high voltage ride through with facts for wind turbines ... · high voltage ride through with facts...

High Voltage Ride Through with FACTS

for DFIG Based Wind Turbines

C. Wessels, F.W. Fuchs

Institute of Power Electronics and Electrical Drives,

Christian-Albrechts-University of Kiel, D-24143 Kiel, Germany,

Phone: +49 (0) 431-880-6105, Email: [email protected]

Acknowledgments

This work has been funded by European Social Funds (ESF) and the state of Schleswig-Holstein, Ger-

many. It has been carried out as part of CEwind Center of Excellence in Wind Energy of Universities in

Schleswig-Holstein, Germany

Keywords

<<Wind Energy>>, <<Doubly-Fed Induction Generator>>, <<High Voltage Ride Through>>,

<<Flexible AC Transmission Systems>>.

Abstract

In future grid codes wind farms will have to fulfill demanding fault ride through requirements. Among

several grid faults the voltage swell is a critical event. Wind farms with doubly-fed induction generators

will not be able to fulfill the new grid code requirements without additional fault mitigation equipment. In

this publication the applicability of the Dynamic Voltage Restorer and the Static Synchronous Compen-

sator to mitigate a three phase voltage swell without phase angle variation at a wind turbine is investigated

and compared. The mitigation equipment adds the missing high voltage ride through capability to the

wind energy system in order to become grid compliant. The function, control structure and design and

the performance is presented and verified by simulations.

1 Introduction

The amount of installed wind power connected to the grid worldwide is increasing notably. This devel-

opment has forced the grid operators to tighten their grid connection rules [1] in order to limit the effects

of wind power parks on network quality and stability. Important issues are the steady state active and

reactive power feed in capability, continuously acting voltage control and fault ride through behavior like

low- and high voltage ride through (LVRT, HVRT). This means, that grid codes demand wind farms to

stay connected to the grid and stabilize the grid voltage in case of grid faults.

Among the grid faults the voltage swell is a critical event that can be caused by switching off large loads

or switching on capacitor banks [2]. The occurance of a symmetrical voltage swell will be analysed in

this paper. When using wind turbines with doubly-fed induction generators (DFIG) the operation during

a grid fault is complex because the stator is directly connected to the grid while the rotor is connected

via converter, which enables control. With no mitigation of the voltage fault the danger of destruction is

imminent. In order to avoid these problems most of the installed wind turbines are automatically discon-

nected from the grid and reconnected when the fault is cleared. This behavior does not fulfill the new

grid code requirements in some countries concerning HVRT capability and though these wind turbines

High Voltage Ride Through with FACTS for Wind Turbines with Doubly Fed InductionGenerator

WESSELS Christian

EPE 2009 - Barcelona ISBN: 9789075815009 P.1

would not be allowed to be connected to the grid. This is the reason why the HVRT capability of wind

turbines with doubly fed induction generators must be implemented.

There are different ways to mitigate voltage swells in distribution systems. Instead of designing all

system components to be tolerant against voltage swells or changing the conventional control methods,

Flexible AC Transmission Systems (FACTS) can be used to lower the voltage at the wind power gener-

ator. FACTS can be classified by series and shunt connected compensation devices. Among these the

Dynamic Voltage Restorer (DVR) and the Static Compensator (StatCom) are the most effective devices,

both based on the voltage source converter concept [3]. A DVR is a power electronic apparatus that pro-

tects a sensitive load from disturbances in the supply voltage by injecting voltage in series with the load.

A detailed description of the structure can be found in [4] and [5]; the control structure is described in [6]

and [7]. The mitigation of unbalanced voltage dips using a DVR is presented in [8]. The StatCom injects

a reactive shunt current into the grid system to correct the voltage swell. The description of a StatCom to

correct the voltage in a power system can be found in [9]. A study on a StatCom at a wind farm can be

found in [10] and the StatCom control structure can be found in [10]. In [12] and [13] comparative in-

vestigations of DVR and StatCom to mitigate voltage dips in power systems can be found. Nevertheless,

adequate investigations for operation of wind turbines with doubly fed induction generators protected by

FACTS devices have not yet been performed in detail.

