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AbstractWith high penetrations of wind energy, differenttypes of wind farm generators do not have the same fault

transient characteristics that can complicate the selection and

configuration of wind farm grid protection equipment. Incorrect

selection of protection equipment will affect the stability and of

the power systems. This paper investigates and analyzes the

transient characteristics of doubly-fed induction generator

(DFIG) under various short-circuit fault conditions for a typical

distribution network. Firstly a detailed model of the DFIG and

its principles is descried. A realistic of a 99 MW wind farm with

66 units of 1.5 MW DFIG is modeled using RTDS/RSCAD. The

transient characteristic of the wind farm with the DFIG units

have been analyzed under different short-circuit fault

conditions. Results from the fault performance study on the

distribution network are presented and discussed.

Index TermsDoubly-Fed induction generator, Wind farm,Fault transient analysis.

I. INTRODUCTIONHE technique advance and wide applications of power

electronic devices make it possible for manufactures to

develop large Variable-Speed Constant-Frequency

(VSCF) wind turbine generators. It is expected that the VSCF

wind power generators will gradually replace constant

frequency wind components [1]. Since most VSCF windturbine generators are induction machines which can't provide

a longer enough stable short-circuit current to operate the

relays for the networks. One of solutions is that relay can use

the fault transient characters by the relay to protect the

network against the fault [2,3]. In order to allow relays to use

correct fault transient information, it is thus necessary to study

the fault characteristics, the security and stability of system

under the network fault conditions.

There are many studies on modeling and simulation of

VSCF wind turbine generators which often consists of a

wound-rotor generator, a brushless exciter and a low-rating

controlled power converter. Mainly from three aspects[4]: (i)

Generator model: VSCF wind turbine generators alwayschoose DFIG which can connect wind turbines and power

system flexibly[5]. Reference [6] takes the mechanism of a

DIFG as a starting point and derives the DFIGs dynamic

mathematical model according to the relationship among the

flux, electric potential and current. The only drawback is the

need to add the relationship between the control variables

Yang Beige and XueHui are with Shanxi Electric Power Co., Ltd., Datong

037008, China.Bai Dandan, Hu Wei and He Jinghan are with the School of Electrical

Engineering of Beijing Jiaotong University, Beijing 100044, China. (Prof. He'semail: 11121572@bjtu. edu.cn).

characteristics of rotor winding excitation voltage and electric

controlled variables of generator; (ii) Converter and control

implementation [7-10]: In [10] a dual PWM converter control

strategy is proposed in order to achieve stability of the DC

voltage and adjust the power factor; and (iii) Pitch control:

There are a variety of improved programs at present so as to

solve that a fixed set of PID parameters in different wind

speed is difficult to get good control results. Reference [11]

puts forward a control pitch on a fuzzy combination of

feed-forward and a new fuzzy PID control to ensure good

control effect at different wind speed levels.

This paper investigates and analyzes the fault transient

characteristics of doubly-fed induction generator (DFIG)

under various short-circuit fault conditions for a typical

distribution network. Results can be used to provide a

reference to improve the effectiveness of the protection

devices for the network with wind farms.

II. MODELING OF DFIGSAND DISTRIBUTIONNETWORKSA. Analysis and Modeling of DFIGs

A DFIG model [13] as shown in Fig.1 consists of a wind

turbine model, a gearbox, a double-fed induction generator

model, an AC/DC and a DC/AC inverters, the controller and

pitch control.

DFIGransmission

Control ofrotor sideconverter

Master controllerPitchcontrol

Control ofstator sideconverter

grid

dcV

mT

sV

Fig.1 Structure of a DFIG and its control system

1) Basic equations of DFIG at two-phase rotated coordinate

system: The control strategy of the DFIG is the vector control

which can achieve a decoupling control of the reactive and

reactive power. The voltage equation of the DFIG at

synchronous rotating coordinate system can be expressed as:

Fault Analysis of Distribution Network with

Wind Turbines of DFIG

Yang Beige, Xue Hui, Bai Dandan, Hu Wei, and He Jinghan

T

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s

s s s 1 s

rr r r 1 r r

dV R I j

dt

dV R I j( )

dt

= + +

= + +

(1)

