Page 180 - Industrial Power Engineering and Applications Handbook

W160 Industrial Power Engineering and Applications Handbook

parameters and sends out warning signals or stops the machine windings take over and the generator may still operate
when operating conditions exceed the permissible limits. and feed back to the power grid at a slightly lower
output (in the ratio of motor outputs at the two speeds).
7 Gear system Since the wind speed is never the To achieve this, two switching circuits may be
same, and the rotor is never permitted to rotate at a provided, one for each speed as shown in Figure
higher speed than its designed parameters, a gear 13.59 (Table 6.1).
system is provided between the blades and the The changeover is obtained by measuring the
gcnerator rotor to protect it from excessive wear and average power generated during a particular time
tear. The gear system helps the generator rotor to run period, say, one minute or so, rather than the speed
at the generator’s rated speed. The normal gear ratio of the wind. When this average power falls below a
to boost the speed of the prime mover to the generator preset level the machine changes over to the lower
speed is from roughly 30-40 r.p.m. to a little over speed windings and vice versa. Due to the unpredic-
the synchronous speed of the induction generator. table nature of the wind speeds, this may require
The gear system is a two- or three-stage fixed ratio frequent changeovers and may affect the reliability
gearbox giving fixed output speeds. Fine adjustment of a double-speed system.
is achieved through pitch control of the blades. As soon as the wind speed reaches the minimum
required speed (cut-in speed), i.e. a lower speed mode,
8 Sensors in dual-speed machines the brakes release and the
Anemometer - to monitor wind speed prime mover picks up speed. At about 95% of the
Temperature - to monitor the operating rated speed of the machine, the stator of the induction
sensors temperature of the gearbox, generator is supplied with the necessary reactive power
generator windings and other from the grid through an inverter, to generate the
important parts of the machi- required magnetic field. At this speed, which is almost
nery the rated speed of the motor, the machine draws just
Wind vane - to monitor wind direction enough reactive power from the grid to meet its
Vibration sensors - to monitor correct alignment, magnetizing requirements and a small amount of
loose or worn-out parts etc. mechanical power from the wind power to meet its
Pressure sensors - for the hydraulic system that friction and windage losses (area DIP, of Figure 6.62).
actuates the yawing mec- The magnetizing current is now much less than the
hanism, brakes and pitch of no-load losses of the machine when it operates as a
the blades motor. The inverter is normally thyristor controlled
to provide the machine with a soft start and to control
9 Electrical system the starting current to almost the rated current in
Main power panel to receive power from the genera- case of excessive machine speed. It also avoids an
tor and feed this back to the power grid excessive voltage dip at the generator terminals and
Step-up transformer if the voltage of the generator time to hook up to the grid. As soon as the machine
is different from that of the grid exceeds its synchronous speed, it starts generating
Related switchgears and feeding the active power back to the grid, similar
Control and relay panel (which may be micro- to the output curve of the machine in Figure 6.64.
processor based), to record, display, monitor and (The curve is provided by the machine supplier.)
control the generated power, voltage and frequency The generator, when hooked up to the grid, will follow
and also detect fault conditions. the grid voltage and frequency. The generator is
Frequency inverters to regulate the voltage and regulated not to overspeed bcyond 101% of its
frequency etc. synchronous speed. Small speed variations are camed
out through pitch control to optimize the power output
10 Induction generator This is a standard squirrel cage and running hours of the machine. The power
motor with additional treatment to weather the site generated is proportional to thc negative slip at which
conditions. The normal specifications are generally the machine operates (area PIPz of Figure 6.62).
the same as for a standard motor. The permissible The total reactive power that it draws from the grid
voltage and frequency variations are, however, wider
as noted below: VAr = V, . I, watts
Voltage f13% where
V, = rated voltage of the machine in volts, and
Frequency -3% + 3%
I, = magnetizing current of the machine in amps.
or as may be required to suit a particular grid system. (Figure I. IS)
A frequency limit of +3% signifies that the machine
should not overspeed beyond 101.5% of its synchro- Use of capacitors
nous speed. This is to avoid overloading of the machine
as well as retaining synchronism with the grid. For Power capacitors are installed to compensate for the
better use the machines are often wound for a dual reactive power, particularly that which the machine draws
speed, such as 416 pole, so that when the wind speed from the grid. The capacitors must be suitable for a
falls below the minimum cut-in speed, the lower speed minimum voltage of V, + 13% (generally V, + 15%).