In this paper the applicability of the DVR and the StatCom to mitigate voltage swells at DFIG based

wind farms is presented. It will be shown that the compensation equipment can add the missing high

voltage fault ride through capability to the wind energy system in order to become compliant to special

grid codes. The function, control and power rating of each device is presented and verified by simula-

tion results using MATLAB / Simulink and PLECS (Plexim GmbH). First results have been published

in [14] before. A deeper analysis including the switching behavior of the full back-to-back converter, a

DC chopper, controller design and simulation results of the wind turbine under a voltage swell without

additional protection equipment are presented here.

The paper is structured as follows: in section 2 the grid integration of wind turbines and its requirements

concerning HVRT are shown. Section 3 describes the wind turbine system components and control and

in section 4 FACTS devices to compensate voltage swells are introduced. Simulation results for both

mitigation systems protecting a single wind turbine are presented and analysed in section 5. Finally, a

conclusion closes this publication.

2 HVRT Requirement

Wind turbines must fulfill certain grid codes to be connected to the power grid in order to maintain the

overall stability. They have to supply a definite reactive current depending on the actual voltage. The

main dynamic requirements concern the fault ride through capability of wind turbines. Among the grid

faults the voltage swell is less common than the voltage sag, but also usually associated with system fault

conditions. A swell can occur due to a single line-to-ground fault on the system, which can result in a

temporary voltage rise on the unfaulted phases. Swells can also be generated after sudden load drops.

The abrupt interruption of current can generate a large voltage. Switching on large capacitor banks may

also cause a voltage swell.

Traditionally, wind turbines were disconnected from the grid in case of an abnormal grid voltage. In

some areas the amount of wind energy has increased essentially, so that disconnection of an entire wind

farm would affect the system stability negatively. Transmission system companies have specified their

recent requirements, so that wind turbines must offer ride through capability under abnormal conditions.

Instead of disconnecting from the grid the turbines have to stay connected and supply power to the grid

continuously. The australian grid code [1] ascertains a HVRT capability of 30 % for a time period of 60

ms (see Figure 1).


WESSELS Christian


Figure 1: Percentage of allowable overvoltage in Australian Grid Code [1]

3 Wind Turbine System Description

The investigated wind turbine system shown in Figure 2 consists of the basic components like the turbine,

a gear (in most products), a doubly-fed induction generator and a back-to-back voltage source converter

with a DC link. A chopper to limit the DC voltage across the DC capacitor is included. The back-to-back

converter consists of a machine-side converter and a line-side converter connected to the grid by a line

filter to reduce the harmonics caused by the converter. It is common to use an inductance or a LCL filter

that provides higher damping. In this paper a LCL filter is used. The wind turbine system is connected

to the high voltage grid by two transformers. The system parameters are given in Table I.

Figure 2: Wind turbine system with DFIG

Due to the short period of time of a voltage swell the mechanical part of the turbine will be neglected

and the mechanical torque Tmech brought in by the wind is assumed to be constant. For deriving control

laws for the DFIG, the one phase equivalent circuit will be used. Here the space vector notation is

used. The three-phase values are transformed into stationary reference frame and further, using the line

voltage vector, into rotating dq coordinates in order to perform the control. From control point of view

it is advantageous to control DC values since Proportional plus Integral (PI) controllers can achieve

reference tracking without steady state errors. As disadvantage the coordinate transformation leads to

current dynamics coupling. A dq reference frame will be used. In these coordinates the DFIG system

voltage and flux can be described as:

us = Rsis +dψs

dt+ jωsψs; (1)

ur = Rrir +dψr

dt+ j(ωs −ωr)ψr (2)

ψs = (Lsσ + Lh)is + Lhir; (3)

ψr = (Lrσ + Lh)ir + Lhis; (4)

The equation of motion and the electrical torque Te are described as:

d

dtωmech =

1

θ(Te −Tmech); Te =

3

2p Im [ψsi

∗

r ] (5)


WESSELS Christian


The stator power of the DFIG can be derived by:

Ps + jQs = −

3

2usi

∗

s (6)

In the equations the following notation is used: ψ,u and i represent the flux, voltage and current vectors

respectively. Subscripts s and r denote the stator and rotor quantities respectively. The superscript ∗

means conjugate complex. Lsσ and Lrσ represent the stator and rotor leakage inductance, and Lh is the

mutual inductance. Rs and Rr are the stator and rotor resistance. θ represents the rotor inertia and p the

generator pole pairs.

