This document compares two control strategies - SVM based voltage oriented control and hysteresis current control - for interfacing a permanent magnet synchronous generator (PMSG) to the grid. Simulation results show that under constant and variable wind speeds: 1) The SVM control strategy regulates the grid voltage, current and DC link voltage more precisely with faster settling time compared to hysteresis control. 2) Both controllers effectively transfer maximum power from the PMSG to the grid while maintaining unity power factor. 3) Under variable wind speeds, the SVM control provides smoother transition of the generator speed, grid current, and active/reactive power compared to hysteresis control.

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Comparative Study of SVM and Hysteresis Control Strategies for Grid Side
Converter of PMSG
Conference Paper · December 2014
DOI: 10.1109/INDICON.2014.7030546
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2. Comparative Study of SVM and Hysteresis Control
Strategies for Grid Side Converter of PMSG
R.K. Soni, Avneet K. Chauhan, Raja Ram Kumar, S.K. Singh
Department of Electrical Engineering
Indian Institute of Technology (Banaras Hindu University), Varanasi
Varanasi, India
ravi.soni.eee08@itbhu.ac.in , avneet.kumar.eee13@itbhu.ac.in, rajaram.rs.eee@itbhu.ac.in
Abstract—This paper deals with dynamic simulation of a
directly driven Permanent Magnet Synchronous Generator
(PMSG) with a full scale converter interfaced to the grid. A
comparative assessment of two control strategies is the main
focus of this paper. The first is the SVM based voltage oriented
control strategy and the second is the hysteresis current control
strategy. Maximum power point tracking and pitch angle
control is also modeled. The controller performance is analyzed
through simulation results with various changes in wind velocity.
Index Terms- PMSG, pitch control, maximum power point
tracking, voltage oriented control, hysteresis current control.
I. INTRODUCTION
The wind energy extracted by a wind turbine is not constant,
from the point of view of utilities, due to fluctuation of
generator output, it is necessary to use a controller to manage
the output produced by the wind turbine generator. The
control technology relating to wind turbine is sophisticated in
terms of generator speed and torque control, pitch angle
control, reactive power control, maximum power point
tracking (MPPT), and so on.[2]India has the fifth largest
installed wind power capacity in the world. By the end of
August 2012, wind power installations in India had reached
17.9 GW.[1]
This paper addresses the generator control of a small wind
turbine for grid connection. Because of the reduction in price
of magnet and magnetic characteristic improvement [3],
PMSG has numerous advantages over other machines. In case
of induction generator stator currents contain not only the
torque-producing currents, but also the magnetizing
components. With the use of the permanent-magnet in the
rotor, the stator currents need only be toque-producing. It
means the permanent-magnet synchronous generator will
operate at higher power factor because of the absence of
magnetizing currents. So the permanent-magnet synchronous
generator will be more efficient than that of induction
generator. In PMSG there is no need of excitation for the
rotor, otherwise which need brush and slips rings, which
means no rotor losses and brush maintenance [4]. In other
Wind energy conversion systems, the gearbox is one of the
most critical turbine components, since its failure is highly
expected, and it requires careful and regular maintenance. In
PMSG due to high number of poles gear box can be omitted.
There are two popular power electronic configurations
employed for interfacing PMSG with gird, first PMSG system
with passive diode rectifier with boost chopper followed by
IGBT inverter, and the second configuration is PMSG system
with two fully controlled full-size IGBT PWM converters [5].
The first configuration does not have the full capability to
control the rotational speed of the PMSG, which, in due
course, deteriorate the capability of maximum wind power
capturing. However, the second configuration allows flexible
control of the rotational speed of the PMSG and consequently
can provide maximum wind power tacking, if proper
switching signals are applied.
This paper studies the performance of grid connected direct
driven PMSG based wind turbines. The PMSG is connected to
the grid at the point of common coupling (PCC) via an AC-
DC-AC back-to-back converter set. One control schemes form
a machine side converter is developed and two different
control strategies for grid-side converter are also compared.
