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Student Name: Callum McBride

Student Number: 17143268

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Module Number: NG3S851

Assignment Title: Induction Generator Schemes for Wind














efficiency of harnessing wind power over the last few decades has improved a
significant amount. From the basic fixed speed “Danish Concept” using
asynchronous generators, to the more complex and efficient variable speed
turbine consisting of a double fed induction generator with pitch and yaw
control mechanisms. This report will explain in detail how energy from the wind
is converted into electrical energy that can be supplied directly to the gird or
in remote areas, how self-excited turbines use capacitor banks for their
reactive power in order to operate off-grid, as stand-alone systems.

















Table of Contents
Abstract. 2
Induction Generator. 4
A.. 4
1.Introduction. 4
Power. 5
Concept. 6
Blades and Stall Control 6
5.Gearbox. 7
Switched Asynchronous Generator. 7
6.1Rotor. 7
6.2Stator. 7
Configuration & Operation. 8
starter (Ac voltage converter) 9
it Works. 9
Formulas. 10
8.1 Synchronous
Speed. 10
Percentage. 10
Stator Copper Loss. 11
Rotor Copper Loss. 11
circuits. 11
.. 12
B. 13
A)          Self-Excited Induction
Generator. 13
B) 14
Doubly-Fed Induction Generator (DFIG). 16
1.Introduction. 16
2.Control 16
Flow.. 17
A)          Sub-Synchronous Power Flow.. 17
B)          Super-Synchronous Power Flow.. 17
4.Converters. 18
Side Converter. 18
Side Converter. 19
Conclusion. 20
Bibliography. 21



1)Cage Induction Generator

Part A        


turbines are used extensively to convert the kinetic wind energy into
mechanical power to rotate generators, thereby generating electrical power onto
the grid. In 2016 wind energy was the second largest form of power generation
in the EU, with a capacity of 153.7GW (Wind Europe , 2016) as can be seen
from figure 1.  A common form of wind turbine, the Danish
concept uses an asynchronous generator to harvest the power from the wind.

Figure 1 (Wind Europe , 2016)








2.Wind Power

Only so
much energy can be harvested from the wind due to the fact that the energy or movement
created is applicable to the flow of the wind through the turbine. If all of
the wind energy is used, then there would be no flow of wind and ultimately, no
energy produced. In addition to this point, according to Betz’ Law, the maximum
efficiency of any wind turbine independent of the design, would be 59.3% (REUK, 2010)

energy in the wind can be estimated by calculating the mass flow of particles
in a specific area (MIT Wind Energy Group, 2010):

Energy KE=

Power is KE per unit time


flow of air through a disc area-A)


The formula above demonstrates that
the energy in the wind is cubically related to the wind speed velocity, so the higher
the wind velocity, the greater the energy available to convert into
electricity. It is also important to take from this formula that a turbine with
a larger swept area, will also produce more electricity.








3.Danish Concept

                  Figure 2 shows the design of a “Danish Concept”
wind turbine, and the components used within it to convert the power from the
wind into a form of electrical energy that can be supplied onto the grid.

Figure 2 (Machinery
Equipment Online, 2016)

4.Rotor Blades and Stall Control

design of the rotor blade (figure 3) causes the movement of air over the top of
the blade to be quicker than that of the underside, thereby causing a lower pressure
on top of the blade.  This in turn
results in a lifting motion, creating the rotation of the rotor blades. The
most common configuration for wind turbines is the three rotor blades design. This
is known to reduce the vibration and chatter from the blades when in operation,
also providing the best rotational speed whilst delivering the least amount of
stress to the mechanical parts.

well as creating the lift for rotation, the rotor blades play another key role
in protecting the mechanics of the turbine. The geometry of the rotor blade
profile has been “aerodynamically designed to ensure that the moment the wind
speed becomes too high, it creates turbulence on the side of the rotor blade
(Figure 4) that is not facing the wind, causing stall and preventing any
further “lift” of the rotor blade” (Danish Wind Industry Association, 2003). The steady rather than abrupt stall is achieved by the gradual
twisting of the blade from the base to the tip.



