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IJESR/August 2013/ Vol-3/Issue-8/413-418 e-ISSN 2277-2685, p-ISSN 2320-9763

International Journal of Engineering & Science Research

SIMULATION AND CONTROL OF DFIG WIND ENERGY CONVERSION SYSTEM

WITH PI-R CONTROLLER

Pooja Dewangan*1, SD Bharti

2

1M.E. Student, Department of Electrical Engineering, Rungta College of Engineering & Technology, Bhilai, India.

2Assoc Prof, Department of Electrical Engineering, Rungta College of Engineering & Technology, Bhilai, India.

ABSTRACT

A doubly-fed induction generator (DFIG) applied to wind power generation system, driven by wind turbine is under

study for low voltage ride-through application during system unbalance. Use of DFIG in wind turbine is widely

spreading due to its control over DC voltage and active and reactive power. An improved control and operation of DFIG

system under unbalanced grid voltage conditions is done in this paper by coordinating the control of both the rotor side

converter (RSC) and the grid side converter (GSC). Conventional dq axis current control using voltage source

converters for both the grid side and the rotor side of the DFIG are analyzed and simulated. Current control scheme

consisting of a proportional integral (PI) controller and a resonant (R) compensator. The PI plus R current regulator is

implemented in the positive synchronous reference frame without the need to decompose the positive and negative-

sequence components. The MATLAB software is used to simulate all the components of grid connected DFIG-based

wind energy conversion system (WECS). DFIG consists of a common wound rotor induction generator with slip ring and

a back-to-back voltage source convertor.

Keywords: DC-link voltage, Doubly Fed Induction Generator (DFIG), Grid side convertor (GSC), PI-R controllers.

1. INTRODUCTION

The conventional energy sources are limited and have pollution to the environment, thus more attention and interest have

been paid to the utilization of renewable energy sources. In recent years, wind energy has become one of the most

important and promising sources of renewable energy and that is not harmful for the environment. One of the most

significant developments of the late 20th century was the re-emergence of the wind as a potential source of energy

generation [1]. Wind energy conversion is a fast-growing interdisciplinary field that encompasses many different

branches of engineering and science [2]. Wind energy is one of the most available and exploitable forms of renewable

energy. The presented system is a variable speed wind generation system based on DFIG. DFIG is one of the most

popular wind turbines which include an induction generator, a back-to-back voltage source converter and a common DC-

link capacitor. The stator of the generator is directly connected to the grid while the rotor is connected through a back-to-

back converter. The back-to- back converter has two main parts; grid side converter (GSC) and rotor side converter

(RSC). The back-to-back power convertor has full controllability over the system [3,4].

The great advantage of the DFIG is that it only requires a ‘partial’ roughly 35% of the generator’s rated capacity

because only 25%-30% of the input mechanical energy is fed to the grid through the converter from the rotor, the rest

going directly to the grid from the stator. The efficiency of the DFIG is very good for the same reason; little power is lost

via the converter. The back-to-back power convertor has full controllability over the system [4].

The control system is an important issue for the WECS. It maximizes the extracted power from the wind through all the

components and also makes sure that the delivered power to the grid complies with the interconnection requirements [3].

This paper deals with the control of dc-link voltage, active and reactive power. The controlling schemes are applied on

both RSC and GSC. The common DC-link voltage is controlled by grid side converter and control of DFIG’s stator

output active and reactive power is controlled by rotor side converter. In this paper dq axis current control scheme is

used. Current control schemes with PI–R controllers in the positive synchronous reference frame for the two converters

are implemented [5]. PI-R controllers are applied to current regulation loop of both convertors (RSC and GSC), RSC and

GSC are designed in the dq reference frame. For current regulation the positive and negative sequence currents must be

controlled, it requires decomposition of positive and negative –sequence components of the current. The positive and


Copyright © 2013 Published by IJESR. All rights reserved 414

negative –sequence components of the current have to be decomposed from measured signals involving time delay and

resulting in amplitude and phase errors. During transients the systems cannot be fully decomposed. The PI plus R current

regulator is implemented without the need to decompose the positive and negative-sequence components.

2. MODELLING OF DFIG SYSTEM

Fig.1 represents the operation of DFIG and the way connected to the grid. DFIG is basically a standard rotor-wounded

induction machine in which stator is directly connected to the grid, and the Connection of the rotor to the grid is via a

back-to-back convertor [3]. The back-to-back converter is divided into two components: the rotor-side converter RSC

and the grid-side converter GSC. RSC and GSC are Voltage-Sourced Converters that use forced-commutated power

electronic devices (IGBTs) to synthesize an AC voltage from a DC voltage source.

