Connect Wind Turbines

8/8/2019 Connect Wind Turbines

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504 IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 24, NO. 2, JUNE 2009

A Novel Scheme to Connect Wind Turbinesto the Power Grid

Nitin Joshi, Student Member, IEEE, and Ned Mohan, Fellow, IEEE

AbstractIn this paper, a new scheme is proposed to connectwind turbines to the power grid. This scheme helps in limitingthe fault currents as well as in voltage unbalance compensation.Voltage is injected in series with the transmission line to limit thefault currents as well as to balance the voltages. This scheme allowswind turbines to remain synchronized to the grid during faults orduring low voltages and is useful for doubly fed as well as squirrelcage induction generators.

Index TermsDoubly fed induction generators (DFIGs), faultcurrent limitation, voltage unbalance compensation.

I. INTRODUCTION

WIND IS THE fastest growing energy source in the world.

In the past 56 years, wind energy use has grown 28%

annually, becoming the most developed technology among vari-

ous renewable energy technologies. Today, wind power capacity

of the world is approximately 50 GW and is expected to reach

160 GW by 2012 [1]. Wind turbines of approximately 4500 kW

and rotor diameters of more than 100 m are now available [2].

However, the interconnection of wind turbines to the power

grid poses many problems. Reactive power compensation, low-

voltage ride-through, and the grid voltage unbalance are some

of these issues.

Wind turbine systems are classified into two types: fixed-speed wind turbines and variable-speed wind turbines. Fixed-

speed wind turbines are directly connected to the grid. Though

these are the cheapest wind turbines, they have a number of

drawbacks. In these turbines, reactive power cannot be con-

trolled; furthermore, any turbulence of wind results in power

variations, thus affecting the power quality of the grid [3].

In variable-speed wind turbines, power fluctuations caused by

wind variations can be absorbed by changing the rotor speed.

These turbines produce 815% more energy output, as com-

pared to fixed-speed wind turbines; however, they necessitate

power electronic converters.

Faults in the power system cause voltage to dip at the point of

interconnection of the wind turbine. This situation results in an

increaseof the current in the stator as well asthe rotor winding of

the generator. Most induction generators are disconnected from

the grid when such faults occur to avoid damage to the converter

[4]. High-fault currents can also damage the generator stator and

rotor windings. Thus, it is important to limit the fault currents.

Manuscript received July 12, 2006; revised April 22, 2007. Current versionpublished May 19, 2009. This work was supported by the National ScienceFoundation under Grant ECS-0245550. Paper no. TEC-00344-2006.

The authors are with the Department of Electrical and Computer Engi-neering, University of Minnesota, Minneapolis, MN 55455 USA (e-mail:[email protected]).

Digital Object Identifier 10.1109/TEC.2008.926040

Fig. 1. Low-voltage ride-through requirement.

The American Wind Energy Association (AWEA) has proposed

low-voltage ride-through requirements for the interconnection

of large wind generators [5]. The proposed voltage requirement

are described by Fig. 1, which shows the grid voltage drop to

0.15 pu due to a line fault with a slow buildup back to 1 pu.

During this period, the wind generators must remain connected

to the grid.Thyristor-controlled resistors have been considered as a

means to suppress short circuit currents [6]. This paper de-

scribes a new method to limit the fault currents in doubly fed

induction generators (DFIGs), as well as in squirrel cage in-

duction generators (SCIGs) connected to the power grid. In this

method, voltage is injected in series with the transmission line

to limit the fault current.

Another issue in wind power is voltage unbalance compensa-

tion. Wind generators experience overheating and torque pulsa-

tions following the grid voltage unbalance, resulting in stress on

mechanicalcomponents [7]. Therefore, beyond a certain amount

of unbalance (e.g., 6 %), induction generators are disconnected

from the grid [8].This study uses a proven method [9] for voltage

unbalance compensation, which completely eliminates negative

sequence voltage.

Reactive power compensation is another requirement for

wind turbines. In the case of squirrel cage induction generators,

this requirement is generally met by shunt capacitors. However,

DFIGs can be operated at any power factor, depending on con-

verter ratings. The scheme proposed here uses vector control for

reactive power and speed control, as in the conventional scheme.

Section II of this paper describes induction generator mod-

eling and vector control. Section III describes the proposed

new scheme for a DFIG. Section IV extends the scheme to the

SCIGs. Comparison of the conventional DFIG scheme and the

0885-8969/$25.00 2009 IEEE


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JOSHI AND MOHAN: NOVEL SCHEME TO CONNECT WIND TURBINES TO THE POWER GRID 505

proposed scheme is given in Section V. The paper is concluded

in Section VI.

