The continuously increasing energy penetration from wind farms into the grid raises concerns regarding power quality and the stable operation of the power system. The Grid Code´s requirements give strict guidelines for a wind farm´s behaviour under faulty or abnormal operating conditions.The primary purpose of this project is the application of a STATCOM for wind farm integration complying with the Grid Code. Towards that, an outer control strategy for the converter is designed so as to regulate the voltage at the point of common coupling by providing reactive power compensation. Thus the safe operation of the grid will be ensured since the wind farm will follow the Grid Code´s standards.The existing Grid Code requires only a positive sequence current controller. This study attempts to investigate whether this is sufficient or not and to examine the possibility of extending the Grid Code requirements so as to incorporate a negative sequence current controller as well. The results support the latter suggestion. Also, the use of SiC devices was also considered in this project. / Den ständigt ökande penetrationen av vindenergi i elnätet väcker farhågor om elkvalitet och stabil drift av kraftsystemet. Nätkoden (Grid Code) ger strikta riktlinjer för en vindkraftsparks beteende i felfall och under onormala driftsförhållanden.Huvudsyftet med detta projekt är att använda en STATCOM för integration av vindkraftsparker så att nätkoden uppfylls. I detta projekt utformas en yttre reglerstrategi för omriktaren för att reglera spänningen vid anslutningspunkten för vindkraftsparken genom att tillhandahålla reaktiv effektkompensering. Därigenom uppnås en säker drift av nätet eftersom vindkraftparken kommer att följa nätkoden.Den befintliga nätkoden kräver endast styrning av plusföljdskomponenten av strömmen. Denna studie försöker undersöka om detta är tillräckligt samt undersöka möjligheten för att utvidga nätkoden genom att införa ett krav på styrning av negativ-sekvens ström. Resultaten stöder det sistnämnda förslaget. Även användningen av halvledarkomponenter av kiselkarbid-SiC studerades i detta projekt.

The power converter is one of the key components in power system applications such as high voltage direct current (HVDC) systems and the grid connection of intermittent sources such as wind and solar power. However, the increased penetration of converter-based generation introduces challenges, such as sub-synchronous interaction between the converter control system and the grid. These control interactions are characterized by fast-growing, subsynchronous oscillations (SSO). This thesis deals with the analysis of sub-synchronous control interactions (SSCI) between doubly-fed induction generator (DFIG)-based wind farms and series compensated transmission lines. Moreover, the thesis aims to identify a method for mitigating the sub-synchronous oscillations using a static synchronous compensator (STATCOM), with a supplementary damping controller. The study is based on work in PSCAD/EMTDC and uses a system based on the IEEE first benchmark model, acting as a grid, and the scaled power output of a DFIG turbine model, modeling a wind farm. Initial impedance-based analysis in the frequency domain shows that the DFIG wind farm has a negative resistance throughout the sub-synchronous frequency range. A negative resistance may result in negative damping of the system and further introduce the risk of instability. The wind farm resistance and, in turn, system stability is affected by the current control loop of the DFIG-converter. The transmission line compensation factor largely impacts the system stability, while the power output has a minor effect. A time-domain analysis is performed to verify the result of the frequency domain analysis. Further on, a grid-forming STATCOM is added to the system for VAr compensation. Additional stability analysis shows that even though improvingthe stability, the STATCOM alone is not adequate to mitigate the SSCI. The proposed damping strategy is based on modifying the STATCOM voltage reference andcan be divided into three steps: detecting the SSO, estimating the sub-synchronous component, and modifying the extracted signal. The detection algorithm is based on a half-cycle comparator, while the modification is done through a proportional gain. When estimating the sub-synchronous components, two methods are proposed and compared. The first estimation method is based on a conventional power system stabilizer (PSS) method, and the second is afilter-less method.

