Development Of Nonlinear Control Schemes For Electric Power System Stabilization

ABSTRACT

Power system stabilizers and other controllers are employed to damp oscillations in power systems, thereby guaranteeing satisfactory dynamic performance following major network disturbances. However, the parameters of these controllers are often tuned based on the power system linearized model which generally is a function of the system operating point or state. These controllers suffer from poor performance when the system state changes. The aim of the research work reported in this Thesis is to develop nonlinear synchronous generator excitation control schemes with control laws for providing improved transient stability when the system is subjected to wide parameter variations due to network disturbances. The study employed fourth-and third-order models of a single-machine-connected-to-an-infinite-bus system to design two nonlinear sliding mode control laws (CLs) and one finite-time homogeneous control law (CL), which were constructed based on a well-chosen output function of the system. The parameters of the control laws were properly selected and/or tuned to give desirable dynamic characteristics using well established linear control methods. Justifications for the selection of the fourth-and third-order synchronous generator models to design the aforesaid controllers are presented. Dynamic simulations of the system under the action of the control laws were carried out using MATLAB® /SIMULINK. In order to test the performance of the laws, several simulation studies were performed when the voltage magnitude (V) of the infinite bus and the transmission line reactance (XE) of the system changed due to an applied three-phase symmetrical fault at the infinite bus and generator terminals. Results obtained from these studies show that the dynamic characteristics of the system being investigated have improved significantly, in terms of the rotor angle and rotor speed first peak, damping of low-frequency mechanical oscillations in rotor angle following fault clearance, and settling times of key stability indicators (rotor angle and rotor speed). For instance, for application of each of 5-cycle, 7-cycle, and 9-cycle fault at the infinite bus, the system rotor angle settled to its stable steady values within 1 - 2.2s with minimal control effort that varied between -5pu and 5pu before settling at the prefault value of 1.5603pu in 4.32s (CL1), in 1.92s (CL2), and in 3.32s (CL3). Whereas, CL3, which is a contribution to the improvement of the existing general higher-order sliding mode control structure for synchronous excitation control, was able to make the system withstand greater fault duration than CL1, CL2, which has a new positive parameter (called the dilation gain) incorporated into it, furnished the system with the greatest fault-retaining capability. In practice, the implementation of the three control laws can be carried out in a static exciter configuration with a very fast response.