Fig.1 Interfacing PMSG based wind turbine to grid
The dynamics of the system and control action is simulated
with detailed model using MATLAB/SIMULINK.
II. MODELING AND CONTROL OF MECHANICAL
SYSTEM
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3. A. Wind turbine model:
The power inherent in the wind, which is converted into
mechanical power by the rotor blades, can be determined by
Eq. (1) [6].
1
2 , (1)
where ρ is the density of air, A is the wind turbine swept area,
is the wind speed, is the power coefficient and it is the
function of the pitch angle β and the tip speed ratio λ .
The tip speed ratio is defined as[7]:
(2)
where R is rotor radius m andω is turbine angular speed
In order to calculate the Cp curve, equ. 3is used. [8]
, (3)
Where
(4)
Values of to have been taken from [9]
0.5176, 116, 0.4, 5, 21,
0 .0068, 0.08, 0.035
Fig. 2 shows how power coefficient (Cp) depends on tip speed
ratio(λ) and pitch angle(β) Power Co-efficient(Cp) is
maximum for β=0, and decreases as β increases. So turbine
power can be controlled by controlling the pitch angle(β).
Fig.2 power coefficient (Cp) vstip speed ratio(λ) curve
B. Drive train model
In stability analysis, when the system response to heavy
disturbances is analyzed, the drive train system must be
approximated by at least a two-mass spring and damper
model. This yields a more accurate response of the wind
turbine’s dynamic behavior during fluctuating wind conditions
or during grid faults[10].
Fig.3 Two-mass shaft model of wind turbine
The mathematical equations of the two-mass model of a drive
train, by neglecting the turbine and the generator self-
damping, are given by Eq.(5-8). [11]
2 (5)
2 (6)
where and (7) Jt and Jg are the turbine and
the generator inertia constant, respectively. Ks is the shaft
stiffness, ωt and ωr denote the turbine and the generator rotor
speed, respectively. t and r denote the turbine and the
generator rotor angle, respectively. The model can be
simplified by removing the shaft stiffness. Hence, there is only
a single inertia which is the sum of the generator rotor and the
turbine inertia expressed as follows:
2 (8)
C. Pitch angle control
The pitch angle is kept constant while the turbine’s rotational
speed is adapted, when the wind speed changes. Variable
speed operation is therefore assured by the control of the
electrical system and the pitch control is inactive below rated
wind speeds and at wind speeds above rated wind the
extracted wind power has to be limited by means of blade
pitching. [12]
Fig.4 Pitch angle control
In order to get a realistic response in the pitch angle control
system, a servomechanism model accounts for a servo time
constant T and the limitation of both the pitch angle (0 to
45deg angle limit) and its gradient (± 10 deg/s rate limit). The
rate-of-change limitation is very important especially during
grid faults, because it decides how fast the aerodynamic power
can be reduced in order to prevent over speeding during faults.
III. MACHINE SIDE CONVERTER CONTROL
The dynamic machine model in the magnet flux reference
system is as follow[13]:
(9)
(10)
Where Uqs and Uds are the stator phase voltages, and
are stator phase currents in d-q frame, is the generator
inductance and is the generator resistance.
The electromagnetic toque in d-q frame is given by,
(11)
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4. Power Equations: The expression of the active and reactive
power are
(12)
(13)
The control of the MSC consists of two strategies:
A. MPPT algorithm:
The Maximum Power Point Tracking, MPPT, block generates
the reference speed which maximizes the power extracted
from the turbine. As shown in Fig. (5) for different wind speed
maximum power corresponds to different generator speed.
Fig.5 Wind turbine power vs. Turbine rotor speed
So a look-up table is formed which gives the optimal
generator speed according to given wind speed to maximize
the power. Speed reference is obtained from look-up table to
control the generator speed.
B. Vector Control of Machine
The control strategy for generator is shown in fig 6. This
scheme will generate the stator current references and, thus,
the voltage references which are compared to a reference
voltage. The obtained errors are applied to voltage regulators
in order to generate the control signals for the inverter IGBTs.
