Download 3. Power Variation of the PMSG Wind Turbine

HARMONIC MITIGATION IN WIND TURBINE ENERGY CONVERSION SYSTEMS
Fernando Soares dos Reis, Member, IEEE, Kelvin Tan Student Member, IEEE, and Syed Islam, Senior Member, IEEE
*School of Electrical and Computer Engineering
Curtin University of Technology
GPO Box U1987, Perth, Western Australia 6845
Australia
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. This paper presents a comparative simulation study
between three different approaches applied to harmonic mitigation on PMSG WECS. The studied
techniques are: a) harmonic trap filters, b) single-switch three-phase boost rectifier and c) three-phase
boost type PWM rectifier;
Keywords:
Harmonic mitigation; PFC; PWM rectifier; permanent magnet synchronous generator; wind energy
converters system; variable speed operation.
Corresponding author:
Fernando Soares dos Reis
School of Electrical and Computer Engineering
Curtin University of Technology
GPO Box U1987, Perth, Western Australia 6845 - Australia
E-mail: [email protected]
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. This paper presents a
comparative simulation study between three different
approaches applied to harmonic mitigation on PMSG
WECS. The studied techniques are: a) harmonic trap
filters, b) single-switch three-phase boost rectifier and c)
three-phase boost type PWM rectifier;
Keywords
Harmonic mitigation; PFC; PWM rectifier; permanent
magnet synchronous generator; wind energy converters
system; variable speed operation.
1. 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 converters 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 sensorless scheme developed by Tan et al in [1] will be used in
this work for extracting desired output power from the
WTG over a wide range of wind speeds. 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 full bridge rectifier 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.
Figure 1. Wind Energy Conversion System
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
three well-known approaches to harmonic mitigation in
three-phase AC-DC energy conversion systems [2, 3]: a)
harmonic trap filters, b) single-switch three-phase boost
rectifier and c) three-phase boost type PWM rectifier;
Using these approaches is possible to minimize or to
eliminate the current harmonic content. A software
simulation model developed in [1] using PSIM
software, which allows easy performance evaluations is
used to estimate the behaviour of these three different
schemes associated with the PMSG WECS. Simulation
results showed the possibility of achieving maximum
power tracking, output voltage regulation and harmonic
mitigation simultaneously.
2. 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 power
factor rectifiers; and a VS-CCI. The entire system is
shown in Fig. 1. A brief description of each element of
the system is given below.
2.1.
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 . This relationship is usually provided by the
turbine manufacturer in the form of a set of non-
dimensional 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
rω m
(1)
(2)
Uw
may, however, interfere with the control system
modelled in this paper.
2.2.
PMSG model
Theoretical models for generator producing power from a
wind turbine have been previously developed [6, 7]. The
outer rotor 20kW CRESTA PMSG described in [8] 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) [7]:
(4)
v  (R  pL )i  ω L i  ω λ
q
q q
r dd
r m
v d   (R  pLd )i d  ω r L q i q
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. Cp max 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.
dω m
dt

1
J
Tm  Te
(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 ρ = d/dt. r=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 
2.3.
p
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 Cp max. 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.
Therefore, the controller has to be designed to keep the
operating point inside the desired region. For a variable
speed wind turbine with pitch control, optimum power
can easily be obtained using appropriate control.
However, for small machines that use a fixed pitch, this
mechanism is not possible. The current paper looks at
fixed pitch machines. The use of pitch machines control
Figure 3. Implemented sensor-less VS-CCI WECS
3. 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.
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.
Figure 4. Predicted DC power characteristics the WECS.
4. 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
using a conventional six-pulse rectifier shown in Fig. 3
which, is normally employed in PMSG WECS is
presented, 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 threephase PFC presented previously will be presented in the
final paper.
4.1.
Figure 6. Harmonic content of the PMSG output current.
In Fig. 7 a detail of the harmonic content amplitude of
the PMSG output current in percent of the fundamental is
shown. The fundamental component was omitted in this
figure, in order to, remark the harmonic content.
Three-Phase Full Bridge Rectifier
A detail of the PMSG WECS output currents, for the
maximum power deliver situation at 12 m/sec wind
speed, is shown in Fig. 5. In order to evaluate the quality
of these currents an objective study was made using the
Fourier analysis, the harmonic content and the total
harmonic distortion (THD) of the output PMSG current
were obtained, the results are summarized in Fig. 6 and
Fig. 7. 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 = 13.47 % which is quite high.
Figure 7. Harmonic content of the PMSG output current.
The amplitude of the 5th harmonic is 12.4% of the
fundamental, which is greater than the 10% allowed by
IEEE 519 standard. Of course, the IEEE 519 standard it
is not applicable to this situation but it is a guideline.
5.0.
HARMONIC MITIGATION
The classical passive trap filters are always associated to
the idea of harmonic mitigation. Also the PFC schemes
will need high frequency filters to mitigate the high
frequency current ripple generated by them selves. A
high frequency filter design criterion will also be
presented.
5.1.
Figure 5. Three phase PMSG output current at 20 kW.
At full load, the harmonic content of the output current is
minimized by the influence of the machine stator
Passive Harmonic Trap Filters
Looking for this classical power electronics problem the
first solution is to use passive harmonic trap filters as
shown in Fig. 8 in fact a reduction of the 5th and 7th
harmonic could be enough to turn the THD to acceptable
levels. If the main idea is to found a compromise solution
to track maximum power at the best wind condition the
trap filters could be used.
necessary. To solve this problem an active solution is
needed, there are basically two actives approaches to
solve this power electronics problem the first solution
have different names as: factor correctors (PFC), power
factor rectifiers (PFR), power factors preregulators
(PFP), PWM rectifiers or resistor emulators the second
possible solution is to use active power filters. The study
of the active power filters applied to PMSG WECS is out
of the scope of this paper. However, is under study at the
moment.
