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MPPT CONTROL FOR WECS
C.Manoj Kumar 1,N.Beaulah 2
Student 1,Assistant Professor 2
1, 2
Department of Electrical and Electronics Engineering, RVS Padhmavathy College Of Engineering & Technology, Chennai
[email protected]
[email protected]
Abstract- A PMSG based variable speed WECS for rectifier
topology using MPPT consisting of two three phase diode
bridges and three thyristors is proposed in the paper for
variable-speed high-power permanent-magnet synchronous
generator (PMSG) wind energy conversion systems WECSs).
The proposed rectifier has several prominent features such as
low cost, low power loss, and simple control. Its ability to
cascade the input voltages allows it to properly regulate
generator speed even when the wind velocity drops to half of the
rated value. Consequently Maximum power-point-tracking
algorithms can be applied to optimize power capture in a wide
range of wind velocities. The operating principle of the rectifier
is elaborated. Its use and control in the WECS is presented. The
converter and control are verified by simulation and
experimental results.
Index Terms—Maximum power-point tracking (MPPT),
permanent-magnet synchronous generator (PMSG), rectifier,
wind-energy-conversion system (WECS).Impedance source
inverter (ZSI)
I. INTRODUCTION
Wind turbine usage as sources of energy has increased
significantly in the world. With growing application of
wind energy conversion systems (WECSs), various
technologies are developed for them. With numerous
advantages, permanent-magnet synchronous generator
(PMSG) generation system represents an important trend in
development of wind power applications. Extracting
maximum power from wind and feeding the grid with highquality electricity are two main objectives or WECSs. To
realize these objectives, the ac–dc–ac converter is one of
the best topology for WECS.
Variable speed operation yields 20 to 30 percent more
energy than the fixed speed operation, providing benefits in
reducing power fluctuations and improving variable
supply. Falling prices of the power electronics have made
the variable speed technology more economical and
common. Such a wind turbine system as other types of
dispersed generation is mostly connected to distribution
feeders and the generation system cannot be easily
connected to the electric power network without
conducting comprehensive evaluations of control
performance and load impacts. This requires a reliable tool
for simulating and assessing dynamics of a load connected
Variable speed wind turbine. The purpose of the work is to
provide the capability of design, modelling, simulating and
analyzing the dynamic performance of a variable speed
wind energy conversion system using MATLAB /
SIMULINK. The modelled system includes a fixed-pitch
type wind blades, a direct-drive Permanent Magnet
Synchronous Generator without a gear-box, and a
controllable power electronics system, which consists of a
two-three phase rectifier and three phase inverter. Models
of the elements and the system control scheme are
proposed in the form of mathematical equations and
graphical control blocks and implemented in
MATLAB/SIMULINK. The simulation results demonstrate
the modelling work provide a reliable and useful simulation
tool for evaluating the dynamic performance of a variable
speed wind turbine integrated into load.
1. 2-3 Phase Rectifier Topology
PMSG based variable speed WECS for rectifier topology
using MPPT consisting of two three phase diode bridges
and three thyristors is proposed in the paper for variablespeed
high-power
permanent-magnet
synchronous
generator (PMSG) wind energy conversion systems
(WECSs). The proposed rectifier has several prominent
features such as low cost, low power loss, and simple
control. Its ability to cascade the input voltages allows it to
properly regulate generator speed even when the wind
velocity drops to half of the rated value. Consequently
Maximum power-point-tracking algorithms can be applied
to optimize power capture in a wide range of wind
velocities. The operating principle of the rectifier is
elaborated. Its use and control in the WECS is presented.
The converter and control are verified by simulation and
experimental results.
2. Wind Energy Conversion System
The impedance source inverter has been reported recently
as a competitive alternative to existing inverter topologies
with many inherent advantages such as voltage boost. This
inverter facilitates voltage boost capability with the turning
ON of both switches in the same inverter phase leg (shootthrough state).
system where a PWM-VSI is employed as the grid-side
converter, the rectifier provides boosted dc voltage when
the wind speed is lower than rated,such that the PWMVSI’s voltage rating can be well-matched with the
generators while its current rating does not need to be
resized Operating principle of the rectifier and its use in the
WECS will be explained in detail in the following sections.
