This work makes a detailed analysis on the principles and characteristics of Direct Power Control (DPC) and Model Predictive Control (MPC) applied to three-phase half-bridge PWM Rectifier. Model predictive control has merits of forecast and real-time optimization. MPC controller computes the optimal space vector of input voltage to the PWM rectifier in the dq frame and then this desired space vector is modulated via space vector pulse width modulation. The simulation results from Matlab/Simulink illustrate the flexibility and effectiveness of MPC-SVPWM approach.

A hypothesis regarding temperature dependant discharge levels was the starting grounds for this student work. Based on calculations and conversations with experienced people in this field, there should be cavity-induced discharges at relatively low voltages when dealing with a paper-mass cable at room temperature.PD measurements with a paper-mass HVDC cable under AC stress have been carried out. To get an understanding on the cavity characteristics at different temperature levels, the cable has been heated to temperatures of 20, 30, 40, 50 and 60 ̊C. Discharges over a large range of magnitude, from 101 to 104 pC was observed. The largest discharges measured coincide with rather large portions of a butt gap being drained of oil.Partial discharges are a stochastic variable, thus one cannot expect values to coincide from measurement to measurement, but rather focus on finding tendencies. The maximum discharge levels showed a temperature dependency, but not as easily interpretable as expected. During the first half of the temperature cycles the maximum discharge levels varied as anticipated; small to non existing discharges at high temperatures and larger discharges at room temperature. This correlation became less clear over time. The deviation from the anticipated characteristics could be caused by: •Temperature gradients over the insulation due to insufficient time to stabilize at the wanted temperature level.•Axial oil flow in the test object.•Irreversible deterioration of the insulation caused by large discharges mid-test. The discharge characteristics did show two clear tendencies throughout the lab work:•Whenever the temperature was raised from one level to a higher level, the maximum discharge size was reduced.•The inception voltage increases with increasing temperature and decreases with decreasing temperatureThe cable was installed in a test rig prior to start-up of the student project. Initially there was PD activity in one of the terminations which had to be modified to some extent to avoid interferences on the measurements. A new test object is under preparation and new terminations have been designed. The new terminations are based on the old design and the modifications made on it. Drawings for the new terminations have been made for the workshop and the fabrication process followed up. Measurements to locate the discharge sites were performed at room temperature at the end of the lab work. These measurements indicated an even distribution of the discharge sites along the length of the cable, meaning the terminations can be deemed healthy.

Denne hovedoppgaven tar for seg avansert måle- og styresystem (AMS) og innføringen av strupefunksjonen som skal kunne bryte og strupe effektuttaket i det enkelte målepunkt for å redusere effektuttaket ved perioder med knapphet i nettet. Formålet har vært å kartlegge og teste mulige styresystemer som finnes på markedet, og som kan benyttes for å oppfylle strupefunksjonens funksjonalitet. Det har i tillegg blitt sett på hvilke nettariffer som kan benyttes til å øke motivasjonen og fremheve nytteverdien for sluttbrukeren ved innføringen av AMS. Det har vist seg at dagens nettariffer ikke er tilpasset bruken av strupefunksjonen, og det er nødvendig å innføre en nettariff som motiverer sluttbruker til å investere i en slik strupefunksjon, noe den tidsvariable nettariffen med energiledd gjør. Strupefunksjonen innebærer å koble ut deler av forbrukslastene ved ulike utkoblingskriterier, for å redusere energi- og effektforbruket. De utkoblingskriteriene som er lagt til grunn er knapphet i overføringsnettet, forbruksmønster og effektforbruk. Gjennom arbeidet med kartleggingen av ulike styresystemer, har det vist seg at det finnes styresystemer som kan benyttes til å begrense effektuttaket, men disse tar kun høyde for maksimalt effektuttak hos sluttbrukeren.Behovet for et styresystem som tar høyde for samtlige utkoblingskriterier var til stede, og det ble utviklet et styresystem basert på teknologi fra PLS og KNX. Gjennom målinger og lønnsomhetsberegninger har det vist seg at styresystemet gir et redusert energiforbruk og effektuttak i høylasttimer, og at det vil gi sluttbrukeren en økonomisk besparelse. Resultatene indikerer en reduksjon i energiforbruket på 1,6 kWh/døgn og en gjennomsnittlig respons på 1,1 kWh/h i høylasttimer. De økonomiske lønnsomhetsberegningene indikerer en total årlig besparelse på kroner 554,-. Resultatene inneholder noe usikkerhet, spesielt den årlige økonomiske besparelsen, grunnet faktorer som for eksempel måleperiodens lengde og nevnte feilkilder.

