Overview of different wind generator systems and their comparison
1. Class-6: Overview of different wind
generator systems and their
Course: Distribution Generation and Smart Grid
Prof. (Dr.) Pravat Kumar Rout
Subhasis Panda (Research Scholar)
Department of EEE, ITER,
Siksha ‘O’Anusandhan (Deemed to be University),
Bhubaneswar, Odisha, India
2. Principle of operation
Kinetic energy present in the
wind speed to electrical energy
Wind power consists of
converting the energy
produced by the movement of
wind turbine blades driven by
the wind into electrical
Wind energy is the world’s fastest
growing renewable energy
The average annual growth rate of
wind turbine installation is around
30% during last 10 years.
The overall capacity of all wind
turbines installed worldwide by the
end of 2018 reached 600
Gigawatt, according to preliminary
statistics published by WorldWind
Energy Association (WWEA)
Wind energy benefit
Reduces the use of fossil
Reduces energy imports
Creates wealth and local
Contributes to sustainable
9. Wind turbine generator types
Referring to the rotational speed, wind turbine concepts can be classified
into fixed speed, limited variable speed and variable speed.
For variable speed wind turbines, based on the rating of power converter
related to the generator capacity, they can be further classified into wind
generator systems with a partial scale and full scale power
Considering the drive train components, the wind turbine concepts can
be classified into geared drive and direct drive wind turbines.
In geared drive wind turbines, one, one conventional configuration is a
multistage gear with a high speed generator; the other one is the
multi-brid concept which has a single stage gear and a low speed
10. Site Selection for Wind Power
The WECS should be located where high average wind velocities
available are in the range of 6 m/s to 30 m/s throughout the year.
TheWECS must be located far away from cities and forests since
buildings and forests offer resistance to wind.
Wind velocities must be measured at several heights as the velocity
of wind increases with height.
Tower design must be adequate to withstand maximum wind speeds
observed in the last few years in the area of installation.
11. Advantages of wind power plant
It is a free and un-exhaustible source of energy.
It is a clean and non-polluting source of energy.
It has a low maintenance cost.
It has a low cost of power generation (about Rs. 2.25/kWH).
12. Disadvantages of wind power plant
At present capital cost of wind power plants is high. It is about Rs.
Wind energy is very fluctuating in nature. It is very difficult to
design a wind energy system due to these fluctuations.This
problem also requires the provision of a suitable storage device to
ensure continuous power supply.
Large variations in wind speed during storms may cause damage to
The efficiency of the system is in the range of 35 to 44%.
Windmill causes sound pollution. A large unit can be heard
few kilometers away.
13. Type-1: Fixed speed concept
The generator operating slip changes slightly as the operating power
level changes and the rotating speed therefore not entirely constant.
However, because the operating slip variation is generally less than
1% , this type of wind generator is normally referred to as fixed speed.
Have been used with a multiple stage gearbox (used to
match the speed difference).
Squirrel cage induction generator (SCIG) directly
connected to the grid through a transformer. The SCIG
operates only in a narrow range around the synchronous
SCIG always draws reactive power from the grid and so
accompanied with the capacitor bank for reactive power
Smoother grid connection was also achieved by
incorporating a soft starter .
The function of the soft starter unit is to built up the
magnetic flux slowly and so minimizing transient currents
during energization of the generator or system start-up
Pole changeable SCIG has been used which leads two rotation
Robust, easy and relatively cheap for mass production.
It enables stall-regulated machines to operate at a constant
speed when it is connected to a large grid, which provides a
stable control frequency.
Although the stall control method is usually used in combination
with the fixed speed SCIG for power control, the active stall
control or pitch control have also been applied.
The speed is not controllable and variable only over a very
narrow range, in which only speeds higher than the synchronous
speed are possible for generator operation.
Wind speed fluctuations are directly translated into
electromechanical torrque variations, causing high mechanical and
fatigue stresses on the system. This may result also swing
oscillations between turbine and generator shaft.
