Advanced Control of Variable Speed Wind Energy Conversion System with DFIG

Djilali Kairous Dept. of Electrical Engineering

Chlef University –UHBC- Chlef, 02002, Algeria

djilali.kairous@uqat.ca

René Wamkeue, Member, IEEE Dept. of Applied Sciences

Université du Québec en Abitibi-Témiscamingue-UQAT- QC, Canada

rene.wamkeue@uqat.ca

Bachir Belmadani Dept. of Electrical Engineering

Chlef University –UHBC- Chlef, 02002, Algeria belmadani_dz@yahoo.fr

Abstract—As a result of the increasing wind power penetration on modern power systems, the wind farms are today required to participate actively in grid operation by an appropriate generation control. This paper presents a comparative study of two control strategies for doubly fed induction generator based wind energy conversion system (DFIG-based WECS). The study focuses on the regulation of the DFIG active and reactive powers using the so-called sliding mode control and classical PI controllers. Control strategies are developed in the dq co-ordinates. The robustness and reliability of implemented controllers are tested with large grid disturbances due to an overvoltage and voltage sags. The DFIG control strategies are evaluated through simulations using MATLAB/SIMULINK software.

Keywords-; Doubly Fed Induction Generator; Sliding Mode Control; Variable Speed Wind Energy System

I. INTRODUCTION

Nowadays, the most widely used wind turbine in wind farms is based on doubly fed induction generator (DFIG) due to noticeable advantages: the variable speed generation, the decoupled control of active and reactive powers, the reduction of mechanical stresses and acoustic noise, and the improvement of the power quality [1].

Different DFIG vector control schemes have been proposed in literature developed. In [2] and [3] authors have developed stator flux control of the DFIG using conventional PI regulators, while in [4] and [5] direct power control of the DFIG is outlined. Recently, L.M. Fernandez and all [6] present a comparative performance study of three control strategies for the active and reactive powers regulation.

However, the DFIG has to endure two types of interference during system operation: (a) external interference, which refers to load torque oscillations on the driving chain due to random variation of the wind speed and direction; (b) internal interference, which refers to the mismatch of controllers and machine parameters caused by changes in the generator temperature, magnetic saturation level and skin effects [7]. These two kinds of interference affect the working efficiency, dynamic performance, and robustness of the wind power generation system. In such circumstances, conventional controllers based on PI-regulators cannot follow the changes in system parameters and are thus useless.

An efficient and more suitable control approach based on sliding-mode control (SMC) is proposed in this paper for power generation control. The SMC is a kind of nonlinear control method based on a state space model of a DFIG. Control characteristics of the SMC could force the controlled variables to move along the scheduled track [8]. Compared to PI control, the proposed approach offers a faster response and characteristics insensitive to parameter variation and interference [9].

II. DESCRIPTION OF THE SYSTEM The basic configuration of the whole system is presented

in Fig.1. The rotor of the DFIG is connected to the grid through two back-to-back bridge converters. The PWM grid side converter (GSC) is used to control the DC-link voltage and keep it constant regardless of the magnitude and direction of the rotor power. The PWM rotor side converter (RSC) is used to generate the optimal active power depending on the wind speed and turbine characteristics. This converter will be disconnected to the DFIG at transient state when the rotor current reaches high value then the nominal value. In this case the rotor will be short circuited at the additional resistance called Crowbar

The magnetic saturation is not taken into account in this study. The rotor side converter model reproduces the reference voltages generated by the control scheme. The DFIG model is associated with a turbine emulator which is controlled with MPPT (Maximum Power Point Tracking) strategy and pitch angel control [10].

