Real Time Implementation of Grid-connection control using Robust PLL for WECS in Variable Speed DFIG-based on HCC

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The 5th International Conference on Electrical Engineering – Boumerdes (ICEE-B) October 29-31, 2017, Boumerdes, Algeria.

Real Time Implementation of Grid-connection control using Robust PLL for WECS in Variable Speed

DFIG-based on HCC

Fayssal AMRANE ^{*, 1, 2}, Azeddine CHAIBA ^{1, 3}, Bruno FRANCOIS ² and Badreddine BABES ⁴

*, 1 LAS Research Laboratory Department of Electrical Engineering, University of Setif 1, Setif, Algeria.

2 Laboratoire d’Electrotechnique et d’Electronique de Puissance de Lille (L2EP), Ecole Centrale de Lille, Lille, France.

3 Department of industrial Engineering University of Khenchela, Algeria.

4 Research Center in Industrial Technologies CRTI, (P.O). Box 64, Cheraga 16014 Algiers, Algeria E-mail : [email protected].

Abstract— This paper presents the experimental real time implementation of a grid-connection field-oriented control (FOC) for wind turbine based on a doubly fed induction generator (DFIG). A control law is synthesized using a hybrid FOC- Hysteresis Current Controller (HCC) in Rotor side converter (RSC) and the stator is connected to grid via robust PLL (phase locked loop). The regulation is achieved below the synchronous speed (Hypo-synchronous mode). The implementation is realized using dSPACE1104 single board card control and acquisition interface. The obtained results of the proposed control present high performance in steady and transient states with low THD of the stator injected current to the grid (<5%).

Keywords—Doubly fed induction generetor (DFIG), Field oriented control (FOC), Hysteresis current controller (HCC), Rotor side converter (RSC), dSPACE1104.

I. INTRODUCTION

Today, most of wind turbines are equipped with Doubly fed induction Generator (DFIG) associated with the AC/DC/AC converter to convert the kinetic energy of the wind into the electrical energy [1]. However, most of these machines are directly connected to the grid to avoid the presence of a converter [2-3]. The main advantage of these installations lies in the fact that the inverters rated power is around the 30% of the generator power [4-5]. A DFIG consists of a wound rotor induction generator (WRIG) with the stator windings connected directly to a three-phase power grid. DFIG’s control strategies have been discussed in literatures [6-7]. Control of DFIG through the Field Oriented Control (FOC) which is performed by rotor currents control has been developed in [8].

Two famous methods for DFIGs employment in wind power systems are used, such as: Stand-alone installation and Grid-connected usage. There are many papers, which have concent-rated on doubly fed induction generators operation as grid-connected and stand-alone systems [9-10]. A comparative study on islanded and grid-connected operation of DFIG based wind generator has been presented in [11]. Most of the researchers in this area are concentrated on fault conditions, system transients, analyzing network dynamics, or grid disturbances’ [12-13-14]. A schematic diagram of variable speed wind turbine system with a DFIG is shown in fig. 1.

Vector control utilizes the dynamic state relationships of DFIG to define angular speed, amplitude and instantaneous position of current, vol-tage and flux linkage vectors [11-12]. In [15],

Ω^me

c Gear box

Pem

The power Converter Grid Side

Hysteresis Current FOC

Controller

Power Grid

Power Converter Machine Side DC/AC

AC/DC

Ω^mec P^em DFIG

Stator

Rotor 3

3 3

3 Wind

3

Fig.1 Schematic diagram of wind turbine system with a DFIG.

the authors propose enhanced hysteresis-based current regulators in the field-oriented vector control (VOC) of doubly fed induction generator (DFIG) wind turbines. This proposed hysteresis-based technique has excellent steady-state performances. In this work, we are interested by the network- connection usage; we used the Hysteresis current controller (HCC) to obtain the switching command signals. The main contribution of this paper is the experimental validation of the proposed control FOC-HCC based on grid-connection control using robust Phase locked loop (PLL) to ensure good tracking of the predefined references regardless the wind speed changing.

This paper is organized as follows; mathematical model of DFIG is presented in section II. In section III, presents the field oriented control of DFIG which is based on the orientation of the rotor flux vector along the axis ‘d’. Section IV describes in details the topology of Hysteresis current controller. The case study diagram of the implementation is described in details in section V. In section VI, experimental results are shown and discussed. Finally, the reported work is concluded.

