The deterioration in power quality due to the increase in non linear loads has sparked a new interest in the filtering techniques used in transmission and distribution systems. Unlike passive filters, active filters are adaptable to rapidly changing source impedance and provide the necessary harmonic compensation for varying non-linear loads. This thesis models a Voltage Source Converter (VSC) based shunt active filter (SAF) to filter harmonics due to large non linear loads. SAF compensates the harmonics by injecting a compensating current which is equal in magnitude but opposite in phase to the disturbance in the system. The power circuit of this SAF consists of a three-phase VSC and the switching signals for this converter is generated by hysteresis based current modulation method. The controller uses the sinusoidal current control strategy of the generalized instantaneous p-q control theory to calculate the reference compensating current. Proposed SAF is simulated using EMTP-RV simulation package under steady state and dynamic conditions and its effectiveness in mitigating harmonics is tested. The stability and response of the SAF is also tested satisfactorily under transient load and severe AC / DC fault conditions. / UOIT

The interest in the utilization of offshore wind power is increasing significantly. Due to the shortage of in-land locations for wind farm and the wind speed offshore is potentially higher than that of onshore, which leads to a much higher power production. In this thesis a large offshore wind farm is modeled using Matlab simulation package. In the simulations active stall regulated wind turbines driving fixed speed asynchronous generators are used. Two different types of interconnections are modeled and compared, one is the Voltage Source Converter (VSC) based HVDC link and the other one uses high voltage AC (HVAC) cable interconnection. Transmission faults are simulated in each system and the transient response are examined. Three phase fault and single line to ground fault are used to compare the performance of the VSC based HVDC interconnection system and HVAC interconnection. It is found that compared to the traditional HVAC transmission, the VSC based HVDC transmission would have better performance under various system disturbances.

Distributed energy resources like solar power (PV Panels) are usually connected to the AC grid through a single phase voltage source inverter (VSI). The major drawback associated with single phase grid tie inverters is the double frequency component of the grid that appears on the DC bus link. Large electrolytic capacitors are generally employed in the inverters to eliminate the ripple component. However, their bulkiness and relatively short lifetime are motivational factors to replace them with small film capacitors. This paper presents a synchronous boost/buck based active power decoupling circuit in parallel with the dc-bus link capacitor and discusses the different types of control strategies implemented. Simulation results are presented for each control technique and it is shown that the ripple on the DC bus link is largely reduced due to inclusion of this circuit along with an expected extension of the lifetime due to the reduction in the amount of dc-bus capacitance used. / text

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The performance of the High voltage dc systems is dependent on the stiffness of the ac bus, it is connected to. With the traditional synchronous reference frame-phase locked loops (SRF-PLL), voltage source converters (VSC) systems with large PLL gains, connected to weak ac networks are shown to be prone to instabilities, when subject to disturbances. In this thesis a new Adaptive PLL is designed with a pre-filter topology which extracts the fundamental positive sequence component of the input voltage, to be fed into the SRF-PLL for tracking of its phase angle. Compared with other traditional PLL topologies, this Adaptive PLL shows superior immunity to voltage distortions, and also has a faster dynamic performance. The thesis presents a comparative analysis of the performance of the traditional SRF-PLL with the Adaptive PLL in a VSC control system, and its impact on stability for VSCs connected to weak ac systems (up to SCR=1.3).

Due to its unique features such as controllable power factor, controllable bi-directional power flow, and rapid dynamic response, Voltage Source Converters (VSCs) have been widely used in various industrial applications such as distributed generation systems, power distribution systems, uninterruptible power supplies (UPS), AC motor drives, etc. To optimize the performance of the VSC, many control algorithms have been proposed. This thesis investigates development of the nonlinear control for the VSC in two applications: power factor control and active power filtering. A detailed description of the dynamic model of the VSC system is presented in different reference frames. A linearization-based control scheme is introduced for power factor regulation and verified by switched simulation and real-time experiment on a test stand which has been constructed at the Applied Nonlinear Control Lab (ANCL), University of Alberta. In addition, an internal model-based control scheme is introduced to perform active power filtering. This algorithm is verified by simulation. / Controls