Electrical equations of the wound rotor induction machine in the (d-q) reference frame are presented in [2], [3], [11]

Figure1. Schematic representation of DFIG-based wind turbine

DFIG Gear

AC

DC

Control

RSC GSC DC

Ω AC

Grid

Crowba

III. DISIN OF SLIDING MODE CONTROL (SMC) The basic idea behind the SMC theory is the specification

of sliding surface. It is chosen in a way that the control feature in to maintain a system within the surface and hence, assuming desired system behavior. Unlike state feedback, where the control doesn't change during system operation, variable structure control can switch at any time from one structure to another. The problem of this control is to determinate the parameters of each structure and the commutation functions. Switching from one structure to another permits to have the benefits of each structure. The system can then have interesting properties as becoming stable while it is elaborated with unstable structures.

In general, four basic steps are required when designing the SMC: (a) the selection of a sliding surface; (b) the design of a test to verify the sliding mode existence; (c) the stability analysis within the surface; (d) the determination of the control input nU with the objective that every state which is outside the commutation surface should be joined in a finite time. Thus, the system will take the dynamics of the selected surface, and moves to the equilibrium point.

In an aim to decouple the active and reactive power, the stator flux vector will be aligned with the d-axis. So, by neglecting the stator resistance and assuming that the stator flux is maintained constant, the DFIG model must be written under state form as:

( ) ( )UtxgtxfX ,,.

+= (1)

The desired control vectors force the trajectory of the system to converge towards the surface defined by:

0)( =xdqσ (2)

So we define the sliding surface in the Park reference as follows:

)()( __ mesdqrrefdqrdqIIx −= λσ

(3)

Where refdqrI _ and mesdqrI _ are the reference and the measured rotor current vectors respectively. The choice of parameter λ guarantees the stability of the system in a given time. In order to guarantee the attraction of the system throughout the surface the following condition must be fulfilled:

0*

<dqdq σσ (4)

The equivalent command (in the equation (1) corresponding to the ideal sliding regime is obtained by

imposing 0),(*

=txdqσ

)5()),(),((),(*

eq

TT

dq Utxgtxfxt

xx

tx +∂∂

=∂∂

∂∂

= ⎟⎠⎞

⎜⎝⎛

⎟⎠⎞

⎜⎝⎛ σσσ

From (4), Ueq can be written as:

1),(),(

−

∂∂

∂∂

−= ⎥⎦⎤

⎢⎣⎡

⎟⎠⎞

⎜⎝⎛

⎟⎟⎟

⎠

⎞

⎜⎜⎜

⎝

⎛txg

xxtxfU

T

eqσσ

(6)

With .

( , ) 0x tσ = , the equivalent control vector eqU can expressed by:

)7(

⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥

⎦

⎤

⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢

⎣

⎡

⎟⎟⎟⎟⎟

⎠

⎞

⎜⎜⎜⎜⎜

⎝

⎛

⎟⎟⎠

⎞⎜⎜⎝

⎛

⎟⎟⎠

⎞⎜⎜⎝

⎛

−

⎟⎟⎟⎟⎟

⎠

⎞

⎜⎜⎜⎜⎜

⎝

⎛

⎟⎟⎠

⎞⎜⎜⎝

⎛

⎟⎟⎠

⎞⎜⎜⎝

⎛

−

++

+

++

+−

=

+

+

qsss

drrrqsr

dsss

qrrr

dsss

qrrrdsr

qsss

drrr

eq

VL

MILcR

VL

MILbR

VL

MILcR

VL

MILbR

U

ωσωϕ

ωσ

ωσωϕ

ωσ

Subscripts d and q refer to the d- and q-axes respectively and subscripts s and r to the stator and rotor of the DFIG respectively; sω ,and rω (in rad/s) are the stator and rotor

variable pulsations respectively; , and dq dq dqV I ϕ are voltage current and flux vectors respectively in the d-q reference frame; sR and rR are stator and rotor resistances; sL and

rL are the stator and the rotor leakage inductance and M is the magnetizing inductance. The constants defined as:

Figure 2. Global diagram of simulation and control of DFIG the by SMC

dmesI

qmesI

DFIG

+ -

s

s

VM

L−

s

s

VM

L−

-+

)σSign(K

refP

+

*I q

*dI

eqqU

eqdU+

+

++

SMC

refQRSC

sL

ssV ϕ−

drV

)σSign(K

qrV

-

DFIG

DC bus

)1(2

rs LL

M−=δ ,

sL

aσ

1= ,

rL

bσ

1= ,

rL

sL

Mc

σ=

(8)