II. MATHEMATICAL M^{ODEL OF}DFIG

The generator chosen for the conversion of wind energy is a double-fed induction generator, DFIG modeling described in model of the induction machine obtained using Park the two-phase reference (Park). The general electrical state is given by the following equations, [16], [17], [18] and [19]:

Stator and rotor voltages:

sd s sd d sd s sq

V R I

dtφ ω φ

= + − (1)

(2)

sq s sq d sq s sd

V R I

dtφ ω φ

= + + (2)

( )

rd r rd d rd s rq

V R I

dtφ ω ω φ

= + − − (3)

( )

r q r rq d r q s r d

V R I

d tφ ω ω φ

= + + − (4)

Stator and rotor fluxes:

sd L Is sd L Im rd

φ = + (5)

sq L Is sq L Im rq

φ = + (6)

rd L Ir rd L Im sd

φ = + (7)

rq L Ir rq L Im sq

φ = + (8)

The electromagnetic torque is given by:

C_e=PL I I_{m rd sq}( −I I_{rq sd}) (9) And its associated motion equation is:

e r d

C C J f

− = dtΩ + Ω (10)

2 turbine

g

J J J

= G + (11) where: is the load torque is total inertia in DFIG’s rotor, Ω is mechanical speed and G is gain of gear box.

III. F^IELDO^RIENTEDC^ONTROL(FOC)

In this section, the DFIG model can be described by the following state equations in the synchronous reference frame whose axis d is aligned with the rotor flux vector as shown in fig. 2, (φ_rq=0) and (_φ_rd =_φ_r) [10].

rq m sq

r

I L I

= −L and _sq ^r _rq

m

I L I

= −L (12)

Fig. 2 Rotor flux vector in the synchronous d-q frame.

The rotor flux and the electromagnetic torque can be given by:

sd s r rq

m

L L I

φ = −σ L and C_em=PL I I_{m sq rd}( −I I_{sd rq})=P Iφ_{rd rq} (13) with:

If : φ_sq=0 (Via stator) ≡φ_rd =L L_{m sd} (14) If : I_sd =0 (Via rotor) ≡φ_rd =L L_{r rd} (15) (Via stator and rotor) ≡φ_rd =L L_{r rd}+L L_{m sd} (16) where:

sq sd φ

φ ^, are stator flux components, φ_rd ,φ_rq are rotor flux components, _V_sd_,_V_sq are stator voltage components, V_rd,V_rq are rotor voltage components. Rs,Rr are stator and rotor resistances, Ls,Lr are stator and rotor inductances, Lm is

mutual inductance,

σ

is leakage factor, P is number of pole pairs, f is the friction coefficient.

IV. HYTERESIS CURRENT CONTROLLER

and are switches connected, one to the output of a hysteresis comparator, the other to this same output via an inverter. The sign of the difference of change between the reference and the measured current does not instantaneously results in the tilting of the comparison. The comparator switches over either switch comes into conduction in turn as

∆I < h illustrated in (fig.3.b). The switching conditions are defined in terms of logical states corresponding as below (fig.3.a) [9-10-20].

( ) 1

u t = + for Îâct^{( )}^t ^Î^ref^{( )}^t ⁻^h or e t( )+h (17)

( ) 1

u t = − for Îâct^{( )}^t ^Î^ref^{( )}^t ⁺^h or e t( )−h (18) Fig.4 presents simulation results of two hysteresis band cases, the second one (∆I=0.01 (A)) presents good tracking current and very low THD value equal to 2.89 % , so this one is experimentally validated.

Ira Hysteresis band ΔIr

+Vdc / 2

-Vdc / 2

Ua

ωt ωt

Output voltage 0

0

+

+ -

- Ira*

Irb*

Irc* Irc_meas Irb_meas Ira_meas - +

u

e

+1

-1

-h +h

b

Real current a

Reference current

Fig. 3 Hysteresis current control topology.

4.2 4.3 4.4 4.5 4.6 4.7 4.8

-10 0 10 20 30

4.2 4.3 4.4 4.5 4.6 4.7 4.8

-10 0 10 20

30 THDi= 2.89%

THDi= 2.91%

4.62 4.64468 4.66 4.68 1012 14

4.62 4.64 4.66 4.68468 1012 14

Case I:ΔI = 0.1 (A): Case II:ΔI = 0.01 (A):

Fig. 4 Hysteresis band performances under two cases.