                  The rotation of the wind
turbine rotor is   transferred to the generator through the use
of a gearbox. The gearbox, having an extremely high ratio (usually around 1:50
for larger turbines) allows the slowly rotating, high in torque movement from
the turbine rotor to be transformed into a high speed low torque rotation which
is required for the generator to rotate at a sufficient speed to produce the electrical

6.Pole Switched Asynchronous Generator

                  Most Wind turbines use a three-phase asynchronous (cage
wound) generator, otherwise known as an induction generator to generate
alternating current. Originally designed as an induction motor, the induction
generator consists of two main parts, the construction of which is explained

Figure 5 (Alternative Energy Tutorials, 2010)









of a set of slotted steel laminations pressed together to form a cylindrical
shape. Copper bars are fixed into the slots and shorted at both ends to
complete an electrical circuit/ current pathway.   








                  The stator is made from stacking several thin,
slotted highly permeable steel laminations inside a cast iron frame. The
magnetic path is laminated to reduce eddy currents and other losses. Three
phase windings pass through the slotted steel within the stator.  Figure 7 shows the layout of both the stator
and rotor laminations.

Figure 7 (Edvard, 2011)







6.3Winding Configuration & Operation

                  Shown opposite (Figure 8) is the
winding configuration of a 2-pole induction generator, consisting of 6 poles or
3 pole pairs. Each pair of poles is 120 degrees out of phase to one another.

The Generator, working on
the same principles as an induction motor, will have a magnetic flux rotation within
the stator according to the incoming supply frequency from the grid, this is known
as the synchronous speed. The rotor rotation, shown opposite, will have a speed
less than that of synchronous speed, this difference is known as slip. In order
to generate electricity, the speed of the rotor must be increased greater than
that of the synchronous speed, this is achieved through the use of wind power.

                  The Turbine, or prime mover,
will increase the rotational speed of the rotor past that of the synchronous
speed (ns) of the motor. As can be seen by figure 11, when this occurs, there
will be both a negative torque and negative slip, ultimately generating electrical
energy onto the grid by means of direct connection, generating at the same
frequency and voltage without the use of any rectifiers or inverters.

switched generators are used in fixed speed wind turbines as they alter the
synchronous speed of the generator, in turn affecting the slip. As the number
of pole pairs increases, the synchronous speed of the generator reduces. This
can be utilised at lower wind speeds to increase the production of power.

In order
to set up the rotating magnetic field previously explained, reactive power is
required which is supplied directly from the grid. Capacitor banks (shown in
figure 2) are used in circuit to prevent any voltage fluctuations due to the
setting up of the magnetic field. Soft starters are also used during the
start-up period of the turbine to prevent in order to limit the inrush current.

7.Soft starter (Ac voltage converter)         

The Soft Starter or Ac voltage
controller is used to reduce the in-rush current by slowly building the
magnetic flux within the generator. Without it, the in-rush current during
start up can be several times the rated current causing excessive starting
torque from the generator, in turn producing severe voltage fluctuations on the
grid. A normal switch used in this situation would cause a brownout due to the
setting up of the magnetic field in the generator, shortly followed by a power
peak due to the current surging onto the grid.

7.1How it Works

                  The soft starter (Figure 12) varies the RMS value
of the ac supply voltage to the load. This is achieved by phase angle control. Dependant on when (the angle) the
thrysitors are triggered, will result in a different waveform output from the
soft starter, ultimately slowing initial start-up and reducing inrush current
and torque.

            Figure 13 shows the different firing angles in respect to the amount
of power delivered to the generator from the soft starter. For example, for %25
of power the firing angle of the thyristors would be around 135 degrees, and
the for half of the power output the firing angle would be 90 degrees. The
thyristors within the soft starter are line commutated, only one pulse is
required to fire the thyristor. Once the waveform is reversed, the thryristor
turns off.


8.Relevant Formulas


8.1 Synchronous Speed

Synchronous speed
(Ns) is calculated by:








8.2Slip Percentage

The Slip (S) can be
calculated using the following formula:















8.3 Stator Copper Loss




8.4 Rotor Copper Loss





9.Equivalent circuits

The equivalent circuit
of a transformer can be used to model an induction motor/generator: (McFadyen,






The rotor side EMF
(sE2) is affected by the slip (increase in speed, reduced slip and less EMF is


The current in the
rotor circuit (I2) is given by:


Which can be rewritten as:

The above equality
allows the equivalent circuit to be drawn as:


As the circuit shown
previously removes the dependence on the slip being required to determine the
secondary voltage and frequency, the circuit can be simplified to the
following. Referring the rotors resistance and reactance to the primary.












