Generator and Electrical System - Small Wind Turbines: Analysis, Design, and Application - page 233

Fig. 11.5 Schematic of an
induction generator (IG) with
excitation capacitors

AC

IG

DC

excitation
capacitors

rectifier

which has electrical terminals for the stator and the rotor. DFIGs are used on many
large turbines. In practice, the main difference from PMGs is that IGs need
excitation capacitors to self-excite before they can generate power, e.g. Chan [ 8 ],
as shown in Fig. 11.5 . Otherwise the topology of Fig. 11.2 is the same for both
types. The capacitance must be chosen to ensure excitation at a reasonable blade
speed by resonance with the induction generator's magnetising inductance; ideally
at the end of the starting sequence as discussed in Chap. 6 . Unfortunately, the
required capacitance can change with cable length and between identically-rated
generators from different manufacturers. Furthermore, capacitors are prone to
failure, which can result in uneven or no excitation. Then the turbine may over-
speed as shown in Fig. 11.1 and it would be foolish to rely on the generator
frequency to indicate the blade speed (using a correction for slip). In other words, a
zero frequency is no guarantee that the blades connected to an IG are stationary, as
it is for a PMG, and a shaft encoder may be required to separately measure X. For
PMGs, by contrast, the blade rpm is always 60 times the generator frequency in Hz
and there is no slip.

11.3 Gearboxes

A gearbox adds complexity, noise, and maintenance. Far more important, how-
ever, is that all gearboxes have a resistive torque significantly larger than any
resistance in an induction generator when it is not producing power. 2 Gearbox
resistive torque, therefore, plays the same role in starting as cogging torque. The
analogy extends further: the ratio of gearbox resistive torque on the low speed
shaft to rated torque (on the low-speed side) turns out to be comparable to the ratio
of cogging to rated torque for PMGs. For example, Fig. 6.10 shows a failed start
for the Aerogenesis 5 kW turbine with a gearbox resistance of 1.9 Nm. The rated
power occurs at X = 320 rpm, giving a rated torque of 149.2 Nm (all values relate
to the low-speed or blade side of the gearbox). Thus the ratio of resistive to rated
torque is 0.013 which is comparable to those shown in Fig. 1.12 for PMGs.

2 Since PMG cogging torque would be multiplied by the gearbox ratio it is unlikely that a
gearbox would be used in conjunction with a low-pole PMG for small wind turbines.