Download A. Power from wind turbine

Using PWM Boost Rectifier for Harmonic Mitigation in
Wind Turbine Energy Conversion Systems
Abstract—Permanent magnet synchronous generators
(PMSG) wind energy conversion system (WECS) using
variable speed operation is being used more frequently in low
power wind turbine application. Variable speed systems have
several advantages over the traditional method of operating
wind turbines, such as the reduction of mechanical stress and
an increase in energy capture. To fully exploit the last
mentioned advantage, many efforts have been made to
develop maximum power point tracking (MPPT) control
schemes for PMSG WECS. To allow the variable speed
operation of the PMSG WECS a conventional three-phase
bridge rectifier with a bulky capacitor associated with voltage
source current controlled inverter (VS-CCI) is used. This
simple scheme introduces a high intensity low frequency
current harmonic content into the PMSG and consequently
increases the total loses in it. Subsequently, decreases the
power capability of the system. In this paper a simulation
study using a three-phase boost type PWM rectifier applied to
harmonic mitigation in these systems is presented.
I.
INTRODUCTION
The amount of energy capture from a WECS depends not
only on the wind at the site, but depends on the control
strategy used for the WECS and also depends on the
conversion efficiency. Permanent magnet synchronous
generators (PMSG) wind energy conversion system
(WECS) with variable speed operation is being used more
frequently in low power wind turbine application. Variable
speed systems have several advantages such as the
reduction of mechanical stress and an increase in energy
capture. In order to achieve optimum wind energy
extraction at low power fixed pitch WECS, the wind
turbine generator (WTG) is operating in variable-speed
variable-frequency mode. The rotor speed is allowed to
vary with the wind speed, by maintaining the tip speed ratio
to the value that maximizes aerodynamic efficiency. The
PMSG load line should be matched very closely to the
maximum power line of the WTG. MPPT control is very
important for the practical WECS systems to maintain
efficient power generating conditions irrespective of the
deviation in the wind speed conditions. To achieve optimal
power output, a sensor-less scheme including a wind
turbine model was developed by Tan et al in [1]. The
developed wind turbine model will be used in this work in
order to evaluate the different harmonic mitigation
approaches. In spite of, all this complex control theory to
get MPPT on PMSG WECS the standard way to implement
a grid connected PMSG WECS at variable speed is using
two conversion stages: the first one an AC-DC stage and
the second one a DC-AC stage. To realize the first one a
classical three phase bridge rectifier (BR) associated to a
bulky capacitor is used and the second stage could be
implemented by two types of converters schemes Voltage
source current controlled inverter (VS-CCI) and Line
commutated inverter (LCI) as shown in Fig. 1. This paper
has the main focus in the first energy conversion stage the
AC-DC converter, which is responsible by an injection of a
high harmonic current content into the PMSG. The
circulation of these currents into the machine will generate
losses. This work applies a well-known approach to
harmonic mitigation in three-phase AC-DC converters to
WECS [2, 3, 4]: The three-phase boost type PWM rectifier.
Using this converter is possible to minimize the current
harmonic content.
Fig. 1. Wind Energy Conversion System.
A software simulation model developed in [1] using
PSIM software, which allows easy performance
evaluations is used to estimate the behaviour of these two
different schemes associated with the PMSG WECS.
Simulation results showed the possibility of achieving
maximum power tracking, output voltage regulation and
harmonic mitigation simultaneously.
II.
WECS MODEL
The WECS considered in this work consists of a PMSG
driven by a fixed pitch wind turbine; an AC-DC energy
conversion stage implemented using two different
approaches and a VS-CCI. The entire system is shown in
Fig. 1. A brief description of each element of the system is
given below.
A.
Power from wind turbine
The output mechanical power of the wind turbine is given
by the usual cube law equation (1). Where Cp is the power
coefficient, which in turn is a function of tip speed ratio 
and blade angle . This relationship is usually provided by
the turbine manufacturer in the form of a set of nondimensional curves, the Cp curve for the wind turbine used
in this study is shown in Fig. 2. The tip speed ratio is given
by equation (2). A= wind turbine rotor swept area [m2],
Uw= wind speed [m/s], = air density [kg/m3], r= radius of
the rotor [m], m= mechanical angular velocity of the
generator [rad/s].
1
3
P  ρC AU
[Watts]
m 2
p
w
λ
rω m
Uw
(1)
(2)
the flux linkages established by the permanent magnet and
r is the angular frequency of the stator voltage. The
expression for the electromagnetic torque in the rotor is
written as:
Te 
 3  P 
  
 2  2 
Ld  Lq  iqid  λ miq 
(6)
The relationship between r and m may be expressed as:
ωr 
C.
