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Nonlinear control applied to power systems

When large disturbances occur in interconnected power systems, there exists the danger
that the power system states may leave an associated region of stability, if timely corrective action
is not taken. Open-loop remedial control actions such as field-forcing, line-tripping, switching of
series-capacitors, energizing braking resistors, etc., are helpful in reducing the effects of the
disturbance, but do not guarantee that the post-fault power system will be stabilized. Linear
controllers are widely used in the power industry, and provide excellent damping when the power
system state is close to the equilibrium. In general, they provide conservative regions of stability.
This study focuses on the development of nonlinear controllers to enhance the stability of
interconnected power systems following large disturbances, and allow stable operation at high
power levels.
There is currently interest in the power industry in using thyristor-controlled series-capacitors
for the dual purpose of exercising tighter control on steady-state power flows and
enhancing system stability. This device is used to implement the nonlinear controller in this
dissertation. A mathematical model of the power system controlled by the thyristor-controlled
series-capacitor is developed for the purpose of controller design.
Discrete-time, nonlinear predictive controllers are derived by minimizing criterion
functions that are quadratic in the output variables over a finite-horizon of interest, with respect to
the control variables. The control policies developed in this manner are centralized in nature. The
stabilizing effect of such controllers is discussed. A potential drawback is the need to have large
prediction horizons to assure stability. In this context, a coordinated-control policy is proposed, in
which the nonlinear predictive controller is designed with a small prediction horizon. For a class of
disturbances, such nonlinear predictive controllers return the power system state to a small
neighborhood of the post-fault equilibrium, where linear controllers provide asymptotic
stabilization and rapid damping. Methods of coordinating the controllers are discussed. Simulation
results are provided on a sample four-machine power system model.
There exists considerable uncertainty in power system models due to constantly shifting
loads and generations, line-switching following disturbances, etc. The performance of fixed-parameter
controllers may not be good when the plant description changes considerably from the
reference. In this context, a bilinear model-based self-tuning controller is proposed for the
stabilization of power systems for a class of faults. A class of generic predictive controllers are
presented for use with the self-tuning controller. Simulation results on single-machine and
multimachine power systems are provided. / Graduation date: 1995
Date05 August 1994
CreatorsVedam, Rajkumar
ContributorsMohler, Ron
Source SetsOregon State University
Detected LanguageEnglish

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