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Title:
Using PFC for Harmonic Mitigation in Wind Turbine Energy Conversion Systems
Authors:
Fernando Soares dos Reis, Member, IEEE
Reinaldo Tonkoski Jr., Student Member, IEEE
Jorge Villar Ale, Dr. Eng.
Fabiano Daher Adegas, Mestrando
**Syed Islam, Senior Member, IEEE
**Kevin Tan, Student Member, IEEE
Address:
Pontifícia Universidade Católica do Rio Grande do Sul
Faculdade de Engenharia
LEPUC – Laboratório de Eletrônica de Potência da PUCRS
Av. Ipiranga, 6681
CEP: 90619-900, Porto Alegre, RS - Brasil.
** Curtin University of Technology
Department of Electrical and Computer Engineering
Tel:
+55 (51) 3320 3686 / 3320 3500
Branch: 4156, 4571, 3686 Sub: 216, 224 and 225
Fax:
+55 (51) 3320 3625
E-mail:
[email protected]
Contact author:
Fernando Soares dos Reis
Topic area:
TPC2- Power Electronics and Electrical Drives
Abstract – Permanent magnet synchronous generators (PMSG) wind energy conversion system
(WECS) using variable speed operation is being used more frequently in 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 allow the variable speed operation of the PMSG WECS a conventional three-phase
bridge rectifier (BR) 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 single-switch three-phase boost rectifier (PFC) applied to harmonic mitigation in this
systems is presented.
Using PFC for Harmonic Mitigation in Wind
Turbine Energy Conversion Systems
F. S. Dos Reis, Member, IEEE, R. Tonkoski Jr., Student Member, IEEE, J. V. Alé, Dr. Eng.,
F. D. Adegas, **S. Islam, Senior Member, IEEE, **K. Tan, Student Member, IEEE
Pontifícia Universidade Católica do Rio Grande do Sul, Porto Alegre, Brazil.
**Curtin University of Technology, Perth, Australia.
Abstract - Permanent magnet synchronous generators
(PMSG) wind energy conversion system (WECS) using
variable speed operation is being used more frequently
in 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
allow the variable speed operation of the PMSG WECS
a conventional three-phase bridge rectifier (BR) 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 singleswitch three-phase boost rectifier (PFC) applied to
harmonic mitigation in this systems is presented. In the
final version, experimental results will be presented.
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 single-switch three-phase boost
rectifier (PFC). Using this converter is possible to minimize
the current harmonic content.
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
Figure 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. In the final version,
experimental results will be presented.
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 [r/sec].
1
3
P  ρC AU
Watts
m 2
p
w
λ
(1)
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. PMSG model
Theoretical models for generator producing power from a
wind turbine have been previously developed. The outer
rotor 20kW CRESTA PMSG described in 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]:
rω m
Uw
(2)
v
di
q
  Ri  L
ω L i ω λ
q
q
q dt
r d d
r m
di
d
v d   Ri d  L d
 ωr Lqiq
dt
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. Cpmax 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 (Tm) produced by the wind
turbine and the electrical torque (T e) load from the
generator (3).
dω m
dt

