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ABSTRACT
In recent years, wind energy has become one of the most important and promising sources of
renewable energy, which demands additional transmission capacity and better means of
maintaining system reliability. The evolution of technology related to wind systems industry
leaded to the development of a generation of variable speed wind turbines that present many
advantages compared to the fixed speed wind turbines. These wind energy conversion systems
are connected to the grid through Voltage Source Converters (VSC) to make variable speed
operation possible. The studied system here is a variable speed wind generation system based on
Doubly Fed Induction Generator (DFIG). The stator of the generator is directly connected to the
grid while the rotor is connected through a back-to-back converter which is dimensioned to stand
only a fraction of the generator rated power.
To harness the wind power efficiently the most reliable system in the present era is grid
connected doubly fed induction generator. The DFIG brings the advantage of utilizing the turns
ratio of the machine, so the converter does not need to be rated for the machine’s full rated
power. The rotor side converter (RSC) usually provides active and reactive power control of the
machine while the grid-side converter (GSC) keeps the voltage of the DC-link constant. The
additional freedom of reactive power generation by the GSC is usually not used due to the fact
that it is more preferable to do so using the RSC. However, within the available current capacity
the GSC can be controlled to participate in reactive power generation in steady state as well as
during low voltage periods. The GSC can supply the required reactive current very quickly while
the RSC passes the current through the machine resulting in a delay. Both converters can be
temporarily overloaded, so the DFIG is able to provide a considerable contribution to grid
voltage support during short circuit periods. This report deals with the introduction of DFIG,
AC/DC/AC converter control and finally the SIMULINK/MATLAB simulation for isolated
Induction generator as well as for grid connected Doubly Fed Induction Generator and
corresponding results and waveforms are displayed.
NOMENCLATURE
Pm Mechanical power captured by the wind turbine and transmitted to the rotor
Ps Stator electrical power output
Pr Rotor electrical power output
Pgc Cgrid electrical power output
Qs Stator reactive power output
Qr Rotor reactive power output
Qgc Cgrid reactive power output
Tm Mechanical torque applied to rotor
Tem Electromagnetic torque applied to the rotor by the generator
Wr Rotational speed of rotors
p derivative symbol
Vqs ,Vds are the three-Phase supply voltages in d-q reference frame, respectively
iqs ,ids are the three-Phase stator currents in d-q reference frame, respectively
λqs ,λds are the three-Phase stator flux linkages in d-q reference frame, respectively
Vqr ,Vdr are the three-Phase rotor voltages in d-q reference frame, respectively
iqr ,idr are the three-Phase rotor voltages in d-q reference frame, respectively
λqr ,λdr are the three-Phase rotor voltages in d-q reference frame, respectively
Rs ,Rr are the stator and rotor resistances of machine per phase, respectively
Lls ,Llr are the leakage inductances of stator and rotor windings, respectively
θ s ,θ r are the stator and rotor flux angle, respectively
Te ,Tm are the electromagnetic and mechanical torques, respectively
Ps ,Qs are the stator-side active and reactive powers, respectively
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Pr ,Qr are the rotor-side active and reactive powers, respectively
RON ,ROFF are the IGBT ON and OFF resistances, respectively
D, J are the moment of inertia and damping coefficient, respectively
P are the Number of poles
M1,M2 are the stator and rotor modulation depths, respectively
Vtri is the triangular Voltage Signal
R,L are the resistance and inductance of input filter, respectively
V1, I1 are the input filter line voltage and current, respectively
E is the DC-link voltage
s is the Laplacian Operator
C is the DC-Link capacitance
PDC is the DC-link active power
J Combined rotor and wind turbine inertia coefficient
Ws Rotational speed of the magnetic flux in the air-gap of the generator, this speed is named
synchronous speed. It is proportional to the frequency of the grid voltage and to the number of
generator poles
INTRODUCTION
With increased penetration of wind power into electrical grids, DFIG wind turbines are largely
deployed due to their variable speed feature and hence influencing system dynamics. This has
created an interest in developing suitable models for DFIG to be integrated into power system
studies. The continuous trend of having high penetration of wind power, in recent years, has
made it necessary to introduce new practices. For example, grid codes are being revised to
ensure that wind turbines would contribute to the control of voltage and frequency and also to
stay connected to the host network following a disturbance.
