Medium Voltage Generation System with Five-level NPC Converters for Kite Tidal Power

Offshore power generation has emerged as a prominent source of energy and the installed capacity of new plants has been steadily increasing in recent years. Tidal power specifically is a promising renewable energy source which has not been highly exploited yet, despite its distinctive advantages of being predictable and independent of weather conditions. The main objective of this Licentiate thesis is to analyze and propose solutions for two common problems in offshore power production, which are the power variations due to the non-steady speed profile of the water speed flowing through the turbine and the efficient transportation of the produced power to the shore.The tidal power application utilized in this thesis is the subsea kite, which is a recently developed tidal energy conversion technology that can increase the generated power compared to a traditional static tidal turbine. A turbine is mounted on a submerged kite and the kite moves inside the sea following a predefined trajectory and generating electric power from the tidal currents. The speed and torque of the turbine varies periodically due to the periodic movement of the kite in the sea and, therefore, the control of the generator needs to be able to handle this variable generated power. The kite studied in this thesis has rated active power of 500 kW.In the first part of the thesis, the power generation system of the subsea kite is modelled and the profile of the generated power is extracted given a specific tidal current and turbine geometry. The control of the power converters is described and tested for the specific profile of the generated power. The speed of the generator is controlled by a properly designed Maximum Power Point Tracking algorithm, which ensures that the generator extracts the maximum power from the tidal stream. Experimental verification of the model of this innovative system is also conducted on a laboratory 35 kVA emulator of the tidal power generator.The second part of the thesis deals with the design of a medium voltage generator\ua0drive. The use of medium voltage in the power generation system is highly advantageous for the tidal kite application, since it can reduce the current flowing through the undersea cables connecting the tidal plant to the local grid. Therefore, the size of the cables can be reduced. The drive proposed here uses two 5-level Neutral Point Clamped (NPC) converters connected back-to-back. The 5-level NPC converters can operate with high voltage, while using multiple low-voltagerated power switches. Contrarily, the typical 2-Level converters have limited voltage capability, since they would require more expensive high-voltage-rated power switches. The increased operating voltage of the power conversion system results to lower current and losses in the cables. Another advantage of the NPC converter is the low harmonics at the ac side, which reduces the requirements for passive grid filters. However, the voltage balancing of the dc-link capacitors in this converter topology is a challenge which has not been effectively solved in previous studies. Therefore, a novel voltage balancing strategy is proposed here that uses advanced Space-Vector-Modulation techniques and hardware-based voltage balancing schemes with reduced number of components and lower power losses. Finally, a laboratory prototype of the NPC-converter-based power conversion system is developed with rated power 50 kVA. SiC MOSFETs are used on theconverters to further increase the system’s efficiency and voltage capability.This thesis presents the model, control and laboratory emulator of a kite-based tidal power generator. The experimental set-up can be utilized for conducting research on other renewable sources, such as wind power, that have similar performance. Also, the developed multilevel drive is suitable for various applications where medium voltage grid-connected drives are used and particularly in distributed renewable power generation