Fig.6 Field Oriented control of machine side converter
The aim of the control is to run the machine at desired speed
to produce maximum power at given wind speed. This control
strategy contains two cascaded loops. The inner loops control
the torque and flux as the d and q axis current component
represents the components of flux and torque. The outer loops
control the speed of machine.
Speed Controller loop:The outer loop of the control system
controls speed of the generator. In this ,control speed is sensed
and compared with its reference speed which is generated
from MPPT and error signal is passes through PI controller
which gives the reference of q-axis current. This outer loop is
slower than inner loops.
Torque Control Loop: According to equation (11), the
electromagnetic torque is controlled through the control of the
q-axis current. The reference of q-axis current generated from
the speed control loop is compared with generator q-axis
current and error is passes through PI controller which
controls the torque of machine.
Flux Control Loop: It is necessary to keep the rotor flux value
constantly equal to the nominal value and to impose the d-axis
current to zero [14]. This method minimizes the losses of the
generator and improves the power factor. Compensation terms
are added to improve the transient response
IV. GRID SIDE CONVERTER CONTROL
The dynamic model of the grid side converter connection, in
reference frame rotating synchronously with the grid voltage
is as follows[15]
0 (14)
(15)
Where Ud and Uq are the d-q axis output voltages of the
inverter, ω is the grid frequency in rad/sec, Ld and Lq are the
inductance in d-q axis which is equal to Ls and Id and Iq are the
d-q axis currents.
The equations of active and reactive power converted to grid
are shown in equ. (16), (17). It’s shown that to control active
power the d-axis current must be controlled and to control
reactive power the q-axis current is needed to be
(16)
(17)
The two different control strategies compared in this paper
are:
A. SVM base voltage oriented control (VOC)
Fig.7 Voltage Oriented Control of grid side converter
The aim of the control as shown in Fig. 7 is to transfer all the
active power produced by the wind turbine to the grid and also
to produce no reactive power so that unity power factor is
obtained, unless the grid operator requires reactive power
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5. compensation. In this control the phase locked loop (PLL) is
used to synchronize the three phase voltage with the grid
voltage.
DC-link voltage control loop: In order to transfer the active
power generated by the wind turbine to grid the DC-link
voltage must remain constant. The outer loop of the control
system regulates the DC bus voltage constant to be greater
than the amplitude of the grid line to line voltage. In this
control DC voltage is sensed and compared to its reference
voltage and error signal is passes through PI controller which
gives the reference of d-axis current.
Active power control loop: From Equations (16) it is clear that
active power can be controlled by controlling the d-axis
current. So d-axis current of grid is controlled by PI controller
to its reference value which is generated by the voltage control
loop.
Reactive Power control: Reactive power depends on the q-
current component as can be seen from Equation (17). So it
can be controlled by controlling q-axis current. Reference q-
axis current is set to be zero so that no reactive power flows
into grid and obtain unity power factor. However in case of
reactive power demand by grid under grid unbalance condition
it can be changed.
A compensation terms will be added to improve the transient
response of the system which can be concluded from
Equations (14) and (15)
B. Hysteresis current control (HCC)
The hysteresis current control system shown in fig. 8 [17] is
designed to maintain the DC-link voltage at a constant value.
This DC-link voltage magnitude is adjusted by controlling the
amount of current supplied to the electrical grid. The
hysteresis controls the grid current by keeping the current
wave in the range of the defined hysteresis band. When the
current wave reached the band limits, the hysteresis
controllers generate a control signal (0 or 1), which defines the
PWM gate signal [16]. The current controllers adjust the
output current Id, tracking the current reference Id
∗. Comparing
the instantaneous currents on the grid with the reference
signal, the controller adjusts the duty cycle of the PWM of the
converter.
