Power Plant Engineering

Chapter 12
Chapter 12
Electrical System
Electrical System
Electrical System
Electrical System
Electrical System
12.1 INTRODUCTION
Without having knowledge about electrical equipment. Power generation from the power plant is
difficult to understand. Hence it is necessary to have an idea about role of electrical equipment. The
purpose of this chapter is to introduce the students to the electrical equipments used in power plant. The
main electrical equipments are as follows;
(1) Generator and generator cooling
(2) Transformers and their cooling
(3) Bus bars
(4) Excitors
(5) Reactors
(6) Circuit breakers
(7) Switch board
(8) Control room equipment
12.2 GENERATORS AND MOTORS
In a generator, an e.m.f. is produced by the movement of a coil in a magnetic field. The current
produced by the e.m.f. Interacts with the field to produce a mechanical force opposing the movement,
and against which the essential movement has to be maintained. The electrical power ei is produced
therefore from the mechanical power supplied.
In a motor, we may suppose a conductor or coil to lie in a magnetic field. If current is supplied to
the coil; a mechanical force is mani-fested and due to this force the coil will move. Immediately that
relative movement takes place between coil and
field, however, an e.m.f. is induced, in opposition
to the current.
To maintain the current and the associated
motor action, it is therefore necessary to apply
to the coil, from an external source, a voltage
sufficient to overcome the induced e.m.f. Thus the
motor requires electrical power to produce a
corresponding amount of mechanical power.
e Gen
Motor e
l
i
B
Force
f
f
Motor
Gen
′
Fig. 12.1

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ELECTRICAL SYSTEM
387
The directions of flux, current and movement in generator and motor action are given in Fig.
12.1. The coil is free to move about the axis O. The component fields are shown, the direction of the
mech-anical force, and the directions of rotation for motor and genera-tor action. The direc-tion of the
e.m.f. is such as to maintain the current in a generator and to oppose it in a motor. The action is revers-
ible: i.e. the same arrangement may act either as generator or motor.
The two-pole and four-pole machines differ considerably in con-struction. At 50 c/s. the former
run at 3000 r.p.m. and the latter at 1500. The useful range of two-pole machines has been extended to
300 MVA, and in consequence the four-pole construction is obsolete.
12.2.1 ROTORS
Rotors are most generally made from solid forgings of alloy steel. The forgings must be homoge-
neous and flawless. Test pieces are cut from the circumference and the ends to provide information
about the mechanical qualities and the microstructure of the material. A chemical analysis of the test
pieces is subsequently made. One of the most important examinations is the ultrasonic test, which will
discover internal faults such as cracks and fissures. This will usually render the older practice of trepanning
along the axis unnecessary.
The rotor forging is planed and milled to form the teeth. About two-thirds of the rotor pole-pitch
is slotted, leaving one-third unslotted (or slotted to a lesser depth) for the pole centre.