Part B

A)     Self-Excited
Induction Generator

                  It is often the case that wind
turbines are situated in remote areas as these provide the highest and most
consistent form of wind power. In these areas however, it becomes unfeasible to
provide the ability to connect directly onto the grid to supply the generator
with the required reactive power to produce the rotating flux within the
stator. In these circumstances, a different approach is needed. It is possible
for the induction generator to become “self-exciting” by introducing capacitors
onto the stator terminal windings of a standard induction generator, this
capacitance supplies the necessary reactive power to achieve generation in
remote areas.










400V, 3-phase cage induction machine has a rated current of
35A and a full-load power factor of 0.86 lagging. The machine is to be run as a
self-excited generator. Calculate the minimum capacitance required per-phase if
the machine is connected in delta.

E=400                     I=35A                     


Apparent power:




Active power:




Reactive power:




=4.12kvar per phase

Operating as a
self-excited asynchronous generator, the capacitor bank must supply at least
4.12kvar per phase. Connected in Delta the voltage per phase would be 400v.






Capacitive current per phase:


Capacitive reactance per phase:




Therefore, the minimum capacitance per phase must be at least:












2)Wound-Rotor Doubly-Fed Induction Generator (DFIG)


Due to a large increase in wind power being delivered onto
the electrical gird, turbines comprising of double-fed induction generators
(DFIG) are being used extensively due to their ability to generate at variable
wind speeds having a typical speed control range  ±30% of synchronous speed. In a DFIG, the
stator is connected directly to the gird whilst a wound rotor is fed with a
variable frequency  by using AC/DC/AC
converters connected onto the rotor through slip rings. This setup (figure 15)
allows two different modes of generation, sub-synchronous (below synchronous speed)
and super-synchronous (above synchronous speed).










The control mechanism generates the
pitch angle of the turbine blades as well as voltage signals Vr and
for Crotor
and Cgrid
respectively at different wind speeds to control the power of the wind turbine
as well as the DC bus voltage and the reactive power/voltage at the terminals
of the grid. The AC/DC/AC converter uses insulated bipolar transistors to control
both the rotor and the grid currents, allowing the rotor frequency to differ
from that on the grid also making it possible to adjust the active and reactive
power fed to the grid from the stator independent from the generators turning





3.Power Flow

power flowing in the rotor of a DFIG machine has three main components:

Electromagnetic power transferred between stator
and rotor through the air gap, otherwise known as air gap power (Ps)

The mechanical power transferred between the
rotor and the shaft (Pm)

The slip power which is transferred between the
rotor and any external source or load through the slip rings (Pr)

Using these components along with the diagrams and equations
to follow. The actual flow of power can be understood in both generating modes.

A)     Sub-Synchronous Power Flow

During sub-synchronous speeds, a voltage at slip frequency,
with a controlled magnitude and phase angle can be supplied to the rotor from
the converters to allow power generation.

B)      Super-Synchronous Power Flow






Mechanical power &
stator electric power output is calculated by the following:

Pm = Tm?rPs = Tem?s


Mechanical power captured by the captured by the wind turbine and transmitted
to the rotor.

Mechanical torque applied to the rotor.

Rotational speed of the rotor

electric power output

Electromagnetic torque applied to the rotor by the generator

Rotational speed of the magnetic flux in the air-gap of the generator.
Otherwise known as synchronous speed, it is proportional to the frequency on
the grid voltage and the number of generator poles.

In steady-state conditions at fixed speed for a lossless

Tm = Tem and Pm= Ps+ Pr.

Following from this:


·      s- slip of the generator


4.1Grid Side Converter

                  The grid side converter carries
current at the same frequency supplied by the grid. It is used to maintain a
constant DC voltage link between the converters. A capacitor is connected
between the two converters as a form of energy storage. The GSC is also used to
provide reactive power to the grid for power factor control by varying the
firing angle of the thyristor switching.





4.2Machine Side Converter

                  Carries current at slip
frequency and provides a variable AC voltage and frequency to control the
torque and speed of the machine. The MSC takes DC power from the DC link and AC
out power at the slip frequency to the rotor to increase its speed past the
synchronous. Conversely, if the rotor is running too fast causing the generator
frequency to be too high, the MSC extracts AC power at the slip frequency to
slow/ reduce the generator slip and converts the rotor output to DC where the
GSC will then supply the grid with power at the correct voltage and frequency.













                  The variable speed wind turbine is currently the
preferred wind power technology. Phasing out the older Danish concept turbines due
to its variable wind speed capabilities, increased efficiency and maximum power
generation. Self-excited type wind turbines however are still used extensively to
supply active power to remote areas where it becomes impracticable to supply
direct connections to the grid.
























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Induction Generator

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