Civil Engineering Reference
In-Depth Information
FIGURE 9-6
Equivalent electrical circuit and phasor diagram of synchronous machine.
increased by increasing the power angle up to a certain limit, beyond which
the rotor would no longer follow the stator field and will step out of the
synchronous mode of operation. In the nonsynchronous mode, it cannot
produce steady power.
9.3.2
Excitation Methods
The synchronous machine excitation system must be designed to produce
the required magnetic field which is controllable to control the voltage and
the reactive power of the system. In modern high power machines, X
can
be around 1.5 units of the base impedance of the machine. With reactance
of this order, the phasor diagram of
Figure 9-6
can show that the rotor filed
excitation required at rated load (100 percent load at 0.8 lagging power
factor) is more than twice that at no load with the same terminal voltage.
The excitation system has the corresponding current and voltage ratings,
with capability of varying the voltage over a wide range of 1 to 3 or even
more without undue saturation in the magnetic circuit. The excitation power,
primarily to overcome the rotor winding I
s
1
2
R loss, ranges from to 1 percent
of the generator rating. Most excitation systems operate at 200 to 1,000 Vdc.
For large machines, three types of excitation systems — DC, AC and
static — are possible. In the DC system, a suitably designed DC generator
supplies the main field winding excitation through conventional slip rings
and brushes. Due to low reliability and high maintenance requirement, the
conventional DC machine is seldom used in the synchronous machine exci-
tation system.
Most utility scale generators use the AC excitation system shown in
Figure 9-7
.
The main exciter is excited by a pilot exciter. The AC output of
a permanent magnet pilot exciter is converted into DC by a floor standing
rectifier and supplied to the main exciter through slip rings. The main
exciter's AC output is converted into DC by means of phase controlled
rectifiers, whose firing angle is changed in response to the terminal voltage
variations. After filtering the ripples, this direct current is fed to the synchro-
nous generator field winding.
An alternative scheme is the static excitation, as opposed to the dynamic
excitation described in the preceding paragraph. In the static excitation
2
⁄
