Voltage stability assessment in power systems by using pseudo thevenin equivalent parameters

Stability continues to be one of the major problems in growing power systems in consequence of the increment in demand for electrical energy. In addition to the well-known angle stability problem, voltage stability has also been faced as an increasingly important problem in recent years and a number of scientific studies has been conducted. In terms of voltage stability investigations, the number of unknown parameters increases and the analysis of the system becomes more complex due to the growth in power system. Since transmission systems are dynamic systems, it is not possible to predict at which load the instability limit will be reached. For this reason, it is necessary to monitor the system in real time and evaluate its stability status instantly, in order to operate the system in a stable state. In order to be able to monitor the bus of interest of the power system real time, it is necessary to know the Thevenin Equivalent circuit parameters seen from the relevant bus of the system. In order to obtain the equivalent circuit parameters, in a system with all parameters known, two bus equivalent of the system can be used by utilizing the bus reduction method, all the elements except the bus of interest and slack bus are added in the bus admittance matrix as passive elements.This is a method that requires all system data as well as a lot of processing power. With the recently used phasor measurement units, it is possible to take measurements from different busses of the system and obtain information about the equivalent circuit parameters of the relevant bus in the system, however, there are difficulties in processing a large number of data synchronously. In this thesis study, the Thevenin equivalent circuit of the bus to be analyzed for voltage stability in the power system has been estimated with the optimization approach using local bus measurements and a constant impedance angle. The equivalent circuit parameters obtained from the estimation represent the power system with sufficient accuracy to make voltage stability evaluation from the related bus. The Thevenin equivalent circuit parameter sets representing the power system with sufficient accuracy are so called Pseudo Thevenin Equivalent Circuit Parameters (STEDP). The Pseudo Thevenin Equivalent circuit parameters were estimated with the sequential voltage, current and power factor measurements made from the related bus, and the critical voltage of the related bus was calculated instantaneously. With the calculated critical voltage and bus voltage, the voltage stability index has been calculated instantaneously and real-time information about the stability status of the bus has been obtained more easily to understand the stability state of the bus. The proposed method is applied to different load buses of IEEE 9 and IEEE 30 bus test systems. Conclusively, it was observed that the voltage stability limits obtained from the proposed method coincided with the voltage stability limits obtained from the power flow analysis.