Electro-mechanical modelling of tidal arrays

The aim of this study is to present, compare and improve the options of power transmission for tidal current arrays. The potential to generate low or zero carbon power from the world’s tides is increasing as technology moves forward. The technically available tidal current energy resource, the resource that can be captured using existing technology, in the United Kingdom can supply a significant amount of the UK electricity demand. Even though tidal current devices have similarities to offshore wind turbines in many aspects, a number of characteristics differentiate the approach needed regarding power transmission and drive-train design. Some of these characteristics are: predictable direction and speed of the tidal current, predetermined available area in a tidal channel, less swept area due higher density of water, continuous underwater operation and smaller distances to shore. This thesis is based on the hypothesis that tidal current energy can be harnessed using today’s technology in an efficient manner. Technology progression never stops and as new materials and methods become available the cost of utilising tidal current energy will drop in the years to come. However, the research question that has to be asked is whether using today’s technology tidal arrays can be an alternative source of electrical power. In order to respond to this research question electromechanical models of tidal current devices have been developed in detail, from resource to the grid connection, using mathematical linear and non-linear programming in MATLAB/Simulink. The tidal models developed include the tidal resource, the tidal turbine with pitch control, geared induction and synchronous generators, the power electronics with the generator controller, the grid side controller, the cables for power transmission, the filters and the grid connection. All the modelling aspects of this study are presented in Chapter 3. Single tidal current devices were compared using different generator technologies, squirrel cage induction generator or permanent magnet synchronous generator, and different location of the power converters, in the nacelle near the generator or many kilometres apart from the generator. Regarding the generator technology, results showed that even though differences are minor, the permanent magnet synchronous generators are more efficient. Regarding the location of the power converters results showed that positioning the power converters in the nacelle always yields fewer electrical losses but component accessibility is minimised due to the underwater operation of the tidal current device. A key focus aspect of the study is the power transmission option with onshore converters which is presented in detail. Using this concept it is possible to generate electricity from tidal current devices but at the same time keep the highest possible system reliability despite the continuous underwater operation. This concept has been used in the first demonstration tidal current arrays developed by Andritz Hydro Hammerfest. What is more, data provided by Andritz Hydro Hammerfest were utilised in order to validate the simulation models. In this study a step forward is taken regarding the concept of keeping the converter dry and controlling the tidal current generator from afar. An algorithm is developed to design power harmonic filters for systems that use long distance controls. Power harmonic filters allow the long distance control system to operate reliably under all conditions but generate significant electrical losses. The power harmonic filter design algorithm presented in this thesis estimates the exact filter parameters so that the filter ensures maximum system reliability and generate minimum possible losses. In addition tidal array topologies using this concept are developed. The final part of this thesis compares a number of different tidal array topologies based on resource to grid efficiency and component accessibility for maintenance. Results showed that when tidal current devices are clustered per four turbines on offshore platforms it is efficient to use as many clusters as possible connected to a single cable whose both ends are connected to the grid. Locating the power converters in the nacelle yields fewer electrical losses compared to locating the power converters on the offshore platform. However, the difference is minimised because the distance between the tidal current device and the offshore platform is the least possible. Having the power converters on an offshore platform is beneficial in terms of accessibility for maintenance and operation because they are not underwater. The results and the methodology from this thesis can be extended to other offshore renewable energy systems such as the wind and wave. In addition, this study can be used as a stepping stone for decision making by tidal current developers

Techno-economic analysis of an off-grid micro-hydrokinetic river system as a remote rural electrification option

Thesis (M. Tech. (Electrical Engineering )) - Central University of Technology, Free State, 2014Remote rural electrification via grid-extension is a challenging solution due to high connection costs and low electricity consumption rate. As a result, it is difficult to recover the initial investment costs. Therefore, electrification is made possible by means of the commonly used off-grid approaches such as solar, wind, diesel generator and conventional micro-hydro. However, owing to non-continuous availability of sunlight and wind, high cost of diesel fuel, and requirements for construction of diversion weirs, these off-grid approaches might not offer a cost-effective and reliable solution to low income rural residents. There are many rural communities throughout the world without access to grid electricity and with access to flowing water. An off-grid micro-hydrokinetic river (MHR) system is one of the promising technologies to be used in remote rural areas with flowing water. It can bring sustainable improvement to their quality of life due to its high energy density and minimal environmental impact. This technology is still in the development stage and there is a lack of application, especially in rural areas. Hence, this study investigates the current status of MHR technology in rural applications. To demonstrate the economic feasibility of an off-grid MHR system, a rural site with multiple energy sources within South Africa has been used. The economic benefit offered by this proposed system at the selected site is compared to the economic benefits offered by other commonly used standalone systems such a solar, wind and diesel generator (DG). This economic comparison has been performed by making use of a Hybrid Optimization Model for Electric Renewable (HOMER) simulation tool. Grid extension has also been used as a comparison method for obtaining an economical distance between grid lines and the remote rural site. The results highlighted the acceptable economic performance of the MHR system. Finally, most of the available modelling and simulation tools for mechanical and electrical systems are not equipped with hydrokinetic modules. Hence, an MHR system model has been developed in MATLAB/Simulink in order to study its dynamic performance as submitted to variable water resource. Its performance has then been compared to the performance of a wind system counterpart for generating the same amount of electrical power. This proved/verified that the proposed system can generate electricity markedly cheaper than a wind system even in areas with adequate wind resource within South Africa

