Power Balancing in Variable Speed Wind Energy Systems Using Vector Control of Front End Converters

8/9/2019 Power Balancing in Variable Speed Wind Energy Systems Using Vector Control of Front End Converters

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Power Balancing in Variable Speed Wind-Energy Systems

Using Vector Control of Front-End Converters

Roberto Cárdenas, Rubén Peña, Marcelo Pérez, Fernando Vargas Greg Asher, Jon ClareUniversity of Magallanes School of Electrical Engineering.

P.O Box 113-D University of Nottingham

Punta Arenas Nottingham University Park, NG7, 2RD

CHILE [email protected] [email protected]

Abstract – This paper presents a novel power balance control

method for wind energy systems feeding an isolated grid. The

system is based on a variable-speed Wind Energy Conversion

System (WECS) connected to an ac load using a power

converter. An energy storage system, connected to the ac load

using an additional converter, is used to balance the power

generated by the WECS with the load. In this paper the vector

control systems for both interfacing power converters are

discussed. The proposed control methods are implemented in a

2kW experimental prototype and the experimental results are

fully analysed and discussed in the paper.

I. INTRODUCTION

Variable speed operation of wind turbines has many

advantages that are well documented in the literature [1-3].

Torque peaks in the gearbox and shafts are reduced, the wind

turbine can operate with maximum aerodynamic efficiency

and power fluctuations can be absorbed as inertial energy inthe blades. In some applications, the wind turbine is

augmented by an additional source, usually a diesel generator

[4-7]. These generation schemes are called wind-dieselsystems.

In wind-diesel systems, wind speed variations may

produce not only power fluctuations but also frequentstart/stop cycles of the diesel engine in response to periods of

unacceptably low wind speed. Simulation results presented in

[5] illustrated than only two minutes of storage (i.e. to be

able to supply the load for two minutes without diesel

generation) can reduce the number of diesel starts from 30 per hour to two per hour. Therefore, in wind-diesel systems

an energy buffer is very important in order to avoid

unnecessary deterioration of the diesel engine.

Fig. 1 shows the wind-diesel system studied in this

paper. A variable speed WECS is used to supply electrical

energy into a stand-alone load. A power converter is used in

the WECS side, to control the electrical torque of thegenerator, driving the wind turbine to the curve of maximum

power capture for a given wind speed. The control of the

WECS electrical generator and its associated power converter can be found in [1-2,8] and will not be repeated

here.

In Fig 1, the control of an energy buffer is also

considered. Energy is stored and released from the energy buffer, via an Energy Storage System (ESS) and interfacing

converter, to match the power absorbed from the wind with

the load power. A flywheel or batteries are used for energy

storing [9]. For sustained periods of low wind power, the

flywheel speed or battery charge will drop below a thresholdand the diesel generator is started, synchronised and

connected to the load. When the flywheel speed or battery

charge is above an upper threshold, the surplus energy has to be dissipated using a resistive load or the energy capture has

to be reduced using pitch control of the blades. The control

of the diesel engine is considered outside the scope of this paper; it is assumed throughout the paper that the diesel

generator is disconnectedThis paper addresses the control systems of the two

interfacing front-end converters. The control of the electrical

machine for the operation of the flywheel energy storagesystem has been discussed elsewhere [10] and it is

considered outside the scope of this paper.

Front-end

converter

Front-end

converter

Machine

converter

E C 3CIGG

G D3

Diesel based generation systemVariable Speed Wind Energy Conversion System

WECS

ω

iC

Flywheel

ElectricalMachine

E

Energy Storage System

ESS

iG

Machine

converter

Load

IG IWIC If

Fig. 1. Proposed control system.

24630-7803-9252-3/05/$20.00 ©2005 IEEE



va, vb, vcvα

sθ -je

ia, ib, ic

PWM2/3 sθ je

P+I

ωsL

ωsL

*α v

* β v

*av

*bv

*cv

vdp*

P+I

2/3

sθ -je

v β

iα

2/3i β

GcE

idC *

vd

idC

*

G E +

-

+

-+

-

+

+

-

+

+

+

θ s

L f (from WECS)

G E Angle

Calc.

C E

Variable

Gain

iqC *

vqp*

iqC

∼Load / WECS

Fig. 3. Control of the ESS front-end converter.

