Large off-shore wind farms raise the concern of widespread tripping of off-shore wind generator in the presence of system faults and corresponding voltage dips that could potentially cause system wide blackout. In this thesis an offshore wind farm and three different types of power transmission are modeled and studied using simulation software. Off-shore wind farm composed of fixed speed induction generators and HVAC interconnection, HVAC interconnection plus STATCOM and HVDC interconnections are studied. Onshore grid faults are simulated for each interconnection. Voltage tolerance curves are established to assess fault ride through capability of each interconnection and compared with different grid transmission ride through capacity required by grid operator.

Due to growing concerns over climate change, more and more countries are looking to renewable energy sources to generate electricity. Therefore, wind turbines are increasing in popularity, along with doubly-fed induction machines (DFIGs) used in generation mode. Current grids codes require DFIGs to provide voltage support during a grid fault. The fault ride-through (FRT) capability of DFIGs is the focus of this thesis, in which modifications to the DFIG controller have been proposed to improve the FRT capability. The static synchronous compensator (STATCOM) controller has been applied with proposed method to study its influence on the voltage at the point of common coupling (PCC). The proposed method was also compared with other FRT capability improvement methods, including the conventional crowbar method. The simulation of the dynamic behaviour of DFIG-based wind turbines during grid fault is simulated using MATLAB/Simulink. The results obtained clearly demonstrate the efficacy of the proposed method.

The increased integration of distributed and renewable energy resources (DERs) has motivated the evolution of new standards in grid interconnection requirements. New standards have the requirement for the DERs to remain connected during the transient grid fault conditions and to offer support to the grid. This requirement is known as the fault ride-through (FRT) capability of the inverter-based DERs and is an increasingly important issue. This dissertation presents the FRT capability of the DERs that employ a voltage control strategy in their control systems. The voltage control strategy is increasingly replacing the current control strategy in the DERs due to the fact that it provides direct voltage support. However, the voltage control technique limits the ability of direct control over the inverter current. This presents a challenge in addressing the FRT capability where the problem is originally formulated in terms of the current control. This dissertation develops a solution for the FRT capability of inverters that use a voltage control strategy. The proposed controller enables the inverter to ride through the grid faults and support the grid by injecting a balanced current with completely controlled real and reactive power components. The proposed controller is flexible and can be used in connection with various voltage control strategies. Stability analysis of the proposed control structure is performed based on a new linear time-invariant model developed in this dissertation. This model significantly facilitates the stability and design of such control loops. Detailed simulation, real-time and experimental results are presented to evaluate the performance of the proposed control strategy in various operating conditions. Desirable transient and steady-state responses of the proposed controller are observed. Furthermore, the newly established German and Danish grid fault ride-through standards are implemented in this research as two application examples and the effectiveness of the dissertation results are illustrated in the context of those two examples.

The increasing penetration of inverter based renewable generation in the form of solar photo-voltaic (PV) or wind has introduced numerous operational challenges and uncertainties. Among these challenges, one of the major ones is the impact on the transient stability of the grid. On the other hand, the direct methods for transient stability assessment of power systems have also fairly evolved over the past 30 years. These set of techniques inspired from the Lyapunov's direct method provide a clear insight into the system stability changes with a changing grid. The most attractive feature of these types of techniques is the heavy reduction in the computational burden by cutting down on the simulation time. These advancements were still aimed at analyzing the stability of a non-linear autonomous dynamical system and the existing power system perfectly fits that definition. Due to the changing renewable portfolio standards, the power system is undergoing serious structural and performance alterations. The whole idea of power system stability is changing and there is a major lack of work in the field of direct methods in keeping up with these changes. This dissertation aims at employing the pre-existing direct methods as well as developing new techniques to visualize and analyze the stability of a power system with an added subset of complexities introduced by PV generation. / Ph. D.