II. MODES OF OPERATION
Fig.2. Two-Three phase rectifier topology
A. Current Paths and Output Voltages Of The Rectifier
The proposed topology is shown in Fig.1 with this
topology; boost converter is omitted without any change in
the objectives of WECS. Moreover, reliability of the
system is greatly improved
3. Two-Three Phase Rectifier Topology
Fig.2 shows the basic structure of the proposed rectifier
topology. The main circuit is made up of two three-phase
diode rectifier bridges and three individual thyristors .Each
diode ridge is fed by a three-phase power supply and their
outputs are connected in parallel. The three thyristors are
used to connect the corresponding input phases of the two
diode bridges.
Fig.3(a) Current paths in Section I. (a) Thyristors off.
Fig.3 (a), the two diode bridges have separate current paths
and maintain their own rectification when the thyristors are
off. Assuming that we have an inductive dc load that
behaves like an ideal current source, the rectifier’s output
voltage vd is equal to the input line-to-line voltage vab.
However if the thyristors (T2 ) connecting the minimum
voltage phase in the upper supply (phase b) and the
maximum voltage phase in the lower supply (phase y) is
turned on at any time within Section I, the two diode
bridges are forced into a series connection through the
thyristors. The dc voltage vd then becomes the sum of vab
and vyx, equalling to 2vab in magnitude.
Fig.2 Simplified circuit diagram of the two-three phase rectifier topology
In normal operation, when the thyristors are off, the
rectifier’s dc output voltage is equal to the output of a
single diode bridge; when the thyristors are controlled and
turned on at the right instants ,the outputs of the two diode
bridges can be cascaded and the resultant dc voltage is
doubled. For the rectifier to operate properly, the two threephase input power supplies should be equal in magnitude
and have 180◦phase displacement with each other. This
constraint guarantees that doubled output voltage is
obtained when the thyristors are turned on .
The voltage-doubling feature of the proposed rectifier
adapts it to be used in variable-speed WECSs to interface
the generators .Either a PMSG with dual stator windings or
two PMSGs,each having one set of three-phase winding,
can be used as the ac power supplies for the rectifier. In a
Fig.3 (b). Current paths in Section I. (b) Thyristors on.
Fig.3 (b) The other five sections have situations similar to
Section I and are not described further for the sake of
brevity. The illustration of all the current paths in all
sections can be found.
III. MPPT CONTROL METHODS FOR PMSG
BASED WECS
Permanent Magnet Synchronous Generator is favoured
more and more in developing new designs because of
higher efficiency, high power density, availability of highenergy permanent magnet material at reasonable price, and
possibility of smaller turbine diameter in direct drive
applications. Presently, a lot of research efforts are directed
towards designing of WECS which is reliable, having low
wear and tear, compact, efficient, having low noise and
maintenance cost; such a WECS is realisable in the form of
a direct drive PMSG wind energy conversion system.
There are three commonly used configurations for WECS
with these machines for converting variable voltage and
variable frequency power to a fixed frequency and fixed
voltage power. The power electronics converter
configurations most commonly used for PMSG WECS are
shown in Fig.4 (a),(b),(c)
Depending upon the power electronics converter
configuration used with a particular PMSG WECS a
suitable MPPT controller is developed for its control. All
the three methods of MPPT control algorithm are found to
be in use for the control of PMSG WECS.
IV. IMPEDANCE SOURCE INVERTER (ZSI)
Several types of power electronics interfaces have been
investigated. In this study, system is interfaced with a six
diode rectifier and three phase Impedance source inverter
which is less expensive than others and commonly put into
industrial use, has been modelled for AC-DC–AC
conversion a rectifier model and inverter model. The six
diodes rectifier converts ac power generated by the wind
generator into dc power in an uncontrollable way and it is
given to Impedance source inverter it convert into ac and
inverter output is given Voltage grid system. The ZSI is a
voltage harmonic source in the point view of ac system and
a harmonic filter need be placed appropriately to reduce the
voltage harmonics it generates. AL-C harmonic filter
consisting of a series interconnection inductor and a
parallel capacitor is located at the ZSI terminal.