Figure 2. Power coefficient vs. Tip seed ratio with =0.
It can be seen that if the rotor speed is kept constant, then
any change in wind speed will change the tip-speed ratio
(), leading to change of Cp as well as the generated power
out of the wind turbine. If the rotor speed is adjusted
according to the wind speed variation, then the tip-speed
can be maintained at the optimum points, which yield
maximum power output from the system. C pmax is the
maximum torque coefficient developed by the wind turbine
at the optimum tip-speed ratio max. The rate of the rotor
speed is proportional to the inverse of the inertia and
difference between mechanical torque (T m) produced by the
wind turbine and the electrical torque (T e) load from the
generator (3).
dω m
dt

1
J
Tm  Te
2
ωm
(7)
Input Bridge Rectifier (AC-DC converter)
The complete grid connected sensor-less PMSG WECS
scheme using a well-known three-phase six-pulse bridge
rectifier and two bulky capacitors are shown in Fig. 3.
(3)
The wind turbine output mechanical torque is affected by
the Cp. In order to maximize the aerodynamic efficiency,
the Te of the PMSG is controlled to match with the wind
turbine Tm to have maximum possible Cpmax. With a power
converter, adjusting the electrical power from the PMSG
controls the Te; therefore, the rotor speed can be controlled.
For the system to operate at maximum power at all wind
speeds, the electrical output power from the power
converter controller must be continuously changed so that
under varying winds speed condition the system is matched
always on the maximum power locus. From the power
curve of the wind turbine, it is possible to operate the wind
turbine at two speeds for the same power output. In
practice, the operating range at region 1 is unstable as the
rotor speed of the WTG belongs to the stall region. Any
decrease in the tip speed region will cause a further
decrease until the turbine stops.
B.
p
Figure 3. Implemented sensor-less VS-CCI WECS.
D.
Power Variation of the PMSG Wind Turbine
The loading characteristic of the PMSG WECS can be
easily simulated by connecting an adjustable load resistor
to the PMSG and rectifier terminal. Fig. 4 shows the
calculated corresponding output power of the PMSG for
wind speeds ranging from 4 to 12 [m/s], where the
generator maximum power curves show the different
operating DC voltages and currents over a range of wind
speeds. In order to extract the peak power from the WTG at
a given wind speed, the WECS has to match closely to the
maximum power curve.
PMSG model
Theoretical models for generator producing power from a
wind turbine have been previously developed. The outer
rotor 20kW CRESTA PMSG is used in this WECS
mathematical model. The model of electrical dynamics in
terms of voltage and current can be given as (4) and (5) [1]:
di
v
q
q
  Ri  L
ω L i ω λ
q
q dt
r d d
r m
(4)
di
d
v d   Ri d  L d
 ωr Lqiq
dt
(5)
Where, R and L are the machine resistance and inductance
per phase. vd and vq are the 2-axis machine voltages. id and
iq are the 2-axis machine currents. m is the amplitude of
Figure 4. Predicted DC power characteristics the WECS.
III.
HARMONIC ANALYSIS
First of all it is necessary to understand why this study is
important. Therefore, a briefly remark of the problem is
presented. To do this job a study case is presented showing
the PMSG output currents at full load condition (20 kW
resistive load) using a conventional BR shown in Fig. 3,
which is normally employed in PMSG WECS. The wind
speed in this case is 12 m/s. Harmonic characterization of
these abnormal currents is obtained and the results are
presented in the following section. A complete harmonic
analysis of the two three-phase harmonic mitigation
approaches mentioned previously will be presented in the
following sections.
IV.
THREE-PHASE FULL BRIDGE RECTIFIER
A detail of the PMSG WECS output current and line-toline voltage (divided by 4), for the rated power deliver
situation at 12 m/sec wind speed, is shown in Fig. 5.
Figure 7. Harmonic content of the PMSG output voltage.
V.
HARMONIC MITIGATION
The classical passive trap filters are always associated with
the idea of harmonic mitigation, but they are not a good
solution for this application once the frequency of the
generator changes with the wind. In this context an active
solutions like the three-phase boost type PWM rectifier will
play an import role.
A.
Figure 5. PMSG output currents and line to line voltage div. by 4.
In order to evaluate the quality of current and voltage an
objective study was made using the Fourier analysis, the
harmonic content and the total harmonic distortion (THD)
of the output PMSG current and voltage were obtained, the
results are summarized in Fig. 6 and Fig. 7.
