The electric grid of Gotland is connected to the mainland via a 90 km HVDC Classic bipole of 2 * 130 MW. The HVDC link balances the load and production on the island to maintain the frequency within limits, the load varies between 50 and 180 MW throughout the year. The power production on the island comes mainly from wind power. Today, the installed power is about 170 MW but the wind power production will be further exploited and the plan is to add another 500 MW of wind power capacity to the existing plants. These plants will be connected to a new 130 kV transmission grid which will have a connection to the existing 70 kV grid at a new substation called Stenkumla. Along with the increased wind power production on the island comes the need of increased transmission capacity to the mainland. A VSC HVDC link of 500 MW is planned for this purpose and it will be connected to Stenkumla. As of today, it is not certain whether the two grids will be connected or not. Having connected grids is in the interest of the grid owner Gotlands Energi AB, GEAB since they then could utilize the technology of the new link and thereby ensure stableoperation during faults that today would lead to black out. In this thesis the feasibility of having connected grids was investigated and the study was divided into three main parts. •Reactive power and voltage profiles •Short circuit study •Converter trip study This study shows that under the assumptions made regarding production grid layout and proportion of WTG types there will be no need for adding reactive power compensation equipment. That is provided that demands are set on wind power plant contractors to have their equipment contributing with reactive power compensation, even during no load. A trip of the SvK VSC HVDC converter during full power production causes the most severe stress to the system. The major problem proved to be surviving the first 100 ms after converter trip without loosing angular stability and the most important measure to improve the stability was active power reduction of the wind turbines. The overall conclusion is that it is feasible to have connected grids during normal operation but demands has to be put on wind power plant contractors.

Transformers and storage systems in the electrical grid must be provisioned or sized just as routers and buffers must be sized in the Internet. We prove the formal equivalence between these two systems and use this insight to apply teletraffic theory to sizing the electrical grid, obtaining the capacity region corresponding to a given transformer and storage size. We conduct a fine-grained measurement study of household electrical load. These measurements are essential for two reasons. First, we use them to construct reference models for home loads; these models are used to find the capacity region using the teletraffic theory. Second, these measurements are used in numerical simulations that are done to validate our analysis. More specifically, we compare results of numerical simulations with the results from teletraffic theory. We show not only that teletraffic theory agrees well with numerical simulations but also that it closely matches with the heuristics used in current practice. Moreover, our analysis permits us to develop sizing rules for battery storage electrical grid, advancing the state of the art.

Transformers and storage systems in the electrical grid must be provisioned or sized just as routers and buffers must be sized in the Internet. We prove the formal equivalence between these two systems and use this insight to apply teletraffic theory to sizing the electrical grid, obtaining the capacity region corresponding to a given transformer and storage size. We conduct a fine-grained measurement study of household electrical load. These measurements are essential for two reasons. First, we use them to construct reference models for home loads; these models are used to find the capacity region using the teletraffic theory. Second, these measurements are used in numerical simulations that are done to validate our analysis. More specifically, we compare results of numerical simulations with the results from teletraffic theory. We show not only that teletraffic theory agrees well with numerical simulations but also that it closely matches with the heuristics used in current practice. Moreover, our analysis permits us to develop sizing rules for battery storage electrical grid, advancing the state of the art.

