Fekadu Shewarega (Asso. Professor)Ahadu Hilawie2024-10-282024-10-282024-09https://etd.aau.edu.et/handle/123456789/3512Voltage security issues continue to be the concerns of power systems as the stress on power systems increases due to increasing energy demand. To manage this stress and prevent voltage security problems, online voltage security assessment (VSA) and reactive power management (RPM) strategies are coming to play crucial role. However, selecting suitable approach and developing one’s own method requires rigorous assessment of the gaps in existing approaches and devising a strategy to fill the gaps. In this regard, this dissertation work aims at developing VSA and RPM schemes for online application to help the efforts made to mitigate the increasing voltage security problems.The dissertation has two major components, voltage security assessment (VSA) scheme development and reactive power management (RPM) scheme development, as separate entities and as complementary entities. The work focuses on designing the components of the schemes by devising new and improved contents of each scheme. The developed VSA scheme performs three major tasks; estimating the network Thevenin equivalent impedances, determining the voltage stability indices and interpreting the results of voltage stability indices. Computational efficiency improving strategies, which are necessary for online application, are adopted at different levels of the VSA scheme. This begins from selecting the method of voltage security assessment, i.e. Thevenin equivalent based approach. Then, to address the limitation of previous Thevenin equivalent determination techniques this work comes up with a new Thevenin impedance determination technique. The developed Thevenin impedance determination technique requires two power flow computations in offline analysis case and only one power flow analysis in the case of online estimation. For the voltage stability assessment task of the VSA scheme two types of voltage stability indices are formulated, which measure proximity to voltage instability, one directly using the power margin and the other indirectly as a simple closeness indicator. The third capability of the VSA scheme is PV and QV curve plotting capability for interpreting the process of development of voltage security problems. The Thevenin equivalent determination capability, greatly, simplifies the maximum power transfer capacity estimation and PV or QV curve determination, which was previously a computationally intensive task.The capabilities of the VSA scheme are tested using simple four bus test system, IEEE 14 bus and IEEE 30 bus test systems. The tests produced results meeting the objectives of producing high accuracy Thevenin parameters, tracking system loading changes, identifying weakest buses, showing the impact of reactive power compensation and showing impact of load increments. Then the scheme is applied to existing Ethiopian Electric Power (EEP) system to examine the performance on large power systems. The application of VSA scheme to EEP system revealed a number of important features of the EEP system pertinent to voltage security, including weakest areas, weakest buses and voltage instability contributing lines. The other scheme, the reactive power management scheme, depends on the results of VSA process. In this case, two approaches are devised for reactive power management purpose. The first is fast reactive power management (FRPM) approach and the second is continuous reactive power management (CRPM) approach. FRPM is proposed considering contingent operating conditions. In this approach the voltage stability indices are used as an indicator of voltage security improvement, while reactive power provisions are made. Reactive power provisions cease when the weakest load buses get far enough from voltage instability. On the CRPM side the objective is to adress the optimization needs of reactive power provisions. In this approach, an improved multi objective particle swarm optimization (IMOPSO) algorithm is proposed. Together with voltage deviation objective function, the algorithm uses the indices developed on the VSA scheme for the multi objective function formulation. In this algorithm, the common multi objective particle swarm optimization (MOPSO) is improved by introducing an adapted binary crossover (ABX) to the new positions obtained by the basic PSO algorithm. Additionally, diversity maintenance strategy is added to the algorithm by employing crowding distance (CD) computation. The developed algorithm is, then, tested and compared with standard MOPSO and NSGA II algorithms. The comparison is made based on the degree of closeness to the true pareto front, as measured by the inverted generational distance (IGD), and based on diversity, as measured by the CDs. The test is made using ZDT1, ZDT2, and ZDT3 common test functions. The IMOPSO showed improved performance over MOPSO and NSGA II algorithms in terms of convergence to the true pareto front and in terms of the speed of convergence as well as in maintaining diversity. The algorithm is then implemented to reactive power optimization of IEEE 14 bus test system and the EEP system. This implementation has resulted diverse options of optimal settings of reactive power controlling parameters. The optimal settings proved to produce an improved voltage security as measured in terms of voltage deviation and voltage stabilityen-USTerms--Thevenin impedance determinationonline voltage stability assessmentimpedance matchingvoltage stability indexDissertation