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Investigation of Voltage Stability in Three-Phase Unbalanced Distribution Systems with DGs
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Smart grids draw lots of attention and interests and they are fundamentally changing traditional power grids. One of the key aspects of smart grid is that more distributed generators (DGs) are connected in distribution systems. Distribution systems have changed from passive to active. Stability problems become important issues, one of which is voltage stability problems. To analyze voltage stability problems, many methods are proposed for transmission systems. However, because distribution systems are very different from transmission system, the methods for transmission systems cannot be directly applied to distribution systems. Therefore, effective methods of analyzing voltage stability problems for distribution systems are needed. The main focus of this dissertation is on three-phase unbalanced distribution systems with DGs. Firstly, improvements were made to an existing three-phase continuation power flow (CPF) method so that the maximum loading factor of distribution systems can be found accurately. Various distribution system components and DGs in PQ mode and PV mode with reactive power were modeled. Comparisons with Matpower software were made to validate the correctness of the implemented three-phase CPF program. Secondly, to provide more detailed voltage stability analysis and determine the weak buses of distribution systems, a new voltage stability analysis method, the CPF scan method, was proposed. The weak buses found by this method are the buses that have higher impact on the maximum loadability or the maximum total real load power that the system can support. Extensive case studies were performed and the impact of different distribution components were investigated. Lastly, to determine whether a distribution will experience voltage stability problems and to determine the weak buses, a measurement-based three-phase voltage stability index was proposed. This voltage stability index provides not only a system-wide index but also an individual index for each bus/phase. These proposed methods were applied to 8-bus system and a modified IEEE 13-node test feeder with DG to study the performance of the methods and investigate the impact on weak buses of different factors in distribution systems. The case studies showed that the proposed two methods, CPF scan and VSI, can successfully identify the impact of certain distribution system components. For more complicated components, such as untransposed lines and DG in PV mode, more research is needed. Also the CPF scan results shows good applications to distribution system operation and planning. The applications of the new proposed methods are not limited to identifying the weak buses. These methods have a great potential to be extended to voltage stability preventive and corrective control.
Chou, Hung-Ming (2016). Investigation of Voltage Stability in Three-Phase Unbalanced Distribution Systems with DGs. Doctoral dissertation, Texas A & M University. Available electronically from