京都大学 / 新制・課程博士 / 博士(エネルギー科学) / 甲第23997号 / エネ博第433号 / 新制||エネ||82(附属図書館) / 京都大学大学院エネルギー科学研究科エネルギー社会・環境科学専攻 / (主査)教授 石原 慶一, 教授 白井 康之, 准教授 尾形 清一 / 学位規則第4条第1項該当 / Doctor of Energy Science / Kyoto University / DFAM

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Due to the rapid growth of grid-tied solar photovoltaic (PV) systems in the generation mix, the distribution grid will face complex operational challenges. High PV penetration can create overvoltages and voltage fluctuations in the network, which are major concerns for the grid operator. Traditional voltage control devices like switched capacitor banks or line voltage regulators can alleviate slow-moving fluctuations, but these devices need to operate more frequently than usual when PV generation fluctuates due to fast cloud movements. Such frequent operations will impact the life expectancy of these voltage control devices. Advanced PV inverter functionalities enable solar PV systems to provide reliable grid support through controlled real injection and/or reactive power compensation. This dissertation proposes a voltage regulation technique to mitigate probable impacts of high PV penetrations on the distribution voltage profile using smart inverter functionalities. A droop-based reactive power compensation method with active power curtailment is proposed, which uses the local voltage regulation at the inverter end. This technique is further augmented with very short-term PV generation forecasts. A hybrid forecasting algorithm is proposed here which is based on measurement-dependent dynamic modeling of PV systems using the Kalman Filter theory. Physical modeling of the PV system is utilized by this forecasting algorithm. Because of the rise in distributed PV systems, modeling of geographic dispersion is also addressed under PV system modeling. The proposed voltage regulation method is coordinated with existing voltage regulator operations to reduce required number of tap-change operations. Control settings of the voltage regulators are adjusted to achieve minimal number of tap-change operations within a predefined time window. Finally, integration of energy storage is studied to highlight the value of the proposed voltage regulation technique vis-à-vis increased solar energy use. / Ph. D. / Rapid growth of grid-tied solar photovoltaic (PV) systems poses both opportunities and technical challenges for the electric distribution grid. Significant among them are overvoltage and voltage fluctuations in the network, which may lead to overheating of electrical devices and equipment malfunction. Due to the variable nature of solar irradiance, existing voltage control devices often need to operate more frequently than usual which can cause recurring maintenance needs for these devices. To make solar PV more grid-friendly, changes are taking place in grid codes which encourage developing advanced PV inverter functions. With these functions, a smart inverter, which possesses bidirectional communication capability, can be integrated into a smart grid environment. This work discusses how these inverters can provide active power curtailment and reactive power compensation to maintain voltages at their points of interconnection. The inherent variability and uncertainty in solar energy production can be addressed with solar forecasting. Application of PV generation forecasting as a tool to aid distribution voltage control is proposed in this dissertation. Using solar forecasting, smart inverters can contribute in relieving the stress on other voltage control devices due to PV-induced fluctuations. Integrating storage elements can also aid this voltage regulation method, as they can consume surplus PV generation when needed. This dissertation is designed to provide a systematic approach to manage the overvoltage and voltage fluctuations on a real-time basis for a high PV penetration scenario. Proposed methodology combines smart inverter functionalities with solar forecasting and develops an application which can be realized to ensure seamless PV integration in a growing landscape of renewables.

For planning issues, it is useful to know the upper limit for photovoltaics (PV) in the electrical grid with current design and operation (defined as hosting capacity) and how this limit can be increased. Future costs for grid reinforcement can be avoided if measures are taken to implement smart grid technology in the distribution grid. The aim of this project is to identify challenges in Swedish electricity distribution grids with a high penetration of local generation of electricity from PV. The aim is also to help Swedish Distribution System Operators (DSOs) to better understand hosting capacity issues, and to see which room for PV integration there is before there is need for actions to maintain power quality. Three distribution grids are modelled and simulated in Matlab: Rural area, Residential area and City (Stockholm Royal Seaport). Since the project is a cooperation between Uppsala University and Fortum, three different representative grids from Fortum’s grid software ”Power Grid” have been used as input to a flexible simulation program developed at Uppsala University. The simulation includes Newton-Raphson power-flow computing but has also been improved with a model of the temperature dependency of the resistance. The results show that there is room for a lot of PV systems in the Swedish grids. When using voltage rise above 1.1 p.u. voltage as limitation, the hosting capacity 60% PV electricity generation as a fraction of the yearly load were determined for the rural grid and the suburban grid. For the city grid, which is very robust, the hosting capacity 325% was determined. When using overload as limitation, the hosting capacities 70%, 20% and 25%, were determined for the same grids.