1
J
Tm  Te
(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
(4)
(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
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  i qid  λ miq 
(6)
The relationship between r and m may be expressed as:
ωr 
p
2
ωm
(7)
C. 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.
III. 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 12m/sec, 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.
Figure 5. PMSG output currents and line to line voltage div. by 4.
Figure 3. Implemented sensor-less VS-CCI WECS
Figure 6. Harmonic content of the PMSG output current.
Figure 4. Predicted DC power characteristics the WECS.
IV. HARMONIC ANALISYS
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 FBR 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.
A. Three-Phase Bridge Rectifier (FBR)
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.
V. HARMONIC MITIGATION
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 7. Harmonic content of the PMSG output voltage.
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.
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 PFC will play an import solution.
A. Single-Switch Three-Phase Boost Rectifier
One of the possible power factor correctors (PFC)
approaches, suitable to implement the input rectifier, the
black block shown in Fig. 3, was the three-phase DCM
boost rectifier proposed by Prasad and Ziogas [3]. Because
this converter presents many advantages such as: a very
low THD in the input current, a simple structure using just
one controlled device and could be controlled using a
simple control strategy easily implemented with generalpurpose PWM integrated circuits (IC) controllers. It is also
important remark that most of the WECS working with
variable speed have a conventional Boost converter
connected between the bridge rectifier and VS-CCI. The
transistor can operate at constant switching frequency. The
control of the DC output power is easily made by duty
cycle control. Only a single active device such as a
MOSFET or IGBT is needed. The main disadvantages of
this implementation are the increasing of the power losses
in the devices in comparison with the conventional threephase AC-DC converter and the need of an additional input
filter to remove the high-frequency harmonic components
of the input currents [3, 4]. Therefore, the implementation
of the input rectifier using a single-switch three-phase
boost rectifier will result in harmonic mitigation on the
PMSG output currents. The complete schematic diagram of
the three-phase DCM boost rectifier is shown in Fig. 8.
“Inductors L1, L2, and L3 have the some small value, such
that they operate in the discontinuous conduction mode in
conjunction with diodes D1 – D6. At the end of the
transistor Q1 conduction subinterval, the inductor currents
reach peak values, which are also proportional to the
applied three-phase line-to-neutral voltages. When
transistor Q1 turns off, then diode D7 becomes forwardbiased and the inductors discharge their stored energies to
the DC output. Since the peak input currents are
proportional to the applied input line-to-neutral voltages,
then the average values of the input currents are also
approximately proportional to the input line-to-neutral
voltages.
shown in figure 8. The obtained values are C_F = 50F and
L = 550 H the chosen switching frequency was Fs = 2.5
kHz. The apparently low switching frequency was elected
because the capacitor C_F size in this case is not a function
of the switching frequency but it is a function of the line
frequency once its main function is to keep the line to line
voltage as close as possible to the sinusoidal voltage
generates by the PMSG due to the high internal impedance
of the generator. The high value of the inductors L_F are
not a problem in this design once they are implemented by
the intrinsic inductances of the PMSG. The low switching
frequency also contributes to minimize the commutation
losses and the simulation time, which is quite large, and
finally inductors L became really low in order to maintain
the DCM. The simulated results are presented in figures 9
to 10. The three-phase bridge rectifier input currents are
shown in figure 9 from this figure is easy to observe the
DCM operation as well as the high peak currents which
yields in high RMS value and therefore high losses in the
input inductors L, capacitors C_F, diodes and transistor.
Figure 9. Three-phase bridge rectifier input currents.
The three-phase PMSG output currents and also the line-toline voltage (divided by 6) are represented in Figure 10.
The improvement in the current and voltage wave forms is
clear when compared with the obtained results for the
conventional three-phase bridge rectifier see figure 5.
Figure 8. Single-switch three-phase boost rectifier.
Approximate three-phase input resistor emulation is
obtained. The three-phase DCM boost rectifier does
generate a modest amount of low frequency input current
harmonics. But, increasing the DC output voltage is
possible to reduce the THD” [4]. Using the design
methodology proposed by Prasad and Ziogas [3] it was
possible to determine the passive components C_F and L
Figure 10. Three-phase PMSG output currents and line-to-line voltage
divided by 6 using Three-Phase Boost Rectifier.
The PMSG output current harmonic content is shown in
figure 11.
The influence of the voltage harmonic content in the
magnetic losses can be evaluated using equations (10) and
(11) proposed by Kaboli et al in [5].
P

e   Vi 
 
P
e1 i 1  V1 
2
(10)
2
Figure 11. Harmonic content of the PMSG output current using ThreePhase Boost Rectifier.
The THD presented by the PFC is lower than 3.5% and the
low amplitude of the 5th harmonic under 3.5% reveals the
suitability of this topology to harmonic mitigation on
WECS. An effective improvement in the voltage THD was
obtained results are shown in figure 12. These good results
were obtained for a constant duty cycle around 50%. The
main drawback of this Boost topology is the high output
voltage 1 kV, inasmuch the line-to-line PMSG RMS output
voltage has increased to 354 V at full load and the PMSG
line current has decreased once the output power was kept
constant at 20 kW.
P

