Date of final exam: 18/05/2005

E-mail: verrelli@ing.uniroma2.it

Tutor: Prof.  R. Marino, Università di Roma "Tor Vergata"

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NONLINEAR CONTROL DESIGN FOR INDUCTION MOTORS AND SYNCHRONOUS GENERATORS   ___________________________________________________________________________________________________________

Advisors:

Prof.  R. Marino, Università di Roma "Tor Vergata"

Prof. P. Tomei, Università di Roma "Tor Vergata"

Summary of the thesis:

The thesis incorporates recent advances in the design of nonlinear control laws for induction motors

and synchronous generators: robust, adaptive, state or output feedback control techniques are used for

both these electro-mechanical systems which are modelled by finite dimensional, deterministic ordinary

differential equations and are possibly affected by uncertainties, such as unknown constant and time-varying

parameters.

Induction motors, which, due to their simpler construction, are more reliable and less expensive than

those permanent magnet, switched reluctance and d.c. motors are difficult to control for several reasons:

their dynamics are intrinsically nonlinear and multivariable (two control inputs and two outputs to be

controlled); not all of the state variables and not all of the outputs to be controlled may be available for

feedback; there are critical uncertain parameters such as load torque, which is typically unknown in all

electrical drives, and rotor resistance, which, due to rotor heating, may vary up to 100% during operations.

The availability of low cost powerful digital signal processors and advances in power electronics made

complex algorithms implementable even for medium- and small- size induction motors, which, in this way,

could replace currently used motors provided that high dynamic tracking performance along with high-power

effciency are achieved: this is what motivated intense research efforts in induction motor control

design.

In analogous way, transient stabilization and voltage regulation for power systems are classically difficult

control problems: all the dynamic models which have been developed for a single machine connected to

an infinite bus show an intrinsic nonlinear nature and, consequently, there are several stable and unstable

equilibrium points. Early studies aimed at determining the stability regions of desired operating conditions

by means of Lyapunov functions in order to study the effect of perturbations. In fact, sudden mechanical

and electrical perturbations may drive the system outside its stability region and force the generator to

be disconnected from the network. The transient stabilization and voltage regulation problem consists

in the design of an excitation control which keeps the generator speed close to the synchronous speed

when perturbations occur (transient stabilization) and regulates the output voltage to the corresponding

reference value in the case of permanent constant perturbations (voltage regulation). To this purpose,

linear controllers are actually employed which are designed on the basis of linear approximations around

operating conditions: only small perturbations and deviations from operating conditions can be handled.

It is clear that nonlinear controllers are required to handle the large perturbations that typically occur in

power systems.

The thesis is divided into two parts: Part I (induction motor) consists of Chapters 2, 3 and 4 while

Part II (synchronous generator) consists of Chapters 5 and 6. Chapters 2 and 3 address the problem of

controlling a speed- sensorless induction motor: the existence of a global controller is explored in Chapter

2, while a nonlinear adaptive control scheme is developed in Chapter 3. Chapter 4 is devoted to nonlinear

control design for a sensorless induction motor: an output feedback control algorithm is proposed. Chapters

5 and 6 address the problem of controlling a synchronous generator with parameter uncertainty: a nonlinear

robust adaptive transient stabilizing control is presented in Chapter 5, while Chapter 6 proposes a nonlinear

robust adaptive transient stabilizing and output regulating control algorithm.

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