The development of interconnected power systems together with an increasing number of renewable non-synchronous power sources, create major challenges for the power system to meet the voltage stability and power quality requirements. One way to increase the voltage stability in a sustainable way is to locally implement a STATCOM. By enhancing grid voltage stability under varying network conditions, the active power transfer capability will increase. However, before a STATCOM can be deployed in the power system, the behavior of it needs tobe investigated for the specific network conditions at the point of interface. The thesis develops a software model in RSCAD of a STATCOM along with important control functions for real-time simulations in RTDS without hardware-in-the-loop. The model aims to be sufficient for representing the gross behavior of a STATCOM in real-time simulations in order to get a quick overview of the dynamic response of the system. The model’s overall performance is evaluated through simulations in RTDS. The results indicate that the main control functions are operating in a stable and sufficient way. Hence, the model can perform in different operation modes as well as handling unbalances that are introduced in the system without losing controllability. There is potential for improvements in order to obtain a model with a more sophisticated control system. The main area would be to introduce limiters and anti-windups at appropriate places as well as a fault-ride-through logic to ensure a safe and stable operation during disturbances.

Static compensators (STATCOM) based on high-power converters are widely used for utilitiesand industrial applications in order to enhance the power system reliability. Nowadays,the Chain-Link Modular Multilevel Converter is the best solution for such applications,providing high eciency and reliability, and good harmonic performance. However, thecurrent delta and wye congurations present diculties in controlling negative sequencein unbalanced networks, as well as high capacitance requirements, which results in bulkySTATCOMs.This thesis aims to analyse a new Chain-Link Modular Multilevel Converter for STATCOMapplications. The main feature of the proposed topology is the presence of a commondc-link that will allow the exchange of energy between phases, facilitating the compensationof negative sequence components. Consequently, the required zero sequence componentinjection for the current Chain-Link congurations are avoided, and thus, also theconsequent over-sizing of the converter. Moreover, it is expected to have lower capacitancerequirements, because of the elimination of the second order harmonic (100 Hz)component in most of the capacitors, as they are charged and discharged by a bi-phasecurrent instead of a single-phase one. Additionally, lower conduction losses are expectedby reducing the number of devices in the conduction path.The layout and operation of the new converter have been analysed in this thesis.A STATCOM Simulink model provided by ABB has been modied to satisfy the newtopology. The theoretical benets of the proposed converter are supported by dierentsimulations carried out in Matlab-Simulink. In particular, it is shown a 50 % of negativesequence capability without any zero sequence component injection. The total capacitanceof the converter can be reduced a 30 % in comparison with the delta Chain-LinkModular Multilevel Converter, which is the preferable topology in the market nowadays.Furthermore, the losses are kept quite low if SiC semiconductors are used.On the other hand, the proposed topology requires the usage of bi-directional switchesto do the commutation and allow the exchange of energy between phases. An in-depthanalysis of the commutation process is shown at the end of this thesis, concluding that BidirectionalControlled Thyristors would be a promising solution for this converter topology. / Reaktiv effektkompensering (STATCOM) baserade på högeffektomvandlare används vidaför samhällsservice och industriella användningar för att förbättra elsystemet pålitliga.Nuförtiden, Chain-Link Modular Multilevel Converter är den bästa lösningen för sådanaanvändningar, vilket ger hög verkningsgrad och mycket pålitliga, samt låg harmonisk distortion.Hursomhelst, den nuvarande delta och wye konfigurationer presentera svårigheteratt kontrollera negativ sekvens i obalanserade nätverk, vilket leder till överdimensioneringav