M.Ing. (Electrical and Electronic Engineering) / Long transmission lines in power system require high line loading in order to lower voltage limits due to line losses. For relatively long lines, line charging is high and thus higher voltage limits reached at low loading. It follows then that it is a challenge to maintaining the voltages between the acceptable limits for relatively long lines. This dissertation highlights the problems experienced when load varying from very low to very high is supplied by very long parallel lines of different impedance characteristic. When the load is extremely high, there are low voltages experienced which are solved by use of shunt capacitors and/or adding more lines. When the load is extremely low, there are high voltages experienced which are solved by use of shunt reactors and/or switching some of the lines off. The type of solutions to this two loading extremes as indicated above, can be problematic, in that; new lines requires servitudes which can take too long, shunt capacitors and reactors in this type of the network is not desirable since the introduction of too many of these devices have maintenance implications and they would require continuous switching to maintain acceptable voltages, resulting in complicated operation of the network. This research proposes the use of a phase shifting transformer located on one of two parallel corridors supplying power to a load located remotely from the rest of the system. The transformer is able to rearrange the active power flows to vary loadings of the corridors and the improvements in voltage regulation can be realised during both low and high load conditions.

M.Ing. (Electrical and Electronic Engineering) / Shunt reactors are used in power system amongst other things, to suppress overvoltages in the network during network switching, auto reclosing of transmission lines and under low loading condition of the network. Traditionally, shunt reactors of fixed type have been used and these have been permanently connected to the system. This research work investigated the feasibility of replacing the fixed shunt line reactors with a reactor, whose parameters are automatically varied depending on the system reactive power requirements, in a mature power system overlaid with high voltage lines to improve amongst, other things voltage stability. However, in a mature system overlaid with parallel higher voltage lines, power tend to flow on the matured system rather than higher voltage system as predetermined by various impedances of the power system. It is therefore desirable that loading of higher voltage lines be increased because of their higher power transfer capability and fact that higher voltage networks generate capacitive power which is substantially more than for each line at the original voltage. However, replacing a large number of fixed line reactors, at low loadings of higher voltage lines, even when system collapse is averted by increasing the number of reactors inserted into the system, overvoltage problems may still be an issue, until a certain number of must-run reactors, operating at full reactance, are put into service. If too much inductive reactance is removed from the system when the loading levels are extremely low, the power system will collapse. Therefore, there was no economic benefit in replacing all the fixed shunt line reactors with controllable type when the loading was still relatively low. Thus, the majority of the converted reactors were operating in their full rating as there was still more reactive power to be absorbed. However, when the power was diverted to flow on the higher voltage system through the use of series compensation of the higher voltage system, there was a possibility of making some fixed shunt line reactors on the higher voltage network controllable. This also allowed more power to be transferred in the higher voltage system thus improving its utilisation. Also, a positive impact on active system losses was realised.

