Environmental Engineering Reference
In-Depth Information
Angular relations in the vector diagram provide the following relation:
(5.84)
Hence, the phase current
I
1
and the phase angle
ϕ
depend on load and
excitation. The load angle
increases with rising load, i.e. torque
M
. The
synchronous internal voltage
V
p
depends only on the excitation current.
The electrical active power
P
1
of the stator
ϑ
(5.85)
can be calculated from the active powers of the three winding phases of the
stator. With the synchronous speed
n
S
of the rotating field and
(5.86)
the
torque
that is associated with the active power becomes:
(5.87)
According to this definition, the power
P
and torque
M
are negative in
generator mode with negative load angle
and the machine produces power.
The rotor torque is higher than the torque that corresponds to the active power
because friction, gearbox and other losses reduce the rotor torque.
Figure 5.26 shows the curve of the torque M over the load angle
ϑ
. A
synchronous machine produces the theoretical maximum torque, called the
pull-out torque,
M
p
, at load angles of ±
ϑ
/2. The magnitude of this torque
changes with the internal voltage and therefore with the excitation current. If
the load torque of the machine increases above the pull-out torque
M
p
, the
machine falls out of step. If it operates in motor mode it stops. If it operates in
generator mode, the rotor runs faster than the rotating field in the stator and
the machine overspeeds, in which case high centrifugal forces could destroy
the rotor of a wind generator. Reliable safety systems such as aerodynamic
brakes must avoid this critical operation mode.
π
Synchronization
Before a synchronous generator can be connected to the mains it must be
synchronized with the mains. Up to now it has been assumed that the stator
frequency is equal to the mains frequency. During the start-up of a
synchronous generator, the rotor speed and therefore the stator frequency is
different from the mains frequency. If a synchronous generator were to be
connected immediately to the mains, strong compensation reactions would
appear such as very high compensation currents. This would cause