omvandlaren. Vidare, dessa Chain-Link topologier har hög kapacitans kraven, vilketresulterar i skrymmande STATCOMs.Den här avhandlingen syftar till att analysera en ny Chain-Link Modular MultilevelConverter för STATCOM användningar. Det viktigaste kännetecken av den föreslagnatopologi är närvaron av en gemensam likspänningslänk som gör det möjligt utbytet avenergi mellan faserna, som underlättar ersättning av negativ sekvens komponenter. Pådetta sätt, den behövlig noll sekvens komponent injektion i de aktuella konfigurationernaundviks, och således, också den därav följande överdimensioneringen av omvandlaren.Dessutom, förväntas det att ha lägre kapacitans kraven, eftersom den första övertonens(100 Hz) komponent undviks i några av kondensatorerna, såsom de är laddas och urladdasav en bi-fas ström i stället för en enda-fas ett. Vidare, lägre ledningsförluster förväntasgenom att reducera antalet enheter i ledningsvågen.Layouten och drift av den nya omvandlaren beskrivs i denna avhandling. En Simulinkmodell från ABB har modifierats för att tillfredsställa den nya topologin. De teoretiskafördelarna med det föreslagna omvandlare topologi stöds av simuleringsresultat i Matlab-Simulink. I synnerhet, är det visas en 50 % av negativ sekvens kapacitet utan noll sekvenskomponent injektion. Den totala kapacitansen hos omvandlaren kan minskas på ett 30 %jämfört med den delta Chain-Link Modular Multilevel Converter, som är den föredragnatopologin på marknaden idag. Vidare är förlusterna hålls ganska låg om SiC halvledareanvänds.Å andra sidan, kräver den föreslagna topologi användningen av dubbelriktade omkopplareatt göra kommuteringen och möjliggöra utbytet av energi mellan faserna. Enfördjupad analys av kommuteringsförloppet visas i slutet av denna avhandling, slutsatsenatt Bi-directional Controlled Tyristors skulle vara en lovande lösning för topologin.

Shunt compensation devices have been a powerful candidate for expanding the limits of transmission facilities, allowing more active power transport and supporting voltage and overall stability of the network. An example of such devices is the Static Synchronous Compensator (STATCOM) which is based on Voltage Source Converter (VSC) which synthesizes output voltage and current for mainly reactive power compensation. STATCOMs might be accompanied with a Thyristor Switched Capacitor (TSC) or Reactor (TSR) which are controllable shunt devices for extra VARs required, along with passive filters for absorbing the switching harmonics generated by the VSC. Such STATCOM topology is referred as the Hybrid STATCOM.However, such configuration typically results in parallel resonances with the maingrid’s passive elements at different harmonic frequencies. This leads to amplification of background harmonics, if any exist, at the Point of Common Coupling (PCC) where the Hybrid STATCOM is connected. This thesis deals with the VSC control in the STATCOM to mitigate the harmonic amplification behavior by active filtering,which is based on emulating an impedance by the control. Also, the overall system passivity is essentially studied, which ensures a passive system with respect to harmonics. / Shunt-kompensationsanordningar har varit en kraftfull kandidat för att öka gränserna för överföringsanläggningar, vilket möjliggör högre aktiv effekt och understöderspännings-stabilitet och övergripande stabilitet i kraftnätet. Ett exempel på sådana enheter är Static Synchronous Compensator (STATCOM) som är baserat på Voltage Source Converter (VSC) som syntetiserar utspänning och ström för huvudsakligen reaktiv effektkompensation. STATCOMs kan åtföljas av en Thyristor Switched kapacitor (TSC) eller Reaktor (TSR) som är kontrollerbara shunt-apparater som ökar kapaciteten för reaktiv effekt (VARs), tillsammans med passiva filter för att absorbera övertoner som genereras av VSC. Sådan STATCOM-topologi kallas HybridSTATCOM. Emellertid, en sådan konfiguration resulterar i parallella resonanser med huvudnätets passiva element vid olika övertoner. Detta leder till förstärkning av bakgrundsövertoner, om någon existerar, vid Point of Common Coupling (PCC) där Hybrid STATCOM är ansluten. Denna avhandling behandlar hur VSC kontrolleras i STATCOM för att mildra förstärkningen av nivåer för övertoner genom aktivfiltrering, vilket är baserat på att VSC kontrollen emulerar en impedans. Dessutom studeras övergripande systempassivitet, vilket säkerställer ett passivt system med avseende på övertoner.

