Civil Engineering Reference
In-Depth Information
and inverters can be used, but the modern design in the wind industry appears
to prefer pulse-width-modulated thyristors. The speed ratio is not limited
in theory, but practical considerations limit the ratio to 3:1, which is wider
than that obtainable using the pole-changing method described above and
the Scherbius machine described later. The energy yield of the variable-speed
system is higher. However, the added cost and the electrical loss in the power
electronics partially offset the benefit. The cost and benefit trade is generally
positive for large machines.
The power-electronics based variable-speed system introduces some sys-
tem level issues not found in other systems. It produces high-frequency
harmonics (electrical noise) in the network, which degrades the quality of
power. Alternatively, for the same quality of power, it requires a higher
degree of electrical filtering to meet the grid-quality requirement.
In addition to high annual energy production, the variable-speed power
electronic system offers remotely adjustable and controllable quality of
power. This has two major benefits not available in other systems:
•
opportunity for remote control. This makes it attractive for offshore
applications.
•
fine-tuning for superior grid connection, making it better suited
for meeting the demand of weak grids in developing countries like
China and India.
7.2.5
Scherbius Variable-Speed Drive
Compared to the variable-speed system using power electronics, the Scher-
bius machine offers lower cost and eliminates the power quality disadvan-
tage. It has been used in hoist applications in factories and mines. Extending
the analysis of the equivalent circuit described in Chapter 6, the speed of the
induction machine can be changed by changing the rotor resistance or by
injecting an external voltage of the frequency corresponding to the desired
rotor slip. The squirrel cage construction does not allow such injection.
Therefore, the wound rotor construction with slip rings is used (
Figure 7-9
).
The rotor circuit is connected to an external variable frequency source via
slip rings, and the stator is connected to the grid system. For this reason, the
Scherbius machine is also called the doubly-fed induction machine. It is fed
from both the stator and the rotor. The speed is controlled by adjusting the
frequency of the external source of the rotor current. The range of variable-
speed control using the Scherbius machines is generally limited to 2:1.
The concept was used in early turbines. The decreased reliability due to
the rubbing electrical contacts at the slip rings had been a concern. However,
some manufacturers appear to have resolved this concern, and are imple-
menting the system in turbines with several hundred kW ratings. The need
of the variable frequency source for the rotor adds into the cost and com-
plexity. For large systems, however, the added cost may be less than the
benefit of greater energy production of the variable-speed operation.
