Station Service Voltage Transformer (SSVT) is a special class of single-phase high voltage instrument transformer with a power winding and optional metering winding at the secondary. The SSVT is capable of being connected directly to the transmission line and providing 120 to 480 V at the secondary power terminals and 115 V at the metering terminals. The power rating of the secondary is normally anywhere from 25 kVA up to 333 kVA. The SSVT was initially developed as a means to provide control power in a substation without the need for stepdown and distribution transformers. Over time, the properties of the SSVT made it an ideal power source for many different applications. Today the SSVT is being installed in numerous applications around the world. Some typical applications include but are not limited to: remote cell towers, transmission tower lighting, oil and gas, mining projects, substation auxiliary power and rural electrification. In any of the listed applications, it is still necessary to acquire accurate revenue metering for the power companies to charge for power usage. In many cases this requires the installation of instrument transformers to perform the metering. Unfortunately, the metering and power windings of the SSVT cannot be operated simultaneously. When a load is applied to the power winding on the SSVT, a load current is reflected back into the primary winding. This current generates a drop in voltage through the primary reactance and resistance. It is these values which are reflected back into the metering winding. The accuracy of the metering winding will fall out of acceptable limits when the voltage drop is present. This research proposes a new method to provide both power and accurate metering, simultaneously, in a dual secondary SSVT. The accuracy of the metering winding remains essentially unaffected by the load on the power winding if the load is less than the maximum rating of the compensator. The result is a single transformer capable of replacing the functionally of two separate transformers. This new approach will have a broad impact in the development of future substation designs.

Transformatorstationer i det svenska elnätet innehar en vital funktion för att försörjningen av elenergi från producent till konsument skall fungera. Stationerna finns på olika spännings-nivåer – men de har alla en sak gemensamt: Behov av att lokalkraftsförsörjningen skall fungera enligt angivna krav. Försörjningen av en stations lokalkraftanläggning kan ske på flera olika sätt. I rapporten belyses alternativen stationstransformator, externt abonnemang, ok-lindning och ABB:s SSVT (Station Service Voltage Transformer). De olika lösningarna karaktäriseras av olika tekniska och ekonomiska aspekter och tillhörande för- respektive nackdelar. Studien utvärderar de fyra olika lokalkraftslösningarna ur ett helhetsperspektiv. Först redogörs för den bakomliggande grundläggande teorin kring lokalkraft och elkraftsekonomi. Därefter jämförs de tekniska och ekonomiska skiljaktigheterna gentemot varandra för respektive given transformatorstationstyp; fördelnings-, region- och stamstation. Vattenfall eftersöker de teknisk-ekonomiskt bästa lokalkraftsalternativen för deras transformatorstationer och rapporten skall fungera som underlag. Ur rapporten kan följande resultat och slutsatser erhållas: • Fördelningsstation: Stationstransformator och ok-lindning rekommenderas då de erbjuder teknisk funktionalitet till ett ekonomiskt försvarbart och likvärdigt pris. • Regionstation: Primärt rekommenderas alternativet ok-lindning. • Stamstation: Likväl som på de andra stationerna rekommenderas ok-lindning för försörjning av lokalkraft. Sammanfattningsvis erbjuder alla fyra alternativ de tekniska krav som finns för lokalkraftsförsörjning, men att en avvägning av vilken lösning som lämpar sig bäst för respektive transformatorstation bör utföras från fall till fall. / Substations in the Swedish electric power network have a vital function regarding the supply of electric energy from producer to consumer. The substations are represented on different voltage levels in the network – but they all have one thing in common: The need for auxiliary power to function as intended. There are different options for the supply of substations auxiliary feed. In the thesis the following alternatives are described holistic: local transformer, external power subscription, auxiliary winding on ordinary transformer, and a new (for the Swedish market) alternative from ABB called SSVT (Station Service Voltage Transformer). The distinct solutions are characterized with different technical and economic aspects and associated pros and cons. The study evaluates the four different auxiliary power solutions from a holistic view. First, the underlying essential theory about auxiliary power and electrical power economy is explained. Afterwards, the technical and economical differences are visualized for each of the solutions, and in comparison to each other for each defined substation type; distribution, region and national substation. The company Vattenfall strives for the best technical-economical alternatives for their different types of substations and the report intends to act as a supportive document. From the report, the following results and conclusions can be obtained: • Distribution substation: Local transformer and auxiliary winding is recommended due the technical functionality relative to its equivalent economic aspects. • Region substation: Primarily, the auxiliary winding alternative is recommended, mostly because of the cost-effectiveness. There are fewer alternatives in this type of substation. • National substation: The auxiliary winding is recommended as local power solution. To sum up, all of the four alternatives fill the technical requirements, but consideration for which solution for which substation type must be done on a case to case basis.