Page 48 - Offshore Electrical Engineering Manual

P. 48

CHAPTER

Alternating Current 2

Synchronous Generators

INTRODUCTION
The intention here is not to produce yet another alternator theory text, but to provide
the reader with some practical guidance on the installation and operation of alternat-
ing current (AC) generators offshore.
AC generators on ships and offshore installations are of the normal commercially
available type of construction, with the field system in the rotor, either of salient-
pole or cylindrical rotor design, and the armature windings in the (stationary) frame.
There are still some slip ring excitation systems in use offshore, but by far the major-
ity are now of brushless design, avoiding the need for regular brush replacement and
the buildup of carbon dust.
Note: wind turbine generators are not normally synchronous machine designs
and are covered in PART 6 Chapter 6.

PRINCIPALS OF OPERATION
Referring to the following phasor diagrams, the terminal voltage V is taken as the
phase reference. The resistive and reactive volt drops are then added, giving the open
circuit voltage or induced electromotive force E.
Hence this gives the equation E = V + I R + jI X , where E, V and I are pha-
S
A A
A
sors rotating at the synchronous speed and the subscripts A and S indicate armature
and synchronous values. The angle δ is the load angle of the machine (see also Fig.
2.2.3). I R and jI X are volt drops due to losses, where the synchronous reactance
A A
S
A
X is a combination of armature winding leakage reactance and a reactance used to
S
represent armature reaction.
The three diagrams in Fig. 2.2.1 correspond to the lagging, unity and leading
power factor conditions, respectively.
From the design viewpoint, the size of the generator is governed mainly by the
kilovolt-ampere rating and rotating speed (i.e., number of poles required). However
the actual size used offshore will also depend to some extent on the prime mover rat-
ings commercially available, as well as the reactive power required for the starting
of large motors used for driving major oil or gas process rotating machinery such as
export gas compressors. Oversising of the generator in relation to the prime mover is
common in oil installations because the increased copper and excitation losses of the

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