Since large scale unscheduled tripping of wind power generation could lead to power system stability problem. Thus network interconnection regulations become more rigid when the wind power penetration reaches a non-neglible portion of the total power generation. This thesis presents a comparison of five different low voltage ride through (LVRT) capability enhancement technologies, i.e., additional rotor resistance, DC bus chopper, crowbar on rotor, the combination of above schemes, and grid voltage support by controlling grid side converter. System simulations are performed under Digsilent environment with model and control blocks provided by the package. Additional models are developed to implement the LVRT enhancement schemes studied. A Doubly-Fed Induction Generator (DFIG) with pitch control is used to simulate different system fault scenarios with different voltage sag magnitude and duration time. Simulation results indicate that different enhancement schemes provide various levels in relieving DC bus overvoltage, rotor winding overcurrent, and overspeed problems, and the method combines all tested schemes seems to provide the best result.

Demand is emerging for offshore wind power plant (WPP) that often has favorable capacity factor and high capacity value as compared with onshore wind farms. There are many challenges regarding power losses, economics, protection system and reliability of the wind farm. Collection system design decisions play an essential role to efficient operation of the WPP. Wind generators also have to be able to cope with grid disturbances. Low voltage ride-through (LVRT) capability of wind turbines requires generator units remain in operation for severe voltage drops during ¡@grid system faults, and be able to withstand depressed voltage for a few seconds in a recovery period. Technical requirements set out in grid codes for off shore wind farm normally relate to different connection points. A rigor LVRT requirement would increase the overall investment costs of the wind farm. In most offshore wind farm projects, radial collector systems connecting a number of wind turbines and terminated at the offshore platform have served well the requirements for an economical design. However, due to the lack of redundancy, its reliability is poor. To improve the reliability of the collector system, the inclusion of a cable section that interconnects the remote ends of two adjacent radial feeders has been proposed. The transmission system of a wind farm takes the power generated and sends it to shore. Medium voltage AC transmission is the simplest one, just gathering the cables from the collector system and taking them together until they reach the point of common coupling (PCC).Through wind farm dynamic simulations by using DIgSIENT package, this thesis demonstrates that the ride through capability which occur at the particular wind parks with different collector system topology are greater than those which the wind turbines are capable of riding through, i.e., LVRT curves of different wind farm collection system designs of an offshore WPP and a single wind generator are different. This can be exploited to reduce the cost in complying with LVRT requirement of offshore WPP.

Recently, matrix converter has received considerable interest as a viable alternative to the conventional back-to-back PWM (Pulse Width Modulation) converter in the ac/ac conversion. This direct ac/ac converter provides some attractive characteristics such as: inherent four-quadrant operation; absence of bulky dc-link electrolytic capacitors; clean input power characteristics and increased power density. However, industrial application of the converter is still limited because of some practical issues such as common mode voltage effects, high susceptibility to input power disturbances and low voltage transfer ratio. This dissertation proposes several new matrix converter topologies together with control strategies to provide a solution about the above issues. In this dissertation, a new modulation method which reduces the common mode voltage at the matrix converter is first proposed. The new method utilizes the proper zero vector selection and placement within a sampling period and results in the reduction of the common mode voltage, square rms of ripple components of input current and switching losses. Due to the absence of a dc-link, matrix converter powered ac drivers suffer from input voltage disturbances. This dissertation proposes a new ride-through approach to improve robustness for input voltage disturbances. The conventional matrix converter is modified with the addition of ride-through module and the add-on module provides ride-through capability for matrix converter fed adjustable speed drivers. In order to increase the inherent low voltage transfer ratio of the matrix converter, a new three-phase high-frequency link matrix converter is proposed, where a dual bridge matrix converter is modified by adding a high-frequency transformer into dc-link. The new converter provides flexible voltage transfer ratio and galvanic isolation between input and output ac sources. Finally, the matrix converter concept is extended to dc/ac conversion from ac/ac conversion. The new dc/ac direct converter consists of soft switching full bridge dc/dc converter and three phase voltage source inverter without dc link capacitors. Both converters are synchronized for zero current/voltage switching and result in higher efficiency and lower EMI (Electro Magnetic Interference) throughout the whole load range. Analysis, design example and experimental results are detailed for each proposed topology.

This thesis first develops a medium-voltage grid code outlining stringent requirements for low-voltage ride-through, high-voltage ride-through and ancillary services based on anticipated grid requirements for distributed energy resources (DER)s. A 100 kW generating capacity DER study system is then formulated taking into consideration key design constraints as motivated by the medium-voltage grid code. Local DER system controls are developed that enable existing systems employing conventional current-control for the grid-interfacing voltage-sourced-converters to comply with the grid code. A supervisory controller is proposed that allows multiple DER units and loads to operate collectively as a DER system with a single point of common coupling. The impact of transformer configurations, fault types and fault locations on DER systems are quantified through a comprehensive fault study using the PSCAD/EMTDC software package. A subset of these fault scenarios are identified for rapid screening of DER system compliance against low-voltage ride-through requirements.

