IMPROVING STABILITY FOR INDEPENDENT POWER CONTROL OF DFIG WITH SFOC AND DPC DURING GRID UNBALANCE

IMPROVING STABILITY FOR INDEPENDENT POWER CONTROL OF DFIG WITH SFOC AND DPC DURING GRID UNBALANCE

20 Nguyen Thanh Hai, Vo Viet Cuong

IMPROVING STABILITY FOR INDEPENDENT POWER CONTROL

OF DFIG WITH SFOC AND DPC DURING GRID UNBALANCE

Nguyen Thanh Hai 2 , Vo Viet Cuong 1

Abstract - This paper presents modified Stator Fed Oriented Control

(SFOC)for Doubly Fed Induction Generator (DFIG) in wind turbines

during grid unbalance,and improves stability by using Notch filter to

eliminate second order harmonic components.The proposed

schemes apply multiple PI controllers with Fuzzy and anti-windup

(PI-F) to obtain commanded rotor currents and also introduce extra

commanded values for rotor currents Comparison of the proposed

controller with Direct Power Control (DPC) using Notch filters for

improvement during grid voltage unbalance is also included The

modifications are applied to rotor side converter (RSC) for active and

reactive power controls of wind turbine The turbine, generator and

control units are also described on MATLAB/SIMULINK Simulation

results show improved stability of active and reactive powers stator

Key words - DFIG; Unbalanced Voltage Dip; PI controller;

Anti-windup; SFOC; Notch Filters; Fuzzy

1 Introduction

Doubly fed induction generators have been the popular

choice in wind power generation due to the low rating of

power electronic circuit connected to the rotor side of the

generator and the grid [1] The active and reactive powers

delivered by DFIG can be controlled independently by

Stator Flux oriented Control and Direct Power Control

which are designed for operation with balanced grid

voltage [2] However, most of the grids experience the

problems of voltage unbalance, which raises the winding

temperature and causes pulsation of torque and power [3]

This paper will investigate the stabilities of active and

reactive powers during transient unbalance of grid voltage

for traditional and modified stator flux oriented control and

direct power control of DFIG The modifications are

hybrid PI-Fuzzy controller and Sequence Component

controller The grid unbalance is modeled with a reduction

of 25% of voltage in one phase Wind speed varies

randomly during the process

2 Mathematical Model Of Wind Turbine

The model of wind turbine and its formula of shaft

torque, turbine torque, power transferred to generator and

related parameters are presented in this session Figure

1illustrates the mechanical system of wind turbine which is

often used in large wind turbine systems

The power extracted from the wind is:

(1)

Where:ρ = 31.22 (kg/m 3 ) air density

A=R 2 (m2) the cross-sectional area through which the

wind passes

R(m): length of turbine’s blades

vw (m/s):the wind speed normal to the cross-session area A

C p (): the aerodynamic efficiency which depends on the

tip speed ratio λ, and blade pitch angle β According to Betz’s efficiency, the maximum theoretical efficiency is 59.3% [10] The tip speed ratio λ is defined as the speed at which the outer tip of the blade is moving divided by the wind speed

(2)

Figure 1 Model of the mechanical part of Wind Turbine [9]

3 Direct Power Control and Stator Flux Oriented Control Of DFIG

Structure of control method with DPC for DFIG is shown in Figure 2 and 3 The proposed control structure with SFOC is shown in Figure 4 Appropriate voltage vectors for rotor side converter are selected to control generated active and reactive power in DPC Converters on rotor side of DFIG are controlled by stator flux oriented control to achieve the independent control of active and reactive powers Modification of the control system by using hybrid PI-Fuzzy controller has provided better performance of the generated powers [5] However, this is only verified with balanced grid voltage To improve stability of the powers during voltage unbalance, inclusion

of Notch filter has been suggested by [6, 11] and presented

in Figure 3 and 4 to eliminate second order harmonic components

V sabc

I sabc

I sαβ

V sαβ

θ r

ω r

θ s

P s

DFIG

P s , Q s

Calculation

PWM

e j(θ s -θ r )

e -jθ s

PLL

Encoder

Required Rotor voltage calculation

Balanced control scheme

P sref

Q sref

V sdq

αβ abc

abc αβ

V sabc

ω s

d/dt

V sαβ

V rdq

V rαβ

V rabc

abc αβ

Q s

*

*

*

V DC

Figure 2 DPC for grid-connected DFIG-based wind generator

without Notch filter [11]