När konsekvenserna för miljön av att använda förbränningsmotorer blir allt mer påtagliga ökar behovet av alternativ för transport av människor och varor. Elfordon säljs i större utsträckning än någonsin förr. Men batterierna i elfordonen är även de en belastning på miljön när de produceras. Om batterierna skulle användas ytterligare efter att de inte längre går att använda i fordon skulle den relativa miljöpåverkan från produktionen minskas och det blir lättare att kompensera för den påverkan som redan skett för varje batteri. För att uppnå detta behöver batteriernas karakteristik undersökas. Degraderingen behöver fastställas för att beräkna hur mycket energi de kan cykla under sitt liv. Hur denna energi kan nyttjas effektivt och meningsfullt i ett modernt elnät är även det av vikt. Vad händer sedan vid slutet av batteriernas liv? Återvinning är ett viktigt steg i processen för att minimera klimatförändringar istället för att lämna batterierna på deponi. Resultaten visar att mängden cykler ett batteri klarar ner till en viss kapacitetsnivå beror kraftigt på arbetstemperatur, urladdningsdjup, C-förhålland och batterityp. Hybricons batterier av LTO-typ uppskattas klara som mest 11 459 cykler vid 10 % urladdningsdjup och 2 490 cykler vid 50 % urladdningsdjup. Användningsområden i den här rapporten där batterilager förväntas kunna användas effektivt är som kortslutningsförstärkning och begränsning av spänningshöjningar hos kunder med hög förimpedans i landsbygdsnät samt som UPS:er till telekommunikation. Kostnader för dessa applikationer uppskattas att kosta mellan ca 1700 kr/kWh och 7600 kr/kWh beroende på konstruktion och andrahandspris på batterierna. Det värde ett batteri uppskattas ha i den här rapporten är den mängd energi det kan lagra under sitt liv, vilket varierar beroende på de faktorer som nämnts tidigare och gör att varje enskilt fall behöver analyseras. Slutsatsen blir att det finns förutsättningar som gör det möjligt att effektivt och ekonomiskt fördelaktigt använda batterier i elnätet, särskilt då utvecklingstrenden visar att utrustningen ständigt blir billigare och tekniken bättre. Dock behöver fler analyser utföras, främst på åldringen i batteriernas andra liv. / As the consequences for the environment of using internal combustion engines are becoming increasingly apparent, the need for alternatives for the transport of people and goods increases. Electric vehicles are sold to a greater extent than ever before. But the batteries in electric vehicles are also a burden on the environment when they are produced. If the batteries were to be used further after they could no longer be used in cars, the relative environmental impact of the production would be reduced and it would be easier to compensate for the impact already made by each battery. To achieve this, the characteristics of the batteries need to be investigated. The degradation needs to be determined to calculate how much energy they can cycle during their life. How this energy can be used efficiently and in a meaningful way in a modern grid is also important. What then happens at the end of battery life? Recycling is an important step in the process to minimize climate change instead of leaving the batteries in landfill. The results show that the amount of cycles a battery can handle down to a certain capacity level depends heavily on operating temperature, discharge depth, C-ratio and battery type. Hybricons batteries of LTO-type is estimated to sustain for 11 459 cycles at 10 % depth of discharge och 2 490 cycles at 50 % depth of discharge. Applications in this report, where battery storage is expected to be used efficiently, is short-circuit amplification and limitation of voltage hikes at customers with high impedance in rural networks as well as UPSs to telecommunications. Costs for these applications is estimated to cost between circa 170 USD/kWh och 760 USD/kWh depending on construction type and second hand price of the batteries. The value of a battery is estimated in this report is the amount of energy it can store during its life, which varies depending on the factors mentioned earlier and necessitates analysis of each case. The conclusion is that there are conditions that make it possible to efficiently and with economical gain use batteries in the grid, especially as the trend shows that the equipment is constantly getting cheaper and the technology is getting better. Although more analysis is required, foremost on the aging of the batteries second life.

The main goal of this thesis is to evaluate current status of renewable energy sources in China and to reveal current problems of this sector. In the first chapter, renewable energy sources are theoretically defined. In the second chapter, the whole energy sector of China is analysed with emphasis on the role of renewable energy sources, their share on primary energy consumption and their share in electricity production. The last chapter covers sector of wind energy in a detail. It analyses feed in tariffs or issues of energy grid. It also discusses economic profitability of wind power plants, their manufacture industry and overall future potential.

abstract: The availability of data for monitoring and controlling the electrical grid has increased exponentially over the years in both resolution and quantity leaving a large data footprint. This dissertation is motivated by the need for equivalent representations of grid data in lower-dimensional feature spaces so that machine learning algorithms can be employed for a variety of purposes. To achieve that, without sacrificing the interpretation of the results, the dissertation leverages the physics behind power systems, well-known laws that underlie this man-made infrastructure, and the nature of the underlying stochastic phenomena that define the system operating conditions as the backbone for modeling data from the grid. The first part of the dissertation introduces a new framework of graph signal processing (GSP) for the power grid, Grid-GSP, and applies it to voltage phasor measurements that characterize the overall system state of the power grid. Concepts from GSP are used in conjunction with known power system models in order to highlight the low-dimensional structure in data and present generative models for voltage phasors measurements. Applications such as identification of graphical communities, network inference, interpolation of missing data, detection of false data injection attacks and data compression are explored wherein Grid-GSP based generative models are used. The second part of the dissertation develops a model for a joint statistical description of solar photo-voltaic (PV) power and the outdoor temperature which can lead to better management of power generation resources so that electricity demand such as air conditioning and supply from solar power are always matched in the face of stochasticity. The low-rank structure inherent in solar PV power data is used for forecasting and to detect partial-shading type of faults in solar panels. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2020