IG IW

WECS ESS

Chopper based

system

Current

ControlVoltage

ControlPWM

2

2

ESS

Control

2

PWM

EG

EC

d

LoadV

3φ3φ

IG* vd* EG*

EG

iG iC Chopper

basedsystem

Voltage

Control

EC*

Fig. 4. Experimental System.

III EXPERIMENTAL RESULTS

Fig. 4 shows the experimental system implementing that

of Fig. 1. The diesel generator has been excluded. The

control structures were implemented on a DSP board basedon a TMS320C31 processor. Two vector controlled 2kW

PWM inverters with a switching frequency of 1KHz are

used. A 1Khz chopper-based system is used to emulate thevariable speed wind turbine, supplying a current I G to the

WECS dc link capacitors. A current profile for the demand

current I G* is sent from the host PC to the DSP-board. Four

voltage transducers are used to measure the load voltage, and

the dc link voltages E G and E C . Four current transducers are

used to measure the currents supplied by the front-end

converters. An additional current transducer is used in the

chopper based I G control system. The front-end converters

are connected to the load using 12mH 0.3Ω filter inductances. The capacitor bank is selected for a resonant

frequency of approximately 500Hz. Unless otherwise is

stated, all experimental tests are carried out with the demandcurrent iqC

* =0; all reactive power is supplied from the WECS

front-end converter. The bandwidth of the dc link voltage

controller is ≈4Hz, the control loops for the d-q axis current

control loops are ≈70Hz. The load voltage control loop has a

bandwidth of about 7Hz.A current profile corresponding to a typical wind gust is

shown in Fig. 5a. A succession of these is supplied to the

WECS dc-link capacitors (as shown in Fig. 5b). With a

demand voltage E G* of 500V, and RL chosen for a load power

of ≈720W, Fig. 5c shows the direct component of the load

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voltage vd and the current idG. As expected, the shape of thecurrent waveform is similar to that of the I G current. Fig. 5d

shows idC supplied by the ESS front-end converter. The ESS

supplies a current between ≈1A to 5A. Note that idG + idC isapproximately 5A (the load current) for the entire duration of

the test corresponding to Fig. 5. The regulation of the load

voltage vd and dc link voltage E G is good. The dc-link voltage

variations are below ±5V, (about 1% of the reference

voltage). For the load voltage, the voltage variations are also

below ±5V (about 4.5% of the reference load voltage).

The system of Fig. 1 is also tested using a current profile

obtained from a real wind profile lasting about 60s. Fig. 6ashows the wind profile and its corresponding current I G, the

latter obtained by simulating a variable speed wind turbine

with an artificially zero inertia and applying the wind profileof Fig. 6a. Because wind turbines normally have large

inertia, they behave like a low pass filter, filtering out much

of the wind turbulence. If the wind turbine has no inertia, the

wind speed fluctuations are reflected on the generator output

power. This represents a worst-case condition for power smoothing since the high frequency content of I G is

maximised and the natural smoothing normally associatedwith the turbine inertia is minimised. The current profile of

Fig. 6a has a peak current of 3.5A with an average value

G I of 1.9A and a dispersion coefficient IGσ of 0.76A. The

mean turbulence intensity value, defined as G IG I σ , is

about 40%. This is a very high value of turbulence for a

variable speed wind turbines and represents a worst case for

testing purposes. Fig. 6b shows idG of the WECS front-end

converter and the load voltage vd . The load is ≈720W (about35% of nominal) for t < 10s. At t = 10s, the load is stepped to

≈1900W (95% nominal). After 50s the load is stepped again

to 720V. The shape of the idG is identical to that of the

current I G, with a small disturbance at the sudden loadvariations. The dip and the overshoot on the vd voltage are

≈17V. The response of the voltage controller is quite goodconsidering the magnitude of the load step, and also

considering that the control loops are not decoupled for fast

load changes. When the load is fixed, there is small variationin the voltage vd produced by the I G current fluctuations.