Rapid aging and failure in wind-farm collector systems has become an issue for grid owners. PD measurements of the cable systems can give good diagnostics, detect aging and pinpoint faults. Different PD measuring techniques are used today and the results will vary with applied voltage and frequency. This master thesis deals with preparation, electrothermal aging and characterization of XLPE cable joints. Dissection of the test objects was performed after aging and characterization. For characterization a variable frequency PD measurement setup has been built. The setup is defined by being able to detect PD with high accuracy in test objects at ± 17 kVpeak using Omicron MPD600 measuring equipment at variable frequencies between 10 mHz and 100 Hz. A PD-free low-pass RLC filter had to be used in the setup to reduce noise from the high-voltage source. The most important findings are that PD measurement results will vary significantly with regards to measuring technique. Aging may not be detected when measuring with too low frequency or voltage.

The overall goal of this thesis is to study and understand Open Conductor Faults and to assess their impact on the stability of a power system. In particular, the thesis has investigated the effect of this type of the fault on the dynamic electromechanical behavior of synchronous machine. The thesis has also focused on the effect of generator and transformer grounding as well as the effect of transformer winding configuration on the stability of the power system during this type of fault.Open conductor faults are series faults which involve a break in one or two of the three conductors of a three phase power system. As such, the fault is an unsymmetrical fault and thus, the theory of symmetrical components was revisited. Symmetrical components and symmetrical circuits have been used to analyze both types of open conductor faults in order to understand the phenomenon and ease calculations. A dual approach to this study has been undertaken. The phenomenon is treated analytically through calculations and then the calculated results are confirmed through computer simulations using SIMPOW, a power system simulation software. In either approach, it is evidently clear, through the use of Eigen values and calculated damping coefficients, that the damping of the machine in an open conductor situation is worse than the for the normal case without the fault. A research in to the developed equations for damping has been undertaken. The theory of induction motors is applied in development of both the positive and negative damping power. The equation for negative damping power is developed first, using the symmetrical component concept and secondly, using the single phasing concept. In the investigation of transformer winding configurations and the grounding generators and transformers, different scenarios have been considered and simulated in order to show their effect during the fault. Finally the two open conductor case has also been investigated analytically and through computer simulation. The effect of grounding is shown in the results obtained. It shows that power transfer only occurs when there is a return path from the point of the fault to the generator.

In the master thesis the concept of Programmed Modulation is investigated for motor drives with Three-Level NPC inverter. Programmed Modulation operates with pre-calculated PWM switching-patterns, which enables the facility of off-line optimization of the converter switching-instants. The Optimization objectives are several, two commonly known are Selective Harmonic Elimination and Synchronous Optimal Modulation. The latter optimization focuses on generating a switching-pattern that will reduce the phase current harmonics. A reduction of harmonic components in the phase currents, means that a reduction in switching frequency is possible. The switching loss component, compared to the total loss in IGBTs increases as the voltage level increase. A reduction in switching losses opens for an increase in the nominal current limit in IGBTs. Hence, reductions in switching frequency gives an increase in power density.Conventional current control strategies cannot be used in a switching-pattern based drive system without sacrificing the optimality of the applied switching-pattern. A novel approach is therefore required to obtain dynamic control. For this has the stator flux trajectory control method been chosen and tested. A simulation model, specially built for Programmed Modulation, is proposed in this thesis. The model has a Stator Flux Trajectory Controller (SFTC) that calculates manipulation of the optimal switching-pattern, the manipulations are added to the original optimal switching-pattern to control the actual stator flux in the drive system. This SFTC is also used to eliminates, fast, deviations between the actual stator flux and a calculated optimal stator flux. This effectively eliminates the currents transient that otherwise could arise after a switching-pattern exchange, due to mismatch between optimal flux and actual flux trajectory.The modulation strategy has been simulated, results shows that fast dynamic control is obtained by controlling the alpha- and beta-components of the stator flux, in rotor field oriented coordinates. Combined with the use of Synchronous Optimal Modulation switching-patterns is this a very promising modulation strategy that have the required qualities for medium voltage drives. The simulation model needs further development, suggestions are given in the further work section.