5.2.
Single-Switch Three-Phase Boost Rectifier
(AC-DC converter)
Figure 8. Passive harmonic trap filters.
A design of two trap filters was made to minimise the 5th
and 7th harmonic the results are shown in Figs. 9 and 10.
Figure 9. Three-phase PMSG output current at 20 kW
using 5th and 7th harmonic trap filters.
A remarkable improvement in the current wave form was
obtained using this simple solution. A THD less than 3%
was obtained as shown in Fig. 10. The fundamental
component was omitted in this figure, in order to, remark
the low harmonic content. Note that this solution implies
in bulky components and is matched only for the
maximum power condition, which occurs at 12 m/s wind
speed.
One of the selected power factor corrector (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 [2].
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 general-purpose PWM integrated
circuits (IC) controllers. 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 three-phase AC-DC
converter and the need of an additional input filter to
remove the high-frequency harmonic components of the
input currents [2, 4]. Therefore, the implementation of
the input rectifier using a single-switch three-phase boost
rectifier resulting in harmonic mitigation on the PMSG
output currents. The complete schematic diagram of the
three-phase DCM boost rectifier is shown in Fig. 11.
Figure 11. Single-switch three-phase boost rectifier.
Figure 10. Harmonic content of the PMSG output current
using 5th and 7th harmonic trap filters.
Unfortunately in the real WECS, the wind speed is
constantly varying and hence the PMSG produces
variable-voltage
and
variable-frequency
output.
Therefore, an infinity number of the trap filters would be
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 release 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. 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].
5.3.
Three-Phase Boost type PWM Rectifier
(AC-DC converter)
As mentioned before the behaviour of the two PFC
associated to PMSG WECS are 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 in order to overcome the disadvantages
presented by the single-switch three-phase boost rectifier.
The converter topology has six two-quadrant controlled
switches each one implemented by an active device
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. 12 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 bi-directional power flow. This
converter also has some disadvantage like: requirement
for six active devices when compared with the previous
circuit shown in Fig. 11 which uses just one active
device, the active semiconductor utilization is low, since
the rectifier has a boost characteristic and requires a
much more complex control scheme using either a
multiplying controller scheme employing average current
control, or with some other approach like estimation [3,
4, 5].
waveform is not so important for the proper operation of
the PFC in the Discontinuous Conduction Mode, some
damping effect is necessary to avoid oscillations in the
average input current produced by transient situations,
like load or wind changes. According to the paper
proposition, the filter will be designed considering only
the differential mode EMI noise produced by the
converter. For this filter type, not only the required
attenuation but also other restrictions must be taken into
account. For example, VDE standard specifies a
maximum x-type capacitance of 2.2 F in order to limit
the line current (50/60Hz component) even at no load
situation. The maximum capacitance should be used to
minimise the inductance value. Let us consider the
damped second order filter topology shown in Fig. 10.
This filter attenuates 40 dB/dec. For a given necessary
attenuation, the cut-off frequency is given by:
fx
fc 
A1
10
A2
(8)
Where fc is the cut-off frequency, fx is the frequency in
which the required attenuation (A1) is determined. A2 is
the filter characteristic attenuation. The inductance value
is given by:
1
Lf  2
4 C1f c2
(9)
The maximum C1+C2 value is 2.2 µF [8], and for a
proper damping effect, C2=10C1. The damping resistance
can be calculated as:
Rd 
Lf
C2
(10)
Fig. 13. EMI differential mode filter.
6. CONCLUSION
Figure 12. Three-phase boost type PWM rectifier.
5.4.
EMI Filter Design Considerations
The design of a suitable EMI filter can be done according
different approaches [7-9]. Usually a high-order filter is
used to reduce inductance and capacitance values. Ideally
it could be considered that no harmonic components exist
in the frequency range between the line and the switching
frequencies. This fact would allow centring the filter
resonance at a suitable frequency in order to guarantee
the required attenuation. In this ideal situation the filter
could be undamped. In the Continuous Conduction Mode
the input rectified voltage is usually used to create the
current reference waveform. In this case the presence of
instabilities in the filter output (converter side) can lead
to severe converter malfunction. As the input voltage
This digest presents a review on harmonic mitigation in
three phase rectifiers using trap filters and PFC applied to
PMSG WECS and preliminary simulation results of a
prototype variable speed PMSG WECS. Three possible
harmonic mitigation solutions were discussed. Using
harmonic trap filters is possible to maximize the PMSG
WECS efficiency by mitigation of the harmonics losses.
Regrettably, this solution is effective just at one specific
operation point since the wind is constantly changing as
well the frequency. The single-switch three-phase boost
rectifier requires a very simple changing in the three
phase rectifier topology as shown in Fig. 11, once the
converter work in the DCM the control is as well very
simple. The full paper will present design considerations
and a complete harmonic mitigation study. However, the
conduction losses in the switch Q1 Fig. 11 are high since
the high RMS current value caused by the DCM
operation. The problem could be minimized using proper
switches like IGBT or GTO. An EMI filter must be used
for EMI reduction and for proper operation of the
converter, to avoid CCM once the PMSG stator
inductance is high. The best technical solution is
obtained using three-phase boost type PWM rectifier
which can mitigate the harmonic generation over the
entire range of wind speed. A very low EMI level is
generated since this converter work in CCM. Now, the
high stator inductance value of the PMSG is desirable
and EMI filter could be strongly minimized or
unnecessary. In spite of this, this is the most expensive
solution and requires more complex control circuits. An
EMI filter design criterion is in addition presented.
7.
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