Wind Energy Conversion System Performance Is Improved Using Integrated Multilevel Converter

Wind Energy Conversion System Performance Is Improved Using Integrated Multilevel Converter

Y. Madi, Y. Mokhtari, Pr. T. Rekioua

1,2,3 Laboratory of Industrial Technology and Information, Electrical engineering Department, A.Mira University, Bejaïa, Algeria,

Abstract Frequency converters are used in wind turbines because they make it possible to apply the variable-speed concept. They also make it possible for wind farm to become active element in the power system. The traditional frequency converter is back-to- back connected two-level converter, in which the output voltage has two possible values. In this paper, maximum power control of wind turbine and permanent magnet synchronous generator connected with five level three- phase flying-capacitor multilevel converter to grid are studied. This converter allows higher power handling, potentially lower power loss, lower harmonic distortion and hence less filtering requirements when compared with two-level converter.

Keywords multilevel converter, wind power, grid connection, permanent magnet synchronous generator.

1. Introduction

   Nowadays, the demand for electricity is increasing. Faced with this problem, and so in order to limit the use of the fossil and natural resources such as uranium, hydrocarbons and water, and to reduce pollution, some countries, have turned to new forms of energy called "renewable". These energy sources use directly or indirectly solar energy. Among these, wind is clearly in good place, not to replace conventional sources, but as complementary energy booster [1].

   Until now , there are two categories of wind turbines: fixed speed wind turbines which are directly connected to the grid through the stator and variable- speed wind turbines controlled by the stator or the rotor by means of the electronic power converters. The second category can increase the energy

   fixed-speed wind turbines. However, the power transmitted through the IGBT converters is limited by the characteristics of the IGBT (maximum voltage and current supported). Commercial turbines currently reach 7.5MW and new developments are pushing to the 10MW milestone [2]. At this power level, multilevel converters and medium voltage operation becomes attractive, mainly due to improved power quality and higher efficiency. This is why several configurations proposed recently include multilevel converters.

2. Wind energy conversion systems

   The conversion system wind power studied is shown in figure.1, it includes, in addition the synchronous permanent magnet generator, an IGBT converter, a DC-Link, a multilevel inverter, a connection to the grid via a filter, and a transformer. The IGBT converter is a three-phase PWM-controlled rectifier. This choice is justified by the fact that it can offer a fully reversible control of the instantaneous power, it can control the electromechanical variables such that the electromagnetic torque and the speed of the generator. The multilevel inverter controls the DC-Link voltage and active reactive power exchanged with the grid power.

3. Wind turbine modeling

   The device studied, in this part, consists of a wind turbine including blades length R driving a generator via a gearbox with ratio G total kinetic power of the wind which passes through the wind turbine is given by the following equation:

   efficiency, reduce mechanical loads and improve the quality of the electrical energy produced, compared to

   P 1 R2v3

   w 2

   (1)

   PWM rectifier

   T two-level

   DC-Link

   Multilevel Inverter

   R L

   Grid

   PMSG

   Grid connection

   Fig.1 Wind energy conversion system

   R : is the blade radius of the wind turbine (m);

   : represent air density (it is 1.25 kg / m in normal

   atmosphere);

   v : is the wind speed (m/s);

   Betz proved that the maximum power extractable by an ideal turbine rotor with infinite blades from wind under ideal conditions is 59.26% (0.5926 times) of the power available in the wind. This limit is known as the Betz limit .The extractable power can thus be written as:

   Pcap Cp (, ).Pw

   (2)

   CP : is the power coefficient which represents the aerodynamic efficiency of the turbine and also depends on speed ratio and the pitch angle , the speed ratio is given by:

   t R

   v

   (3)

   Models for power coefficient have been developed. For example [3] models CP as a function of the tip

   speed ratio and the blade pitch angle in degrees as

   :

   Fig.2 Power coefficient characteristic versus speed ratio

   and pitch angle

4. Control of permanent magnet synchronous generator PMSG

   Cp ,

   c c 1

   1

   1

   2

   c3

   c4

   x c

   5 e

   c 1

   6

   6

   (4)

   Figure.3 illustrates the tow control functions of the PMSG:

   In this equation, the parameter also depends

   and .

   • Vector control of PMSG.

   • Control of PWM converter.