h   Vi  1
 
P
h1 i 1  V1  i
(11)
Where: ke and kh are constants, Bmax is peak flux density, f
is the rated frequency, weight represents the core and
copper weight, Ph1, Pe1 and V1 are the hysteresis and eddy
current power losses and the PMSG line to line output
voltage respectively at the nominal condition resistive load
without harmonics, i is the harmonic order and Vi are the
amplitude of the harmonic components of the PMSG line to
line output voltage. In order to evaluate the influence of the
different harmonic mitigation approaches in the PMSG
power losses. The PMSG power losses as well as the
system losses were obtained and the results are summarized
in tables 1 and 2 respectively.
Table 1. PMSG losses (W) and efficiency (%).
Table 2. WECS losses (W) and system efficiency (%).
Figure 12. Harmonic content of the PMSG output line-to-line voltage
using Three-Phase Boost Rectifier.
VI. Power Losses Calculations
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 (P CU) 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. [5, 6]. 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 (Ph) and eddy current
power losses (Pe) in the core the equation (9) represents
these losses [6].
PCu  3 R
a
P
 Pe  Ph  (k e f
core
2

 I2
a_i
i 1
2
2
B max  k h f B max )  Weight
(8)
(9)
When the simple BR scheme is used the total losses in the
PMSG increases about 30% in relationship to the rated
condition using resistive loads directly connected to the
machine. This significant extra power loss will imply in
premature aging and consequently reduction of the PMSG
lifetime. An amazing result is obtained using the PFC once
the losses on the PMSG are reduced about 10% in
relationship to the rated condition. This improvement will
reduce the internal PMSG temperature and therefore will
increase its lifetime. Another positive point comes from
table 2 in it is shown that the system efficiency () remains
practically the some using BR or PFC schemes.
VII. CONCLUSION
In this paper a well-known harmonic mitigation solution
was successfully applied to the PMSG WECS AC to DC
conversion system. The losses study has demonstrated that
the PMSG losses decrease when the PFC is used. The
PMSG efficiency () increases about 3% and the system
efficiency remain practically constant. The single-switch
three-phase boost rectifier has presented encouraged
results, such as: low current and voltage 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
conduction losses in the BR diodes and switch Q1 (Fig. 8
and 9) since the high RMS current value caused by the
DCM operation and b) the high output voltage 1 kV. Both
problems could be minimized using proper diodes and
switch like IGBT. With the actual technology these
problems could be easily solved. In the final version,
experimental results will be presented.
VIII. REFERENCES
[1]
[2]
[3]
[4]
[5]
[6]
K. Tan and S. Islam, "Optimum Control Strategies
in Energy Conversion of PMSG Wind Turbine
System Without Mechanical Sensors", IEEE
Transactions on Energy Conversion, Vol. 19, No.
2, June 2004, pp. 392-400.
Phipps, J.K.;"A transfer function approach to
harmonic filter design", Industry Applications
Magazine, IEEE , Volume: 3 , Issue: 2 , MarchApril 1997, pp.:68 – 82.
A. R. Prasad, P. D. Ziogas, and S. Manias, “An
active power factor correction technique for threephase diode rectifiers,” in PESC’89 Rec., pp. 58–
65.
Robert W. Erickson, “Some Topologies of High
Quality Rectifiers” Keynote paper, First
International Conference on Energy, Power, and
Motion Control, May 5-6, 1997, Tel Aviv, Israel,
pp. 1-6.
Kaboli, Sh.; Zolghadri, M.R.; Homaifar, A.,
“Effects of sampling time on the performance of
direct torque controlled induction motor drive”,
IEEE International Symposium on Industrial
Electronics, 2003. ISIE '03, Volume: 2 , June 911, 2003, pp.:1049 – 1052
Yao Tze Tat, "Analysis of Losses in a 20kW
Permanent Magnet Wind Energy Conversion
System", in the Department of Electrical and
Computer Engineering. Western Australia: Curtin
University of Technology, October 2003. pp. 101.