Control of wind integrated power grid using voltage controllability and vulnerability detection
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Abstract
Modern power grids are the largest and most complex engineered systems. Integration of distributed resources like wind farms and economic constraints push power grids to operate close to their limits. Due to this, power system is subject to various stability threats. Voltage stability is one of the most prominent issues that constraints modern power grid. It is directly linked to the reactive power balance in an area. Generators are the prominent source of reactive power. Besides generators, various other reactive power sources like capacitor and reactor banks are used throughout the grid for reactive power support. In a modern power grid with high penetration of renewable energy, reactive support from sources such as wind farms and PV farms are becoming mandatory. Similar to the excitation controller system in conventional synchronous machine, terminal voltage and reactive power generation is regulated by converter controller system in the renewable sources. These controllers regulate reactive power within their reactive power capability limit for voltage control based on the reference voltage set points. In the North America, generally, operators switch on/off additional capacitor/reactor banks instead of changing set points of generator excitation controllers to adjust the mismatch of reactive power generation and consumption in an area. Lack of access of generator controls to system operators and local nature of reactive power flow are the common arguments provided for this practice. Access of system operators to the generator controls can be improved in the North America as a large number of utility operators in North America owns both transmission and generation system. Hence, the system operators can provide a priority to generators for reactive power and voltage control before switching additional reactive power sources. This will be easier and economically beneficial as generators will be used to its full capacity instead of additional sources. With the advancement of communication technology and measurement units, the generator excitation controller and converter controllers is not only limited to terminal voltage control but used for system wide voltage control as well. Similar to hierarchical levels of frequency control, a hierarchical voltage control architecture with primary, secondary and tertiary voltage control has been proposed in some European countries. The primary voltage control is the voltage control in local sense while the secondary level voltage control is based on areas or zones. Similar to this methodology, a voltage control area concept has been introduced to support reactive power requirements within the areas or zones of power grid where the generators are given priority before switching additional reactive power sources. This is much more meaningful and important when more renewable energy resources are introduced to the grid as these sources will be distributed throughout a grid. This dissertation proposes a method for online identification of VCA based on reactive power sensitivities. Then for voltage control action the bus prone to voltage instability is identified. Voltage stability index (VSI) utilizing maximum loadability margin is proposed to monitor and detect vulnerable buses. Area for voltage control action is decided by identifying an area belonging to the vulnerable bus detected using VSI. Further, a secondary voltage control scheme based on the participation factor of generators in the VCA is proposed for the voltage control of the grid. The effectiveness of proposed technique is demonstrated using IEEE 39 test system and modified IEEE 39 wind power test system.