_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

Fig 1:The wind turbine and the doubly-fed induction generator system

A capacitor connected on the DC side acts as the DC voltage source. A coupling inductor L is used to connect GSC to the

grid. The three-phase rotor winding is connected to RSC by slip rings and brushes and the three-phase stator winding is

directly connected to the grid. The power captured by the wind turbine is converted into electrical power by the induction

generator and it is transmitted to the grid by the stator and the rotor windings. The control system generates the pitch

angle command and the voltage command signals Vr and Vgc for RSC and GSC respectively in order to control the power

of the wind turbine, the DC bus voltage and the reactive power or the voltage at the grid terminals.

2.1 Induction generator model

The equivalent circuit of the induction generator is shown in Fig.2 [5-7,9,10] and the electric and magnetic equations of

the model are described by equations (1.1)-(1.4).

Rs Ls Lr Rr

+ Lm +

Vsdq V+

rdq

- -

- + - +

Jωs ᴪ+

rdq J (ωs-ωr)ᴪ+

rdq

Fig 2: Equivalent circuit of the induction generator

Stator Voltage is given by:

Rotor

Control

AC/DC/AC Converter

Wind

Turbine

Drive

Train Stator

AC AC DC

RSC GSC

Three –

Phase Grid Vr Vgc

Induction

Generator

L



�� = �� − ��Ψ� + �Ψ� ��

�� = �� + ��Ψ�� +�Ψ� �� (1.1)

Rotor Voltage is given by:

�� = �� − ��Ψ� + �Ψ��

�� = �� + ��Ψ�� + �Ψ�� (1.2)

Flux Linkage is given by:

Ψ�� = �� − ��

Ψ� = �� − ��

Ψ�� = −�� − �� (1.3)

Ψ� = �� − ��

Electromagnetic Toque is:

�� = Ψ�� − Ψ�� (1.4)

where vs, is and Ψs are stator voltage, current and flux respectively; vr, ir and Ψr are rotor voltage, current and flux

respectively; ωs is the angular velocity of the chosen frame of reference; d and q represent d and q axis, respectively. Lm

is the mutual inductance; Lsl and Lrl are the stator and rotor leakage inductances, respectively.

2.2 Converter model

With the assumption that the converters are lossless, the equations of converters are as follows:

The power at the rotor side (also called slip power) is given by:

�� = �� + �� = �� − �� (1.5)

And the power at the stator side is given by:�� = �� + ��

�� = �� − �� (1.6)

So the total output power is:

� = �� + �� = �� + �� + �� + ��

� = �� + �� = �� − �� + �� − �� (1.7)

3. PI-R CONTROLLER

For current regulation the positive and negative sequence currents must be controlled precisely. To get the accuracy in

current controlling the accuracy of the d-q components decoupling and the removal of network voltage disturbance is

must. The positive and negative –sequence components of the current have to be decomposed from measured signals

involving time delay and resulting in amplitude and phase errors. During transients the systems cannot be fully

decomposed. Therefore here a strategy is adopted to have a PI-R current controller to overcome from these problems and

this regulator consists of a proportional and integrator term, which contains an R pole.



R

��

�� --- --- --- --- --- ��

+

- PI ��

--- --- --- --- ---

Fig 3: Rotor current control scheme based on PI-R controller in the dq+ reference frame

In order to reduce the sensitivity towards possible grid frequency variation, a component with a cut-off frequency of ωc1

can be inserted into the R part to widen its frequency bandwidth as shown in Fig.3 [5,9].

With a PI-R controller, the DC components are mainly regulated by the PI controller while the double-frequency ac

signals are fully controlled by the R regulator. Hence, a PI-R current controller in the positive synchronous reference

frame can directly regulate both positive and negative sequence components without involving sequential decomposition.

3.1 The modeling of PI-R controller RSC

The modeling of PI-R controller for RSC During network imbalance, a DFIG system can be represented in the dq+

reference frame as

��

� = !"#$

�� − !"#$

�� − !"#$

% &#'#(

)�� − �� − *��+�� , + *��-. + +��

� / (2.1)

The rotor control voltage produced by the PI-R controller without any decomposition of positive and negative-sequence

components, V+rdq is given as.

�� = 0�� + ��

� (2.2)

Where U2�3� is the output from the PI-R controller and σL2 is stator transitory inductance.

�� = 6

67 ��

= &8-9! + :;<=� + �:;>=

�?�@AB=��C@A(D?/%C��∗ − �� D (2.3)

Where E+

rdq is the equivalent rotor back electromagnetic force acting as a disturbance to the PI-R controller and is given

by,

�� = #'

#()��

� − �� − *��+�� , + *��-. ++��

� + �� (2.4)

Where KiP1, KiI1, and KiR1, are the proportional integral and R parameters respectively.