Corresponding flux equation is :

s s s m r

r m s r r

L I L I

L I L I

= +

= +

(2)

Where,sVand

rV are the stator and rotor voltage vectors;

sI and rI are the stator and rotor current vectors ; s and r are

the stator and rotor flux linkage vectors;sR and rR are the

stator and rotor resistance ;sLand

rL are the stator and rotor

winding total self-inductance;m

L ,

L andrL are the mutual

inductance between stator and rotor, the leakage inductance of

stator and rotor; 1 is the synchronous angular speed; r is

the rotor angular speed.

If turning the stator flux linkage to coincide with the d-axis

at synchronous rotating system, the q-axis component of fluxlinkage will be zero. According to the output expression of theDFIG, the stator active and reactive output power can be

described as :

ms s s rq

s

2

s s s s ms rd

s s

L3P i

2 L

L3Q ( i )

2 L L

=

=

(3)

2) The control of the rotor-side inverter: According toequation (3), the d-axis component of rotor current can control

thesP and the rdi can control the sQ .Because the rqi and rdi are

orthogonal to each other, thesP and sQ can achieve

decoupling control. The rotor-side inverter takes the stator

flux-oriented rotor current control methods. Rotor current is

indirectly controlled by controlling the applied voltage on therotor. The equation of rotor current control at the stator flux

oriented coordinates can be derived as:

rdrd r rd r sl r rq

2rq m

rq r rq r sl ms r rd

s

diu R i L L i

dt

di Lu R i L + ( i L i )

dt L

+

+

=

= +

(4)

Where the 2m s r L L/1 L= represents leakage factor.

From equation(4), the rotor active and reactive current are

completed decoupled, but the voltage vector which controls

the current isnt decoupled. Choose the feed-forwardcompensation to solve this question:

rdc sl r rq

2

mrqc sl ms r rd

s

u L i

Lu ( i L i )

L

=

= +

(5)

So, after rdu substracting rdcu and rqu subtracting rqcu

the control to rqi and rdi will be decoupling.

3) The control of the grid-side inverter: Similarly, grid-side

converter selects the grid voltage oriented vector control

method to maintain the DC link capacitor voltage at a

predetermined constant value independent of the direction and

size of the rotor power. And control the reactive power at areference value according to the wind turbine reactive power

requirements. The control voltage of grid-side converter can

be derived as:

gd

gcd g gd g gd e g gq

gq

gcq g gq g e g gd

diu (R i L ) (u L i )

dt

diu (R i L ) L i

dt

= + + +

= +

(6)

The feed-forward compensation is:

gcdD gd e g gq

gcqD e g gd

u u L i

u L i

= +

=

(7)

Where, gR and gL are the resistance and inductance of the

grid-side converter; gdu and gqu are the d- and q-axis

component of three-phase voltage; gdi and gqi are the d- and

q-axis component of three-phase current.

According to equation (1)-(7), the VSCF wind turbine withDFIG model can be established and the control structure and

the control blocks are shown in Fig.2.

Fig.2 Control of DFIG

B. Distribution Network ModelUsing the established DFIG model and a actual wind farm

in Shanxi province, the studied distribution network wasestablished as shown in Fig.3. The number of wind turbines is

66 units and the total installed capacity is 99MVA. Each unit

in wind farm uses the two boost program. Each wind turbine

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export voltage is 0.69kV, and then boosts to 35kV. All the

units pool together through collection line, then access intopower grid. The second boost voltage will be 110kV. Take the

actual operating data of a DFIG in the wind farm and the

variable values are shown in table 1.The substation and

transmission line parameters in the distribution network areshown in table 2 and table 3.