Fig.5.shows a rectifier and ZSI system model that has been
implemented in Matlab/Simulink.
(a)
Fig.5 Two-Three Rectifier and inverter model.
(b)
(c)
The six diodes rectifier converts ac power generated by the
wind generator into dc power in an uncontrollable way and
so control has to be implemented by the power electronics
inverter. Current-controlled ZSI can generate an AC
current which follows a desired reference waveform so can
transfer the captured real power along with controllable
reactive power. For the modelling study, DQ control
method that is widely used for ZSI current control is
employed. Variables in the ABC three phase coordinates
may be transformed into those in the d-q reference frame
rotating at synchronous speed by the rotational d-q
transformation matrix. In the three-phase balanced system,
the instantaneous active and reactive power outputs, P and
Q, of the wind turbine are described by equation,
3
3
2
2
𝑃 = (𝑉𝐷 𝐼𝐷 + 𝑉𝑄 𝐼𝑄 ), 𝑄 = (𝑉𝐷 𝐼𝑄 − 𝑉𝑄 𝐼𝐷 )
Fig.4 PMSG wind energy conversion systems with MPPT
Where,
VD = d-axis voltage at the wind turbine
VQ = q-axis voltage at the wind turbine
ID = d-axis current at the wind turbine
IQ = q-axis current at the wind turbine.
Here, VQ is identical to the magnitude of the instantaneous
voltage at the wind generation system and VD is zero in the
rotating d-q coordinates, by equation
3
3
2
2
𝑃 = |𝑉𝑜 |𝐼𝑄 ,𝑄 = |𝑉𝑜 |𝐼𝐷 ---(1)
where |VO| is the instantaneous voltage magnitude of the
wind turbine system. Since the voltage remains at a level of
the grid AC voltage and the voltage variation is very small
compared to changes in the magnitude of IQ and ID, P and
Q are mainly subject to the d-axis current and q-axis
current respectively. DQ control decouples real and reactive
components and enables real power and reactive power to
be separately controlled by specifying the respective
reference values of P ref and Q ref for the both power outputs
and independently adjusting the magnitude of the d-axis
current IQ and that of the q-axis current ID. The reference
values P ref and Q ref of the wind generation are specified by
what ZSI’s control strategies are taken for real and reactive
power output. The firing signals are generated by the sine
pulse width modulation (SPWM) technique. The desired
current vector IABC_REF and the actual output current vector
IABC_WT of the wind system are compared and the error
signal vector IERR is compared with a triangle waveform
vector to create the switching signals.
Where, CP max = the maximum power coefficient
 opt = value of  where CP max = CP (  opt)
 = electrical loss in generator and inverter
(I) Reactive Power Control
Various control modes can be used for determining the
amount of reactive compensation to provide. Possible
control modes include power factor, current and voltage.
Constant power factor mode and voltage regulation mode
are implemented in this analysis. In constant power factor
control (PFC) mode, the reference value of the reactive
power of the wind turbine, QREF, may be specified by
equation (3).
𝑄𝑅𝐸𝐹 = 𝑃𝑅𝐸𝐹 √1 − 𝑃𝑓 2 /𝑃𝑓 --- (3)
Where Pf is power factor and P ref is the reference value of
real power output of the VSWT.
