09-11-2012, 05:37 PM
Power Quality in Wind Power Systems
[attachment=38835]
Abstract.
Wind power generation systems influence power
quality in a specific way making important the need for detailed
and accurate evaluation of disturbances caused by wind
generators. Wind power system can cause sub- and
interharmonic components to appear in the spectrum of voltages
and currents.
The paper shows different aspects of spectrum estimation in
power systems, including the doubly fed induction generator
(DFIG) connected to low voltage distribution grid. Harmonic
distortions caused by wind generator under various operational
conditions regarding wind speed, active and reactive power are
shown and discussed. Advanced spectral methods (like
ESPRIT) are applied to overcome the drawbacks of Fourierbased
techniques.
Key words
power quality, subharmonics, interharmonics, parametric
spectral estimation.
Introduction
Wind generation is one of the most mature and cost
effective resources among different renewable energy
technologies [1].
There exist various design concepts of wind generators
which allow fulfillment of regulatory requirements [4],
concerning grid stability. One of the most efficient
designs is the doubly fed induction generator DFIG [10],
which allows the regulation of reactive power and the
adjustment of angular velocity to maximize the output
power efficiency. These generators can also support the
system during voltage sags.
However, the drawbacks of converter-based systems are
harmonic distortions injected into the system [1].
The paper is devoted to the evaluation of harmonic
distortion in various operation conditions of the wind
generation units.
Parametric spectral estimation ESPRIT algorithm [4] is
applied to extract information on harmonics parameters.
It is assumed that those advanced methods could give
more detailed information, then traditional FFT.
ESPRIT Algorithm
ESPRIT algorithm [4] belongs to the subspace parametric
spectrum estimation methods. It allows determining the
parameters of harmonic components with high accuracy.
Furthermore, the results of spectral estimation do not
depend, to large extent, on the length of the analysis
window. Extensive investigations shown in [6] confirm
the suitability of this algorithm to provide reliable
estimates. Parametric algorithm show very high accuracy
in frequency estimation independently of the window
length and also relatively high accuracy in amplitude
estimation, without the de-synchronization effects.
Model of wind generator
Simplified model of DFIG was used. The rotor of
induction machine is connected to the grid with a backto-
back voltage source converter which controls the
excitation system. This most significant feature enables
sub synchronous and super synchronous operation speeds
in generator mode and adjustable reactive power
generation.
The wind turbine was modeled with a separately excited
30 kW DC machine. Torque and angular velocity could
be set in a controlled manner in accordance to speedtorque
characteristic of a real three blade rotor adapted
for steady state operation of the 5 kW DFIG model. The
generator in Fig. 1 is a 5 kW, two pole pairs slip ring
induction machine with synchronous speed of 1500 rpm.
[attachment=38835]
Abstract.
Wind power generation systems influence power
quality in a specific way making important the need for detailed
and accurate evaluation of disturbances caused by wind
generators. Wind power system can cause sub- and
interharmonic components to appear in the spectrum of voltages
and currents.
The paper shows different aspects of spectrum estimation in
power systems, including the doubly fed induction generator
(DFIG) connected to low voltage distribution grid. Harmonic
distortions caused by wind generator under various operational
conditions regarding wind speed, active and reactive power are
shown and discussed. Advanced spectral methods (like
ESPRIT) are applied to overcome the drawbacks of Fourierbased
techniques.
Key words
power quality, subharmonics, interharmonics, parametric
spectral estimation.
Introduction
Wind generation is one of the most mature and cost
effective resources among different renewable energy
technologies [1].
There exist various design concepts of wind generators
which allow fulfillment of regulatory requirements [4],
concerning grid stability. One of the most efficient
designs is the doubly fed induction generator DFIG [10],
which allows the regulation of reactive power and the
adjustment of angular velocity to maximize the output
power efficiency. These generators can also support the
system during voltage sags.
However, the drawbacks of converter-based systems are
harmonic distortions injected into the system [1].
The paper is devoted to the evaluation of harmonic
distortion in various operation conditions of the wind
generation units.
Parametric spectral estimation ESPRIT algorithm [4] is
applied to extract information on harmonics parameters.
It is assumed that those advanced methods could give
more detailed information, then traditional FFT.
ESPRIT Algorithm
ESPRIT algorithm [4] belongs to the subspace parametric
spectrum estimation methods. It allows determining the
parameters of harmonic components with high accuracy.
Furthermore, the results of spectral estimation do not
depend, to large extent, on the length of the analysis
window. Extensive investigations shown in [6] confirm
the suitability of this algorithm to provide reliable
estimates. Parametric algorithm show very high accuracy
in frequency estimation independently of the window
length and also relatively high accuracy in amplitude
estimation, without the de-synchronization effects.
Model of wind generator
Simplified model of DFIG was used. The rotor of
induction machine is connected to the grid with a backto-
back voltage source converter which controls the
excitation system. This most significant feature enables
sub synchronous and super synchronous operation speeds
in generator mode and adjustable reactive power
generation.
The wind turbine was modeled with a separately excited
30 kW DC machine. Torque and angular velocity could
be set in a controlled manner in accordance to speedtorque
characteristic of a real three blade rotor adapted
for steady state operation of the 5 kW DFIG model. The
generator in Fig. 1 is a 5 kW, two pole pairs slip ring
induction machine with synchronous speed of 1500 rpm.