V. CASE STUDY

The proposed rotor side converter (RSC) representing by FOC and Hysteresis current topology is represented in details in fig.5. Ird* is the image of flux, and Irq* is the image of torque from the MPPT (maximum power point tracking). The PLL used in the proposed control is described in fig.6 [9-10].

Fig. 7 presents the experimental test bench developed in Automatic Laboratory of Setif (LAS), the DFIG used in this

Stator Axis: is fixing Rotor Axis d-q Frame

θr

θ^s 0

θm

(3)

abc Sin_cos

dq0

mag abc

magnitude

X / Divide abc_to_dq0

Transformation

Integrator

Integrator_1 Kp

Ki 1/s

1/s

2*pi

1/2/pi Fo=25Hz

mod sin

cos

+ + 1

1 2

3 4 freq wt Sin_cos 2^ndOrder

Filter

Fig. 6 PLL implementation (Matlab/Simulink® structure).

real time implementation is a 3.5 kW, whose parameters are indicated in table.1 and DCM (Direct Motor Current) is a 3kW.

DCM 3 [kW]

DFIG 3.5 [kW]

2

9 5 8

4

3 1

10 7 6

11

Fig. 7 Experimental test bench; 1: PC, 2: Speed sensor, 3: Power analyzer, 4:

Oscilloscope, 5: Inverter (Semikron), 6: Current sensor, 7: Voltage sensor, 8:

dSPACE 1104, 9: DCM, 10: DFIG, 11: AC 380 V (Grid).

VI. EXPERIMENTAL RESULTS AND DISCUSSION Fig. 8 shows a diagram of test system. In the upper right is described the DFIG sub-system, composed of a 3.5kW, and in the upper left emulator turbine represented by 3kW DCM, as follows in fig.8.

dSPACE1104Card

Data acquisition interface

PC

V_grid_ab I_grid_ab

V^s_ab I^r_ab I^_dc

V_dc

Speed Sensor

DFIG

C DCM

+

-

3 Grid

Inverter Rectifier

B 3

A

Scope

3

Teta_r

Grid-connection topology

Fig. 8 Test system global topology.

The lower part of fig.8 is composed of a data acquisition system connected to the control board. The FOC algorithm is implemented on real time board (dSPACERTI1104) from Tex- as Instrument with a TMS320F240 DSP (20 MHz) and a mic- roprocessor Power PC 603e (250 MHz); with a sampling time Ts= 50 μs (fixed step); the controller is executed at 20 k Hz.

The connections between the dSPACE card and the power converter are carried out by an interface card, which adapts the control signal levels. The current and voltage are ensured by the Fluke i30S and ST1000-II sensors respectively, whereas, both the rotor position and speed are given by a 1024-pulse incremental encoder implemented on the DC motor shaft (as shown in fig.7).

Fig.5 Rotor side converter control system.

+

- Wind turbine simulator

DFIG DCM Sa Sb Sc abc

abc dq

dq

abc abc dq

dq

Encoder PLL

Synchronisation

Ira*

Irb*

Irc*

Ira_meas Irb_meas Irc_meas

Vdc

Rotor

Stator Vgq Vgd

Vsd Vsq

Park Park

Park Park-¹

Irc_meas Irb_meas Ira_meas Irq* is the image of torque from MPPT

Ird* is the image of flux Ird*

Irq*

Irq_meas Ird_meas

Vga Vgb Vgc

+ + +

-- - -

-

VsaVsbVsc

K

2 Pole pair

Grid connection

topology.

Grid

θs

θ_slip

1 S θr ωr

θr

Ωr Ω_mec

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A. Rotor side converter (RSC):

980 rpm

Rotor speed

Ira 1 sec

Decreasing speed Increasing speed

Sub-synchronous mode

a

Irc Irb

1480 rpm

980 rpm

2 sec 500 rpm

b

c

d

Increasing and decreasing of Irq* amplitude Ira_meas

Ira* 25 msec

25 msec

5 msec -5 A

0 A -7 A

Irq*

Ird*

0 A -5 A

Isa_meas Ira_meas

-5 A -7 A -5 A

Increasing and decreasing of Irq* amplitude Irq_meas

Irq*

Irq_meas

Irq*

Ird*

Ird_meas

0 A 0 A

-7 A

-3 A

Keeping Ird*=0 (A) and varying Irq* from -7 (A) to -3 (A).