Accurate day-ahead grid loss forecasts are, among other things, essential to determine the electricity price for the upcoming day. The more accurate forecast, the closer the trading on the 'day-ahead' electricity market can become the actual operation the next day, which dedcrease the need for correcting production on the balancing market. Followingly, the need for extra imbalance costs, which make the electricity price higher, is reduced with accurate forecasts. This project's purpose was to explore a wide range of mathematical models to increase the energy comapny Fortum's day-ahead grid loss forecasting accuracy, and thereby contribute to lower the risk for high imbalance costs.  Two electrical grids located in Sweden, with different characteristics, were studied. One electrical grid was located in Dalarna and the other one was located in Värmland. Four different model types were tested for each grid. The linear models ARIMAX and SARIMAX were explored and the two artificial neural networks FNN (Feedforward Neural Network) and LSTM-RNN (Long-SHort Term Memory Recurrent Neural Network) were explored. By constructing different model structures of each model type, as well as statistically testing which predictors to include as input to the models, the most accurate model for grid loss forecasting was found. The models' forecasting accuracy were validated based on the MAPE (Mean Absolute Percentage Error). Variables important as predictors were found to be power production, electricity prices and grid losses at previous time steps. For the grid in Dalarna ARIMAX(2,0,2) was the model generating most accurate day-ahead grid loss forecasts and for the grid in Värmland, SARIMAX(1,0,0)(0,0,1)[24] was the most accurate model. That is, different models were found as the most accurate one for grid loss foracsting, as the two studied electricity grids had different characteristics. Hence, this result implies that there is no universal model that is the most adequate at modelling all types of grid losses. To find useful models when forecasting grid losses day-ahead, an analysis of the particular grid losses being studied is therefore not irrelevant.

I denna rapport undersöks om det föreligger ett behov av anpassningar inom energisystemet för att möjliggöra utvecklingen av eldrivna lastbilar i Sverige och mer specifikt Stockholm. En ansats till att projektera utvecklingen av eldrivna lastbilar görs och genom en kvalitativ litteraturstudie samt intervjuer med aktörer som representerar Scania, Volvo, Ellevio, EoN, Powercircle, Forum för smarta elnät, KTH, Energimarknadsinspektionen och BIL Sweden, och anpassningar som skulle kunna utgöra lösningar till dessa identifieras inom kategorierna: ‘’ökad tillgänglig kapacitet’’, ‘’förbättrad planering’’, ‘’d irekt reglerande åtgärder samt ekonomiska incitament’’ och ‘’p olitiska beslut på kommun- samt regional nivå med syfte att öka elektrifieringen av lastbilar ’’. Slutligen görs en jämförelse av anpassningarna med avseende på tillgänglighet och implementeringstid, lagliga hinder, kostnadseffektivitet, miljöpåverkan, replikerbarhet samt intressekonflikter. Utifrån resultatet dras slutsatserna att elektrifieringen kommer möjliggöras av flera lösningar i varierande grad men att anpassningar som inbegriper ökad flexibilitet inom elsystemet, för motverkandet av kapacitetsbrist inom elnätet, sannolikt kommer ingå. Vidare konkluderas att flera av de identifierade anpassningarna begränsas av legislativa- och politiska beslut och förändringar som gör dess bidragande till elektrifieringen av lastbilar svårkvantifierad. Resultaten bör därför betraktas som indikativa. / This study investigates the need for adaptations within the energy system of Stockholm, Sweden in order to improve the prerequisites for the development of electrical trucks. An attempt to project the development of electrical trucks is executed and by the use of a qualitative literature study as well as interviews with representatives for Scania, Volvo, Ellevio, EoN, Powercircle, Swedish Smartgrid, KTH, the Swedish Energy Markets Inspectorate and BIL Sweden, possible adaptations are identified within the categories: ‘’increased available capacity’’, ‘’improved planning’’, ‘’direct regulations and financial incentives’’ aswell ‘’political decisions on municipal and regional level with the aim to increase the electrification of trucks’’. Finally a comparison with respect to availability and implementation, legislation, cost, environmental impact, replicability and conflicts of interest is made between the identified adaptations. Based on the investigation it is concluded that several adaptations, to varying degrees, will play a part in enabling the electrification. It is furthermore concluded that adaptations that involve increasing the flexibility within the electricity system will play a prominent role as solutions. Several of the adaptations are limited in their implementation as they are dependent on the conceiving of legislative and political decisions. This provides uncertainty in terms of projecting their contribution to the electrification of trucks and the results are to be taken as indicative rather than absolute.