   1 1

   0.08

   0.035

   3 1

   (5)

   ired iond

   The values of the coefficients,

   c1 , c2 , c3 , c4 , c5

   PMSG

   et c6

   are given in Table I as:

   Table I

   coefficients	values
   c1	0.5
   c2	116
   c3	04
   c4	0
   c5	5
   c6	21

   coefficients	values
   c1	0.5
   c2	116
   c3	04
   c4	0
   c5	5
   c6	21

   Coefficients defining the evolution of Cp

   Cem _ ref

   Imsdq

   Vector control of PMSG

   Vector control of PMSG

   Control of PWM

   converter

   Control of PWM

   converter

   U

   U

   Vmsd ref

   c

   Vmsq ref

   The difference between the curves of different wind turbines is small and can be neglected in the dynamic simulations. Knowing the speed of the turbine.

   The aerodynamic torque developed (in Nm) can then be calculated:

   Fig.3 Control of permanent magnet synchronous generator PMSG

   Control of AC machines is difficult because the mathematical model of the system is strongly coupled. All control devices are conceived with the aim of finding the ease and quality setting naturally offers DC machine. The similarity between the PMSG and DC machine is made possible by the vector

   aer p

   aer p

   C Paer C .

   t

   .s.v3 1

   2 t

   (6)

   control; the aim of this control is the decoupling of axes d-q. The model of the PMSG in the d-q synchronous reference frame is given by.

   Vmsd

   Rs .imsd

   • Lsd .

   dimsd dt

   • Lsq .r .imsq

     (7)

     Pg vsd .isd

     • vsq .isq

       (10)

       V R .i

       L .

       dimsq L

       . .i

       .

       Qg vsd .isq vsq .isd

       (11)

       sd

       sd

       msq

       s msq

       sq dt

       sd r

       msd f r

       By inversion of these relations, it is possible to

       Cem

       p

   • Lsq

     .i

     msq

     .imsd

     f

     .imsq

     (8)

     impose some references for the active power

     pg ref

     and reactive power

     Qg ref while imposing the

     imsd , imsq

     : The stator currents

     following reference currents:

     Vmsd , Vmsq

     : The stator voltages

     p .v

     • Q .v

       R : Stator resistance

       i g ref

       sd mes

       g ref

       sq mes

       (12)

       s sd ref

       v2 sd mes v2 sq mes

       Lsd , Lsq

       : Stator inductances

       p .v

       • Q .v

       : Permanent magnetic flux

       i g ref

       sq mes

       g ref

       sd mes

       (13)

       f

       p : Number of pole pairs

       sq ref

       v2 sd mes v2 sq mes

       dr r dt

       p dm

       dtdt

       : Represents the electrical speed

       1. Modeling and Control of Three Phase Inverter Multilevel

          of the rotor.

          Cem

          : Electromagnetic torque

          Three-phase five-level structure (n+1, with n=4) of

          Among the strategies applied to vector control of a synchronous machine, which consists in imposing a direct current reference imsd equal to zero is the most widely used. This choice is justified in order to avoid demagnetization of permanent magnets of the

          a flying capacitor inverter studied, has three symmetrical arms of eight switches in series. Each switch consists of a switch IGBT and a diode connected in parallel provides the reversibility of the load current figure .5.

          armature reaction along the axis of d [4]. The electromagnetic torque is given by.

          us1

          T11

          T21

          T12

          T22

          T13

          T23

          Cem f .imsq

          (9)

          T31

          T32

          T33

          To control the generator power, it is enough to control the PMSG electromagnetic torque Cem, by regulation of the stator current and to know the rotational speed of the shaft.

5. Connection to the grid

   The provided energy by the PMSG-based variable- speed wind turbine and transmitted on DC current is applied to a multilevel converter which makes it possible to control the continuous voltage and the active and reactive powers exchanged with the grid

   [5] [6]. An inductive filter RL has been designed to limit harmonic current injection into the grid figure.4.

   The active and reactive powers passed through the grid are given in Park model by the following

   c13

   T41

   c12 c11

   T51 T61 T71 T81

   c23

   c22 c21

   T52 T62 T72 T82

   c33

   um1

   T43

   c32 c31

   T53 T63 T73 T83

   R L

   R L

   R L

   R L

   R L

   R L

   um2

   relations:

   Fig.5 Three-phase five-level structure of a flying capacitor

   inverter.