Fig.8 Grid-side Hysteresis Current Control
This leads to a reduced error signal (delta). In the system, DC-
link voltage is maintained at 660 V. The error between
reference voltage and actual voltage feed the PI controller to
produce a Id* and Iq* is set to be zero because it is responsible
for reactive power. The output current then compared to the
actual current supplied to the electrical grid. The error
difference between the two signals is connected to the
hysteresis current control, which produces the gate signal of
the PWM. As the DC-link voltage increases, reference
currents Id* produced also increase. When the current is lower
than the reference current, the converter connects the positive
side of the DC-link source to the load, thus the current
increases. On the contrary when the current is higher than the
current reference the converter connects the negative side of
the DC-link source to the load, which reduces the currents.
With these two operations, the error of the current can be
maintained within a certain fixed band.
V. SIMULATION
A. Case(1): Wind speed is constant at 12 m/s.
Fig.9 Rotor Speed
Fig.10 a) Grid Voltage b) Grid Current c) DC link Voltage d) Active
power e) Reactive power for SVM based control at constant wind
speed(12 m/s)
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6. Fig.11 a) Grid Voltage b) Grid Current c) DC link Voltage d) Active
power e) Reactive power for Hysteresis current control
B. Case(2):The wind speed profile is considered
varying smoothly with ramp of slope 10.
Fig.12 a) Wind Speed b) Rotor Speed C) Grid Current for SVM based
control at variable wind speed
Fig.13 a) DC link Voltage b) Active power C) Reactive power for SVM
based control at variable wind speed
Fig.14 a) DC link Voltage b) Active power C) Reactive power for
Hysteresis Current Control at variable wind speed
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7. VI. RESULTS
TABLE I. OBSERVATION
Parameters Vector oriented
control
Hysteresis current
control
Settling time 0.02 sec 0.04 sec
Grid current THD 3.07 % 3.40 %
Current Fundamental value 16.29 A 16.33 A
Inverter voltage THD 35.8 % 40.08 %
Voltage Fundamental value 644.5 V 606.8 V
In case (1) simulation for both control strategies are done at
rated wind speed i.e. 12m/s . Negative powers in Fig. 10 and
11 indicates that at the time of stating of generator the power
given by WECS is less therefore the grid supplied the power
to machine. The comparative analysis of both the control
strategies is given in Table 1. In case 2 simulations is done for
wind speed varies in step as shown in fig. 12(a) in which from
4 to 6 sec speed is more than rated wind speed in this time
pitching action of turbine is shown in fig. 12(e) and by
pitching, rotor speed and turbine power are controlled as
shown in fig. 12(b) and 13(b). DC-link voltage is remains
constant to transfer total turbine power to grid in case of both
control strategies shown in fig. 13(a) and 14(a).
VII. CONCLUSIONS
The performed comparative study allows to conclude that both
the implemented control strategies are suitable to PMSG
drives for wind turbines applications. However, with SVM
based voltage oriented control techniques applied to the drive,
it shows a better performance since lower current distortion
higher overall efficiency is obtained. It is also observed that in
than hysteresis current control peak overshoot is high as well
as settling time is more.
APPENDIX
TABLE II. TURBINE PARAMETERS
PARAMETERS RATING
Rated Power 10 kw
Rated wind speed 12 m/s
Air density 1.225 kg/m3
Blade radius 1.7 m
Number of blades 3
Gear ratio 1
TABLE III. GENERATOR PARAMETERS
PARAMETERS RATING
Stator resistance (Rs) 0.425 ohm
Stator inductance(Ls) 8.2 mH
Number of pole pairs (P) 5
Moment of inertia (J) 0.012 kg/m2
Viscous friction (B) 0.0012 N.m.s
Flux linkage (ψ) 0.433 V.s
Rated speed(w) 600 rpm
TABLE IV. GRID AND OTHER PARAMETERS
PARAMETERS RATING
Grid voltage(L-L) 440 V
Grid frequency 50 Hz
DC link voltage 660 V
DC link capacitance 2200 uF
SVM switching frequency 4 kHz
Filter inductance 5mH
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978-1-4799-5364-6/14/$31.00 ©2014 IEEE
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