II. INDUCTION GENERATOR MODELING

AND VECTOR CONTROL

The induction generator is modeled using conventional dq

equations, as follows [10]:

vsd = Rs isd +d

dtsd dsq (1)

vsq = Rs isq +d

dtsq + dsd (2)

vr d = Rr ir d +d

dtr d dA r q (3)

vr q = Rr ir q +d

dtr q + dArd (4)

dA = d m (5)

Tem = p2

(r q ir d r d ir q ) (6)

Jeqd

dtmech + Bmech = Tem TL (7)

sd = Ls isd + Lm ir d (8)

sq = Ls isq + Lm ir q (9)

r d = Lr ir d + Lm isd (10)

r q = Ls ir q + Lm isq . (11)

Here, vsd , vsq , vr d , and vr q are d and q axes statorand rotor winding voltages, as described in [10]. Similarly

isd , isq , ir d , and ir q are d and q axes stator and rotor windingcurrents, andsd , sq , rd , andr q arestator androtor fluxlink-ages in Wb-turns. d is the speed of a reference frame and mis the rotor speed in electrical radians per second. TL and Temare load torque and electromagnetic torque in newtonmeters,

respectively, and Jeq and B are moment of inertia and frictionconstant. Rs , Rr , Ls , Lr , and Lm are the induction generatorparameters. Only DFIG vector control is taken into account, as

shunt capacitors are used in the case of SCIGs for reactive power

control.

For reactive power and speed control, stator-flux-oriented

vector control is used. In this approach, the d-axis is always

aligned to the stator flux space vectors . Therefore, the q-axis

stator flux, i.e., sq , is zero, and sd is kept constant. To find

the reference frame speed d , such that the d-axis is alignedtos , DFIG equations are written as follows, assuming that

sq = 0:

vsd = Rs isd +d

dtsd (12)

vsq = Rs isq + dsd . (13)

From (13), d can be calculated as

d =

(vsq Rs isq )

sd . (14)

Also,

sq = Ls isq + Lm ir q = 0 (15)

from which we can write

isq = LmLs

irq . (16)

From the above equations, d can be written as

d =1

sd

vsq +

Lms

ir q

(17)

where s is the stator time constant Ls /Rs . Similarly, isd canbe written in terms ofir d :

sd = Ls isd + Lm ir d . (18)

Thus, we can write

isd =sd

Ls

LmLs

ir d . (19)

In (17), to find sd

, the following analysis is used:

vsd = Rs isd +dsd

dt. (20)

From (19) and (20)

dsddt

+ Rssd

Ls= vsd + Rs

LmLs

ir d . (21)

After taking Laplace transform,

sd (s) =s vsd (s) + Lm ir d (s)

1 + ss. (22)

All theaforementioned analyses canbe represented by a block

diagram shown in Fig. 2

III. NEW SCHEME FOR DFIG

The proposed new scheme for DFIG is shown in Fig. 3. In

a conventional scheme, the grid-side converter generally func-

tions as a STATCOM. In our scheme, the grid-side converter

is connected in series instead of in parallel. Also, instead of 1

three-phase converter, 3 single-phase converters are used.

The function of the rotor-side three-phase converter is similar

to that in a conventional scheme of a DFIG, i.e., for reactive

power control and speed control. Vector control as described in

the previous section is used for this purpose.

The grid-side converters have three objectives:

1) to maintain the dc link voltage;

2) to compensate for unbalanced voltages;

3) to limit the fault current/low-voltage ride-through.

The single-phase converter in phase A acts to compensate

for the voltage unbalance, and either the phase B or phase C

converter can operate to maintain the dc link voltage. If voltage

unbalance compensation is the only aim, then two converters

are sufficient. However, for a low-voltage ride-through, these

converters will add voltage in series of the faulty phase, such

that the voltage seen by the induction generator will be higher

than the voltage of the faulty phase. In the following analysis, the

function of all converters is interchangeable. This is required,

as the faults can occur on any phase.


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Fig. 2. Block diagram for vector control.

Fig. 3. New scheme for DFIG.

A. Converter A Control

Converter A is used for voltage unbalance compensation.Voltage unbalance compensation is achieved similarly to the

method and the remaining part of this system will be given

in [9]. However, in that scheme, a rectifier is used for the dc

link, while in this scheme, a series converter is used to maintain

a dc link voltage, as shown in Fig. 3.

As per the International Electrotechnical Commission (IEC)

definition, voltage unbalance is given as the ratio of the negative

sequence voltage to the positive sequence voltage. Negative

sequence voltage in a three-phase system is given by

vne g =

1

3 [van + 2

vbn + vcn ] (23)

where = 11200 , and van , vbn , and vcn are phase voltages. If

we inject voltage in phase A given by the equation

vi = [van + 2 vbn + vcn ] (24)

then the new negative sequence voltage will be

vne wne g =1

3

van [van +

2 vbn + vcn ] + 2 vbn + vcn

= 0. (25)

Thus, voltage unbalance compensation can be achieved. It has

been proven in [9] that the rating of this converter and the series

transformer is less than 3% of the generator rating for 10%

voltage unbalance compensation.