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The purpose of this thesis report is to investigate challenges related to railway electrification. Inac-railway systems, trains are most often single-phase loads, which can create significantunbalances in the grid, when fed directly from two adjacent phases. To enhance power quality atthe railway feeding point, one alternative is to design the system such as the power is fed to thecatenary through Static Frequency Converters (SFCs), which can be a costly solution. Anotheroption is to use Static Compensators (STATCOMs) in shunt connections, handling only theunbalanced amount of power, in order to symmetrize the railway load as seen from the grid side.Modular multilevel converters (MMCs) are the state of the art solutions for medium- and highvoltage/power converter applications, such as load balancing and reactive-power compensation.This project will focus on analyzing Modular Multilevel Converter (MMC) topologies to be usedas load balancers in railway systems. However, the specific requirements of this application reveala potential weakness of MMCs. That is the asymmetrical character of the load may require muchhigher device rating in an MMC compared to a conventional converter, due to the symmetrizingcomponents to rebalance the capacitor voltages in the submodules.This project analyzes a reduced MMC-based topology with only two branches in order to meet therailway load-balancing characteristics. This chain-link compensator uses dc-dc converters tofacilitate power channels in order to restore the power imbalance, without overrating thesemiconductors in the main current path. / Syftet med detta examensarbete är att studera de svårigheter som elektrifierade järnvägar står inför.När systemet utgörs av växelström kan tågen ses som en enfas last, vilket kan skapa avsevärdaobalanser i nätet då det matas från två intilliggande faser. En alternativ systemdesign för att uppnåen god effekt leverans uppnås då effekten är matad till kontaktledningen genom statiska frekvensomräknare (SFCs), detta kan dock vara en kostsam lösning. Ett annat alternativ är att användastatisk shunt kompensation (STATCOMs) som selektivt kan hantera den obalanserade effekten föratt uppnå symmetri och balans i järnvägslasten, sett från nätets sida.Modulära multiomriktare (MMCs) är en ledande teknik för omriktare till applikationer på mellanochhögspännings nivå, till exempel för lastbalans och reaktiv kompensering. Detta projektanalyserar modulära multiomriktare för lastbalansering i järnvägssystem. Kraven för dennaapplikation leder till en potentiell svaghet i MMCs där den asymmetriska karaktären på lastenkräver högre märkeffekt jämfört med konventionella omriktare. Detta på grund av de komponentersom återskapar symmetri och balans i kondensator spänningarna i submodulerna.Här analyseras en reducerad variant av MMC-baserad topologi med enbart två grenar för att mötakraven på järnvägens lastbalans. Denna kedjelänk-kompensator använder dc-dc omriktare för attstyra effektflödet till lasten vid obalanserad effekt, utan att för den skull överdimensionerahalvledarna i den ordinarie strömvägen.

Gotland har länge präglats av mycket speciella lösningar och legat i framkant vad gäller ny teknik. Under ca 20 år har Gotland haft en VSC-HVDC-station som har stabiliserat spänningen i det gotländska nätet. HVDC Light har löst en del av de tekniska begränsningarna som hindrat utvecklingen av vindkraftverk. Anläggningen börjar lida mot slutet av sin tekniska livslängd och examensarbetet är ett första steg i utredningen för en eventuell ersättning. Med hjälp av elkvalitetsmätare kartlagdes prestanda och eventuella förbättringsområden. Den svenska stamnätsoperatören Svenska kraftnät har sedan 2016 haft en VSC-HVDC-station som använts för att utväxla energi till asynkrona systemet i Östeuropa. Svenska kraftnät har på senare tid velat utnyttja spänningsregleringsmöjligheterna och utreda mättekniska metoder. Elkvalitetsmätare placerades ut på lämpliga mätpunkter för att utreda anläggningens beteenden. ELSPEC G4500 elkvalitetsmätare installerades på Nordbalt och Gotlands HVDC Light för att mäta under sensommar till hösten 2017. Skillnader mellan CVT och IVT samt Rogowskispole och CT mättes. Valet av ELSPEC lämpade sig bra eftersom att inga triggningsvillkor behövdes som tillåter att man kan upptäcka små men viktiga avvikelser. Gotlands HVDC Light stabiliserar nätet främst mot spänningsdippar efter kortslutningar i 10 kV-elnätet och flimmer ifrån vindkraftsparkerna i ände 2. Märkeffekten för en uppdaterad anläggning kommer baseras på den kortslutningseffekt i 10 kV-nätet som kan utvecklas under anläggningens livstid. Spänningsregleringen ska baseras på