Fig. 9 (a): variation of the rotor speed and rotor currents (Hypo-synchronous mode), (b), (c) & (d): Robustness tests by variation of reference transversal

current to ensure the unity power factor.

In the rotor side converter (RSC), we used several tests, as shown in Fig.9. (a, b, c & d):

Fig.9.a shows the behavior of rotor speed (Ωr (rpm)) and rotor currents (Irabc (A)) under variation condition. It can be seen the Hypo-synchronous mode (<1500 (rpm)) and proportionality between rotor speed and rotor currents in terms of rotor frequency. Fig.9.a displays the speed increasing from 980 rpm to 1480 (rpm) (period of 2 (sec)) and then the speed decreasing with the same value from 1480 (rpm) to 980 (rpm) (in period of 2 (sec)). Fig.9.b illustrates the steady state of the performance of reference direct current. It is clear that the rotor sinusoidal measured current I_ra_meas (A) follows exac-

B. Grid side converter (GSC):

Isa Isb

Isc

Isa Isb Isc

3 A ^{5 msec}

Vsb

Isb

Isa Isc

50 V 3 A

Generator mode

a

b

c

5 msec

Fig. 10 (a): Steady state of the stator currents, (b): generator mode & (c):

Experimental measure obtained from power quality analyzer (THD of stator currents).

-tely its reference I*_ra(A). By using the hysteresis band current controller (described in section IV-case II-) equals to +/-0.01 (A), this later is sufficient to follow the reference sinusoidal current. Ird*(A) is maintain to 0 (A) to ensure an exchange of stator active power and Irq*(A) is varying from - 5(A) to -7(A) (means negative torque Ce(N.m) to make the generator mode. Fig.9.c represents the behavior of measured stator and rotor sinusoidal currents under rotor transversal currents variation; Irq* (A) is varying from -5 (A) to -7 (A) to ensure the generating mode. It can be seen that the stator frequency value is big than the rotor frequency (means the rotor speed is less than the synchronous speed (Ωr (rpm)

<1500 (rpm)). Fig.9.d displays the steady state of magnitude increasing of rotor transversal current Irq* (A) from -7 (A) to -5 (A) and the rotor reference direct currents is keeping at zero value (Ird* (A) = 0 (A)).

In the Grid side converter (GSC), different tests are realized, as shown in fig.10 (a, b & c):

Fig.10.a illustrates the stator current in steady state injected into the grid. It can be seen an excellent sinusoidal form of the current waveforms. Fig.10.b displays the stator currents and voltage in steady state. It is clear that the stator current Isb (A) and the stator voltage Vsb(V) are 180° out-off phase, which proves that only the active power flows to the grid; means generator mode and the factor power equal to the unity.

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Fig.10.c obtained from the power quality analyzer that shows the stator currents (THDI_sbac %) equal to 3.7% satisfies IEEE 519 Standard (5%). It can be seen also via the analyzer power the sinusoidal form of stator currents waveform.

APPENDIX

TABLE.1.PARAMETERS OF THE DFIG.

Rated Power: 3.5 kWatts Stator Resistance: Rs = 2.3Ω Rotor Resistance: Rr = 6.95Ω Stator Inductance: Ls = 0.04 H.

Rotor Inductance: Lr = 0.036 H.

Mutual Inductance: Lm = 0.061 H.

Rated Voltage: Vs = 220/380 V Number of Pole pairs: P= 2

Rated Speed: N=1420 rpm Friction Coefficient: fDFIG=0.00 N*m/sec The moment of inertia: J=0.2 (kg*m)²

VII. CONCLUSION

In this paper present the real time implementation of robust performance FOC in the case of Stand-alone in variable-speed WECS using DFIG. The experimental test bench emulating a wind turbine and driving a DFIG allows validating experimentally the modeling and the control techniques. The performance of a HCC-FOC is implemented in a real time via dSPACE1104 card. The experimental obtained results validate the control strategy, and provide good THD of stator current injected into the grid and excellent tracking of the predefined references regardless the wind speed changing.

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