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B. Converter C Control

Converter C tries to maintain the dc link voltage, as shown in

Fig. 3. The dc link is approximately modeled as follows:

PC = Pr Pse r (26)

Vd CddVddt = Pr Pse r (27)

dVddt

=1

Vd Cd(Pr Pse r) (28)

where PC is the difference between the rotor active power andthe active power injected in series, Pr is the rotor active power,Pse r is the active power injected in the grid by all series convert-ers, Vd is the dc link voltage, and Cd is the dc link capacitance.

As converter C is trying to maintain the dc link voltage, it has

to either inject or absorb real power from the grid, according

to the generator speed. Converter C will not supply or draw

reactive power from the grid. Thus, the voltage injected from

converter C, vcseries , should be in the phase with the line Ccurrent, ics , and the magnitude of the voltage to be injected willbe decided by the active power to be transferred. This converter

will supply the active power required by the rotor, so the rating

of this converter and the series transformer is the same as that

of the rotor-side converter.

C. Fault Current Limitation

The single line to a ground fault case was considered, as

this is the most common fault. This fault is detected by voltage

magnitude. The moment that voltage magnitude goes below a

certain limit, the converter in that phase will inject some voltage,

such that the voltage seen by the generator will be higher thanthe fault voltage, and the fault currents will be limited. Here, it

can be noted that the grid voltage is still the fault voltage, but by

injecting voltage in series with the line, the voltage seen by the

generator is changed. This way the generator operating point on

the curve shown in Fig. 1 can be moved upwards and can achieve

fault ride through. The same principle can be applied in the case

of low-voltage ride-through. During a fault, power in the faulty

phase will be transferred to other phases, as one converter is

still operating to maintain the dc link voltage. It should also be

noted that one converter is still operating for balancing voltages,

which will further help in stabilizing the system. This scheme is

not designed for three-phase faults, but these faults occur rarely.

D. Simulation Results

Simulations were performed on a 750 kW induction generator

using MATLAB/SIMULINK. Figs. 4 and 5 show the results for

speed and the reactive power control. The speed reference was

changed from 1.15 to 0.85 pu, and the reactive power command

was changed from 0 to 0.4 pu.

Fig. 6 shows the results for voltage unbalance compensation,

i.e., unbalanced voltages and achieved balanced voltages. This

case was tested for 11% grid voltage unbalance. At the generator

point, almost 0% unbalance was achieved.

Fig. 4. Speed response.

Fig. 5. Reactive power response.

Fig. 6. Voltage unbalance compensation in DFIG.


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Fig. 7. Torque in unbalanced case (DFIG).

Fig. 8. Torque in balanced case (DFIG).

Figs. 7 and 8 show torque ripples for unbalanced and balanced

cases. The torque ripple was reduced from 20% to almost 0%.

Figs. 9 and 10 show the fault current limiter (FCL) results.

At 10 s, a fault was applied on phase B for 250 ms (i.e., phase

B voltage was dropped to 0.16 pu). When the FCL was used,the fault current peak observed was only approximately 3.5 pu,

while without the FCL, the fault current reached approximately

7 pu.

IV. EXTENSION OF PROPOSED SCHEME TO AN SCIG

As noted earlier, vector control is not a consideration in squir-

rel cage induction generator. However, voltage unbalance com-

pensation and the fault current limitation are still achievable.

Fig. 11 shows the DFIG scheme modified for the squirrel cage

induction generator. Since the rotor in SCIG is short-circuited,

we cannot control reactive power. SCIG modeling is similar to

the modeling given in Section II,except for the fact that the rotor

Fig. 9. Fault current without FCL (DFIG).

Fig. 10. Fault current with FCL (DFIG).

voltages, i.e., Vr d and Vrq , are zero. The single-phase convertercontrols are also the same as in the DFIG.

Figs. 12 and 13 show the results for voltage unbalance com-

pensation and the fault current limitation in the case of an SCIG.

Again, a 100% voltage unbalance was achieved, and the faultcurrents were limited from 20 to 2 pu. It should be noted that,

since vector control is not considered here, the single phase con-

verter rating will be less than the converter rating used in the

case of a DFIG.

V. COMPARISON OF PROPOSED SCHEME WITH CONVENTIONAL

SCHEME

In the conventional scheme of connecting the DFIG to the

power grid, the rating of the grid side converter depends on the

required speed range of the induction generator. It is approx-

imately s Pg , where s is the slip of the machine and Pg is

the generator rating. In the proposed scheme, the rating of the


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Fig. 11. Similar scheme for SCIG.