Fig. 6c shows the dc link voltage E G and the direct

component idC of the current supplied by the ESS front-end

converter. Before the load step the ESS front-end converter is

absorbing energy from the system (idC <0). After the loadstep, the additional load power is supplied from the ESS

converter and the current idC changes from 0 to ≈3A. Whenthe load is disconnected, the current idC is again reduced to a

low value. Notice that at the end of the current profile, because of the low load, the ESS front-end converter is again

absorbing energy from the system.

Fig. 6c shows also the dc link voltage E G, when the load

is connected and disconnected. The dip and the overshoot are

less than 15V (3% of the nominal voltage). During

application of the current profile, there is also a small

variation of ± 3V in the dc link voltage E G. Given the high

turbulence of the I G current profile used, this variation is very

0

0.5

1

1.5

2

2.5

3

Time (s)

I n p u

t C u r r e n

t ( A ) I G

0 1 2 3 4 5 6

Fig. 5a. Current profile corresponding to a wind gust.

0

0.5

1

1.5

2

2.5

3

Time (s)

I n p u t C u r r e n t ( A )

I G

0 5 10 15 20 25 30

Fig. 5b. Current profile at the input of the system.

0 5 10 15 20 25 300 5 10 15 20 25 30-3

-2

-1

0

1

2

3

4

80

120

160

200

240

D i r e c t C u r r e n

t ( A )

D i r e c t V o

l t a g e

( V )

vd

idG

Fig. 5c. Current supplied by the WECS front-end converter andcorresponding load voltage.

1

3

5

7

9

Time (s)

D C l i n k v o

l t a g e

( V )

D i r e c t C u r r e n

t ( A )

0 5 10 15 20 25 300 5 10 15 20 25 30

idC

E G

460

470

480

490

500

510

520

Fig. 5d. Current supplied by the ESS front-end converter and correspondingdc link voltage.

small and shows the high performance of the proposed

control system.

In order to test the robustness of the control method, its

performance is tested using a load with a low pf of 0.53 (20Ωin series with 100mH - both per phase). The shape of the

current profile used in the experimental tests is identical to

that of Fig. 6a, but with an average value I G of approximately1.3A.

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0 10 20 30 40 50 600

2

4

6

8

10

12Wind

I G

0

1

2

3

4

5

6

Time(s)

W i n d S p e e d

( m s - 1

)

I n p u

t C u r r e n

t I G ( A )

Fig. 6a. Wind profile and corresponding current profile.

0 10 20 30 40 50 600 10 20 30 40 50 60

-5

-3

-1

1

3

5

75

175

225

275

125

Time(s)

D

i r e c t C u r r e n t ( A ) idG

vd

L o a d V o l t a g e ( V )

Fig. 6b. Current supplied by the WECS front-end converter and loadvoltage corresponding to Fig. 6a.

0 10 20 30 40 50 600 10 20 30 40 50 60-2

0

2

4

6

8

10

420

440

460

480

500

520

D

i r e c t C u r r e n t ( A )

Time(s)

D C l i n k v o l t a g e ( V )

idC

E G

Fig. 6c. Current supplied by the ESS front-end converter and dc link

voltage corresponding to Fig. 6a.

Fig. 7a shows the d-q currents supplied by the ESS and

WECS converters. For this test the ESS front-end isabsorbing much of the energy supplied from the WECS. This

is because the load has a low power factor and there is a largevoltage drop in the load inductance. Fig. 7b shows the dc-

link voltage E G and the load voltage vd . The performance of

the control system is good even for this low power factor load. There is some increase in the switching noise and the

oscillation magnitude has increased slightly respect to the

performance of Fig. 6. Nevertheless the experimental test of Fig. 7 shows the robustness of the proposed control system.

Even when the power factor is relatively low, the

performance obtained from the control system is still quitegood.

0 10 20 30 40 50 600 10 20 30 40 50 60-4

-2

0

2

4

Time (s)

C u r r e n t s ( A )

iqG

idG

idC

Fig. 7a. d-q components of the front-end converters for a low power factor load.

0 10 20 30 40 50 600 10 20 30 40 50 60

480

485

490

495

500

505

510

100105

110

115

120

125

130

E G

vd

D

C l i n k v o

l t a g e

( V )

L o

a d V o

l a t g e

V d

( V )

Time (s)

Fig. 7b. Load voltage and dc link voltage corresponding to Fig. 7a.