en PI-regulator och ska kunna reglera fullt på 40 ms. Komponentspänningar kan användas för att ge reaktiv effekt på de faser som behöver det. Behovet mot flimmer ska baseras på en mätning i närmaste konsumtionscentrum, två mil från ände 2. Om behovet finns, ska en separat loop för flimmerkontroll som motverkar 1,5 Hz-komponenter implementeras utifrån en punkt ca 1,5 mil ifrån ände 2. Teknikvalet står mellan two-level generation 3 eller MMC, beroende på uppgradering eller ersättning. Många olika framtida scenarion påverkar HVDC Lights roll och oavsett, kommer mycket resurser behövas för Gotlands och HVDC Lights framtid. Det har observerats två beteenden på Nordbalt varav ena är långsam och det andra beteendet är snabbt. Det snabba beteendet uppstår när lågohmiga fel sker som också synkronmaskinerna tar hand om. Nordbalt kan hjälpa till för mindre spänningsvariationer om den varit snabbare likt beteendet vid lågohmiga fel. En stabilitetsbedömning behövs dock eftersom att snabbare beteende ökar risken för instabilitet. Eftersom att data mellan CVT och IVT skiljde sig mycket, upplystes mättekniska problem. Några lösningar diskuteras varav RCVT och PQSensor gås igenom grundligt. Alternativa lösningar som MoW och mobila enheter presenteras också vagt. Samtliga lösningar visade sig ha praktiska hinder, vilket försvårar genomförandet. Det uppmanas därför att man bör testa teknikerna i laborationsmiljö eftersom begränsat med studier gjorts på dessa samt att konkurrerande tillverkare uppger olika uppgifter. / Gotland has long been known for various special solutions and been on the leading edge regarding new technology. For the past 20 years Gotland has had a VSC-HVDC station that has stabilized the voltage in the Gotlandic grid. HVDC Light has solved some of the technical limitations that has hindered the growth of wind turbines. The station has almost reached the end of its technical lifespan and the thesis is a first step in the investigation for an eventual replacement. With power quality analyzers performance and improvements were investigated. The Swedish transmission system operator Svenska kraftnät have since 2016 had a VSCHVDC station that have been used to exchange energy to the asynchronous system in Eastern Europe. Svenska kraftnät has of lately wanted to take advantage of the voltage control capabilities and investigate measurement technologies. Power quality analyzers were installed on suitable connection points to investigate the behavior of the station. ELSPEC G4500 power quality analyzers were installed on Nordbalt’s and Gotland’s HVDC Light to measure during the late summer to fall of 2017. The differences between CVT and IVT, and Rogowski coil and CT were captured. The choice of ELSPEC suited well since no trigger conditions are needed which allows small but important errors to be discovered. Gotland’s HVDC Light stabilizes the grid mostly against voltage sags for faults in the 10 kVgrid and flicker from the wind farms in node 2. The rated power for an upgrade should be based on the 10 kV-grid short-circuit power to be developed during the station’s life span. The voltage control should be based on a PI-controller and should be able to transmit full power in 40 ms. Component voltages can be used to produce reactive power on the phases in need. The need against flicker should be based on measurements in the closest center of consumption, 2 miles from node 2. If it’s needed, a separate loop for flicker-control that prevents 1.5 Hz-components based on a point 1.5 mile from node 2 should be implemented. Depending on an upgrade or replacement, the topology can either be “two-level generation 3” or MMC. Many different future scenarios affect HVDC Light’s role and more resources will be required for Gotland’s and HVDC Light’s future. Two behaviors have been observed on Nordbalt where one is slow and the other behavior is fast. The fast behavior was only observed when low impedance faults occurs, that the synchronous generators also handles. Nordbalt can contribute to small voltage variations if it was faster, like the behavior for low impedance faults. A consideration in stability is needed since a faster behavior can lead to stability problems. Since the data between CVT and IVT differed a lot, metrology problems were discovered. Some solutions were discussed, of which RCVT and PQSensor was thoroughly reviewed. Alternative solutions like MoW and mobile units were also presented vaguely. All solutions showed practical difficulties, which complicates the implementation. It is therefore encouraged to test the technologies in a laboratory environment since few studies have been made on these and competing manufactures state different information.