This thesis first develops a medium-voltage grid code outlining stringent requirements for low-voltage ride-through, high-voltage ride-through and ancillary services based on anticipated grid requirements for distributed energy resources (DER)s. A 100 kW generating capacity DER study system is then formulated taking into consideration key design constraints as motivated by the medium-voltage grid code. Local DER system controls are developed that enable existing systems employing conventional current-control for the grid-interfacing voltage-sourced-converters to comply with the grid code. A supervisory controller is proposed that allows multiple DER units and loads to operate collectively as a DER system with a single point of common coupling. The impact of transformer configurations, fault types and fault locations on DER systems are quantified through a comprehensive fault study using the PSCAD/EMTDC software package. A subset of these fault scenarios are identified for rapid screening of DER system compliance against low-voltage ride-through requirements.

In last decade, there is a large increase in installed capacity of wind power. Asmore wind power is integrated into utility networks, related technologychallenges draw much attention. The doubly fed induction generator (DFIG) isthe mainstream choice for wind turbine generator (WTG) in current market andthe object of this thesis. It is very sensitive to voltage dips. The enhancement oflow voltage ride through (LVRT) is one of the most important issues for DFIG,and many works have already been done to provide solutions.In current works, the voltage dip waveforms that are applied in LVRTrelated works are largely different from waveforms in reality, because they failto consider the the effect of realistic wind farm configurations on waveforms ofvoltage dips and significant influences of additional characteristics of voltagedips. The true impact of the voltage dip needs to be assessed in performanceevaluation and development of LVRT methods. To support the development ofpractical LVRT capacity enhancement solutions, the application of voltage dipknowledge is definitely demanded.In this thesis, the characteristics of realistic waveform voltage dips in windfarm are analyized based on voltage dip knowldege from power quality field,measured voltage dip from industry and realistic wind farm configurations.Classical analysis theory is applied to explain the principles of the impact ofvoltage dip characteristics on dynamic behavior of DFIG. The impacts of manywidely neglected characteristics such as phase angle jump (PAJ), point on wave(POW) of initiation and recovery, voltage recovery process, transformerconfigurations, load effect are revealed and verified by simulations. The impactof many voltage dip characteristics on DFIG are studied for the first time. / De senaste tio åren har sett en stor ökning av installerad effekt av vindkraft.Mer vindkraft i elnäten har lett till större uppmärksamhet om dess tekniskautmaningar. Den dubbelmatad asynkrongenerator (DFIG) är idag denvanligaste förekommande typen i vindkraftverk. Den är mycket känslig förspänningssänkningar. Förbättring av tålighet för spänningssänkningar (LVRT)är en av de viktigaste frågorna för DFIG, och många studier har redan söktlösningar.I befintliga studier om LVRT har spänningssänkningarna skiljt sig väsentligtfrån verkliga vågformer, då de inte har tagit hänsyn till realistiskavindparkkonfigurationer och betydande påverkan av ytterligare egenskaper hosspänningssänkningar. För att stödja utvecklingen av praktiska LVRT lösningarbehövs mer kunskap om spänningssänkningar för att bedöma dess verkligainverkan.Detta examensarbete förbättrar LVRT analysen av DFIG genom att tillämpakunskap om spänningssänkningar från elkvalitetsområdet, tillsammans medrealistiska vindparkskonfigurationer. Inflytandet av ändringar i fasvinkel(PAJ), fasvinkeln vid sänkning och återhämtning (POW), spänningsåterhämtning, transformatorkonfigurationer, last och många andra egenskaperav spänningssänkningar ingår också. Inflytandet av många egenskaper avspänningssänkningar studeras här akademiskt för första gången. Denkaraktäristik av realistiska spänningssänkningar som inträffar vid generatornspoler, och de effekter dessa har, studeras och förklaras genom teoretisk analysoch intensiva simuleringar.