3.2 The modeling of PI-R controller

The modeling of PI-R controller for GSC .The GSC under unbalanced supply voltage can be represented in the dq+

frame as

�� F�� = G(HI

J KLMNMHIJ KOA(#MNMHI

J KGMHIJ

#M (2.5)

Where V+

gdq presents the control voltage produced by the GSC PI-R controller, and is designed as

�F�� = −�F�F�

� + �F�� (2.6)

Where

�8-L!�@ + 2�QB=� + C2��D@

8-�!R

8-.

1R0�� + ��

IJESR/August 2013/ Vol-3/Issue-8/413

Copyright © 2013 Published by IJESR. All rights reserved

�F�� = 6

67 �F��

= &8-9@ + :;<?� + �:;>?

�?�@AB?��C@A(D?/

�F�� = ��

� − �F�F�� − *��F�F�

�

Where KiP2, KiI2, and KiR2, are the proportional, integral and R parameters for the GSC,

4. SIMULATION ANALYSIS

A wind energy generation system based on doubly fed induction generator connected to grid system with PI

on both side is simulated using MATLAB. The DFIG is rated at 1.5 MW and frequency is set to 60Hz. The

voltage is regulated at1200V. A three-phase RLC load at the primary side of the coupling transformer is used to generate

the voltage unbalance. Under voltage unbalance condition in the system, grid supplies unbalance current. In induction

machine negative sequence current produce pulsation in torque, which causes the total output active power and the dc

link voltage, both contain oscillations. With PI

dc-link voltage, active and reactive power oscillations decrease.

In Fig.4 to Fig.6, Shows different simulation results of DFIG wind turbine conversion system under voltage unbalance

with PI-R controller.

Fig 4: Simulation results for DC

Fig 5: Simulation results for Active Power (P) with P

Fig 6: Simulation results for Reactive Power (Q) with P

8/413-418 e-ISSN 2277-2685, p

Copyright © 2013 Published by IJESR. All rights reserved

D /%C�F��∗ � �F�

� D

, are the proportional, integral and R parameters for the GSC, respectively.

A wind energy generation system based on doubly fed induction generator connected to grid system with PI

on both side is simulated using MATLAB. The DFIG is rated at 1.5 MW and frequency is set to 60Hz. The

phase RLC load at the primary side of the coupling transformer is used to generate


negative sequence current produce pulsation in torque, which causes the total output active power and the dc

link voltage, both contain oscillations. With PI-R controller negative sequence current is quickly regulated and as a result,

ve and reactive power oscillations decrease.


Simulation results for DC-link Voltage (Vdc) with P-IR controller

Simulation results for Active Power (P) with P-IR controller

Simulation results for Reactive Power (Q) with P-IR controller

2685, p-ISSN 2320-9763

417

(2.7)

(2.8)

respectively.

A wind energy generation system based on doubly fed induction generator connected to grid system with PI-R controller

on both side is simulated using MATLAB. The DFIG is rated at 1.5 MW and frequency is set to 60Hz. The dc link

phase RLC load at the primary side of the coupling transformer is used to generate


negative sequence current produce pulsation in torque, which causes the total output active power and the dc-

R controller negative sequence current is quickly regulated and as a result,


controller

IR controller



From the fig.4 to fig.6 its clear that with due course of time the oscillations in dc-link voltage, active power and reactive

power reduces, and we are getting a constant dc-link voltage while reactive power reduces to zero, thus we will get

maximum total active power. It proves that the proposed control system controls; dc link voltage, active and reactive

power more accurately.

5. CONCLUSION

Enhanced control and operation of a DFIG-based wind Power generation system under unbalanced supply voltage

conditions have been investigated in this paper. A new coordinated control strategy for the RSC and GSC has been

proposed. Simulation results show that PI-R controller overcomes oscillations in dc link voltage, active and reactive

power. The RSC is controlled to eliminate the electromagnetic torque oscillation while the GSC compensates for the

oscillation of the DFIG stator output active power to eliminate the oscillation in the total active power generated from the

overall system. PI–R current controllers in the positive synchronous rotating reference frame have been proposed for

regulating the GSC’s and RSC’s positive and negative sequence currents. Such controllers can provide precise control of

both positive and negative sequence currents without involving the decomposition of positive and negative sequence

component. The control and operation of DFIG-based wind power system under unbalanced conditions can be

significantly improved by simultaneously eliminating torque and total generated active power oscillations.

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