Figure 6. Harmonic content of the PMSG output current.
The fundamental components were omitted in these
figures, in order to, remark the harmonic content. From
these figures it is possible to observe that the 5th, 7th, 11th,
13th, 17th and 19th harmonics are significant. The obtained
total harmonic distortion was THD = 10.68 % and 29.15 %
for current and voltage respectively, which are quite high.
At full load, the harmonic content of the output current is
minimized by the influence of the machine stator
equivalent inductance and resistance which are L_F1 = 3
mH and R_S1 = 0.432  respectively. Unfortunately this
effect is not so noticeable when the available wind
decreases and therefore, the maximal output power
decreases and the THD increases. The amplitude of the 5 th
current harmonic is 9.2% of the fundamental, which is
greater than the 4% allowed by IEEE 519 standard. Of
course, the IEEE 519 standard it is not applicable to this
situation but it is a guideline.
Three-Phase Boost type PWM Rectifier (ACDC converter)
As mentioned before the behavior of the Three-Phase
Boost type PWM Rectifier associated to PMSG WECS is
studied in this work focused in losses reduction on PMSG
by harmonic mitigation. The three-phase boost type PWM
rectifier has the capability of to work as an ideal PFC, with
very low THD at any operation point. Hence, it is
investigated in this work. The converter topology has six
two-quadrant controlled switches each one implemented by
an IGBT associated to a diode. The inductors and capacitor
filter the high-frequency switching harmonics, and have
little influence on the low frequency AC components of the
waveforms. The switches of each phase are controlled to
obtain input resistor emulation. To obtain undistorted line
current waveforms, the DC output voltage V must be
greater than or equal to the maximal peak line-to-line AC
PMSG output voltage. The three-phase boost type PWM
rectifier shown in Fig. 8 has several attributes the AC input
currents are non-pulsating because the converter works in
continuous conduction mode (CCM), and hence very little
additional input EMI filtering is required. The converter is
capable of bidirectional power flow. This converter also
has some disadvantage like: requirement for six active
devices, since the rectifier has a boost characteristic
working in CCM it requires a complex control scheme
using either a multiplying controller scheme employing
average current control, or with some other approach like
estimation [3, 4].
Fig. 8. Three-phase boost type PWM rectifier.
The three-phase bridge rectifier input currents are shown in
figure 9 from this figure is easy to observe the CCM and
the very good results obtained.
VII.
In this paper a well-known harmonic mitigation solution
was successfully applied to the PMSG WECS AC to DC
conversion system. The Three-phase bridge rectifier has
presented encouraged results, such as: low current THD,
simple power topology and control circuit and can work in
all wind conditions. Which allow expecting an increasing
in the PMSG lifetime without reduction of the power
capability. The main drawbacks of this topology are: a) the
control complexity and b) the converter cost. With the
actual technology these problems could be minimized using
propers switches like IGBT and DSP processors. The losses
study has demonstrated that the PMSG losses decrease
when the Three-phase bridge rectifier is used. The PMSG
efficiency () increases about 2% and the system efficiency
increases about 1%, losses study will be presented in the
final paper.
Figure 9. Three-phase bridge rectifier input currents.
VIII. REFERENCES
The PMSG output current harmonic content is shown in
figure 10.
0.010
[1]
Harmonic amplitude in percent of the fundamental component
0.009
0.008
THD = 0.06 %
[2]
0.007
0.006
0.005
0.004
[3]
0.003
0.002
0.001
0.000
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
Figure 10. Harmonic content of the PMSG output current using ThreePhase Boost Rectifier.
VI.
[4]
PMSG POWER LOSSES CALCULATIONS
[5]
Basically the total power losses generated into the machine
can be divided into two big groups: a) copper losses and b)
core losses. The copper power losses are produced in the
stator winding as function of RMS current in it according
to equation (8), where Ia_I is the RMS value of the ith
harmonic component of the current Ia and Ra is stator
equivalent resistance. [9]. However, operating at higher
current level resulted in temperature rise. The change of Ra
due to temperature rise was not included in the
calculations. The high frequency flux changing generated
by the harmonics causes hysteresis and eddy current power
losses in the core the equation (9) represents these losses
[9, 10].
PCu  3 R
a

 I2
a_i
i 1
Pcore  Pe  Ph  (k e f 2 B 2 max  k h f B
1.5 to 2.5
(8)
max )  Weight
CONCLUSION
(9)
In order to evaluate the influence of the different
harmonic mitigation approaches in the PMSG power losses
a normalized study is presented in the final paper.
[6]
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