In order to obtain good dynamic performances and commutation around the surface, the control vector is modified as shown below:

neq UUU += (9)

nU is the Signum function defined by:

)(dqn signKU σ−= (10)

eqU is valid only in the sliding surface and nU ensures the sliding mode’s existence. The DFIG global sliding-mode control are illustrated in detail in Fig.2

IV. SIMULATION RESULTS To demonstrate the pertinence of the proposed DFIG

sliding-control approach, simulations have been performed using MATLAB/ SIMULINK software on a 20 kW, 380-V, 50-Hz wound-induction machine with the following parameters:

H.M 0340= , Ω= 0455.0Rs Ω= 19.0Rr , HLs 07.0= ,

HLr 0213.0= , 0024.0=Ff , 2.53.0 mkgJs = Two simulation contingencies are considered: first

dynamic behaviour of the system with variation of the wind speed and secondly, transient states at of symmetric dip voltage and overvoltage in the grid. It assumed that the voltage sag reduce the voltage at 10% of their nominal value during 300ms. The three-phase fault occurs between t =2.8s t=3.1s and t=9s -9.3s (in both regions low and high speed respectively). It assumed, also, the Overvoltage increase the voltage at 125% of the nominal value, between t = 15s and t = 15.3 s.

The reactive power reference is fixed at 0VAR. The results (Fig3-Fig10) show the behaviour of the DFIG-based WECS with SMC and PI regulators. Fig.3 shows the variation of the speed between high and low value, 8.7 and 13.5m/s witch enhance the generator speed at 1150 and 1850tr/mn respectively (Fig4).The the pitch angel have a dynamic depend to the power variation (Fig.5).Stator and rotor currents (Fig.6 and Fig7 respectively) chow, clearly, the different reaction at transient stats by the SMC and PI regulator.

As chow Fig8 and Fig.9 The torque and the active power have constant value for the sufficient wind speed, this values depend to the variation of the wind in low speed regions.. At transient state the active power and the torque reach high instantaneous values. These values became much higher at high speed values then low speed as chows the figures. The reactive power dynamic is presented in Fig10. The peaks reach very high values with PI regulators then SMC. This transient stat can disturb system and result disturbances in the grid also, especially for the high penetration wind turbine.

This fault will be more dangerous in the instances of high generator speed.

Figure.3. Wind speed

Figure.4. Generator speed

Figure.5. Pitch Angle

Figure.6a. Phase stator current with PI regulator

Figure.6b. Phase stator current with SMC

Figure.7a. Phase rotor current with PI regulator

Figure.7b. Phase rotor current with SMC

Figure.8. Electromagnetic torque

Figure.9. Active power

Figure 10 Reactive power

CONCLUSION

The present paper presents a comparative study on the performance of two control strategies; PI and SMC for DFIG wind turbines when they operate with power regulation. Both controls achieved by an appropriate control of the direct and quadrature components of the rotor voltage, performed through the rotor side power converter, in collaboration with the blade pitch angle control. The performances of proposed approach have been assessed through simulations in the cases of overvoltage and voltage sag. As results proved, the SMC gives a good powerful tool to enhance wind turbine performances and it gives promising results in decoupled power generation, and hence, improves the system stability of the grid-connected DFIG system. It is observed that this tuning is helpful in not only improving the damping of the oscillatory modes but also in enhancing the fault ride-through capability of the DFIG system.

With the increasing penetration of DFIG-based wind farms into the grid, it is important to study the implications of large scale DFIG systems on grid stability. Nevertheless, this paper does provide a good initial study of the DFIG system with controller. Computations with multi machine DFIG system will be required to confirm the obtained results and determine if it’s possible to quantify the impact of DFIGs on power system stability.

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