Analysis of the impact of closed-loop power flow control strategies on power system stability characteristics.
The demand for electrical energy in industrialised countries continues to increase steadily. As a result of this growing demand for electrical energy, there is a need for optimisation of the power system in terms of transmission and control. One option could possibly be an increase in transmission facilities to handle the increase in growth; however factors such as environmental issues as well as the possible cost incurred could hamper this particular approach. An alternative resides in loading the existing transmission network beyond its present operating region but below its thermal limit, which would ensure no degradation of the system. For this approach to be realised, improved control of the flow of power in an interconnected network would be advantageous so as to prevent unwanted loop flows and inadvertent overloading of certain lines. This approach can be made possible by the use of Flexible AC Transmission Systems (FACTS) technology. The concept of FACTS incorporates power-electronic compensation devices that can be typically used in an ac power system to enhance the system's power transfer and controllability. There exists a number of FACTS devices, where each device can be utilised differently to achieve the broad objective. One such device is the Thyristor Controlled Series Capacitor (TCSC). The TCSC is a class of FACTS device that makes it possible to alter the net impedance of a particular transmission line in an effort to force the flow of power along a "contract path". This thesis identifies, in the published literature, a set of strategies for the scheduling of power flow by use of variable compensation; such strategies are then considered in more detail in the analysis of the thesis. Firstly, a detailed dynamic model of a TCSC is developed together with its various controls and associated circuitry within the power systems simulation package PSCAD. In addition to this, a power flow controller scheme is then implemented, which exhibits the functionality of the power flow controller strategies reviewed in the literature. In order to test the validity and operation of the TCSC model as well as the analysis of the power flow controller scheme, a single-machine infinite bus (SMIB) study system model is developed and used as part of the investigation. This thesis, firstly, presents a theoretical analysis of two particular modes of power flow control in an interconnected ac transmission system. Secondly it confirms the results of an analytical study in previously published work with the implementation of the two control modes, and further extends the scope of the previous study by examining the impact of the power flow controller's design on the small-signal and transient stability characteristics of the study system. The key findings of this extended investigation are that the power flow controller's mode of operation has an important influence on both small-signal and transient stability characteristics of a power system: in partiCUlar, it is shown that one mode can be detrimental while the other beneficial to both system damping and first swing stability. Finally, the thesis applies the understanding of the power flow controller's operation obtained from the SMIB study system to the problem of inter-area mode oscillations on a well-known, two-area, multi-:generator study system. Real-time simulator results are presented to exhibit the effect of the power flow controller modes and controller design on the oscillatory characteristics of the two-area study system.