Turbine speed cannot be adjusted with the wind speed to
optimize the aerodynamic efficiency
Three stage gearbox makes the nacelle more burden and
also a large fraction of the investment cost.
Capacitors are connected in parallel to the generator for
the reactive power compensation.
It is necessary to obtain the excitation current from the
stator terminal of SCIG . This makes it impossible to support
grid voltage control.
Uses a wound rotor induction generator (WRIG) with
variable rotor resistance by means of a power electronics
converter and pitch control method.
The stator is directly connected to the grid. The rotor
winding is connected in series with a controlled resistor.
Variable speed operation can be achieved by controlling
the energy extracted from the WRIG rotor.
Dynamic speed control range depends on the size of
the variable rotor resistance, and the energy extracted
from the external resistor.
Slip rings can be avoided by power converter and
resistors. The control signals are transmitted to the
rotating electronics by an optical coupling.
Reactive power and soft starter are also required
This corresponds to a variable speed wind turbine with a WRIG
and a partial-scale power converter on the rotor circuit.
The stator is directly connected to the grid. The rotor is
connected through a power electronics converter
The power electronics converter controls the rotor frequency
and thus the rotor speed.
The variable speed range is +/- 30% around the synchronous
The ratings of the power electronics converter is only 20-30% of
the generator capacity
The power converter system can perform reactive
power compensation and smooth grid connection
Grid side converter can control can control its
reactive power, independently of the generator operation;
this allows the performance of voltage support towards the
A multistage gearbox is till necessary in the drive train because
the range for DFIG is far from a common turbine speed of 10-25
rpm. A gearbox is inevitable to have some drawbacks, such as heat
dissipation from friction, regular maintenance and audible noise.
The corresponding control strategies are comparatively
Under grid fault conditions, the both stator and rotor currents
are very high and the protection is necessary particularly for
Large stator peak currents under abnormal conditions may
cause high torque loads on the drive train of wind turbines
26. Limitations ….
The slip ring is used to transfer the rotor power by means of a
partial scale converter which requires a regular maintenance,
and may be result in machine failures and electrical losses
28. A variable speed wind turbine with a direct-drive generator
connected to the grid through a full scale power converter.
The direct-drive generator rotates at a low speed, because the
generator rotor is directly connected on the hub of the turbine
The low speed and high torque operation requires multi-
poles design. So direct-drive generators are usually designed with a
large diameter and small pole pitch.
The full scale power converter can perform smooth grid
connected over the entire speed range.
30. Continue ….
Rotor carrying field system provided by DC excitation.
Stator carries a three phase winding similar to that of the
Rotor may have salient poles or cylindrical.
The amplitude and frequency of the voltage can be fully
controlled by the power electronics at the generator side, so
that the generator speed is fully controllable over a wide
range, even to very low speeds
Excitation current can be controlled by means of the
power converter in the rotor side
In order to arrange space for excitation windings and pole shoes,
the pole pitch has to be large enough for the large diameter-specific
design, so a larger number of parts and windings properly make it a
heavy weight and expensive solution.
It is necessary to excite the rotor winding with DC, using slip
rings and brushes, or brushless exciter, employing a rotating rectifier,
and the field losses are inevitable.
The advantages of PM machines over electrically excited
higher efficiency and energy yield
no additional power supply for the magnet field excitation
improvement in the thermal characteristics of the PM
machine due to the absence of the field losses
higher reliability due to the absence of mechanical
components such as slip rings
lighter and there for higher power to weight ratio
In this scheme, a variable speed pitch control wind turbine is
connected to a single stage planetary gearbox that increases
the speed by a factor of roughly 10 and a low speed
permanent magnet generator.
With a multiple gearbox is used in order to reduce the generators
volume and improve the generator efficiency in variable speed wind
turbine concepts with a full scale power converter.