220KV

35KV

110KV

110KV

T3

T5

T6

35KV

110KV

T2

T1

110KV

35KV

G

F1

load4

F2

F3

10KV

load3

load1

TRF1 TRF2

TRF3 TRF4

110KV

load2

TRF5

T4

Fig.3 The studied distribution network based on Shanxi Province

TABLEI

PARAMETERS OF DFIGMODEL

Power Rating 1.5MW Stator resistance 0.0244pu

Stator voltage 690V Rotor resistance 0.01316pu

No. of pole pairs 1 Stator leakage

inductance

0.26619pu

Maximum rotorspeed

1.2pu Rotor leakageinductance

0.45471pu

DC-link voltage 1.5kV Stator and rotor

mutual inductance

16.7495pu

Frequency 50HZ Rated wind speed 15m/s

Cut-in speed 3m/s Cut-out speed 21m/s

TABLEIIPARAMETERS OF THE SUBSTATION NEAR THE WIND FARM

Name X1 X2 X3 Ratio Total

Capacity

(MVA)

Load

Loss

(MW)

TRF1 0.118 0.0074 0.0772 22081.5%/121/385 120 0.132

TRF2 0.119 0.00585 0.0675 22081.5%/121/385 120 0.132

TRF3 0.341 0.0023 0.249 11081.25%/36.7/10.5 31.5 0.026

TRF4 0.549 0.0075 0.3325 11081.25%/36.7/10.5 20 0.025

TRF5 0.210 0.1136 11081.25%/36.7 100 0.1

TABLEIIITRANSMISSION LINE PARAMETERS NEAR THE WIND F ARM

Lines Voltage SB

(MVA)

UB (kV) X0 (p.u) R0 (p.u) X1 (p.u) R1 (p.u.)

T1 110 100 110 0.1608 0.0471 0.0520 0.0124

T2 110 100 110 0.1668 0.0847 0.0477 0.0247

T3 110 100 110 0.1982 0.0911 0.0566 0.0242

T4 110 100 110 0.2747 0.1132 0.0785 0.0265

T5 110 100 110 0.0861 0.0396 0.0246 0.0105

T6 35 20 35 0.0895 0.0545 0.0256 0.0175

III. FAULT ANALYSISWhen the wind farm connects to the power system, the

power flow and the fault contribution in the network will

change. Protections now used must be adjusted and changed to

ensure correct fault clearing with right method, and maintain

the safe and stable operation of the system.

If the power system goes wrong, the short-circuit current issupplied by not only conventional power but also the wind

farm. The change of the short-circuit current will influence the

operation of the relay protection, such as making the

protection devices miss-trip or miss-operate. We need tooptimize the protection devices by analyzing the fault

characteristics of the power system connected with DFIG.

This makes simulation analysis of the fault characteristics by

setting different fault position, different wind speed anddifferent reactive power state.

A. At different fault positionBased on the network in Fig.3, we carried out various fault

simulation and analysis. In this paper only results of the faultson lines T1, T2 and T3 are presented and discussed. The total

installed wind farm capacity is 99MVA, and this wind farm

has 66 units, and rated capacity of every unit is 1.5MVA, the

annual average wind speed is 7.5m/s. Take the three-phaseshort-circuit as the fault type, the operation wind speed is15m/s, fault initial angle is 90 degrees. Monitoring the voltage

and current situation of the bus bar 6, and the results are as

Fig.4 and Fig.5.

From the results we can get that the degree of the voltage

drop and the short current characteristics in the wind farm are

(a)fault on T1 (b) fault on T2 (c) fault on T3

Fig.5 The current waveform at bus 6 with different fault position

(a)fault on T1 (b) fault on T2 (c) fault on T3

Fig.4 The voltage waveform at bus 6 with different fault position

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different when the fault position is different. If the fault is on

T1, the voltage drop is large, and then finally decreases tozero. At the same time, the short circuit current increases at

first and then decreases to zero fast, that causes the generator

unstable. If the faults on T2 or T3, the voltage drop is less than

the voltage drop when the fault is on T1, and then decreases toa stable value larger than zero. The generator is unstable

because of serious voltage dip, and the wind farm could only

supply transitory short circuit current and the amplitude islow.

B. At different wind speedThe output of the wind farm changes with the wind

conditions because of the random, volatile and

non-controllable of the wind power. So it is necessary to studythe characteristics of wind power short circuit failure in

different wind speed. If there is a three-phase short circuit on

T1, make a research on the voltage and current characteristics

at the point where the wind farm access into the power grid.The simulation results are shown in Fig.6 and Fig.7.