A field oriented control with a stator voltage oriented rotor current control loop is performed. By the

conventional linear control the mechanical speed ωmech and the stator reactive power Qs can be controlled

independently. The control structure is shown in Figure 3. Taking into account the system equations (1)-

Q ,control

w PI currentcontrol

PLL

uS,abcuS,dq

uR,abcu2,dq*

w

w

S,abcS,dq ii

DFIGPWM

dq

dq

dq

a,b,c

a,b,c

a,b,c

iR,dq iR,abc

iR,dqQ*, w*mech

mech

mech

mech

* *

g

g

g

gR

R

R

S

QS

SS

Grid

Q,calculation

Figure 3: Control structure of machine-side converter

Figure 4: Control structure of line-side converter

(6) the inner current and outer reactive power and speed controller parameters are tuned as described in

[17]. The line voltage ug is measured for synchronizing the control with the stator frequency.

The line side converter is connected with the grid via LCL filter. In this paper the voltage-oriented PI

control [15] with converter current feedback is used to control the PWM rectifier. The cascaded control

structure is shown in Figure 4. To design the PI controller parameters (kDC,TDC), the inner control loop is

modeled as a first order delay element with the delay time [17] of Tinner = 2 · 1fs

, where fs is the switching

frequency as given in Table I. The dynamics of the DC link are taken into account. The controller is

tuned with the symmetrical optimum [17]:

kDC =2uDCCDC

3aDCTinnerug

; TDC = a2DC ·Tinner; aDC = 2 (7)

The inner current control is performed in rotating dq-coordinates with PI controllers as well. The con-

troller parameter (kI ,TI) are tuned as:

kI =−L f fs

aI

; TI = a2I ·

1

fs

; aI = 2 (8)

For controller parameter design the filter capacitance can be neglected and L f describes the inductance

of the sum of the grid filter inductances and fs describes the converter switching frequency. Grid syn-

chronization is done with a PLL algorithm. In the simulations both the line and generator side converter

are modeled as a two level IGBT converter using PLECS software to include the switching behaviour.

The DC side of the line side converter consists of the DC capacitor and is connected to the generator side

converter. A DC chopper across the DC capacitor is included to limit the DC voltage to a maximimum

level of 115 % of the DC voltage reference. It is controlled by a hysteresis controller with a width of 5

%. So the DC chopper is activated when the DC voltage reaches the maximum level of 115 % U∗

DC and

is deactivated when the voltage sinks lower than 110 % U∗

DC.

The transformer is an important part to connect the wind turbine to the grid. It is common to use the

simplified model of a series impedance for calculations. The transformer system parameters are given in

Table I. The grid is modelled by the Thevenin equivalent (a voltage source in series to a grid impedance)

and is characterized by the X/R ratio of reactance to resistance and the grid short circuit power (see Table

I).


WESSELS Christian


From investigations of the wind turbine system it becomes clear that the hardware system must withstand

high stress in case of a voltage swell (see simulation results without additional protection in Figure 11).

An alternative solution, instead of modifying each component in a plant to be tolerant against voltage

swells or changing the conventional control algorithms, is to install additional HVRT protection equip-

ment. In this paper FACTS devices are investigated for voltage protection purpose.

4 HVRT Equipment

The acronym FACTS (Flexible AC Transmission Systems) identifies alternating current transmission

systems incorporating power electronics based controllers to enhance the controllability and increase

power transfer capability [3]. FACTS controllers with self-commutated static converters used as con-

trolled voltage sources are the most appropriate solution to mitigate voltage swells because of the rapid

controllability and the ability to exchange active and reactive power with the power system indepen-

dently. The group of FACTS controllers can be classified by series and shunt connected controllers. The

operation principles for voltage swell compensation are shown in Figure 5 and Figure 6.

Z

UU U

U WT

110 kV 20 kV 690 V

WTg

l

l

p

WTI

Figure 5: Operation principle of series compensation

Z

UU

U WT

110 kV 20 kV 690 V

WT

WT

g

l

l

pI

I

Figure 6: Operation principle of shunt compensation

The proposed compensation device presented in this paper is placed in between the two transformers

connecting the wind turbine to the grid. The series controller is placed in series with the line and can

directly influence the line voltage at the wind turbine. The shunt controller is connected in shunt and

can control this line voltage indirectly by injecting a current that produces an additional voltage drop at

the line impedance Zl that decreases the voltage at the wind turbine. A Dynamic Voltage Restorer is a

solution to realize a series controller. A Static Synchronous Compensator is a solution to implement a

shunt controller. Further details of both devices are given in the following sections.