In voltage regulation (VR) mode, reactive power
compensation is controlled in such a manner that the
voltage magnitude of the VSWT-connected bus being kept
constant at a specified level. The reference magnitude of
the voltage to be regulated must be set as the nominal
voltage of the AC grid where the wind turbine is
considered as being interconnected. Whether the mode
controls constant power factor or voltage, the reactive
power capability of a VSWT is limited. Such a limitation is
required to be considered in the modelling study. The
reactive capability limits of the wind turbine used in this
study are determined by MVA rating of the inverter which
may be described by equation (4)
2
2
𝑄𝑙𝑖𝑚𝑖𝑡𝑠 = ±√𝑆𝐼𝑁𝑉
− 𝑃𝐼𝑁𝑉
----- (4)
Where Q limits, PINV and SINV are the reactive power limits,
the real power output and MVA rating of the inverter
respectively.
V. SIMULATION RESULT ANALYSIS
Fig.6 Current control scheme of an Impedance source inverter
(I)Capturing the Maximum Power from Wind
The maximum aerodynamic power available from wind
energy can be described by equation (1). This simply
means that the maximum power may be achieved by
varying the turbine speed with varying wind speed such
that at all times it is on the track of the maximum power
curve. One way of enabling the maximum power capture is
to specify the reference value of real power for the inverter
control as the available maximum power multiplied by the
inverter efficiency, as shown in equation (2).
𝑃𝑀𝑀𝑎𝑥 =
1
2
𝑀𝑎𝑥
𝜋𝜌𝑅5 𝐶𝑃
3
/𝜆3𝑜𝑝𝑡 𝜔𝑀
---------- (2)
𝑃𝑅𝐸𝐹 = ɳ𝑃𝑀𝑀𝑎𝑥
The rating capacity is chosen to be 1.5 MVA and real
power 1.5 MW. The rated speed of the rotor is chosen to be
40 rpm. The rated wind speed is 8 m/s. the cut-in and cutout speeds are 4 m/s and 16 m/s
respectively. The
switching frequency of the grid interface inverter is 1.040
kHz. The capacitor value of grid interface rectifier is
2500uF and dc link voltage is
2.5kv. The generated
voltage of Permanent Magnet Synchronous Generator is
0.6kv. The transformer rating of grid connected side is
2.5k/130kv. The p.u voltage magnitude of primary of the
transformer is 0.99p.u. The maximum value of Cp is 1.2.
The proposed system operate the unity power factor control
Simulation waveform real power of step change of wind
velocity from 8 to 11m/s in variable speed wind turbine. It
shows that there is a light overshoot at variation of wind
velocity Fig.7. is observed from the simulation waveform
reactive power of step change of wind velocity from 8 to
11m/s in variable speed wind turbine. Fig.7.shows that
simulation waveform pitch angle in variable speed wind
turbine coupled Permanent Magnet Synchronous Generator
at different velocity. Fig.9. Simulation waveform two-three
phase rectifier step change of wind velocity from 8 to
11m/s in variable speed wind turbine coupled Permanent
Magnet Synchronous Generator. Fig.7. Simulation
waveform generated speed of step change of wind velocity
from 8 to 11m/s in variable speed wind turbine coupled
Permanent Magnet Synchronous Generator in 1.01p.u.
Show that simulation waveform grid voltage in p.u of step
change of wind velocity from 8 to 11m/s in variable speed
wind turbine coupled Permanent Magnet Synchronous
Generator.
Maximum rate change = 2 degree
Controller = PI controller
Fig 7 PMSG Simulation
Fig.11. is observed that Simulation waveform mechanical
torque in p.u of step change of wind velocity from 8 to
11m/s in variable speed wind turbine coupled Permanent
Magnet Synchronous Generator. Fig.12. Simulation
waveform torque in p.u of step change of wind velocity
from 8 to 11m/s in variable speed wind turbine coupled
Permanent
Magnet
Synchronous
Generator.
Fig.4.6.Simulation waveform of Vf in p.u of step change
of wind velocity from 8 to 11m/s in variable speed wind
turbine coupled Permanent Magnet Synchronous
Generator. Fig.7 Simulation waveform of boosted voltage
in shoot through state occur in leg 1. Fig.13. Simulation
waveform of boosted voltage in shoot through state occurs
in leg 2. Fig.7.7.
