| Modern power systems are usually operated near their limits for reasons such as pursuing economic benefits. But this may cause certain stability problems. Among them the voltage stability issue, which directly led to or significantly contributed to several major blackouts in recent years, has received serious concerns. To prevent a voltage collapse, efficient online voltage stability monitoring tool along with suitable voltage stability control techniques are required.This thesis proposed an online voltage stability assessment (VSA) and control method based on the concept of short circuit capacity (SCC). First, the SCC-based voltage stability analysis method and voltage stability index are introduced and further studied. An improved voltage stability index based on SCC (IVSI_SCC) is proposed, which considers the voltage dependency of the power loads. With the help of IVSI_SCC, the relationship between dynamics of electric elements and voltage instability is analyzed. Second, an optimal PMU placement (OPP) method is presented to guarant the data supply for subsequent voltage stability assessment and control. Then, an online VSA method using SCC and Thevenin equivalent is proposed. Further, an online voltage stability control (VSC) method based on IVSI_SCC and particle swarm optimization (PSO) alogrithm is given out. At last, the proposed VSA and VSC method is applied to a practical power system. The details and fruit of this thesis are as follows:(1) The flaw of tranditional SCC-based voltage stability index is studied, and then the IVSI_SCC is proposed. The effectiveness of IVSI_SCC is verified by case studies on a5-bus power system. Employing the IVSI_SCC, Moreover, the impacts of over excitation limiter (OEL) and load tap changer (LTC) to voltage instability are analyzed. It is revealed that the actions of OEL and LTC will reduce the difference between the SCC supplied from system side and the SCC required by power load, consequently accelerate the voltage collapse.(2) The OPP problem is studied and an OPP model considering the controlled islanding (CI) of power system is proposed. In this model the property of zero-injection bus is taken into account, which contributes to reduce the number of required PMUs. Meanwhile, maximizing the measurement redundancy is introduced as the additional objective, thus the global optimal OPP scheme with minimum number of PMUs and maximum measurement redundancy will be found. In addition, single PMU or line loss is also incorporated into the model. At last, case studies on several IEEE standard test systems and a large-scale practical system provide verification of the effectiveness of the presented OPP model.(3) An online VSA method is presented. To avoid the assumption for parameters and growth mode of power loads, an online load indentification method is offered firstly. Then, the Thevenin equivalent of power system based on expended admittance matrix and compensation current method is studied, which successfully includes the detailed model of generators and loads. At last, the strategy for online VSA is developed. The case studies show that the proposed method can effectively trace the variation of voltage stability margin and accurately detect the vulnerable bus.(4) An online voltage stability control method is studied. At first, the SCC-based model for online voltage stability control is established, in which the terminal voltage control of generator, reactive power compensation, and load shedding are used to maintain voltage stability for power system in the case of contingencies. Then, the PSO algorithm is employed to solve the presented control model. The effectiveness of proposed control method is verified on the IEEE39-bus system.(5) The proposed VSA and VSC method is applied to a real power system and compared to the practical Automatic Voltage Control (AVC) system. The result shows that the proposed mothod is superior to AVC in the aspect of adjusting voltage level, reducing additional power loss, and maintaining voltage stability. |
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