   Multilevel Inverter

   vb1

   vrs1

   vls1

   Grid

   DC-Link

   DC-Link

   vm1

   R f

   vm 2

   vm1

   L f

   vs1

   is1

   is 2

   vs 2

   vs3

   Fig.4 Grid connection studied

   Signaux de commande

   Signaux de commande

   Fonction de connexion

   Fonction de connexion

   This inverter uses a common DC bus and (n-1) DC bus in each arm. To produce (n+1) levels of output voltage, (n-1) capacitors are required. The circuits for each phase have an identical structure. Therefore,

   Table II

   Control signals								Switching functions				umc0
   T1c	T2c	T3c	T4c	T5c	T6c	T7c	T8c	f1c	f 2c	f3c	f4c
   1	1	1	1	0	0	0	0	1	0	0	0	us1
   0	1	1	1	0	0	0	1	0	1	0	0	us 2 3us1 4
   1	0	1	1	0	0	1	0	0	1	0	0
   1	1	0	1	0	1	0	0	0	1	0	0
   1	1	1	0	1	0	0	0	0	1	0	0
   1	1	0	0	1	1	0	0	0	0	1	0	us 3 us1 2
   0	0	1	1	0	0	1	1	0	0	1	0
   1	0	1	0	1	0	1	0	0	0	1	0
   1	0	0	1	0	1	1	0	0	0	1	0
   0	1	0	1	0	1	0	1	0	0	1	0
   0	1	1	0	1	0	0	1	0	0	1	0
   1	0	0	0	1	1	1	0	0	0	0	1	us 4 us1 4
   0	1	0	0	1	1	0	1	0	0	0	1
   0	0	1	0	1	0	1	1	0	0	0	1
   0	0	0	1	0	1	1	1	0	0	0	1
   0	0	0	0	1	1	1	1	0	0	0	0	0

   Control signals								Switching functions				umc0
   T1c	T2c	T3c	T4c	T5c	T6c	T7c	T8c	f1c	f 2c	f3c	f4c
   1	1	1	1	0	0	0	0	1	0	0	0	us1
   0	1	1	1	0	0	0	1	0	1	0	0	us 2 3us1 4
   1	0	1	1	0	0	1	0	0	1	0	0
   1	1	0	1	0	1	0	0	0	1	0	0
   1	1	1	0	1	0	0	0	0	1	0	0
   1	1	0	0	1	1	0	0	0	0	1	0	us 3 us1 2
   0	0	1	1	0	0	1	1	0	0	1	0
   1	0	1	0	1	0	1	0	0	0	1	0
   1	0	0	1	0	1	1	0	0	0	1	0
   0	1	0	1	0	1	0	1	0	0	1	0
   0	1	1	0	1	0	0	1	0	0	1	0
   1	0	0	0	1	1	1	0	0	0	0	1	us 4 us1 4
   0	1	0	0	1	1	0	1	0	0	0	1
   0	0	1	0	1	0	1	1	0	0	0	1
   0	0	0	1	0	1	1	1	0	0	0	1
   0	0	0	0	1	1	1	1	0	0	0	0	0

   Correspondence between the control signals and switching functions.

   balanced voltage capacitors terminals c11 , c12 and

   c13

   the phase 1 are independent for the phases 2 and 3. The three arms share a common DC voltage source. This flying capacitor multilevel converter has more degree of freedom and flexibility in level composite aspects than diode-clamped multilevel inverter. The advantages of flying capacitor multilevel converter are flexible switch mode, high protection ability to power devices, to control real power and reactive

   power conveniently [7].

   Each arm is equivalent to a switching circuit to five ideal switches which switching functions are noted

   f rc figure.6 r 1, 2,3, 4,5 and c 1, 2, 3. The

   status of the last switch is complementary to other

   states [8]:

   f5c

   f1c

   . f2c

   . f3c

   . f4c

   (14)

   T1c

   D1c

   Three-phase five-level structure of a flying

   us1

   C13

   T2c T3c

   T4c

   C12 C11

   T5c T6c T7c

   T8c

   D2c D3c D4c

   D5c D6c D7c

   D8c

   umc0

   us1

   is

   us2

   us3

   us4

   im1

   im2 im3

   im4

   f1c

   f2c

   f3c

   f4c

   f

   umc0

   capacitor inverter is equivalent to a 3 5 matrix converter (with ideal switches) figure.7.

   o

   o

   5c

   o

   Fig.7. Equivalent matrix structure of the five-level flying

   capacitor inverter.

   Fig.6 .Equivalent circuit of the C arm using the switching function.

   The modulated voltages are obtained from the capacitor voltage balancing and switching functions

   The switching functions

   f rc

   are related to gate

   as:

   signals of transistors Trc . Each switching circuit c,

   um1 f11 f13 .u s1 f21 f23 .u s2 f31 f33 .u s3 f41 f43 .u s4

   (15)

   there are 16 configurations, allowing connections

   u f

   f .u f

   • f .u

     • f

   • f .u

     • f

   • f .u

     (16)

     between the different voltages balancing and load.

