Planning and Operation of Large Amounts of Wind Power in a Distribution System

Doktorsavhandling, 2015

The global installed capacity of wind power has shown a significant growth, from 24 GW in 2001 to 370 GW in 2014. The trend shows that an increasing capacity of wind power is being connected to the electric power system. Due to lower costs associated with the connection of wind power to distribution systems, their wind power hosting capacity needs to be fully exploited. The hosting capacity of distribution systems is limited due to a number of issues such as voltage flicker and harmonics, overvoltage and thermal overloading, and increased fault level. A further issue that deserves attention is that the effect of wind power on the frequency of tap changes (FTC) of the substation transformer due to highly fluctuating nature of wind power. Thus, the thesis investigates these integration issues of wind power and identifies the limiting factors. Then, the thesis proposes mitigation solutions so as to maximize the hosting capacity of a distribution system. Moreover, the thesis investigates the control and coordination of different mitigation solutions. The investigation of the effect of wind power on the FTC shows that the change on the FTC in a distribution system connected to an external grid with X/R ≥5 is negligible up to a significant level of wind power penetration. However, in a distribution system connected to an external grid with lower X/R ratio, a significant increase in the FTC has been observed as wind power penetration increases. This issue has been effectively mitigated by using reactive power from the wind turbines. Furthermore, the thesis identifies voltage rise and thermal overloading as the two main limiting factors of wind power integration into distribution systems. Thus, active management strategies (AMSs)--such as wind energy curtailment (WEC), reactive power compensation (RPC), and coordinated on load tap changer (OLTC) voltage control-- have been investigated in the thesis to increase the wind power hosting capacity of distribution systems. To facilitate the investigation, an optimization model whose objective function is to maximize the profit gained by the distribution system operator (DSO) and the wind farm owner (WFO) is developed. The results of the analysis show that by using AMSs the wind power hosting capacity of a distribution system can be increased up to twice the capacity that would have been installed without AMSs. Further wind power installation calls for grid reinforcement as a preferred strategy. In order to implement these AMSs, the system states, i.e. bus voltages, need to be known. Thus, state estimation is proposed as a cost efficient way of obtaining information about the system states. The investigation of the state estimation (SE) algorithms for distribution system applications has identified that the node-voltage-based weighted least square SE algorithm is more appropriate in a system where only few real time measurements are available. Moreover, the analysis of measurement types and locations has shown that power injection and voltage measurements from wind turbine sites provide cost-effective solution with superior SE accuracy. Consequently, the control of the OLTC is carried out by relaxing the deadband of the automatic voltage control (AVC) relay so that the AVC relay acts on the network’s maximum or minimum voltage obtained through the SE. This is found to be simpler to realize than adjusting the set point of the AVC relay. Voltage control through RPC and WEC as well as overload mitigation through WEC is actualized by using integral controllers implemented locally at the wind turbine site. Furthermore, RPC from the local wind turbine is also used to mitigate an overvoltage at a remote bus on the same feeder when the remote wind turbine reaches its regulation limit. The coordination of voltage regulation between RPC and WEC is achieved by using slightly varying reference voltages for each controller and they are in turn coordinated to the SE-assisted OLTC voltage control by using time delays. The different control and coordination strategies proposed in the thesis are successfully demonstrated using an actual distribution network, measured load, and wind power data.