When the wind speed is 15m/s, the voltage drops seriouslyduring the fault and the wind farm can only provideinstantaneous short-circuit current. When the wind speed is

9m/s, the voltage during the fault is larger, but the current is

smaller. The reason is that the stator and rotors flux linkage

will increase as wind speed increases.

C. At different reactive stateAs all the wind generators are DFIGs, we can use

converters to control the size and frequency of the excitation

current added in the rotor windings, so as to achieve thepurpose of power control. Thus the reactive power of the wind

farm can be controlled as absorption or output state. In this

article, by controlling the reactive power of the wind farm onabsorption or output state, we can observe the fault voltage

and fault current in the point connected to the power grid. As

the waveforms shown in Fig.8 and Fig.9, the fault location is

still set in the T1 line and the fault type is still the most seriousthree-phase short-circuit.

It can be seen from the waveforms in Fig. 8 and Fig.9,whether the DFIG is absorbing or output the reactive power,

the change trend of voltage and current is basically the same,but the short circuit current during the fault when absorbing

the reactive power is larger than that when output reactivepower. This is mainly because when stator output reactive

power, the generator armature reaction is demagnetizing thesynthesis of flux. After the three-phase short-circuit fault

occurred in the power grid, the rotor flux will increase, inorder to guarantee the stator flux won't mutate, so the short

circuit currents of the stator and the rotor will increase, and

vice versa.

IV. CONCLUSIONBased on theoretical analysis of double-fed wind turbine

working principle, this paper builds a double-fed inductiongenerator model on the RTDS software and a distributionnetwork simulation model based on t a wind farm in Shanxi

Province. Also the fault characteristics of distribution network

connected with DFIG wind turbine generators were studiedwhen simulated with different fault location, different wind

speed and different reactive state. The results show that the

greater the wind speed is, the greater short-circuit current

DFIG provides. By adjusting and controlling converter, theDFIG output or absorb reactive power according to the

network needs, and the short-circuit current is large when

(a) Current waveform (b)voltage waveform

Fig.9 the voltage and current waveform in the point (bus 6) connected to the

power grid when output the reactive power

(a) Current waveform (b)voltage waveformFig.8. the voltage and current waveform in the point (bus 6) connected to the

ower rid when Absorbin the reactive ower

(a)current waveform (b)voltage waveform

Fig.7voltage and current waveform at bus 6 when the wind speed is 9m/s

(a)current waveform (b)voltage waveform

Fig.6voltage and current waveform at bus 6 when the wind speed is 15m/

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DFIG output reactive power. The wind power station will

provide short-circuit current to the point of failure after thewind power is merged into the grid, Therefore, the protection

system designing need to consider the characteristics of the

wind turbine, and regarding its short-circuit characteristics as

a protection device configuration reference.

ACKNOWLEDGMENT

Thanks to the support of Datong Electric Company ,China.

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BIOGRAPHY

YANG Bei-ge is now a senior engineer engaged in Power Supply

Company of Datong. He recently is focused on the application of thepower grid operation mode intelligent core platform and research onthe intelligent communication devices monitoring system of standaccused of layer and software design of infrared thermal imagingwireless video surveillance system client.

Xue Hui is now a professional Engineer in Branch Marsh Center ofPower Supply Company in Datong . His interest is the power system.

HE Jing-han received her master degree from the TianjingUniversity in 1994 and got the PHD degree in Beijing JiaotongUniversity in China. She is now a professor in Beijing Jiaotong

University. Her interests include online power system monitoring

protection and control, power quality and rail electrification.

Bai Dan-dan received her BSc degree from the Beijing JiaotongUniversity in 2011. She is currently working toward the MSc degreein the School of Electrical Engineering, Beijing Jiaotong University,Beijing, China. Her research interest is power system protection and

control.

Hu Wei received his BSc degree from the Henan University of

Technology in 2009. He received the MSc degree in the School ofElectrical Engineering, Beijing Jiaotong University, Beijing, China.

His research interest is power system protection and control.