Environmental Engineering Reference
In-Depth Information
This summary indicates that all renewables, except PV systems, rely on electromechanical
generators for the fi nal stage of conversion from mechanical into electrical energy. This
chapter introduces the principles of operation of two classes of electrical generators, the
'synchronous' and the 'asynchronous' types, both used extensively in RE applications.
Additionally, this chapter deals briefl y with the principle of operation of the transformer, a
ubiquitous device in multivoltage level power systems. Understanding the operation of the
transformer is a necessary prerequisite for the study of the 'asynchronous' type of
generator.
Power electronics plays a vital role in PV and an increasingly important role in the wind
power area. A review of power electronic devices and the converters based on them is covered
in the penultimate section. Finally, the chapter concludes with a description of how electro-
mechanical and/or power electronic converters are used in PV and wind systems.
In what follows a symbol written in regular type indicates that the parameter is a scalar
while bold type is used if it is a vector, phasor or a complex number.
4.2 The Synchronous Generator
4.2.1 Construction and Mode of Operation
In an electrical generator, mechanical input power is converted into electrical output power.
To get an appreciation of how this energy conversion process is carried out it is useful to
look briefl y at the underpinning physics. Faraday's law of electromagnetic induction [1] states
that a conductor of length l (m) moving with a velocity u (m/s) through a magnetic fi eld of
uniform fl ux density B (Tesla), l, u and B being mutually perpendicular, will experience an
induced voltage across it given by
(
)
vB u
=
volt
(4.1)
Nature is such that this mechanical ( u ) to electrical ( v ) conversion process described by
Equation (4.1) is 'mediated' through the presence of a magnetic fi eld ( B ). The equation shows
that to generate a high and therefore useful voltage it is necessary to have a high magnetic
fl ux density, a long conductor length and as high a conductor velocity as possible. All the
above requirements are particularly well satisfi ed if the mechanical - magnetic - electrical
structure of the generator is arranged as a rotating rather than a linear one. In practice it has
been found that it is preferable to have the conductors stationary and move the source of the
magnetic fi eld. With a stationary set of conductors the problems of insulation and electrical
connections are eased and centrifugal forces on the main windings are absent.
Figure 4.1(a) shows in outline an AC generator also known as an alternator or synchronous
generator . Here the conductors that form a winding, known as the stator , are stationary and
the source of magnetic fl ux rotates. The source of fl ux is a rotor with poles marked north and
s outh that carries a fi eld winding as shown in the fi gure. The fi eld winding is fed or excited
from an external DC source through sliding contacts known as slip - rings . An additional
reason why it is preferable to arrange the main windings to be stationary is that the DC power
associated with the fi eld is a small fraction of the power delivered by the stator winding. In
fact, in some designs the fi eld winding can be dispensed with completely and replaced by a
Search WWH ::




Custom Search