IV. CONCLUSIONS

This paper has presented a new control strategy for power

balancing in a variable speed wind generation or wind-diesel

system feeding an isolated grid. The WECS comprises a

variable speed wind turbine and back-to-back power

converters. The ESS system comprises a front-end power converter interfacing an energy storage medium (flywheel or batteries). The WECS is connected to the load using a vector

controlled front-end converter, which regulates the load

voltage and frequency. Power balancing is achieved by

regulating the dc-link voltage of the WECS converter using

the direct component of the current supplied by the ESSfront-end converter. Experimental results, using real wind

profiles, wind gusts and power steps, have been presented

and the results demonstrate excellent performance. For a

typical wind profile the voltage regulation in the ac load isalmost perfect.

The experimental and simulation results obtained in this

work are very promising and illustrate the advantages andimprovements that can be expected when modern control

techniques and power conversion are applied to wind-diesel

systems.

V. ACKNOWLEDGEMENTS

The support of the Chilean Government throughFondecyt grant 1050592 is gratefully acknowledged. The

support of the British Council, through their academic links programme, which has made the collaboration between The

University of Magallanes and The University of Nottingham

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possible, is also kindly acknowledged.

VI. REFERENCES

[1] A. Miller, E. Muljadi, D. Zinger, “A Variable Speed

Wind Turbine Power Control”, IEEE Trans. On Energy

Conversion, Vol. 12, No. 2, June 1997, pp. 181-186.

[2] E. Muljadi, C. Butterfield, “Pitch-Controlled Variable-Speed Wind Turbine Generation”, IEEE Trans. On Industry

Applications, Vol. 37, No. 1, 2001, pp. 240-246.[3] M. Steinbuch, “Optimal multivariable control of a wind

turbine with variable speed”, Wind Engineering, Vol. 11, No.

3, 1987, pp. 153-163.[4] A.J Rudell, J.A.M. Bleij, L. Freris :“A wind diesel system

with variable speed flywheel storage”, Wind Engineering,

Vol. 17, Nr. 3, pp. 129-145, 1993.

[5] R. Dettmer, “Revolutionary Energy; A wind/diesel

generator with flywheel storage”, IEE Review, pp. 149-151,

April 1990.[6] F. Hardan, J. Bleij, R. Jones, P. Bromley, A. J. Rudell,

“Application of a Power-Controlled Flywheel Drive for Wind Power Conditioning in a Wind/Diesel Power System”,

in Proceedings of IEE 9th International Conference on

Electrical Machines and Drives, pp. 65-70, 1999.

[7] I. J. Iglesias, L. Garcia, A. Agudo, I. Cruz, L. Arribas,“Design and Simulation of a Stand-Alone Wind Diesel

Generator with a Flywheel Energy Storage System to Supply

the Required Active and Reactive Power”, in Proceedings of

IEEE Pesc’00, pp. 1381-1386, 2000.[8] R. Cárdenas, R. Peña, "Sensorless Vector Control of

Induction Machines for Variable Speed Wind Energy

Applications", IEEE Transactions on Energy Conversion,

Vol. 19, Nr. 1,pp. 196-205, March 2004.[9] R. Hebner, A. Walls, “Flywheel Batteries Come AroundAgain”, IEEE Spectrum, pp. 46-51 April 2002.

[10] R. Cárdenas, R. Peña, G. Asher, J. Clare, " Power

Smoothing in Generation Systems Using a Sensorless Vector Controlled Induction Machine Driving a Flywheel ", IEEE

Transactions on Energy Conversion, Vol. 19, Nr. 1,pp. 206-

216, March 2004.

[11] T. Wang, Z. Zhilong, G. Sinha, X. Yuan, “Output Filter Design for a Grid-Interconnected Three-Phase Inverter”, in

Proceedings of the IEEE Power Specialist Conference,

PESC’03, June 2003, pp. 779-784, Acapulco, México.

[12] R. Peña, R. Cárdenas, J. Clare, and G. Asher, “Control

strategies for voltage control of a boost type PWMconverter,” in Proc. Power Electron. Specialist Conf., Vol. 2,

Vancouver, BC, Canada, June 2001, pp. 730–735.

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Power Balancing in Variable Speed Wind Energy Systems Using Vector Control of Front End Converters

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