) , ( 2

1 3w p  

w

turb

v R



=

ISSN 1859-1531 - THE UNIVERSITY OF DANANG, JOURNAL OF SCIENCE AND TECHNOLOGY, NO 12(97).2015, VOL 1 21

Figure 3 The typical configuration of a grid-connected

DFIG-DPC with Notch Filter

Figure 4 The proposed control scheme for the RSC

of a DFIG using PI+F controller and Notch filters

In both control scheme in Figure 3, Notch filters are

used to eliminate second order harmonic components in

positive and negative sequences of stator voltage In Figure

4, Notch filters are used with positive sequences of stator

voltage and rotor current

Figure 5 shows the spatial relationships between the

stationary (α,β)s reference frame, the rotor (α,β) r reference

frame rotating at the speed of ω r , and the dq+ and dq−

reference frames rotating at the angular speed of ω s and −ω s,

respectively As shown, the d+

-axis of the dq+ reference

frame is fixed to the positive sequence stator voltage V +

sd+

According to Figure 5, the transformations between (α,β) s ,

(α,β) r and dq+ and dq− reference frames are given by

2

2

I t =I + t +I − t (5)

s

dq

d

dt

2 slip

I+ =I++ +I+− =I++ +I−−e−  [6; 7; 8] (7)

I+ =I+++I+−=I+++I−−e−  [6; 7; 8] (8)

Active and reactive power of stator:

m

s

L

L

s m

V L

PI-Fuzzy controllers as shown in Figure 6 are used to control the errors between the required and actual values of both the active power and reactive power delivered to the grid

by the generator The parameters of the PI-Fuzzy are adjusted

by the fuzzy rules to obtain the best output to drive the errors

to zero The variable parameters of the controllers, which are fixed in traditional PI controllers, will help to achieve the best performance of the system The outputs of these controllers are commanded values of d-q components of rotor current in the stator flux oriented reference frame These commanded values of currents are used to regulate the RSC for provision

of the rotor phase voltage to DFIG

The fuzzy rules for parameters of PI-FUZZY controllers are presented in Table 1 and Table 2 The rules are developed by trial and error method LN, SN, ZE, SP, and LP represents large negative, small negative, zero, small positive and large positive respectively S, M, H stand for small, medium and high respectively

Table 1 rule base of K p [5] Table 2 rule base of Ti [5]

LN SN ZE SP LP LN SN ZE SP LP

e

LN H H H H H

e

LN H H H H H

Figure 5 Relationships between (α,β) s, (α,β)rand dq+ and dq− reference frames [6]

Figure 6 PI-Fuzzy controller

The triangular membership functions of inputs and outputs of PI-Fuzzy controller are shown in Figure 7, 8:

Figure 7 Membership functions of two inputs of fuzzy bloc

22 Nguyen Thanh Hai, Vo Viet Cuong

Figure 8 Membership functions of two outputs of fuzzy bloc

4 Simulation and Results

Simulation implementation of proposed control method

for 2.3 MW DFIG is carried out, Table 3 The grid voltage

unbalance happens after 35 seconds, the commanded

values of reactive power and active power change at 50s

and 60s respectively Comparisons of average values of

active and reactive powers in steady state with different

controllers are presented in Table 4 and 5 Both actual

values and percentage of references are shown Average

electromagnetic torque of the generator is shown in Table

6.The randomly variable wind speed is shown in Figure 9

and Figure 10 is grid unbalance at 35s

Table 3 Parameters of DFIG 2.3MW

Frequencyof the electric system ωS 100π (rad/s)

Inertia of Rotor IWTR 17.10 6 (kg.m 2 )

The simulation results with different controllers are shown

in figures 11 to 16 for active and reactive output power respectively These figures demonstrate the power responses when voltage unbalance happens and when the commanded values of powers change under voltage unbalance Torque response of the generator is shown in Figure 17