Compared with the DFIG system the advantages are
The generator has a better efficiency
the generator can be brushless
the grid-fault ride-through capability is less complex
The limitations are
large, more expensive (100% of the rated power instead of
The losses in the converter are higher because all powers are
processed by the power electronics converter
40. Variable Speed multi-stage geared
concept with a full scale power
Case-2: SCIG system
A variable speed multistage geared SCIG
Full scale converter
41. Type-7: Other Potential Generator types for
different wind turbine concepts
Linear Induction Generators
Switched Reluctance Generators
Brushless DFIGs (BDFIG)
42. Brushless DFIGs
The output of the induction generator (in case of BDFIG) is
directly connected to the grid, and thus the generator
output frequency must be equal to the grid frequency
It does not need slip ring ( in case of BDFIG); however
it requires double stator windings, with different number of
poles in both stator layers
The second stator layer generally has lower copper mass,
because only a part of the generator nominal current flows in the
The second stator winding is connected through a power
electronics converter, which is related at only a fraction of the
wind turbine rating
It (BDFIG) has the capability of realizing the variable speed
operation and independently controls the stator active
and reactive power.
The machine operation principle and its assembly are relatively
44. Advantages of permanent magnet
Flexibility in design allows for smaller and lighter designs
Higher output level may be achieved without the need to increase
Lower maintenance cost and operating costs, bearings last longer
No significant losses generated in the rotor
Generator speed can be regulated without the need for gears or
Very high torque can be achieved at low speeds
Eliminates the need for separate excitation or cooling systems
45. Disadvantages of Permanent magnet
Higher initial cost due to high price of magnets used
Permanent magnet costs restricts production of such generators
for large scale grid connected turbine designs
High temperatures and severe overloading and short circuit
conditions can demagnetize permanent magnets
Use of diode rectifier in initial stage of power
conversion reduces the controllability of overall system
46. Advantages of Asynchronous
Lower capital cost for construction of the generator
Known as rugged machines that have a very simple design
Higher availability especially for large scale grid connected
Excellent damping of torque pulsation caused by sudden wind
Relatively low contribution to system fault levels
47. Disadvantages of Asynchronous
Increased converter cost since converter must be rated at the full
Results in increased losses through converter due to large
converter size needed for IG
Generator requires reactive power and therefore increases cost of
initial AC-DC conversion stage of converter
Many experience a large in-rush current when first connected to
Increased control complexity due to increased number of switches
48. Advantages of Doubly fed Induction
Reduced converter cost, converter rating is typically 25% of total
Improved efficiency due to reduced losses in the power
Suitable for high power applications including recent advances in
Allows converter to generate or absorb reactive power due to
Control may be applied at a lower cost due to reduced converter
49. Disadvantages of Doubly fed
Increased control complexity due to increased number of switches in
Stator winding is directly connected to the grid and susceptible to
Increased capital cost and need for periodic slip ring maintenance
Increased slip ring sensitivity and maintenance in offshore installations
Is not direct drive and therefore requires a maintenance intensive
gearbox for connection to wind turbine
50. Advantages of Wound field
Minimum mechanical wear due to slow machine rotation
Direct drive applicable further reducing cost since gearbox not
Allow for reactive power control as they are self excited
machines that don't require reactive power injection
Readily accepted by electrically isolated systems for grid
Allow for independent control of both real and reactive power
51. Disadvantages of Wound field
Typically have higher maintenance costs again in comparison to
that of an IG
Magnet used which is necessary for synchronization is expensive
Magnet tends to become demagnetized while working in the
powerful magnetic fields inside the generator
Requires synchronizing relay in order to properly synchronize
with the grid
Li, H., & Chen, Z. (2008). Overview of different wind generator
systems and their comparisons. IET Renewable Power
Generation, 2(2), 123-138.
54. Key Questions ….
Compare the performance of various wind turbines used for wind
Enumerates the need of converter for wind generators?
Enumerates the need of capacitors for wind generators?
Classify the wind generators according to the speed of rotation,
gear train used and converter used?
How the converters enhance the performance and help in
integration of the turbine with grid?