Este trabalho de mestrado está relacionado com a análise do comportamento dinâmico de turbinas eólicas equipadas com gerador síncrono com excitação a ímãs permanentes operando durante contingências na rede elétrica e com variação na velocidade do vento. Tal configuração tem muitas vantagens quando comparada com a configuração atualmente utilizada pelos principais fabricantes de turbinas eólicas de grande porte. A principal delas é devido ao fato de não utilizar caixas de engrenagens. Porém, o impacto na rede elétrica com a conexão de tal configuração deve ser minimizado melhorando as condições operativas no ponto de conexão da fazenda eólica. Desta forma, o projeto em questão visa investigar formas de diminuir os impactos negativos causados e melhorar a estabilidade do sistema, principalmente, no ponto de conexão à rede elétrica Portanto, o projeto de pesquisa deste mestrado está relacionado ao desenvolvimento de modelos computacionais que representem tal tecnologia, considerando os principais componentes e controles utilizando o software de simulação PSCAD. Também são incluídos novos métodos de controle e elementos adicionais que melhoram a confiabilidade e o nível de penetração de parques eólicos com geradores a ímãs permanentes. Ressalta-se que tais modelos não estão disponíveis de forma completa no software citado acima, portanto foram desenvolvidos partindo de modelos genéricos existentes. Todas as partes do modelo foram realizadas com elementos básicos acessíveis em qualquer programa de simulação, de modo que qualquer engenheiro ou pesquisador possa programar este mesmo modelo em qualquer software. Esta dissertação começa com uma breve descrição do modelo desenvolvido, incluindo o modelo aerodinâmico das turbinas eólicas, o controle do conversor do lado do gerador, o controle do conversor do lado da rede e o sistema de proteção do link de corrente contínua. Com o modelo desenvolvido, analisa-se a operação do gerador síncrono a imãs permanentes durante contingências na rede. A aplicação de um sistema avançado de armazenamento de energia e a possibilidade de suporte de frequência à rede elétrica também foram desenvolvidos e analisados. Considerando os resultados obtidos, podemos verificar que a conexão de parques eólicos equipados com geradores síncronos com excitação a ímãs permanentes pode trazer benefícios para a rede elétrica, quando o seu potencial de controle e de operação é utilizado. Mais detalhes podem ser verificados nos capítulos a seguir. / This master\'s project aims to study the behavior of wind turbines with Permanent Magnet Synchronous Generator, operating during contingencies and changes in wind speed. This configuration has many advantages compared with configurations currently used by leading manufacturers of large wind turbines. The main one is due to the fact of use smaller gearboxes or even gearless turbines. However, the impact on the electrical grid connection of this configuration should be minimized by improving the operating conditions at the connection point of the wind farm. Thus, this project aims to investigate ways to reduce the negative impacts and improve the system stability, especially at the grid connection point. This work employs the simulation software PSCAD for developing a computational model of this technology, considering the main components and controls. New control methods and additional features are included, for improving the reliability and the level of penetration of wind farms with permanent magnet generators in the power system. It is noteworthy that such models are not available in the simulation software PSCAD; therefore it was developed from existing generic models. All parts of the model were done with basic elements; as a result any engineer or researcher can program this same model in any software. This work begins with a brief description of the developed model, including: the aerodynamic model of the wind turbines, the control of the generator side converter, the control of the grid side converter and the protection system of the Link-DC. With this model, the operation of the permanent magnet synchronous generator was analyzed during network contingencies. The application of a sophisticated energy storage system for frequency support was also both developed and analyzed. The connection of wind turbines with Permanent Magnet Synchronous Generator has advantages when its potential for control and operation is used. More details can be checked in the following chapters.