Thesis (MScEng)--Stellenbosch University, 2012. / ENGLISH ABSTRACT: The implementation of the Caprivi Link Interconnector (CLI) High Voltage Direct Current (HVDC) scheme in 2010 connecting the weak Namibian and Zambian Alternating Current (AC) transmission networks via overhead line is based on Voltage Source Converter (VSC) technology. This world-first combination of attributes presents a unique opportunity to study harmonic interaction between weak AC systems and VSC-based HVDC schemes. Relatively few publications exist that focus on AC and DC harmonic interaction and very few refer to VSC HVDC schemes. Because weak AC systems are much more prone to harmonic distortion than strong AC systems, there is a clear motivation for more detailed work in this field. In order to understand the context wherein AC and DC harmonic interaction exists, the fields of AC power system harmonic analysis and resonance, VSC switching theory, HVDC scheme configurations, Pulse Width Modulation (PWM) techniques and frequency domain analysis techniques are discussed. This thesis then presents the concept of Harmonic Amplitude Transfer Ratio (HATR) by a theoretical analysis of AC and DC harmonic interaction due to the fundamental component, as well as harmonic interaction due to scheme characteristic harmonics and is compared to the simulation results obtained from different software solutions. Simulation and modelling techniques for AC and DC harmonic interaction are discussed including AC and DC systems modelling. The theoretical results and simulation results are compared to the results obtained from a real life case study on the CLI HVDC scheme where a harmonic resonance condition occurred. The correlation of these three sets of results confirms the validity of the theories presented and possible mitigation of the case study resonance problems is explored. The results and conclusion highlight a variety of interesting points on harmonic sequence components analysis, VSC zero sequence elimination, AC and DC harmonic interaction due to the fundamental component and the HATR for different PWM methods, AC and DC harmonic interaction due to scheme characteristic harmonics, modelling techniques and mitigation for the resonance conditions experienced in the analysed real life case study. / AFRIKAANSE OPSOMMING: Die implementering van die Caprivi Skakel Tussenverbinder (CLI) hoogspannings- gelykstroom (HSGS) skema in 2010 wat die swak Namibiese and Zambiese Wisselstroom (WS) transmissienetwerke verbind via „n oorhoofse lyn is gebasseer op Spanningsgevoerde-omsetter tegnologie. Hierdie wêreld-eerste kombinasie van eienskappe verskaf „n unieke geleentheid om harmoniese interaksie tussen swak WS stelsels en Spanningsgevoerde-omsetter Hoogspannings GS stelsels te bestudeer. Relatief min publikasies wat fokus op WS en GS harmoniese interaksie bestaan, en baie min verwys na Spanningsgevoerde-omsetter Hoogspannings GS skemas. Omdat swak WS stelsels baie meer geneig is tot harmoniese verwringing as sterk WS stelsels, is daar „n duidelike motivering vir meer gedetaileerde werk in hierdie veld. Om die konteks te verstaan waarin WS en GS harmoniese interaksie bestaan, word die velde van WS kragstelsel harmoniese analise en resonansie, Spanningsgevoerde-omsetter skakelteorie, Hoogspannings GS skema opstellings, Pulswydte Modulasie (PWM) tegnieke, en frekwensiegebied analiese tegnieke bespreek. Hierdie tesis stel dan die konsep van Harmoniese Amplitude Oordragsverhouding voor deur „n teoretiese analise van WS en GS harmoniese interaksie na aanleiding van die fundamentele komponent, asook harmoniese interaksie a.g.v. harmonieke wat die stelsel kenmerk en word vergelyk met die simulasieresultate verkry uit verskilllende sagteware oplossings. Simulasie- en modelleringstegnieke vir WS en GS harmoniese interaksie word bespreek insluitend WS- en GS stelselmodellering. Die teoretiese resultate en simulasieresultate word vergelyk met die resultate wat verkry is uit „n werklike gevallestudie op die CLI HSGS skema waar „n harmoniese resonansie toestand voorgekom het. Die ooreenkomste tussen hierdie drie stelle resultate bevestig die geldigheid van die teorieë soos uiteengeset voor, en die moontlike verbetering van die gevallestudie resonansie probleme word verken. Die resultate en samevatting beklemtoon „n verskeidenheid punte aangaande harmoniese volgorde-komponent analiese, Spanningsgevoerde-omsetter zero-volgorde uitskakeling, WS en GS harmoniese interaksie na aanleiding van die fundamentele komponent en die Harmoniese Amplitude Oordragsverhouding vir verskillende PWM metodes, WS en GS harmoniese interaksie na aanleiding van skema-kenmerkende harmonieke, modelleringstegnieke, asook verbetering van die resonansie toestande soos ervaar in die analise van die werklike gevallestudie.

