Firstly, this dissertation proposes a new characterization and classification method for daily sky conditions by using the daily sky clearness index (KD) and the daily probability of persistence (POP-KD) that can be derived from ground-based irradiance measurement data. Quality of daily solar irradiance is characterized by a newly proposed parameter, POP-KD. This characterized daily quality is varying and uncertain at the middle level of the quantity, but high and more certain at very high and low quantity levels. In addition, the proposed characterization method shows interesting results for KD and POP-KD: a statistical consistency for multiple years and similarity for their seasonal trends. The classification results also indicate an existence of dominant classes, and transitions between the dominant classes are significant for all locations. This dissertation also generates annual synthetic sequences of KD and POP-KD using a Markov approach. The generated sequences show statistical similarities with observed sequences. Secondly, this dissertation proposes methodologies to estimate day-ahead solar irradiance using the National Weather Service (NWS) sky cover forecast. For model development, this paper splits up a direct estimation process from the sky cover forecast to solar irradiance into two stages: forecast verification and cloud-to-irradiance conversion. Uncertainty for each stage and for the overall estimation process is quantified. NWS forecast uncertainty (about 20%) is identified as the main source of uncertainty for the overall process. In addition, verification of the sky cover forecast shows approximately 20% overestimated bias at days with a high irradiance level. Thus, the NWS sky cover forecast needs to be adjusted based on the type of day. This dissertation also proposes a conversion equation relating daily quantity of cloud information and daily quantity of solar irradiance. The proposed conversion equation achieves accuracy with simplicity. Five day-ahead solar irradiance quantity estimation methods are proposed in this dissertation. The proposed methods incorporate different schemes for dealing with the bias discovered in the cloud forecast. The observed data are regularly found within the 95% confidence intervals of the estimated values. Estimation results demonstrate the effectiveness of the conditional adjustment schemes at different irradiance levels. Lastly, this dissertation proposes a methodology to estimate day-ahead solar irradiance using fluctuation information of the NWS sky cover forecast. POP-KD was used as a parameter for the quality of daily solar irradiance. POP-KD efficiently represents the quality of daily solar irradiance. In addition, POP-KD indicates the probability that solar irradiance variability is within the ramp rates of common generators in power systems at a certain photovoltaic penetration level. This dissertation also proposes a new equation for the conversion from cloud fluctuation information to daily quality of surface solar irradiance. The proposed equation achieves accuracy. The proposed day-ahead solar irradiance quality estimation method is based on fluctuation information provided by the NWS sky cover forecast. This method uses a normalization approach to relate fluctuation of cloud forecast and fluctuation of cloud observation. The observed data are regularly found within the 95% CIs of the estimated values. / Ph. D.

The power grid is a complex system that requires detailed planning and operation of the power sources. As renewable resources are increasing their share of the power mix, there are new criterion and aspects to take into account. Renewable resources such as wind and solar power are intermittent and challenge the planning and operation further. Simultaneously, the power demand increases and will continue to increase in the future. As the power resources and demand are changing in the power system, the power grid needs to adapt and be more flexible. A flexibility tool for the power system is Dynamic Rating which can dynamically increase the capacity in the power grid components depending on the environmental conditions. This can enable for higher transmission in overhead lines which facilitates the operation planning of power units. Dynamic rating can be applied to various power system components, but the focus in this project is Dynamic Line Rating (DLR). It is interesting to study the implementation of dynamic rating in the realtime market. The real-time market optimal power flow can be done in singleinterval or multi-interval. In this project, dynamic rating is implemented into multi-interval real-time market optimal power flow to investigate the effect on the generation costs. The results indicate that DLR can reduce the generation costs in power grids that are sometimes congested. / Kraftnätet är ett komplext system som kräver detaljerad planering och verksamhet av energikällor. Allt eftersom andelen förnybara energikällor ökar tillkommer nya aspekter att ta hänsyn till. Förnybara energikällor såsom vindkraft och solenergi har en mer ojämn produktion då de är beroende av vädret, vilket utmanar drift och planering av elproduktion mer än tidigare samtidigt som elbehovet ökar och kommer att fortsätta öka i framtiden. Det växande elbehovet och utökningen av förnybara energikällor kräver att elnätet kan anpassas och vara flexibelt. Ett sätt att öka flexibiliteten är att värdera kapaciten i transmissionsledningar dynamiskt (DLR) istället för statiskt där kapaciteten beräknas efter värsta tänkbara scenario. Dynamisk värdering kan därför möjliggöra högre kraftöverföring i ledningarna under gynsamma väderförhållanden vilket underlättar driften och planeringen av kraftkomponenterna. I detta examensarbete studeras dynamisk värdering av transmissionsledningar i realtidselmarknaden med flera intervall (MIRTM) där driftplaneringen optimeras så att driftkostnaderna minimeras under bivillor för kraftsystemet. Målet är att undersöka hur driftkostnaderna påverkas då bivillkoret för kraftflödet i transmissionsledningarna är mer flexibelt med DLR. Resultaten från detta projekt visar att DLR kan minska kostnaderna i elnät som är överbelastade.

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