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.

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.

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.

The trend of future wind farms moving further offshore requires much higher reliability for each wind turbine in order to reduce maintenance cost. The drive-train system and power electronic converter system have been identified as critical sub-assemblies that are subject to higher failure rates than the other sub-assemblies in a wind turbine. Modern condition monitoring techniques may help schedule the maintenance and reduce downtime. However, when it comes to offshore wind turbines, it is more crucial to reduce the failure rates (or reduce the stresses) for the wind turbines during operation since the harsh weather and a frequently inaccessible environment will dramatically reduce their availability once a failure happens. This research examines the mechanical, electrical and thermal stresses in the sub-assemblies of a doubly-fed induction generator (DFIG) wind turbine and how to reduce them by improved control strategies. The DFIG control system (the rotor-side and the grid-side converter control) as well as the wind turbine control system are well established. The interactions of these control systems have been investigated. This research examines several further strategies to reduce the mechanical and electrical stresses. The control system's coordination with the protection schemes (crowbar and dc-chopper) during a grid fault is presented as well. An electro-thermal model of the power converter has been developed to integrate with the DFIG wind turbine model, for the evaluation of the thermal stresses under different operating states and control schemes. The main contributions of this thesis are twofold. A first contribution is made by providing all the control loops with well-tuned controllers in a more integrated methodology. The dynamics of these controllers are determined from their mathematical models to minimize the interference between different control-loops and also to reduce the electrical transients. This thesis proposes a coordination strategy for the damping control, pitch control and crowbar protection which significantly reduces the mechanical oscillations. On the other hand, an integrated model of the wind turbine and converter electro-thermal system is established that can illustrate the performance integration with different control strategies.

<p>U ovoj doktorskoj disertaciji razvijena je tehnika upravljanja za<br />pretvarač energetske elektronike pri nesimetričnim naponskim<br />prilikama u elektroenergetskom sistemu. Kao što je pokazano,<br />primenom tehnike poništavanja signala kašnjenjem moguće je<br />izdvajanje komponenti struje inverznog redosleda i njihovo<br />potpuno poništenje, što će omogućiti pouzdanu kontrolu<br />komponenti struje direktnog redosleda upotrebom klasičnih<br />tehnika upraljanja, uz adekvatno unapređenje tehnike za<br />sinhronizaciju sa vektorskim reprezentom napona. Predložena<br />je i upotreba algoritama za poboljšanje parametara kvaliteta<br />električne energije bez dodatnih pasivnih elemenata.</p> / <p>This PhD thesis presents an improved control technique for grid<br />connected converter under asymmetrical voltages at the point of<br />common coupling. As presented, using delay signal cancellation<br />technique it is possible to differentiate and completely mitigate the<br />negative sequence current, offering the possibility of reliable positive<br />sequence current control using classical control algorithms. The<br />improvements made in synchronization offered adequate<br />phase angle estimation under voltage asymmetry. Furthermore, a<br />technique for the improvement of power quality indices without<br />passive elements between the grid and</p>

Technological improvements on wind energy systems with governmental supports have increased the penetration level of wind power into the grid in recent years. The high level of penetration forces the wind turbines stay connected to the grid during the disturbances in order to enhance system stability. Moreover, power system operators must revise their grid codes in parallel with these developments. This work is devoted to the modeling of variable speed wind turbines and the investigation of fault ride trough capability of the wind turbines for grid integration studies. In the thesis, detailed models of different variable speed wind turbines will be presented. Requirements of grid codes for wind power integration will also be discussed regarding active power control, reactive power control and fault ride through (FRT) capability. Investigation of the wind turbine FRT capability is the main focus of this thesis. Methods to overcome this problem for different types of wind turbines will be also explained in detail. Models of grid-connected wind turbines with doubly-fed induction generator and permanent magnet synchronous generator are implemented in the dedicated power system analysis tool PSCAD/EMTDC. With these models and computer simulations, FRT capabilities ofvariable speed wind turbines have been studied and benchmarked and the influences on the grid during the faults are discussed.

As new requirements for grid connected generators were implemented, the requirements for evaluating the Fault Ride Through capabilities of the generators became stricter. When refurbishing a power unit, proof that the power unit meet the requirements must be submitted to the authorities. Performing simulations of the Fault Ride Through of a production unit is an extensive process and requires advanced simulation models and tools. Therefore, the need for a simplified tool for estimating the Fault Ride Through capability arose, which is what the project produced. Also, knowledge of which production module characteristics (such as the generator parameters, transformer and excitation system) have the largest effect on the Fault Ride Through time, would also be beneficial. Through the creation of Simulink simulation models of two hydropower stations and a sensitivity analysis of station parameters, the relative effect of the parameters on the Fault Ride Through time is estimated and implemented into a time independent Fault Ride Through time estimation tool, based on the Equal Area criterion. The purpose of the time-independent Fault Ride Through time estimation tool is to provide an insight into the approximate transient stability of the hydropower station and which parameters affect the performance the most. Simulations show that the transient reactance of the generator, the generator inertia, the transient time constant and the transformer inductance have the largest effect on the Fault Ride Through capability. The results show that a simplified tool cannot estimate the Fault Ride Through as accurately as a time-domain simulation model can.