Simulation waveforms of boosted voltage in shoot through
state occur in leg 3. Fig.Simulation output of speed vs time
in m/s. Fig.14Simulation output of power vs time in m/s.
Fig.14.Simulation
output
of
power
vs
time
inm/s..Simulation output o f rated power vs wind speed in
m/s..Simulation output of real and reactive power in m/s.
Fig 8.MPPT control using PMSG
Simulation Model for 2-3phase Rectifier Topology
SIMULATION PARAMETERS
Rating capacity = 1.5MVA
Real power = 1.5MW
Rated speed = 40rpm
Rated wind speed = 16 m/s
Cut-in speed = 4 m/s
Cut-out speed = 25 m/s
DC link voltage = 1800 V
Fig 9.2-3phase rectifier topology
Maximum pitch angle = 45 degree
Fig.14 Output Speed of VSI
Fig 10 Dc output voltage from 2-3phase rectifier topology
Fig. 15 Output torque of ZSI
Fig 11.PMSG Voltage
SIMULATION OF ZSI
Fig 12. PMSG current
Fig 16 Simulation of ZSI
SIMULATION OF VSI
Fig 13 VSI Output Voltage
Fig 17 Output voltage of ZSI
turbines in networks must ensure their systems meet the
requirements for grid connection. Therefore, the work done
in this study provides a reliable tool for evaluating the
performance of variable speed wind turbines and their
impacts on power networks in terms of dynamic behaviours
as a preliminary analysis for their actual integrations and
operations.
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Fig 18 Output speed of ZSI
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dynamics of wind turbine into account,” Proc. IEEE PESC, pp. 1–6, 2006.
4. K. E. Johnson, L. J. Fingersh, M. J. Balas, and L. Y. Pao, “Methods for
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J.Solar Energy Eng., vol. 126, p. 1092, 2004.
Fig19.Output torque ZSI
5.S. Morimoto,H.Nakayama, M. Sanada, andY. Takeda, “Sensorless
output maximization control for variable-speed wind generation system
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using IPMSG,” IEEE Trans. Ind. Appl., vol. 41, no. 1, pp. 60–67,
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ZSI
VSI
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Voltage =1800
Voltage =1500
Speed =1800
Speed = 1500
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Torque = 100
Torque =100
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Harmonics is reduced
Harmonics is more
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VI.CONCLUSION
9. D. Xu and Z. Luo, “A novel AC–DC converter for PMSG variable
speed wind energy conversion systems,” in Proc. IEEE IPEMC, 2009, pp.
A dynamic model of a variable speed wind generation with
power electronic interface was proposed for computer
software tool simulation study and implemented in
Matlab/Simulink. Component models of a VSWT and its
control scheme have been built by using Matlab function
block and control block provided in the software. A wind
model was integrated into the modelling to see the wind
impact. Dynamic responses of the wind turbine to varying
wind speeds and under different reactive control schemes
were simulated and analyzed based on the modelled
system. In the view point of electric utilities, grid interface
of intermittent generation sources such as wind turbines has
been a challenge that can cause lower power quality in
power systems. So comprehensive impact studies are
absolutely necessary before wind turbines being added to
real networks. Also, users who intend to install wind
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for direct-drive variable-speed wind power unit,” IEEE Trans. Energy
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Authors Biography
N.Beaulah received her B.E degree in Electrical
& Electronics Engineering from Karpaga
Vinayaga College of Engineering & Technology
(Affiliated to Anna University-Chennai) and
M.E (Power Electronics & Drives) from Sri Sai
Ram Engineering College (Affiliated to Anna
University),Chennai. At present she is working with RVS
Padhmavathy College Of Engineering & Technology, Chennai as
Assistant Professor. Her area of interest includes Renewable
Energy and Power Electronics
C.Manoj Kumar currently pursuing B.E
degree in Electrical & Electronics Engineering
from RVS Padhmavathy College Of
Engineering & Technology (Affiliated to Anna
University-Chennai) .His area of interest
includes Power Electronics, Renewable
Energy & Drives.