In order to identify the weakest bus in a power system so that the Unified Power Flow Controller could be connected, an investigation of static and dynamic voltage stability is presented. Two stability indices, static and dynamic, have been proposed in the thesis. Multi-Input Multi-Output (MIMO) analysis has been used for the dynamic stability analysis. Results based on the Western System Coordinate Council (WSCC) 3-machine, 9-bus test system and IEEE 14 bus Reliability Test System (RTS) shows that these indices detect with the degree of accuracy the weakest bus, the weakest line and the voltage stability margin in the test system before suffering from voltage collapse. Recently, Flexible Alternating Current Transmission systems (FACTs) have become significant due to the need to strengthen existing power systems. The UPFC has been identified in literature as the most comprehensive and complex FACTs equipment that has emerged for the control and optimization of power flow in AC transmission systems. Significant research has been done on the UPFC. However, the extent of UPFC capability, connected to the weakest bus in maintaining the power flows under fault conditions, not only in the line where it is installed, but also in adjacent parallel lines, remains to be studied. In the literature, it has normally been assumed the UPFC is disconnected during a fault period. In this investigation it has been shown that fault conditions can affect the UPFC significantly, even if it occurred on far buses of the power system. This forms the main contribution presented in this thesis. The impact of UPFC in minimizing the disturbances in voltages, currents and power flows under fault conditions are investigated. The WSCC 3-machine, 9-bus test system is used to investigate the effect of an unsymmetrical fault type and position on the operation of UPFC controller in accordance to the G59 protection, stability and regulation. Results show that it is necessary to disconnect the UPFC controller from the power system during unsymmetrical fault conditions.

The interaction of an HVDC converter with the connected power system is of complex nature. An accurate model of the converter is required to study these interactions. The use of analytical small-signal converter models provides useful insight and understanding of the interaction of the HVDC system and the connected system components. In this thesis analytical models of the HVDC converters are developed in the frequency-domain by calculating different transfer functions for small superimposed oscillations of voltage, current, and control signals. The objective is to study the dynamic proprieties of the combined AC-DC interaction and the interaction between different HVDC converters with small signal analysis. It is well known that the classical Bode/Nyquist/Nichols control theory provides a good tool for this purpose if transfer functions that thoroughly describe the 'plant' or the 'process' are available. Thus, there is a need for such a frequency-domain model. Experience and theoretical calculation have shown that voltage/power stability is a very important issue for an HVDC transmission link based on conventional line-commutated thyristor-controlled converters connected to an AC system with low short circuit capacity. The lower the short circuit capacity of the connected AC system as compared with the power rating of the HVDC converter, the more problems related to voltage/power stability are expected. Low-order harmonic resonance is another issue of concern when line-commutated HVDC converters are connected to a weak AC system. This resonance appears due to the presence of filters and shunt capacitors together with the AC network impedance. With a weak AC system connected to the HVDC converter, the system impedances interact through the converter and create resonances on both the AC- and DC-sides of the converter. In general, these resonance conditions may impose limitations on the design of the HVDC controllers. In order to improve the performance of the HVDC transmission system when it is connected to a weak AC system network, a reactive compensator with a voltage source converter has been closely connected to the inverter bus. In this thesis it is shown that the voltage source converter, with an appropriate control strategy, will behave like a rotating synchronous condenser and can be used in a similar way for the dynamic compensation of power transmission systems, providing voltage support and increasing the transient stability of the converter. / QC 20100708