Fig. 12. Voltage unbalance compensation in SCIG.

converter that is trying to maintain the dc link voltage depends

on the speed range and is s Pg . The rating of the convertertrying to compensate for the voltage unbalance is small; it is 3%

for a 10% voltage unbalance compensation [9].

Previous efforts to reducetorqueripple duringthe grid voltage

unbalance have not been very effective, i.e., in these schemes,

complete voltage balancing cannot be achieved. In the proposed

scheme, it is possible to completely eliminate the negative se-

quence voltage and significantly reduce the torque ripple. In the

conventional scheme, during the fault, there is a risk of higher

currents in converters, and it may be necessary to disconnect the

generator from the grid. However, in the proposed scheme, the

fault currents can be limited, and faulty power can be transferred

to other phases. Thus, the new scheme is helpful in eliminating

Fig. 13. Fault current (isd ) in SCIG.

almost all problems related to the grid connection of wind tur-

bines, although it requires six additional switches. The proposed

scheme can also be applied to an SCIG.

VI. CONCLUSION

In this paper, a new scheme is presented to connect wind

turbines to the power grid. With this scheme, it is possible to

keep wind turbines synchronized to the grid, even in the case of

faults or low grid voltages. Voltage unbalance can be completely

removed by using this scheme. The scheme is applicable for

doubly fed as well as SCIGs. Simulation results are presented

for all the cases.


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APPENDIX

INDUCTION GENERATOR PARAMETERS

REFERENCES

[1] Global Wind Energy Council Brochure [Online]. Available: http://www.gwec.net/fileadmin/documents/GWEC_Brochure.pdf

[2] J. G. Slootweg, Wind power: Modelling and impact on power systemdynamics, Ph.D. dissertation, Tech. Univ. Delft, Delft, The Netherlands,2003.

[3] A. Petersson, Analysis, modeling and control of doubly-fed inductiongenerators for wind turbines, Ph.D. dissertation, Chalmers Univ. Tech-nol., Goteborg, Sweden, 2005.

[4] W. Janssen, H. Luetze, A. Buecker, T. Hoffmann, and R. Hagedorn, Lowvoltage ride through for wind turbine generators, U.S. Patent 6,921,985B2.

[5] AWEA Petition before Federal Energy Regulatory Commission[Online]. Available: http://www.awea.org/policy/documents/AWEAR-mkgPetition.pdf

[6] J. Morren and S. W. H. de Haan, Ridethrough of wind turbines withdoubly-fed inductiongenerator during a voltage dip, IEEE Trans. EnergyConvers., vol. 20, no. 2, pp. 435441, Jun. 2005.

[7] E. Muljadi, T. Batan, D. Yildirim, and C. P. Butterfield, Understandingthe unbalanced voltage problem in wind turbine generation, in Proc. Ind.

Appl. Conf., 1999, vol. 2, pp. 13591365.[8] T. Brekken and N. Mohan, A novel doubly-fed induction wind generator

control scheme for reactive power control and torque pulsationcompensa-tion under unbalanced grid voltage conditions, in Proc. Power Electron.Spec. Conf., 2003, vol. 2, pp. 760764.

[9] V. B. Bhavaraju and P. N. Enjeti, An active line conditioner to balancevoltages in a three-phase system, IEEE Trans. Ind. Appl., vol. 32, no. 2,pp. 287292, Mar./Apr. 1996.

[10] N. Mohan. (2001). Advanced Electric Drives, MNPERE [Online].Available: http://www.mnpere.com

Nitin Joshi received the B.E. degree from the IndianInstitute of Technology-Mumbai, Mumbai, India, in2000, and the M.Tech. degree from Dr. Babasa-heb Ambedkar Marathwada University, Aurangabad,India, in 2003, both in electrical engineering. He iscurrently working toward the Ph.D. degree at theDepartment of Electrical Engineering, University ofMinnesota, Minneapolis.

Ned Mohan received the B.Tech. (Honors) degreein electrical engineering from the Indian Institute ofTechnology-Kharagpur, Kharagpur, India, in 1967,the M.S. degree in electrical engineering from theUniversity of New Brunswick, St. John, NB, Canada,in 1969, and the M.S. degree in nuclear engineeringand the Ph.D. degree in electrical engineering fromthe University of Wisconsin, Madison, in 1972 and1973, respectively.

He is currently the Oscar A. Schott Professor of

power electronics at the University of Minnesota,Minneapolis, where he has been teaching since 1976. He has numerous patentsand publications in the field of power electronics, electric drives, and powersystems. He is the author of five textbooks.

Dr. Mohan was the recipient of the Distinguished Teaching Award presentedby the Institute of Technology, University of Minnesota.

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