. For this reason, a real-time dynamic software was developed in the Matlab-Simulink environment including all the SOFC system components (the fuel cell stack with the calculation of the electrochemical aspects considering also the real losses, the reformer, and a cathodic recirculation based on a blower, etc.). This tool was coupled with the real system utilizing a User Datagram Protocol (UDP) data exchange approach (the model receives flow data from the plant at the inlet duct of the cathodic vessel, while it is able to operate on the turbine changing its set-point of electrical load or turbine outlet temperature). So, the software is operated to control plant properties to generate the effect of a real SOFC in the rig. In stand-alone mode the turbine load is changed with the objective of matching the measured Turbine Outlet Temperature (TOT) value with the calculated one by the model. In grid-connected mode the software/hardware matching is obtained through a direct manipulation of the TOT set-point. This approach was essential to analyze the matching issues between the SOFC and the micro gas turbine devoting several tests on critical operations, such as start-up, shutdown and load changes. Special attention was focused on tests carried out to solve the control system issues for the entire real hybrid plant emulated with this HIL approach. Hence, the innovative control strategies were developed and successfully tested considering both the Proportional Integral Derivative and advanced approaches. Thanks to the experimental tests carried out with this HIL system, a comparison between different control strategies was performed including a statistic analysis on the results The positive performance obtainable with a Model Predictive Control based technique was shown and discussed. So, the HIL system presented in this paper was essential to perform the experimental tests successfully (for real hybrid system development) without the risks of destroying the stack in case of failures. Mainly surge (especially during transient operations, such as load changes) and other critical conditions (e.g. carbon deposition, high pressure difference between the fuel cell sides, high thermal gradients in the stack, excessive thermal stress in the SOFC system components, etc.) have to be carefully avoided in complete plants

An approach to the characterisation of the performance of a tidal stream turbine

In order to better manage and maintain deployed Tidal Stream Turbine (TST) devices their response to complicated and severe loading mechanisms must be established. To aid this process the research presented details a methodology for mapping TST operational data, taken under a variety of operating conditions, to a set of model parameters. The parameter sets were developed based on a TST rotor torque model which, as well as providing means of characterising turbine behaviour, can be used to create TST simulations with minimal computation expense. The use of the model in facilitating parameter surface mapping is demonstrated via its application to a set of rotor torque measurements made of a 1/20th scale TST during flume testing. This model is then deployed to recreate the known rotor behaviour which is compared with the original flume based measurements. This is a flexible tool that can be applied to investigate turbine performance under conditions that cannot be readily replicated using tank-based experiments. Furthermore, Computational Fluid Dynamics simulations of such conditions could be time consuming and computationally expensive. To this end, the use of the model in creating drivetrain test bed based simulations is demonstrated. The model, which can be calculated in real-time, is used to develop representative turbine simulations at high turbulence intensity levels which were not achievable during flume experimentation. The intention is to provide a test-bed for future turbine performance monitoring under more realistic, site specific conditions. The work will also support the deployment of performance surfaces in real-life turbine applications

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

Improving the performance of hybrid wind-diesel-battery systems

Off-grid hybrid renewable energy systems are known as an attractive and sustainable solution for supplying clean electricity to autonomous consumers. Typically, this applies to the communities that are located in remote or islanded areas where it is not cost-effective to extend the grid facilities to these regions. In addition, the use of diesel generators for electricity supply in these remote locations are proven to be uneconomical due to the difficult terrain which translates into high fuel transportation costs. The use of renewable energy sources, coupling with the diesel generator allows for the diesel fuel to be offset. However, to date, a common design standard for the off-grid system has yet to be found and some challenges still exist while attempting to design a reliable system. These include the sizing of hybrid systems, coordination between the operation of dissimilar power generators and the fluctuating load demands, optimal utilisation of the renewable energy resources and identifying the underlying principles which reduce the reliability of the off-grid systems. In order to address these challenges, this research has first endeavoured into developing a sizing algorithm which particularly seeks the optimal size of the batteries and the diesel generator usage. The batteries and diesel generator function in filling the gap between the power generated from the renewable energy resources and the load demand. Thus, the load requirement is also an important factor in determining the cost-effectiveness of the overall system in the long run. A sensitivity analysis is carried out to provide a better understanding of the relationship between the assessed renewable energy resources, the load demand, the storage capacity and the diesel generator fuel usage. The thesis also presents the modelling, simulation and experimental work on the proposed hybrid wind-diesel-battery system. These are being implemented with a full-scale system and they are based on the off-the-shelf components. A novel algorithm to optimise the operation of a diesel generator is also proposed. The steady-state and dynamic analysis of the proposed system are presented, from both simulation and an experimental perspective. Three single-phase grid-forming inverters and a fixed speed wind turbine are used as a platform for case studies. The grid-forming inverters adopt droop control method which allows parallel operation of several grid-forming sources. Droop control-based inverters are known as independent and autonomous due to the elimination of intercommunication links among distributed converters. Moreover, the adopted fixed speed wind turbine employs a squirrel cage induction generator which is well known for its robustness, high reliability, simple operation and low maintenance. The results show a good correlation between the modelling, the experimental measurements, and the field tested results. The final stage of this research explores the effect of tower shadow on off-grid systems. Common tower designs for small wind turbine applications, which are the tubular and the lattice configurations, are considered in this work. They generate dissimilar tower shadow profiles due to the difference in structure. In this research, they are analytically modelled for a wind turbine which is being constructed as a downwind configuration. It is proven that tower shadow indeed brings negative consequence to the system, particularly its influence on battery lifetime within an off-grid system. This detrimental effect occurs when power generation closely matches the load demand. In this situation, small frequent charging and discharging cycles or the so called microcycles, take place. The battery lifetime reduction due to these microcycles has been quantified and it is proven that they are not negligible and should be taken into consideration while designing an off-grid hybrid system