Control of turbine-based energy conversion systems

This thesis investigated the modelling and control of wind and hydrokinetic turbine-based energy conversion systems. Wind turbines are a mature technology and the technical challenges are associated with their connection to the grid. However, hydrokinetic energy conversion systems are fairly new and their design is usually based on knowledge transferred from the wind industry. Variable-speed wind turbines are either fully or partially decoupled from the frequency of the grid. Therefore, as conventional plants are decommissioned, wind turbines have to comply with requirements issued by the transmission system operator of each country. To investigate this, vector control schemes of a doubly fed induction generator (DFIG) and of a fully rated converter (FRC)-based wind turbine were modelled using MATLAB/Simulink. Simulations showed that in case of a fault at the point of connection to the grid there is a larger impact on the torque of a DFIG than a FRC-based wind turbine. In addition, the FRC-based wind turbines can increase their output to contribute to the restoration of the grid frequency. Technical knowledge from the design, control and the modelling of variable-speed wind turbines was used for the design of an electrical subsystem for a hydrokinetic energy conversion system for man-made waterways. An FRC-based configuration based on a dc-dc converter was used for the control of the laboratory prototype of a hydrokinetic energy conversion system and the derivation of its characteristic power curves. Very high efficiencies of the system were observed due to the restricted flow conditions. Similarly to wind turbines, the variable-speed operation of the hydrokinetic energy conversion system enabled its maximum power point tracking (MPPT). A gradient-based method was analysed and a ‘perturb and observe’ algorithm-based control scheme was used for the maximum power extraction. The technical challenges are associated with the selection of the sampling time of the algorithm according to the inertia of the system and the convergence speed coefficient according to the voltage constant of the generator. The laboratory prototype and the projected full-scale system were modelled and simulated. Simulation and experimental results show good agreement on achieving the MPPT of the hydrokinetic energy conversion system. These findings are very important for the future design of heuristic MPPT control schemes for hydrokinetic energy conversion systems

Control of an axial flow tidal stream turbine

Tidal stream turbines have the potential to generate electricity from a renewable source that is highly predictable and non-polluting. The United Kingdom’s geographical location means that it has one of the most significant wave and tidal resources in the world. The implications of controlling horizontal axial flow, variable speed tidal stream turbines for the purpose of optimising energy capture, regulating power and reducing mechanical loads has been investigated. The research was separated into two main areas: control of the turbine in below rated flow speeds where the objective was to maximise energy capture, and control of the turbine in high flow speeds where the aim was to regulate power, shed load and ensure the survivability of the turbine. A control strategy that maximises the power output of a tidal stream turbine in below rated flow speeds, by allowing the rotor speed to be varied, was implemented. The performance of the control strategy was assessed in simulation and then implemented on a hardware in the loop test rig. In both cases the controller performed well and was able to maximise power output from the turbine by varying the rotor speed to maintain the optimum tip speed ratio. Two control strategies for regulating the output power from a tidal stream turbine in high flow speeds were investigated and compared. The first strategy relies on adjusting the pitch angle of the rotor blades to regulate power and the second assumes fixed pitch rotor blades and relies on the stall characteristic of the blades to regulate power. Models of both the pitch and stall regulated turbines were developed and simulated using the commercially available software GH Tidal Bladed®. The control system design behind both strategies was developed in Matlab. Following control implementation a comparison of each model was carried out in terms of performance under turbulent flows, loading and energy yield. Following the comparison it was concluded that the performance of the pitch regulated tidal stream turbine was superior to that of the stall regulated turbine. The research also revealed that the dynamics of each turbine model are significantly different for above rated flow speed operation. This has implications on controller design and loading experienced by each tidal stream turbin