A. Dynamic Voltage Restorer

The function of the Dynamic Voltage Restorer (DVR), used for series compensation, is to inject dynam-

ically the controlled inverse fault voltage to keep the wind park voltage constant. The main components

of a DVR are the voltage source converter with an energy source at the DC side, an output filter, a cou-

pling transformer and a bypass switch, i.e. a thyristor if the DVR is supposed to be bypassed in normal

operation without grid voltage disturbances. A closed loop control was implemented using a rotating

dq reference frame aligned to the grid voltage as described in [4]. The control structure of the DVR is

shown in Figure 7.

Figure 7: Control structure of the DVRFigure 8: Control structure of the StatCom

The grid voltage and the voltage across the coupling transformer are measured. The DVR reference

voltage u∗p is calculated by subtracting the actual grid voltage ug from the constant reference voltage u∗g.

The system dynamics allow the control with an integral controller, that controls the actual DVR voltage


WESSELS Christian


up to the reference voltage. To design the I controller gain the delay time of the voltage source converter

caused by sampling and computation is modeled as a first order delay element with the delay time of

TDV R and the gain is set to kDV R [17].

kDV R =1

2TDV R

; TDVR =1

fs

(9)

After transformation of the dq values into three phase coordinates a PWM algorithm creates the pulses

to control the voltage source converter. The DVR is synchronized to the grid voltage with a PLL. For

simplicity it is modelled as a sinusoidal voltage source in each phase in the simulations. If unity power

factor of the wind turbine is assumed the complex power consumption of the DVR can be calculated as:

SDV R = Up · I∗

WT = (Ug −UWT ) · I∗WT (10)

= (Ucos(δ)+ jUsin(δ)−1) ·PWT

where U describes the relative amplitude and δ the phase angle jump of the voltage swell. The active

and reactive power are calculated by building real and imaginary part of the complex power. In case of

no phase angle jump of the voltage swell δ = 0 the DVR only compensates active power in the size of a

fraction of the rated wind turbine power PWT , dependent on the amplitude of the voltage swell U .

B. Static Synchronous Compensator

The Static Synchronous Compensator (StatCom) influences the transmission system by shunt current

compensation (see Figure 6). Depending on the control strategy the StatCom can be used for flicker or

harmonic mitigation. Here it is used to control the transmission line voltage by injecting a reactive shunt

current that causes an additional voltage drop at the system impedance Zl that consists of the grid and the

transformer impedance. If the system impedance is low, the current that should be drawn by the StatCom

can be very high. Note that the Static Var Compensator (SVC) is another FACTS device that can control

continuously the shunt reactive power at the bus where it is connected by using thyristors and linear

reactors. Hence it can control the amplitude of the voltage at the controlled ac bus. The variation of

current is obtained by control of the gate firing instant of the thyristors. But the normally used six pulse

configuration produces substantial 5th and 7th harmonic currents [3] which requires the use of harmonic

filters and is not investigated here.

The power circuit of the StatCom usually consists of a three-phase voltage source converter, a line filter

and a coupling transformer. Here, a cascaded control structure in rotating coordinates aligned to the

grid voltage angle is used as described in [9] and [10]. The structure of the StatCom control is shown

in Figure 8. The grid voltage ug and the shunt current ip are measured. The inner current control loop

forces the voltage source converter to behave as a controlled current source. The current is controlled

by two PI controllers for d- and q-axis that are modeled as shown for the line-side converter in equation

(8). A power transformer with the same size as the system transformer 2 (see Table I) is simulated to

connect the StatCom to the grid. The outer voltage control loop controls the grid voltage by injecting

reactive current. By controlling the d-component of the current and interchanging active power with the

grid, the DC voltage of the voltage source converter can be kept constant, which is not performed here

