Download A. Power from wind turbine

PWM Boost Rectifier versus Trap Filters Applied to 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
Department of Electrical and Computer Engineering
Curtin University of Technology
GPO Box U1987, Perth, Western Australia 6845
AUSTRALIA
e-mail: [email protected]
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 two different harmonic
mitigation approaches applied to PMSG WECS. The studied
techniques are: a) harmonic trap filters and b) three-phase
boost type PWM rectifier.
I.
*
FENG - Department of Electrical Engineering
Pontifical Catholic University of Rio Grande do Sul
90619-900, Av Ipiranga, 6681, Porto Alegre, RS
BRAZIL
e-mail: [email protected]
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 two well-known approaches to
harmonic mitigation in three-phase AC-DC converters to
WECS [2, 3]: a) harmonic trap filters (HTF) and b) threephase boost type PWM rectifier. Using these approaches is
possible to minimize the current harmonic content.
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
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
(1)
and
λ
rω m
(2)
Uw
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  iqid  λ miq 
(6)
The relationship between r and m may be expressed as:
ωr 
Figure 2. Power coefficient vs. Tip seed ratio with =0.
C.
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]:
v
di
q
  Ri  L
ω L i ω λ
q
q
q dt
r d d
r m
(4)
Figure 4. Predicted DC power characteristics the WECS.
III.
HARMONIC ANALYSIS
di
d
v d   Ri d  L d
 ωr Lqiq
dt
(5)
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.
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 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. Also the PFC schemes will
need high frequency filters to mitigate the high frequency
current ripple generated by them selves.
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.
Passive Harmonic Trap Filters
Looking for this classical power electronics problem a first
design could be to use passive HTF 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 point at the best wind condition, the performance of
the trap filters must be investigated. A design of two trap
filters was made to minimise the 5th and 7th harmonic at
nominal power. The designed components are C5 = 135F,
C7 = 69F and L5 = L7 = L_F1 = 3mH. As the focus of this
work is in the field of the losses minimization is important
to remark that there is no ideal components.
Figure 8. Passive harmonic trap filters.
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 5th
current harmonic is 9.2% of the fundamental, which is
Hence, the resistance series equivalent (RSE) of each filter
component must be considered. The RSE of each branch
were estimated equal to the armature resistance (RSE =
0.432 ) once the branch inductances are equal to the
armature inductance. The simulated results are shown in
figures 9 to 12. A remarkable improvement in the current
and voltages waveforms was obtained using this simple
solution. Figure 9 shows the PMSG output currents and the
line to line output voltage (divided by 5), which are almost
sinusoidal. A current THD less than 2.5% was obtained as
shown in Fig. 10. However, this solution implies in bulky
components and is matched only for the maximum power
condition, which occurs at 12 m/s wind speed. In figure 11
the PMSG output line to line voltage div. by 5 is shown in
the frequency domain.
Figure 9. Three-phase PMSG output currents and output line to line
voltage div. by 5 using 5th and 7th harmonic trap filters.
Figure 11. Harmonic content of the PMSG output line-to-line voltage
using 5th and 7th harmonic trap filters.
Harmonic amplitude in percent of the fundamental component
1.6
1.4
1.2
1.0
THD = 2.26 %
0.8
0.6
0.4
0.2
0.0
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 5th and 7th
harmonic trap filters.
From these figures is possible to observe an improvement
in the PMSG output line-to-line voltage waveform in
relationship with the first case without any filter shown in
figure 5. The voltage THD was drastically reduced from
29.15% to 9.20%. On the other hand its RMS value has
increased from 293 V to 343 V respectively. This occurs
because the chosen comparison parameter, which was: the
constant output power (20 kW). The resistive load is
adjusted in order to keep the output power constant. Once
the harmonic core losses are related with the harmonic
content of the voltage [5, 6] this characteristic can not be
neglected. In order to evaluate the system losses is
necessary to know the RMS value of the currents in PMSG
output ia, HTF currents ia5th and ia7th and the BR diodes
current i_Ret_a. Figure 12 shows the time domain evolution
of these currents. The high current values will generate
important losses in the system. On the other hand the
copper losses inside the PMSG will be slightly reduced
since the HTF has change the WECS operation point
increasing the line-to-line voltage and decreasing the line
current. Unfortunately in the real WECS, the wind speed is
constantly varying and hence the PMSG produces variablevoltage and variable-frequency output. Therefore, an
infinity number of the trap filters would be 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 is to use the factor
correctors (PFC) and 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.
Figure 12. PMSG output, bridge rectifier and 5th and 7th harmonic trap
filters current at 20 kW.
B.
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. 11 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].
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
(11)
Fig. 12. Three-phase boost type PWM rectifier.
VI.
PMSG POWER LOSSES CALCULATIONS
The maximum C1+C2 value is 2.2 µF, and for a proper
damping effect, C2=10C1. The damping resistance can be
calculated as:
Lf
C2
Rd 
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
(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.
VII.
EMI FILTER DESIGN CONSIDERATIONS
The design of a suitable EMI filter can be done according
different approaches. 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
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. The maximum capacitance should be
used to minimise the inductance value. Let us consider the
damped second order filter topology shown in Fig. 13. This
filter attenuates 40 dB/dec [11]. For a given necessary
attenuation, the cut-off frequency is given by:
fx
fc 
A1
10
A2
Fig. 13. EMI differential mode filter.
VIII.
(8)
max )  Weight
(10)
(12)
CONCLUSION
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.
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. The full paper will present
design considerations and a complete harmonic mitigation
study for this PFC. 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. An
EMI filter design criterion is in addition presented.
IX.
[1]
[2]
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