Figure 9 Random variation of wind speed

Figure 10 The grid voltage unbalance happens after 35 seconds Table 4 Average value of P s[MW]in steady state for 3 controllers

Grid

Voltage

P sef =2

DPC WITHOUT NOTCH

SFOC WITH PI-F & NOTCH

FILTER

Balanced

(11-19s)

2.002 0.1%

2.086 4.3%

1.920 -4%

2.002 0.1%

2.08 4.2%

1.919 -4%

2.001 0.1%

2.138 6.9%

1.908 -4.6% Unbanced

(31-49s)

2.001 0.05%

2.113 5.7%

1.904 -4.8%

2.001 0%

2.1 5%

1.915 -4.2%

2.02 1%

2.225 11.3%

1.867 -6.7%

During the unbalanced voltage, best performances of

active power are observed for DPC with Notch Filter, then

the traditional DPC without Filter In detail, the lowest

value of PMax for DPC with Notch filters is 5.0% of the commanded The highest value of PMin for DPC with Notch Filter is -4.2% of the commanded value

Table 5 Average value of Qs [MVAR] in steady state for 3 controller

Grid

Voltage

Q sref =1

DPC WITHOUT NOTCH

SFOC WITH PI-F & NOTCH

FILTER

Balanced

(11-19s)

1.007 0.1%

1.073 7.3%

0.928 -7.2%

1.00 0%

1.073 7.3%

0.928 -7.2%

1.00 0%

1.114 11.4%

0.889 -11.1% Unbanced

(31-49s)

1.051 5.1%

1.09 9%

0.879 -12.1%

1.00 0%

1.057 5.7%

0.891 -10.9%

0.997 0.3%

1.117 11.7%

0.881 -11.9%

During the unbalanced voltage, best performances of

active power are observed for DPC with Notch Filter, then

the traditional DPC without Filter In detail, the lowest

value of QMax for DPC with Notch filters is 5.7% of the commanded The highest value of QMin for DPC with Notch Filter is -10.9% of the commanded value

29.95 29.96 29.97 29.98 29.99 30 30.01 30.02 30.03 30.04 30.05 -800

-600 -400 -200 0 200 400 600 800

Time [s]

%) (

%)

(

Psref Psref P Deviation = −

%) (

%)

(

Qsref Qsref Q

ISSN 1859-1531 - THE UNIVERSITY OF DANANG, JOURNAL OF SCIENCE AND TECHNOLOGY, NO 12(97).2015, VOL 1 23

Table 6 Average value of generator’s torque in steady state for the 3 controllers