In response to the new grid code requirements, several DFIG models have been suggested
recently, including the full-model which is a 5th order model. These models use quadrature and
direct components of rotor voltage in an appropriate reference frame to provide fast regulation of
voltage. The 3rd order model of DFIG which uses a rotor current, not a rotor voltage as control
parameter can also be applied to provide very fast regulation of instantaneous currents with the
penalty of losing accuracy. Apart from that, the 3rd order model can be achieved by neglecting
the rate of change of stator flux linkage (transient stability model), given rotor voltage as control
parameter. Additionally, in order to model back-to back PWM converters, in the simplest
scenario, it is assumed that the converters are ideal and the DC-link voltage between the
converters is constant. Consequently, depending on the converter control, a controllable voltage
(current) source can be implemented to represent the operation of the rotor-side of the converter
in the model. However, in reality DC-link voltage does not keep constant but starts increasing
during fault condition. Therefore, based on the above assumption it would not be possible to
determine whether or not the DFIG will actually trip following a fault.
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In a more detailed approach, actual converter representation with PWM-averaged model has
been proposed, where the switch network is replaced by average circuit model, on which all the
switching elements are separated from the remainder of network and incorporated into a switch
network, containing all the switching elements. However, the proposed model neglects high
frequency effects of the PWM firing scheme and therefore it is not possible to accurately
determine DC-link voltage in the event of fault. A switch-by-switch representation of the backto-back
PWM converters with their associated modulators for both rotor- and stator-side
Converters has also been proposed. Tolerance-band (hysteresis) control has been deployed.
However, hysteresis controller has two main disadvantages: firstly, the switching frequency does
not remain constant but varies along the AC current waveform and secondly due to the
roughness and randomness of the operation, protection of the converter is difficult. The latter
will be of more significance when assessing performance of the system under fault condition.
In order to resolve the identified problems, a switch-by-switch model of voltage-fed, current
controlled PWM converters, where triangular carrier-based Sinusoidal PWM (SPWM) is applied
to maintain the switching frequency constant. In order to achieve constant switching frequency,
calculation of the required rotor voltage that must be supplied to the generator is adopted.
Various methods such as hysteresis controller, stationary PI controller and synchronous PI
controller have been adopted in order to control current-regulated induction machine. Among
which, synchronous PI controller has been acknowledged as being superior.
Power quality is actually an important aspect in integrating wind power plants to grids. This is
even more relevant since grids are now dealing with a continuous increase of non-linear loads
such as switching power supplies and large AC drives directly connected to the network. By now
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only very few researchers have addressed the issue of making use of the built-in converters to
compensate harmonics from non-linear loads and enhance grid power quality. In, the current of a
non-linear load connected to the network is measured, and the rotor-side converter is used to
cancel the harmonics injected in the grid. Compensating harmonic currents are injected in the
generator by the rotor-side converter as well as extra reactive power to support the grid. It is not
clear what are the long term consequences of using the DFIG for harmonic and reactive power
compensation. some researchers believe that the DFIG should be used only for the purpose for
which it has been installed, i.e., supplying active power only .
Doubly Fed Induction Generator
Wind turbines use a doubly-fed induction generator (DFIG) consisting of a wound rotor
induction generator and an AC/DC/AC IGBT-based PWM converter. The stator winding is
connected directly to the 50 Hz grid while the rotor is fed at variable frequency through the
AC/DC/AC converter. The DFIG technology allows extracting maximum energy from the wind
for low wind speeds by optimizing the turbine speed, while minimizing mechanical stresses on
the turbine during gusts of wind. The optimum turbine speed producing maximum mechanical
energy for a given wind speed is proportional to the wind speed. Another advantage of the DFIG
technology is the ability for power electronic converters to generate or absorb reactive power,
thus eliminating the need for installing capacitor banks as in the case of squirrel-cage induction generator.