Rapporten omhandlar utviklinga av relévernmodellar for bruk i ATPDraw. ATPDraw er eit simuleringsprogram for elektriske kretsar, og inneheld per i dag ikkje modellar av relévern.Fyrste del av rapporten tek for seg grunnleggande teori og virkemåte til dei vanlegaste typane relévern, samt diskré signalbehandling. Relévern omtala er overstraumvern, differensialvern og impedansvern. Av signalbehandling er det sett på omgjering av diskré signal til effektivverdiar og frekvenskomponentar, med amplitude og fase.I andre del av rapporten er resultata frå utviklingsarbeidet presentert. Kjeldekode for kvar modell er forklart, og flytskjema er vist for dei modellane med omfattande kode. Her er det også forklart kort om korleis modellane kan nyttast i ATPDraw.Til slutt er nokre av modellane verifisert mot eit fysisk relévern. Relévernet som vart nytta til verifisering var eit impedansvern levert av Siemens, modellnummer 7SA610. På bakgrunn av relévernets funksjonalitet vart kun modellane av impedansvern utan effektpendlingsfunksjon og overstraumvern samanlikna.Modellane presentert i denne oppgåva har som funksjon å etterlikne reelle relévern. Verna vart bygd opp frå grunnleggande teori om virkemåte funnen i diverse litteratur og artiklar. Denne virkemåten, presentert i fyrste del, er oppnådd på alle modellar. Av verifiserte modellar viser det seg at overstraumverna og signalbehandlingsmodellane har ein virkemåte tilsvarande reelle relévern, medan impedansvernmodellen har eit lite avvik. Dette kjem truleg av at relévernet nyttar seg av ytterligare filtrering.

The society's dependency of a reliable power supply is increasing, and properly identifying the distance to the stability limits of the power system and avoiding large disturbances is thus becoming increasingly important. In this thesis, a system integrity protection scheme (SIPS) for voltage instability is proposed. First, six voltage stability indicators are studied and compared, and their performance is tested in several power system models. They are first tested in a two-bus system where the load impedance is gradually increased until the load-side voltage reaches zero. The performance of the indicators is then tested under circuit contingencies in models of the IEEE Reliability Test System and of the Norwegian power system. From the results, the most reliable indicators seem to be the ones that are based on local measurements (SDI, ISI and VSIscc).Several actions to mitigate voltage instability are described and tested in the power system models, including load shedding, switching of reactive compensation equipment, increasing AVR set points and increasing the active power generation. Of the unconventional actions, increasing AVR set points appears to be the most effective mitigation action.A SIPS is proposed based on the above mitigation actions, voltage stability indicators and signals from activation of OELs. The principle behind the scheme is to avoid load shedding as far as possible by using indicator values and OEL activation signals to initiate preventive mitigation actions to relieve the situation when the system is approaching instability. This also reduces the necessary amount of load to shed to stabilise the system.Simulations show that the proposed SIPS works as long as it has an adequate amount of mitigation actions available. In the simulations in the model of the Hammerfest/Skaidi region in Northern Norway, there was a general lack of possible mitigation actions, providing no alternative other than shedding large amounts of load to prevent voltage collapse after critical contingencies.