One of the challenging issues with the Microgrid is that the bidirectional power flow providedby the distributed generator (DG) which modify the fault current level. Furthermore, theinverter based-renewable energy source (IB-RES) limits the total fault current contributionto the grid due to its thermal capability. Since Microgrid should be able to operate in gridconnectedand islanded mode, protection strategies are needed to solve this challenging issue.By only having IB-RES and battery storage system, the fault condition and normaloperation cannot be distinguished. Apart from fault clearing issue, there is a consideration tostudy the fault isolation in the Microgrid under the limited fault current provided by IB-RES.To have fault isolation capability, the intelligent electrical device (IED) is needed. The firststep is to find a method that can detect a fault under the fault level modification constraint.This thesis presents a zero and negative sequence current protection to detect a fault.However, to make it selective, this protection will be applied directionally. It is common thatthe distribution grid has unbalanced load operation, thus providing zero and negativesequence component in the grid. To apply the directional zero and negative sequence currentprotection, the unbalanced load flow is simulated to distinguish the fault and normaloperation under unbalanced load condition.Safety and regulation are discussed briefly in this thesis. It is important that each of theIB-RES has fault ride-through (FRT) capability that follows a regulation. However, thisregulation is expected to have a coordination with the proposed protection in the Microgridso the reliability, selectivity, and sensitivity can be achieved in grid-connected and islandedmode. This thesis shows the coordination between fuses, IED, and inverter FRT capability.After providing a protection strategy, the adaptability of the proposed protection isassessed regarding of Microgrid expansion. The result shows that by applying the schemeand following the grading margin requirement that is presented in this thesis, the Microgridexpansion will not disrupt the proposed protection coordination. Since it is known that thedistribution grid is expanding its load capacity and microgeneration in continuous basis, it isconvenient that the proposed protection in the IED is expected to be adaptable, means that ithas a fixed IED setting when the grid is expanded. The analysis is performed by electrical transient analysis program (ETAP) and MatlabSimulink. The short circuit analysis, sequence-of-operation, and unbalanced load flow aresimulated by ETAP, while the protection stability is simulated by Matlab Simulink. / Ett problem som finns med microgrid är att de distribuerade produktionsgeneratorerna harett dubbelriktat effektflöde som modifierar felströmmen. Dessutom, inverterbaseradeförnyelsebara energikällor (IB-RES) begränsar det totala felströmsbidraget på grund av desstermiska kapacitet. Eftersom microgrids ska vara operativ vid både anslutning till externt nätsamt önätsdrift behövs skyddsstrategier för att kunna hantera fel, speciellt vid önätsdrift.Om endast IB-RES och batterilager används kan feldrift och normal drift inte särskiljas.Bortsätt från felhantering är det viktigt att studera felbortkoppling för microgrid underbegränsad felström som fås av IB-RES. För att kunna åstadkomma felbortkoppling behöveren IED (från engelskans Intelligent Electronic Device). Det första steget är att finna en metodför att kunna detektera fel under fel nivå modifiering. Denna avhandling tittar på att användanoll- och minusföljds ström sekvensskydd för att detektera fel. För att göra skyddet selektivtkommer det att titta på riktningen av effektflödet. I distributionsnät är det vanligt att haobalanserade laster vilket medför noll- och negativa sekvenskomponenter i nätet. För atttillämpa riktningsskydd för noll och negativ sekvens ström simuleras ett obalanserateffektflöde för att särskilja på feldrift och normal drift vid obalanserad last.Säkerhet och förordningar diskuteras kortfattat i denna avhandling. Det är viktigt att varjeIB-RES har en feltålighet som följer vissa förordningar. Denna förordning förväntassamordna det föreslagna skyddet i micronåt så att pålitlighet, selektivitet och känslighet kanåstadkommas vid nätanslutning och önätsdrift. Denna avhandling visar samordningen mellansäkringar, IED och feltåligheten för växelomriktare.Anpassningsförmågan för det föreslagna skyddet bedöms med avseende på expansion avmicrogrid. Resultatet visar att en expansion av ett microgrid inte kommer att störa denföreslagna samordningen om skyddsmetoden och tidsfördröjningskravet som presenteras idenna avhandling följs. Eftersom det är känt att distributionsnätet kommer att fortsätta ökasin lastkapacitet och mikrogenerering, är det lämpligt att skyddet förväntas varaanpassningsbart vilket innebär att det har en fast IED inställning när nätet expanderas.Analysen genomförs med mjukvarorna electrical transient analysis program (ETAP) ochMatlab Simulink. Kortslutningsanalysen, arbetssekvensen och obalanserad lastflödesimuleras av ETAP, medan skyddsstabiliteten simuleras av Matlab Simulink.