(i∗d = 0), because ideal sinusoidal voltage sources are used to model the StatCom in simulations instead

of a discrete converter with DC link. The grid voltage is controlled by an integral controller and gives the

reference signal for the q-current controller. To design the I controller gain (kStat), the inner control loop

is modeled as a first order delay element with the delay time of Tinner and the grid control path yields the

transfer function G(s) =ug,d

ip,q= ωLl . The I controller is tuned to [17]:

kStat =1

2Tinner ·ωLl

; Tinner = 4 ·1

fs

(11)


WESSELS Christian


Again, a PLL is used to synchronize the control to the grid voltage. The complex power consumption of

the StatCom can be derived as:

SStat = UWT · I∗p = UWT ·

(Ug −UWT )

Zl

(12)

=Ucos(δ)−1+ jUsin(δ)

Rl + jXl

U2g ;

where again U describes the relative amplitude and δ the phase angle jump of the voltage swell. In case of

no phase angle jump the StatCom consumes only reactive power and is dependent on the line impedance

Zl that consists of the grid and the transformer impedance. Anyway, the active power is controlled to

zero by the control implemented here.

5 Simulation Results

To verify the HVRT capabilities of the wind turbine system protected by the DVR and StatCom, simu-

lations have been performed using MATLAB / Simulink. The complete power circuit (see Figure 2) is

built in the PLECS environment, which is a toolbox to simulate rapidly electrical and power electronic

circuits. The control structures are embedded in Simulink.

0.75 0.8 0.85 0.9 0.95 1 1.05

−1

−0.5

0

0.5

1

t in [s]

U i

n [

p.u

.]

Ug

UWT

Up

Figure 9: Operation principle of DVR (one phase of: voltage at grid, WT and DVR)

0.75 0.8 0.85 0.9 0.95 1 1.05−4

−3

−2

−1

0

1

2

3

4

t in [s]

U,I

in

[p

.u.]

Ug

UWT

Ip

Figure 10: Operation principle of StatCom (one phase of: voltage at grid and WT; current at StatCom)

Performance of the system without additional protection and with HVRT equipment during operation

of a symmetrical 30 % grid voltage swell without phase angle jump of a time period of 200 ms are ex-

amined. Simulations are performed for a single 2 MW wind turbine system with doubly-fed induction

generator connected to a typical high voltage grid (see system parameters in Table I). The operation

principle of the DVR is shown in the following Figure 9 and the operation of the StatCom is shown in

Figure 10 by presenting one phase of the voltage at the grid and the wind turbine; and the voltage of the

DVR or current for the StatCom, respectively. The machine is rotating at rated speed of n = 1800 min−1,

generating the rated power while the power factor is kept unique. At the time of t = 0,8 s the symmetrical

voltage swell occurs and is cleared at the time of t = 1 s.


WESSELS Christian


The signals are given in pu values, whereas the rated voltage or rated current is taken as a base. During

normal operation the DVR voltage is controlled to zero. When the voltage swell occurs at t = 0,8 s the

DVR injects the same amplitude of the voltage swell inverse into the system and the voltage at the wind

turbine is reduced to the rated voltage. In Figure 10 the operation principle of voltage swell mitigation

with shunt compensation is shown. The voltage of one phase at the grid and the wind turbine, as well as

the StatCom current are presented. During normal operation with no voltage swell the StatCom current

ip is controlled to zero. When the swell occurs an inductive current is drawn by the StatCom and the

voltage at the wind turbine is mitigated to the rated voltage.

Figure 11: Simulation results for

operation without mitigation device

(a)voltage at WT and grid);b)WT

power;c)DC chopper signal;d)DC

voltage of systems back-to-back

converter

Figure 12: Simulation results for

operation with DVR (a)voltage at

WT and grid);b)WT power;c)DVR

power;d)DC voltage of systems

back-to-back converter

Figure 13: Simulation results for op-

eration with StatCom (a)voltage at

WT and grid);b)WT power;c)DVR

power;d)DC voltage of systems

back-to-back converter

To show the high stress in the wind turbine system simulation results without mitigation equipment are

shown in Figure 11. Strong oscillations occur in the wind turbine power due to the response of the stan-

dard linear control applied here to the step in the grid voltage. The DC chopper limits the DC voltage

into the tolerance range and is often triggered. To obtain controllability of the line side converter the

converter voltage must not exceed a maximum level or the DC voltage will rise and the power flow in the


WESSELS Christian


converter will be reversed. Here, the DC voltage does not rise to 1,3 p.u. due to the additional voltage

drop across the line filter, but the control is pushed towards its controllability limit and the DC voltage

rises to approximately 1,03 p.u. after the oscillations are damped. In Figure 12 the operation with DVR

is presented. In a) the d-component of the voltage at the wind turbine stator and the grid in rotating dq

coordinates aligned to the grid voltage angle are shown. It can be seen that the voltage swell is mitigated

after approximately t = 10 ms to the rated voltage. After the grid voltage swell is cleared at t = 0,7 s a

voltage dip occurs at the wind turbine because of the same DVR control delay time.