Performance of pitch and stall regulated tidal stream turbines

Controllers for a pitch and a stall regulated horizontal axial flow, variable-speed tidal stream turbine are developed, and a performance comparison is carried out. Below rated flow speed, both turbines are operated in variable-speed mode so that the optimum tip-speed ratio is maintained. One of the turbines has variable pitch blades, which above rated speed are pitched to feather in order to regulate power. The other turbine has fixed pitch blades and uses speed-assisted stall to regulate power. The control system design behind both strategies is examined in MATLAB, with the performance under turbulent flows, loading and energy yield analysis being evaluated in GH Tidal Bladed. Both strategies provide a satisfactory performance, but the out-of-plane loads on the stall regulated turbine were higher over the entire range of operation. In addition, the dynamic characteristics of the stall regulated turbine require a more complex control design. The results suggest that the pitch regulated turbine would be a more attractive solution for turbine developers

Inverter Design for SiC-based Electric Drive Systems with Optimal Redundant States Control of Space Vector Modulation

The need for inverters with ever increasing power density and efficiency has recently become the driving factor for research in various fields. Increasing the operating voltage of the whole drive system and utilizing newly developed SiC power switches can contribute towards this goal. Higher operating voltage allows the design of drives with lower current, which leads to lower copper losses in cables and machine, while SiC switches can drastically increase the inverter efficiency. Offshore renewable power generation, such as tidal power, is a typical application where the increase of operating voltage can be highly beneficial. The ongoing electrification of transportation calls also for high power electric powertrains with high power density,where SiC technology has key advantages.In the first part of the thesis, suitable control schemes for inverters in synchronous machine drive systems are derived. A properly designed Maximum Power Point Tracking algorithm for kite-based tidal power systems is presented. The speed and torque of this new tidal power generation system varies periodically and the inverter control needs to be able to handle this variable power profile. Experimental verification of the developed control is conducted on a 35 kVA laboratory emulator of the tidal power generation unit.Electric drives using multilevel inverters are studied afterwards. Multilevel inverters use multiple low-voltage-rated switches and can operate at higher voltage than standard two-level inverters. The Neutral Point Clamped (NPC) converter is a commonly used multilevel inverter topology for medium voltage machine drives. However, the voltage balancing of its dc-side capacitors and the complexity of its control are still issues that have not been effectively solved. A new method for the optimal utilization of the redundant states in Space Vector pulse-width-Modulation (SVM) is proposed in this thesis in order to control its dc-link voltages. Experimental verification on a 4-kV-rated prototype medium-voltage PMSM drive with 5-level NPC converters is conducted in order to validate the effectiveness of the proposed control technique.Low switching and conduction losses are typical characteristics of SiC switches that can be utilized to build inverters with high power density, due to the increased efficiency and smaller form-factor. Due to the above, SiC power modules have been particularly attractive for the automotive industry. The design approach of 2-level automotive inverters has been studied in this project. Moreover, a new design approach for the cooling system of automotive inverters has been developed in this thesis, which fine-tunes the inverter heatsink utilizing standard legislated test routines for electric vehicles. Multiple conjugate-heat-transfer (CHT) computation results showcase the iterative optimization procedure on a test-case 250 kW (450 A) automotive SiC inverter.Finally, the experimental testing of high power machine drives in order to verify the control and the hardware design is an important step of the development process. Thus, the performance of the prototype 450 A SiC 2-level inverter has been been experimentally validated in a power hardware-in-the-loop (P-HIL) set-up that emulates an automotive drive system. Several challenges have been addressed with respect to the accurate modelling of the motor and the control of the circulating power in the system. A new control technique utilizing the redundant states of the SVM has been developed for this set-up to effectively suppress the zero-sequence current to 3.3 % of the line current at rated power

Development of a Control System for a Tidal Turbine

La tecnologia delle turbine sottomarine è stata sviluppata per sfruttare l'energia cinetica presente nelle correnti marine. Questa tesi presenta lo sviluppo di un sistema di controllo per una turbina sottomarina che permette di migliorare la qualità dell'energia prodotta e di ridurre le forze assiali prodotte. La turbina è stata interamente modellata in MATLAB. Numerose simulazioni sono state effettuate sottoponendo il sistema a diversi campi di moto e i risultati sono stati analizzat