SFOC WITH PI-F & NOTCH FILTER

During the unbalanced voltage, best performances of

active power are observed for DPC with Notch Filter, then

the traditional DPC without Filter In detail, the lowest value

of TMax for DPC with Notch filters is 15899 (N.m) of the

commanded The highest value of TMin for DPC with Notch

Filter is 10195 (N.m) of the commanded value

5 Discussion

DPC has shown good steady state of active power

responses during the voltage balance and unbalance as

shown in Table 4 The deviation of the mean value of active

power from the reference value is almost zero percent with

the inclusion of Notch filter SFOC also gives good

performance with small deviation (about 1%) The

fluctuation of active power is smallest for DPC with Notch

filter during the unbalance

Steady state responses of reactive power are also very

good when Notch filters are included The deviations are

0% and 0.3% respectively for DPC and SFOC The

deviation is much higher without Notch filter during the

voltage unbalance as shown in Table 5 There is no

significant difference observed between the responses

during the voltage balance, with or without Notch filters

The fluctuation is observed to be smallest for DPC with

Notch filter SFOC however gives smallest torque

variation during voltage unbalance as shown in Table 6

The results obtained in Table 4 are further

demonstrated in Figure 11 SFOC’s active power response

when voltage unbalance happens has higher ripples while

the responses obtained with the two DPC schemes are not

significantly distorted The responses to change in the

commanded values during the unbalance are good for the

three control scheme as shown in Figure 12 DPC schemes

give faster responses as shown in Figure 13

Figure 11 Active output power of DFIG when voltage

unbalances happen

Figure 12 Active output power of DFIG during the transient states

Figure 13 Dynamic responses of DFIG’s active output power

during the change of commanded value

Figure 14 Reactive power of DFIG when voltage unbalances happen

Figure 15 Reactive power of DFIG during transient states

Figure 16 Dynamic responses of DFIG’s reactive power during

the change of commanded value

Figure 17 Torque of DFIG

Higher ripples are also observed in reactive power responses of SFOC when voltage unbalance occurs as shown in Figure 14 The observation is consistent with statistics presented in Table 5 Reactive powers in the three control scheme follow the commanded values under the

1.8 2 2.2 2.3

DPC WITHOUT NOTCH FILTER

2 2.2 2.3

SFOC WIT H PI+F& NOT CH FILT ER

Time [s]

1.8

2

2.2

2.3

Time [s]

DPC WITH NOTCH FILTER

20 40 60 0.8

1.1 1.4 1.7 2 2.3

DPC WITHOUT NOTCH FILTER

1.1 1.4 1.7 2 2.3

SFOC WIT H PI+F& NOT CH FILT ER

Time [s]

20 40 60

0.8

1.1

1.4

1.7

2

2.3

Time [s]

DPC WITH NOTCH FILTER

0.8 1.1 1.4 1.7 2 2.3

DPC WITHOUT NOTCH FILTER

Time [s]

0.8 1.1 1.4 1.7 2 2.3SFOC WITH PI+F& NOTCH FILTER

Time [s]

0.8 1.1 1.4 1.7 2 2.3

Time [s]

DPC WITH NOTCH FILTER

0.8 0.9 1 1.1 1.2

Time [s]

DPC WITH NOTCH FILTER

0.8 0.9 1 1.1

1.2

DPC WITHOUT NOTCH FILTER

Time [s]

0.8 0.9 1 1.1

1.2

SFOC WITH PI+F&NOTCH FILTER

Time [s]

.7 1 1.3 1.6 1.9 2.2

Time [s]

DPC WITH NOTCH FILTER

0.7 1 1.3 1.6 1.9 2.2

DPC WITHOUT NOTCH FILTER

Time [s]

.07 1 1.2 1.6 1.9 2.2

SFOC WITH PI+F&NOTCH FILTER

Time [s]

.7 1 1.3 1.6 1.9 2.2

Time [s]

DPC WITH NOTCH FILTER

0.7 1 1.3 1.6 1.9 2.2

DPC WITHOUT NOTCH FILTER

Time [s]

.07 1 1.2 1.6 1.9 2.2

SFOC WITH PI+F&NOTCH FILTER

Time [s]

0 3 6 9 12 15 18 20

Time [s]

DPC WITH NOTCH FILTER

0 3 6 9 12 15 18 20

DPC WITHOUT NOTCH FILTER

Time [s]

0 3 6 9 12 15 18 20

FOC WITH PI+F& NOTCH FILTER

Time [s]

24 Nguyen Thanh Hai, Vo Viet Cuong condition of voltage unbalance as shown in Figure 15 Time

responses of reactive power in DPC control schemes are also

less than those of SFOC as shown in Figure 16

Torque responses observed in Figure 17 are also

consistent with the statistics shown in Table 6

6 Conclusion

The proposed SFOC scheme for DFIG with the

inclusion of PI-Fuzzy controllers and Notch filters has

improved the stability of independent control of active and

reactive power during grid voltage unbalance The

responses of active and reactive power are compared with

a traditional DPC and modified DPC using Notch filters to

increase the stability The observations are made during the

occurrence of voltage dip in one phase, transient states as

well steady states of the powers under unbalanced

condition In all the observations, the independent control

of the powers is maintained for the proposed scheme

However, high fluctuations in active and reactive

powers are present in the responses obtained with the

proposed scheme although lower ripples are observed for

generator’s torque

Experimental verification of the new control scheme

should be carried out to validate the results obtained with

simulation

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(The Board of Editors received the paper on 15/05/2015, its review was completed on 05/07/2015)