Electrical Power Systems

Synchronous Machine: Steady State and Transient Operations 79
Synchronous Machine: Steady State
and Transient Operations
4.1 INTRODUCTION
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One of the important elements of a powers system is the synchronous generators, which greatly
influences the system behaviour during both steady-state and transients conditions. The
synchronous machine model in steady-state and transient conditions are presented in this
chapter.
4.2 SYNCHRONOUS GENERATOR
Synchronous generator is the most important component of a power system. It converts
mechanical power into electrical form and feeds it into the power network. Large scale power
is generated by three-phase synchronous generators (alternators), driven either by steam
turbines, hydro turbines or gas turbines. The armature windings are placed on the stator
(stationary part). The armature windings are designed to generate balanced three-phase voltages
and are arranged to develop the same number of magnetic poles as the field winding that is on
the rotor.
The field is placed on the rotor and requires a relatively small power (0.2–3 per cent of the
machine rating) for its excitation. The rotor is also equipped with one or more short-circuited
windings known as damper windings. The rotor is driven by a prime mover at constant speed
and its field is excited by direct current. The generator excitation system maintains generator
voltage and controls the reactive power flow.
Synchronous machine rotor may be of cylindrical or salient construction. The cylindrical
type of rotor (round rotor) has one distributed windings and a uniform air gap. There generators
are driven by steam turbines and are designed for high speed 3000 or 1500 rpm (two and four
pole machines respectively with frequency 50 Hz) operation. The rotor of these type of generators
has a large axial length and small diameter to restrict the centrifugal forces. Nearly 65 to 70%
percent of large synchronous generators are cylindrical rotor type ranging from about 100 MVA
to 1500 MVA. A salient pole synchronous-machine is distinguished from a round rotor machine
by constructional features of field poles which project with a large interpolar air gap (non
uniform air gap). It has a relatively large number of poles, short axial length and large
diameter. The generators in hydroelectric power stations are driven by hydraulic turbines and
they have salient-pole rotor construction.