Page 387 - Wind Energy Handbook

FIXED SPEED, TWO-SPEED OR VARIABLE-SPEED OPERATION 361

improvement can be obtained by running the turbine at one of two fixed speeds so that
the tip speed ratio is closer to the optimum than with a single fixedspeed.
Noise considerations are often of greater significance than energy capture in the
decision to opt for non-fixed speed operation. As noted in Section 6.4.4, the
aerodynamic noise generated by a wind turbine is approximately proportional to
the fifth power of the tip speed. Both variable speed and two-speed operation allow
the rotational speed to be substantially reduced in low winds, thus reducing turbine
aerodynamic noise dramatically when it could otherwise be objectionable because
of low ambient noise.

6.9.1 Two-speed operation

At one time, two-speed operation was relatively expensive to implement because
separate generators were required for each speed of turbine rotation. Either
generators of differing numbers of poles were connected to gearbox output shafts
rotating at the same speed, or generators with the same number of poles were
connected to output shafts rotating at differing speeds. The rating of the generator
for low-speed operation would normally be much less than the turbine rating.
The development of induction generators with two sets of independent windings
has allowed the number of poles within a single generator to be varied by con-
necting them together in different ways. Standard generators of this type are now
available which can be switched between four- and six-pole operation, giving a
speed ratio of 1.5. Given correct selection of the two operating speeds, this ratio
produces close to the optimum increase of energy capture.
It is important to note that, in the case of stall-regulated machines, only limited
energy gains are to be had by converting from fixed-speed to two-speed operation
for a given rated power, because the maximum rotational speed of the two-speed
machine is restricted to the rotational speed of the fixed speed machine in order to
limit the power to the required value. Energy gains are only of the order of 2 or 3
percent, but, nevertheless, two-speed operation is often deemed worthwhile on stall
regulated machines because of noise considerations.
In the case of pitch-regulated machines, the energy gain obtainable by moving to
two-speed operation depends on the power rating and rotational speed of the
baseline fixed speed machine. Where these parameters have been chosen to be close
to the optimum, in relation to rotor diameter and rotor chord respectively, an
energy gain of only about 3 percent is attainable. However, energy gains of up to 10
percent are possible when the baseline design is sub-optimal.
Some disadvantages of two-speed operation are:

• additional generator cost,
• extra switchgear is required, which is subjected to a demanding duty in terms of
frequency of operation,
• control of turbine speed is required at each speed change,
• energy is lost while the generator is disconnected during each speed change.