The UK is facing two major challenges in the development of its electricity network. First, two thirds of the existing power stations are expected to close by 2030. Second, is the requirement to reduce its CO2 emissions by 80% by 2050. Both of these challenges are significant in their own right. The fact that they are occurring at the same time, generates a significant amount of threats to the existing power system, but also provides many new opportunities. In order to meet both these challenges, significant amounts of offshore wind generation has been installed in the UK. For the wind generation with the longest connections to land, Voltage Source Converter (VSC) based High Voltage Direct Current (HVDC) transmission has to be used. Due to the high power rating of the offshore wind farms, compared to the limited transmission capacity of the links, a large number of point-to-point connections are required. This has lead to the concept of HVDC grids being proposed, in order to reduce the amount of installed assets required. HVDC grids are a new transmission environment and the fundamental question of how they will protect themselves must be answered. Several new technologies are under consideration to provide this protection, one of which is the HVDC circuit breaker. As HVDC circuit breakers are a new technology, they must be tested in a laboratory environment to prove their operation and improve their Technology Readiness Level (TRL). This thesis is concerned with how such HVDC circuit breakers are operated, rated, and tested in a laboratory environment. A review of the existing circuit breaker technologies is given, along with descriptions of several novel circuit breakers developed in this thesis. A standardized method of rating DC circuit breaker and their associated test circuit is developed. Mathematical analysis of several circuit breakers is derived from first principles and low power prototypes are developed to validate these design concepts. A high power test circuit is then constructed and a semiconductor circuit breaker is tested. The key learning outcomes from this testing are provided.

Thesis submitted in fulfilment of the requirements for the degree Doctor of Technology: Electrical Engineering in the Faculty of Engineering at the Cape Peninsula University of Technology / This research focuses on the predictive current control of multilevel converters with the aim of providing an optimized system for three-phase, multilevel inverters (MLIs) so that the load current and the voltage of the capacitors can be controlled. A model predictive current control algorithm is proposed, specifically directed at the utilisation of power obtained from renewable energy systems (RESs). The model was developed for three-phase, multilevel voltage source inverters (MLVSIs), three-phase, three-level diode-clamped converters (DCCs) and flying capacitor converters (FCCs). In this study the renewable energy systems model is used to investigate system performance when power is supplied to a resistiveinductive load (RL-load). The proposed control method was split into two different control algorithms. Firstly, a finite set-model predictive current control (FS-MPCC) method was developed to control the output current of three-phase, MLIs. This control method was selected to reduce the calculation effort for model predictive control (MPC) and to increase the possible prediction horizon. Secondly, to solve the flying capacitor voltage balance problem in an FCC, as well as to solve the DC-link capacitor voltage balance problem in a DCC, a hysteresis-voltage alancing algorithm based on predictive control, was designed—this algorithm was used to keep the flying capacitor voltages and DC-link capacitor voltages within their hysteresis bands. Finally, for some classes of power converters, a performance evaluation of the FS-MPCC method for three-phase, three-level MLIs was investigated in terms of power quality and dynamic response. The improvement was assessed in terms of total harmonic distortion (THD) of the output voltage for the RL-load. The modelling and co-simulation were carried out using MATLAB/Simulink with PSIM software. The co-simulation results indicated that the proposed control algorithms achieved both high performance and a high degree of robustness in RESs applications.

Nowadays, the need for more contributions from renewable energy sources is rapidly growing. This is forced by many factors including the requirements to meet the targeted reductions of greenhouse gas emissions as well as improving the security of energy supply. According to the International Renewable Energy Agency (IRENA) report 2016, the total installed capacity of solar energy was at least 227 GWs worldwide by the end of 2015 with an annual addition of about 50 GWs in 2015, making solar power the world’s fastest growing energy source. The majority of these are grid-connected photo voltaic (PV) solar power plants, which are required be integrated efficiently into the power grids to meet the requirements of power quality standards at the minimum total investment cost. For this, multilevel voltage source inverters (VSI) have been applied extensively in recent years. In practice, there is a trade-off between the inverter’s number of levels and the required size of output filter, which is a key optimisation area. The aim of this research is to propose a generic model to optimise the design number of levels for the Cascaded H-Bridge Multilevel Inverter (CHB-MLI) and the size of output filter for medium voltage – high power applications. The model is based on key measures, including inverter power loss minimisation, efficient control for minimum total harmonic distortion (THD), minimisation of total system cost and proposing the optimum size of output filter. This research has made a contribution to knowledge in the optimisation of CHB-MLI for medium-voltage high-power applications, in particular, the trade-off optimisation of the inverter’s number of levels and the size of the output filter. The main contribution is the establishment and demonstration of a sound methodology and model based on multi-objective optimisation for the considered key measures of the trade-off model. Furthermore, this study has developed a generic precise model for conduction and switching loss calculation in multilevel inverters. Moreover, it applied Genetic Algorithm (GA) optimisation to provide a complete optimum solution for the problem of selective harmonic elimination (SHE) and suggests the optimum size of output passive power filter (PPF) for different levels CHB-MLIs. The proposed trade-off optimisation model presents an efficient tool for finding the optimum number of the inverter’s levels and the size of output filter, in which the integration system is at its lowest cost, based on optimisation dimensions and applied system constraints. The trade-off optimisation model is generic and can be applied to any multilevel inverter topologies and different power applications.