Figure 13 shows the simulation results for the system protected by a StatCom device. The mitigation of

the overvoltage is slightly slower compared to the performance of the DVR because of the larger control

delay time of the cascaded control. Because of the low system impedance Zl that consists of the grid and

transformer impedance the current to mitigate the 30 % voltage swell must be very high. Here it is 2,6

times the rated current of the wind turbine. Hence, the StatCom reactive power shown in Figure 13 c)

is 2,6 times the rated wind turbine power. Note that the realization of reactive power capability can be

done by a combination of StatCom, SVC and reactive power capability of the wind turbine generator, as

described in [16]. Here, the single StatCom is investigated. The active power consumption is controlled

to zero by the StatCom control. The power is calculated by using the p-q power theory in three phase

systems [18].

In contrast to that, the DVR as a solution of series compensation, draws only active power from the sys-

tem to mitigate the symmetrical voltage swell without phase angle jump (see Figure 12 c)). The reactive

power is controlled to zero. The oscillations are due to the oscillating currents in the DFIG as a response

of the standard field oriented control to the stator voltage step. Note that these oscillations may not occur

in reality due to additional damping not considered in the simulations or by advanced control algorithms,

which are not considered here. The oscillations can also be seen in the wind turbine power in Figure

(12) and (13) b), but they are damped after several grid periods. In Figure (12) and (13) d) the voltage

of the back-to-back converters DC link is shown in pu values. The rated DC-link voltage is taken as a

base. The DC voltage overshoot is 9 % for DVR and 15 % for StatCom caused by the slightly slower

performance of control. The DC chopper has to be triggered to limit the DC voltage, but not as often as

without mitigation equipment.

6 Conclusion

Addidional HVRT equipment makes wind turbine systems comply to the new HVRT grid code require-

ments without designing all turbine components to be tolerant against the higher voltage level. FACTS

devices for voltage swell mitigation at a wind turbine with doubly fed induction generator are analysed.

The power electronic devices DVR as a solution of series compensation and StatCom as shunt com-

pensation solution are selected. A three phase voltage swell without phase angle variation is analysed

theoretically and by means of simulation. Both investigated FACTS devices can protect the wind turbine

system from voltage swells at the point of common coupling. A fast compensation of the overvoltage

in about 10 ms is the result. This very short time overvoltage has to be compensated by means of other

methods. For voltage swells without phase jump the StatCom unit draws an inductive current that leads

to favourable consumption of reactive power. However, if the system impedance is low the current that

must be drawn can be a multiple of the nominal system current, making the StatCom ineconomical for

the system analysed (1 wind turbine generator at a transformer with same nominal apparent power). In

contrast to that the DVR is consuming active power in the range of a fraction of the wind turbine power

depending on the maximum amplitude of the voltage swell which makes it the more economical solu-

tion for the protection of a single wind turbine. The performance of the devices for unbalanced voltage

swells will be analysed in future work. Also a combined solution of reactive power sources like StatCom

and SVC might be applied. In order to meet all of the grid code requirements, especially the HVRT

requirements investigated in this paper, FACTS devices will play an increasingly role in the future.


WESSELS Christian


Table I: System parameters

Symbol Quantity Value

Ug grid voltage (phase-to-phase, rms) 690 V

ω Line angular frequency 2 π 50 Hz

PWT Wind turbine rated power 2 MW

n Rated speed 1800 min

St1 Wind turbine transformer rated power 2,5 MW

uk1 relative short circuit voltage of wind turbine transformer 6 %

St2 Wind park transformer rated power 2,5 MW

uk2 relative short circuit voltage of wind park transformer 10 %

Ssc Short curcuit power of the grid 2 GW

X/R ratio of reactance to resistance 5

fS switching frequency for LSC,DVR and StatCom 2 kHz

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