     Table II shows the possible combinations of switches

     m2 12 13 s1 22 23 s2 32 33 s3 42 43 s4

     and voltage levels corresponding.

     These equations can be rewritten as:

     um1 m11.us1 m21.us2 m31.us3 m41.us4

     um2 m12 .us1 m22 .us2 m32 .us3 m42 .us4

     (17)

     (18)

     This expression is used to define modulation function

     m1c

     f1c

     f13

     , m2c

     f 2c

     f 23

     , m3c

     f3c

     f32

     (19)

     iCref

     C Cref

     UC

     (26)

     i

     i

     U

     U

     C

     C

     m4c f 4c f 43 ,

     c 1 et 2

6. Simulation results

   The purpose of the control system is to generate the equivalent mean value of modulated voltage

   references (written umc t ) inside each modulation

   period. The mean value of a modulated voltage during the modulation period Tm is expressed as [9]:

   Simulations have been performed using a wind generator based on a permanent magnet synchronous

   machine 800KW connected to the network via two

   kinds of inverters (two and five levels.) This chain conversion was simulated for a profile of mean wind around (12m / s) for a period of 20s figure.8.

   1 k 1.Tm

   (20)

   umc t T . umc t dt, aveck N. c 1, 2

   m k .Tm

   Wind speed (m/s)

   Wind speed (m/s)

   This quantity is linked to modulation functions and voltage sources, which are assumed to be nearly constant during the modulation period Tm :

   umc

   4

   4

   mrc .usr , r 1, 2, 3, 4et c 1, 2

   r1

   (21)

   For inverter operation, we wish to impose the following phase-to-neutral reference:

   Fig.8 Wind speed profile

   c _ ref max n

   c _ ref max n

   v v .si .t c 1.

   2

   , with

   3

   c 1,2,3

   (22)

   Figures 9 to 12 show simulation results using a

   two-level converter. Figure.9 shows the active power

   delivered by the wind and the active power sent to the

   The reference voltages are calculated by:

   grid connection, we see that the two powers following which shows the good performance of the system.

   um1_ ref

   um2 _ ref

   v1_ ref

   v2 _ ref

   • v3 _ ref

   • v3 _ ref

   (23)

   Figure.10 shows the reactive grid power. So the generation system can operate at unit power-factor, absorbs or provides reactive power, figure.11 shows the first-phase line current end line voltage .The

   modulated voltage is sown in figure.12.

   1. Controlling the DC bus voltage:

   The electrical equations of DC bus voltage are given by this expression.

   t0 t

   1

   Uc C

   So:

   iC UC t0

   t0

   (24)

   Fig.9. Wind power Pmec and active power transited to the grid connection Pg .

   Uc t0 : is the value of the DC voltage at the initial

   time.

   The capacitor current is given by:

   iC ired iond

   With:

   (25)

   ired

   iond

   : rectified current;

   : ripple current;

   Fig.10 Reactive power transmitted to the grid connection

   1

   Adjusting the DC bus is composed of a control loop to maintain a constant DC bus voltage, with PI

   i

   i

   controller C

   C

   and generating the reference current to

   be injected into the capacitor.

   Fig.11 Voltage and current of the first phase of the grid connection.

   Fig.12. Modulated voltage um12 .

   Figures 13 to 16 show the simulation results using a five-level of a flying capacitor inverter. Figure.13 shows that the total active power generated is sent to

7. Conclusion

   The modelling of a wind turbine with a permanent magnet synchronous generator connected to the grid via five-level of a flying capacitor inverter has been treated. This wind system was modelled using d-q rotor reference frame and is interfaced with the power system through an inverter and a filter modeled in the power system reference frame. The inverter control allowed, through grid current regulation, to achieve a

   decoupled active and reactive power control for

   the grid connection and practically Pg

   Pmec .

   operate with unitary power factor.

   Reactive power is imposed zero figure.14. The modulated voltage has five levels figure.15. The phase between the grid voltage and current is figure.16 and so the reactive power is null. The current is better than the current obtained using a two- level converter figure.11.

   Fig.13 Wind power Pmec and active power transited to the grid connection Pg .

   Fig.14 Reactive power transmitted to the grid connection

   Fig.15 Modulated voltage um12 .

   1

   Fig.16 Voltage and current of the first phase of the grid connection.

   From the simulation results, the multilevel inverter topology can overcome some of the limitations than the standard two-level inverter. Harmonics decreases as the number of levels in the output voltage is increased.

8. RÉFÉRENCES

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