The maximum length of an high voltage underground cable (HV UGC) cable is oftenconstrained by the criterion that the cable cannot have more than 50% reactivepower compensation. If this limit is exceeded the current in the circuit breaker maynot have a zero crossing after energization, which is referred to as the zero missingphenomenon. This is problematic if a fault occur shortly after energization. Inthe past 10 years, different methods have been proposed which would allow greaterreactive power compensation. These methods either prevent the zero missing phenomenon(preventive methods) or provide a way to open the circuit breaker if afault occurs (handling methods).A new 200 km, 220 kV line has been proposed in Iceland referred to as Sprengisandslína.One proposed option is to build it as an OHL-UGC-OHL line as the voltagecriteria is not fulfilled if Sprengisandslína is built as an UGC with a 50% reactivepower compensation. The aim of this thesis is to see whether the zero missing phenomenoncan be avoided by implementing countermeasures - this gives a preliminaryresults whether Sprengisandslína can be built as an UGC. In this thesis the four differentpreventive methods are analyzed with a transient study for Sprengisandslína:(1) Pre-insertion resistor, (2) simultaneous synchronized switching, (3) synchronizedswitching where the cable is energized before the shunt reactor, and (4) synchronizedswitching where the shunt reactor is energized before the cable.Preliminary steady state studies were performed to determine the minimum numberof shunt reactors needed to fulfill the voltage criteria. The results showed thatthe minimum number needed were three assuming they are all of equal size locatedevenly along the cable (one at each end and one in the middle). Additionally, it isnecessary to see whether the generators would become underexcited if the cable isiiienergized with 100% reactive power compensation as it can reduce the lifetime ofthe generators. The results showed that the generators did not become underexcited.The countermeasure of synchronized switching where the shunt reactor is energizedbefore the cable and the countermeasure of simultaneous synchronized switchingwere shown to eliminate the zero-missing phenomenon when the cable was energizedwith 100% reactive power compensation. Synchronized switching where theshunt reactor is energized before the cable was seen to have lower switching overvoltages,21% higher than the nominal value, and the lower inrush current of 2.38kA. However, the method of simultaneous synchronized switching is cheaper andthe switching overvoltages and inrush current were within an acceptable margin(switching overvoltages were 35:9% higher than the nominal value and the inrushcurrent was 4.01 kA).The results of the study indicate that Sprengisandslína can be energized as an UGCwith 100% reactive power compensation if either the countermeasure of simultaneoussynchronized switching or synchronized switching where the shunt reactor isenergized before the cable are used. However, a detailed frequency study must beperformed before either of the countermeasures can be recommended. / Högsta längden på en högspänning underjordisk kabel begränsas ofta av de kriteriersom kabeln inte kan ha mer än 50% reaktiv effektkompensation. Om denna gränsöverskrids kan strömmen i strömbrytaren inte ha noll genomgang efter aktivering,kallad noll saknad fenomen. Detta är problematiskt om ett fel inträffar strax efteraktivering. Under de senaste 10 åren har olika metoder föreslagits, vilket skullemöjliggöra större reaktiv effektkompensation. Dessa metoder hindrar antingen detnollbristande fenomenet (förebyggande metoder) eller ger ett sätt att öppna strömbrytarenom ett fel uppstår (hanteringsmetoder).En ny 200 km, 220 kV linje har föreslagits på Island kallad Sprengisandslína. Ettföreslaget alternativ för att den här linjen ska byggas är att bygga den som en OHLUGC-OHL-linje, eftersom spänningskriterierna inte är uppfyllda om Sprengisandslínaär byggt som en UGC med en 50% reaktiv effektkompensation. Syftet meddenna avhandling är att se huruvida det saknade fenomenet kan undvikas genomatt genomföra motåtgärder - detta ger ett preliminärt resultat om Sprengisandslínakan byggas som en UGC. I denna avhandling analyseras de fyra olika förebyggandemetoderna med en övergående studie för Sprengisandslína: (1) Förinsättningsresistor,(2) Synkroniserad samtidigkoppling, (3) Synkroniserad inkoppling där kabelnaktiveras före shuntreaktorn och (4) ) synkroniserad inkoppling där shuntreaktornaktiveras före kabeln.Preliminära steady state studier utförs för att bestämma det minsta antalet shuntreaktorersom behövs för att uppfylla spänningskriterierna. Resultaten visade att detminsta antalet som behövdes var tre förutsatt att de alla är lika stora som liggerjämnt längs kabeln (en i varje ände och en i mitten). Dessutom är det nödvändigtatt se om generatorer skulle bli underexiterad om kabeln är energiserad med 100%iiiivreaktiva effektkompensation eftersom det kan minska generatorns livslängd. Resultatenvisade att generatorer inte blev underexiterad.Motståndet för synkroniserad omkoppling där shuntreaktorn aktiveras före kabelnoch motmätningen av samtidig synkroniserad omkoppling visades för att elimineradet nollmissande fenomenet när kabeln aktiverades med 100% reaktiv effektkompensation.Synkroniserad omkoppling där shuntreaktorn aktiveras innan kabeln visadesig ha lägre omkopplingsvolymer, 21% högre än nominellt värde och den lägre inbrusningsströmmenpå 2,38 kA. Metoden för samtidig synkroniserad omkopplingär emellertid billigare och omkopplingsvolymen och inströmmen var inom en acceptabelmarginal (omkopplingsvolymer var 35; 9% högre än nominellt värde ochinströmningsströmmen var 4,01 kA).De resultaten av studien indikerar att Sprengisandslína kan energiseras som en UGCmed 100% reaktiv effektkompensation om antingen motspelet av samtidig synkroniseradomkoppling eller synkroniserad omkoppling där shuntreaktorn aktiveras innankabeln installeras. En detaljerad frekvensstudie måste dock utföras innan någon avmotåtgärderna kan rekommenderas.

As spacial restrictions become more and more of an issue for renewable energy parks onshore, more developers are turning offshore to develop future production. The desire for a diversity of energy sources combined with a desire to utilize space efficiently has sparked an interest in renewable energy parks where more than one energy source occupy the space. This master thesis took up the task of modeling the grid connection of such an offshore multi-source park consisting of wind turbines and floating solar panels from a techno-economical perspective. The grid connection consists of everything from the wind turbine and solar output to the Point of Common Connection (PCC) onshore. The park consists of a 4x4 set of 15MW wind turbines accompanied by three large floating platforms of solar panels. Two different ways of creating the grid connection have been investigated, including many sub-cases. HVAC-transmission to shore has been compared with HVDC-transmission at different line voltages and transmission distances. The use of shunt reactors at the middle point of the transmission lines have been investigated to try extending the distance where HVAC was viable. A radial AC-collection grid has also been compared with a parallel DC-grid to see if DC-grids shows any promise. All of these different cases have been evaluated based on their total cost over a 25 year period. The result has shown that HVAC-transmission is desirable for shorter distances, but eventually reaches a break-even point where HVDC makes more sense economically. Adding a middle shunt was shown to be able to make HVAC competitive, if not desirable, compared to HVDC as the distance becomes long. Changes in the collection grid have significantly less of an impact on the cost of the entire system compared to the transmission. It was concluded that a multi- source park of wind and solar shows similar results compared to offshore parks consisting only of wind power and similar conclusions could be drawn. The biggest challenges for multi-source parks are not in the electrical domain, but in structural design where floating solar face a lot of challenges such as mechanical stress from strong waves. / EU-SCORES