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8/11/2019 Wind Generator and Types
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Sanjeev Kumar Singh
M-Tech (EPES)
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Three types of typical wind generator system for large wind
turbines exist.
The first type is a fixed speed wind turbine system using a
multistage gearbox and a standard squirrel cage inductiongenerator(SCIG).
The second type is a variable speed wind turbine system with a
multistage gearbox and a doubly fed induction generator (DFIG)
.In this case the power electronics converter feeding the rotor
winding has a power rating of 30% of the generator capacity;
stator winding of the DFIG is directly connected to grid.
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The third type is also variable speed wind turbine but it is a
gearless wind turbine system with a direct drive generator,
normally a low speed high torque synchronous generator and a
full scale power electronics converter are used. Additionally a variety innovative concept of wind turbine appear
for ex., a smaller low speed permanent magnet synchronous
generator with a gearbox
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Referring to the rotational speed wind turbine concept can be
classified into fixed speed, limited variable speed and variable
speed
For variable speed wind turbine based on the rating of the powerconverter related to the generator capacity , they can be further
classified in to wind generator system with a partial scale and
full scale power converter.
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In addition considering the drive train component , the wind
turbine concept can be classified in to gear drive or direct drive
wind turbine.
In gear drive wind turbines, one conventional configuration is amultistage gear with a high speed generator; the other one is
multibridge concept which has a single stage gear and a low
speed generator .
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The squirrel cage induction generator operate only in a narrow
range around the synchronous speed ,the wind turbine equipped
with this type of generator is often called the fixed speed wind
generator system . Since the SCIG always draws reactive power from the grid .this
concept was extended with a capacitor bank for reactive power
compensation .
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The well known advantages of SCIG are it is robust easy and
relatively cheap for mass production . In addition it enables stallregulated machines to operate at a constant speed when it is
connected to a large grid which provide a stable control
frequency
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The speed is not controllable and variable over a vary narrow
range in which only speed higher then the synchronous speed are
possible for generator operation because a higher slip means a
higher dissipation of electrical energy in the rotor bars, forexample the slip is normally not higher then 1% for one MW
wind turbine. Additionally the fixed speed concept means that
wind sped fluctuation are directly translated in to
electromechanical torque variation this cause high mechanicaland fatigue stress on the system and may result in swing
oscillation.
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A three stage gearbox in the drive train is necessary for the wind
turbine concept. Gearbox represent a large mess in the nacelle
and also a large fraction of the investment cost .
Starting is necessary to obtain the excitation current from thestator terminal of SCIG this makes it impossible to support the
grid voltage control.
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This wind turbine concept uses a wound rotor induction
generator (WRIG) with variable rotor resistance by means of a
power electronics converter and the pitch control method.
The stator of WRIG is directly connected to the grid where as therotor winding is connected in series with a controlled resister .
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Variable speed operation can be achieved by controlling the
energy extracted from the WRIG rotor ;however this power must
be dissipated in the external resister .With the increase in the
variable speed range a higher slip means a high power extractedby the rotor and the lower generator efficiency , so that the
rating of the resistor must be also higher therefore the dynamic
speed control range depend on the size of the variable rotor
resistance and the energy extracted from the external resistanceis also damped as heat loss in the controllable rotor resistance
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In this configuration (DIFG)the stator is directly connected to
the grid where as the rotor is connected through a power
electronics converter .the power converter control the rotor
frequency and thus the rotor speed .this concept support the wide
speed range operation depending on the size of the frequency
converter .typically the variable speed range is +-30% around
the synchronous speed .the rating of the power electronics
converter is only 25-30%of the generator capacity .which makes
this concept effective an popular from the economics point ofview .the largest capacity for the commercial wind turbine with
DIFG has been up to 5 MW from repower
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Since this case the rotor energy ,instead of being dissipated can
be fed in to grid by the power electronics converter. More ever
the power converter system can perform reactive power
consumption in smooth grid connection , for example ,the gridside converter can control its reactance power in dependently of
the generator operation ;this allow the performance of voltage
support towards the grid
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A multistage gearbox is still necessary in the drive train because
the speed range for DIFG is far from a common turbine of 10-
25 rpm.
The slip ring is use to transfer the rotor power by means of apartial scale converter , which requires a regular maintenance
and may be results in machine failure and electrical losses
Under the grid fault condition ,on the one hand , large stator
current result in large rotor currents so that the power
electronics converter needs to be protected from destroy
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The difference between gear drive wind turbines and direct drive
type is the generator rotor speed .the direct drive generator
rotates at low speed ,because the generator rotor is directly
connected to the hub of the turbines rotor . To deliver a constant power ,the low speed makes it necessary to
produce a higher torque
A higher torque means a large size of the generator.
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Therefore for direct drive generator , the low speed and high
torque operation require multi poles ,which demand a larger
diameter for implementation of large number of poles with a
reasonable pitch .
Direct drive generator can be classified as-:
electrically excited synchronous generator
permanent magnet synchronous generator
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The EESG is usually built with a rotor carrying the field system
provided with the dc excitation .the stator carries three phase
winding quit similar to that of the induction machine .the rotor
may have salient poles or may be cylindrical (low speed ). The amplitude and frequency of the voltage can be fully
controlled by the power electronics at the generator side ,so that
the generator speed can be fully controlled over a wide range
even to vary low speed .
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Higher efficiency.
No additional supply for the field excitation .
Lighter and therefore higher power to weight ratio.
Implement in thermal characteristics of the PM machine drive to
the absence of field losses.
Higher reliability du to absence of mechanical component such
slip rings.
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High cost of PM material .
Difficulties to handles in manufacture .
Demagnetization of PM at high temp.
Types Radial flux PM machine .
Axial flux PM machine.
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