Det er ønskelig å øke driftsspenningen for krafttransmisjon ved bruk av høyspenningskabler, da en høyere driftsspenning vil gi lavere tap. Om man beholder samme tykkelse på isolasjonen og samtidig øker driftsspenningen, vil også det elektriske feltet i isolasjonen øke. Et økt felt vil gi en økt belastning på isolasjonen og kan føre til fremskyndet aldring og sammenbrudd. Det er flere faktorer om påvirker aldringen og dermed levetiden i et isolasjonssystem basert på polymerisolasjon (som her PEX) [1]. De viktigste faktorene under normal drift er forurensninger og urenheter. Dette er velkjente problemer og det tas strenge forhåndsregler under produksjon av slike isolasjonssystemer. På tross av dette er det praktisk umulig å unngå forurensinger når det produseres kabelisolasjon i store mengder.En annen angrepsvinkel er derfor å øke isolasjonssystemets toleranse ovenfor slike forurensninger. En måte å øke toleransen på er å sørge for god kontakt mellom isolasjons-materialet og en eventuell forurensning. Det er forsøkt oppnådd i dette prosjektet ved å øke kohesjonen mellom isolasjonsmaterialet og forurensningen ved å tilføre et heftemiddel. Dette er gjort ved å produsere testobjekter med tilsatt partikkelforurensning, for så å teste holdfastheten. Testobjektene har blitt tilsatt partikler behandlet på fire forskjellige måter:1)Referanse – Testobjekt uten tilsatte partikler.2)Ubehandlet – Testobjekt tilsatt ubehandlede partikler.3)Heftemiddel – Testobjekt tilsatt partikler behandlet med nanokompositter.4)Slippmiddel – Testobjekt tilsatt partikler behandlet med Chemlease PMR-90.Da det fra tidligere forsøk var kjent at denne typen testobjekt var hyppig utsatt for en defekt, ble laboratorie-arbeidet innledet med feilsøking av testobjektene [11]. Feilkilden viste seg å være vanskelig å finne, og nesten hele prosjektperioden ble benyttet på feilsøking. Under feilsøkingen ble flere av produksjonsparameterne variert, uten store forskjeller på resultatet. Defektene var stort sett av samme karakter uansett hvilke parametere som ble forandret.Det kan konkluderes med at ujevnheter i grenseoverflaten mellom indre halvleder og isolasjonsmaterialet medfører gjennomslag ved langt lavere påtrykt spenning enn det isolasjonssystemet er designet for å motstå. Når det gjelder feilkilder er det ennå ikke mulig å konkludere med noe, selv om høy mekanisk belastning på overflaten ved produksjon av råkopper virker som en sannsynlig feilkilde. Dette kommer høyst sannsynlig fra små ujevnheter som riper eller hakk i overflaten på støpeformene som følge av slitasje.Den resterende tiden ble benyttet til måling av PD, men det ble ikke nok tid til å gjennomføre tilstrekkelig med målinger. Det er og få testresultater fra de målingene som ble gjennomført, sannsynligvis grunnet de samme defektene som forhindrer måling av gjennomslagspenning. Det kan derfor ikke konkluderes med om heftemiddelet har noen effekt basert på PD målingene.

This dissertation presents an analytical study on virtual synchronous machine-based power control in active rectifiers for micro grids supported by prototype modelling, simulation results and discussions.Popularity and demand of the distributed energy resources and renewable energy sources are increasing due to their economic and environmental friendliness. Concept of micro grid with an active rectifier (AR) interface has been found to be promising for smart integration of such distributed generation units. Having the presence of a synchronous generator (SG) in a micro grid introduces several advantages in terms of stability and reliability in the power system. This is mainly owed to the inertia, damping and load sharing properties of SG. This in return, gives rise to the question if an AR of a micro grid can imitate the behaviour of a synchronous generator, can the stability and reliability introduced by SG be replicated in a micro grid. A research on the state-of-the-art for uninterruptible power supplies (UPS) has been carried out to identify the implementation and the control strategies of redundancy and parallel operation as UPS has been an established technology over the last decades. The theoretical study on virtual synchronous machine (VSM) concept in the fall, 2011, has been extended in developing a model with classical inner current control and outer voltage control loops based on the synchronous reference frame.The complete active rectifier model has been able to emulate the inertia, damping and load sharing properties of a SG and redundancy and expandability of parallel UPS systems. It must be emphasized that due to the flexibility of the virtual machine parameters and the absence of magnetic saturation and eddy current losses, a much improved performance have been achieved with a VSM compared to a synchronous generator.Simulations have been carried out for single and parallel operation of active rectifiers in island and grid-tied modes with satisfactory stability, damping and power sharing features.Key words – Active rectifier, virtual synchronous machine, micro grid, uninterruptible power supply, load sharing, redundancy, island mode, grid-tied mode, synchronous reference frame