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.