Este trabalho de mestrado está relacionado com a análise do comportamento dinâmico de turbinas eólicas equipadas com gerador síncrono com excitação a ímãs permanentes operando durante contingências na rede elétrica e com variação na velocidade do vento. Tal configuração tem muitas vantagens quando comparada com a configuração atualmente utilizada pelos principais fabricantes de turbinas eólicas de grande porte. A principal delas é devido ao fato de não utilizar caixas de engrenagens. Porém, o impacto na rede elétrica com a conexão de tal configuração deve ser minimizado melhorando as condições operativas no ponto de conexão da fazenda eólica. Desta forma, o projeto em questão visa investigar formas de diminuir os impactos negativos causados e melhorar a estabilidade do sistema, principalmente, no ponto de conexão à rede elétrica Portanto, o projeto de pesquisa deste mestrado está relacionado ao desenvolvimento de modelos computacionais que representem tal tecnologia, considerando os principais componentes e controles utilizando o software de simulação PSCAD. Também são incluídos novos métodos de controle e elementos adicionais que melhoram a confiabilidade e o nível de penetração de parques eólicos com geradores a ímãs permanentes. Ressalta-se que tais modelos não estão disponíveis de forma completa no software citado acima, portanto foram desenvolvidos partindo de modelos genéricos existentes. Todas as partes do modelo foram realizadas com elementos básicos acessíveis em qualquer programa de simulação, de modo que qualquer engenheiro ou pesquisador possa programar este mesmo modelo em qualquer software. Esta dissertação começa com uma breve descrição do modelo desenvolvido, incluindo o modelo aerodinâmico das turbinas eólicas, o controle do conversor do lado do gerador, o controle do conversor do lado da rede e o sistema de proteção do link de corrente contínua. Com o modelo desenvolvido, analisa-se a operação do gerador síncrono a imãs permanentes durante contingências na rede. A aplicação de um sistema avançado de armazenamento de energia e a possibilidade de suporte de frequência à rede elétrica também foram desenvolvidos e analisados. Considerando os resultados obtidos, podemos verificar que a conexão de parques eólicos equipados com geradores síncronos com excitação a ímãs permanentes pode trazer benefícios para a rede elétrica, quando o seu potencial de controle e de operação é utilizado. Mais detalhes podem ser verificados nos capítulos a seguir. / This master\'s project aims to study the behavior of wind turbines with Permanent Magnet Synchronous Generator, operating during contingencies and changes in wind speed. This configuration has many advantages compared with configurations currently used by leading manufacturers of large wind turbines. The main one is due to the fact of use smaller gearboxes or even gearless turbines. However, the impact on the electrical grid connection of this configuration should be minimized by improving the operating conditions at the connection point of the wind farm. Thus, this project aims to investigate ways to reduce the negative impacts and improve the system stability, especially at the grid connection point. This work employs the simulation software PSCAD for developing a computational model of this technology, considering the main components and controls. New control methods and additional features are included, for improving the reliability and the level of penetration of wind farms with permanent magnet generators in the power system. It is noteworthy that such models are not available in the simulation software PSCAD; therefore it was developed from existing generic models. All parts of the model were done with basic elements; as a result any engineer or researcher can program this same model in any software. This work begins with a brief description of the developed model, including: the aerodynamic model of the wind turbines, the control of the generator side converter, the control of the grid side converter and the protection system of the Link-DC. With this model, the operation of the permanent magnet synchronous generator was analyzed during network contingencies. The application of a sophisticated energy storage system for frequency support was also both developed and analyzed. The connection of wind turbines with Permanent Magnet Synchronous Generator has advantages when its potential for control and operation is used. More details can be checked in the following chapters.