New control strategy for variable speed wind turbine with DC DC converters

New Control Strategy for Variable Speed Wind Turbine with DC-DC converters Vladimir Lazarov*, Daniel Roye†, Dimitar Spirov* † and Zahari Zarkov* * Technical University-Sofia, Bulgaria,

New Control Strategy for Variable Speed Wind

Turbine with DC-DC converters

Vladimir Lazarov*, Daniel Roye†, Dimitar Spirov* † and Zahari Zarkov*

* Technical University-Sofia, Bulgaria, e-mail: vl_lazarov@tu-sofia.bg, d_spirov@tu-sofia.bg, zzz@tu-sofia.bg

† G2ELab, INP Grenoble, Saint-Martin-d'Hères, France, e-mail: daniel.roye@g2elab.grenoble-inp.fr

Abstract— The paper studies the performance of variable

speed wind turbine (VSWT) configuration with

non-inverting buck-boost converter The wind turbine systems

consist of permanent magnet synchronous generator

(PMSG) connected to diode rectifier, DC chopper and load

New control strategy, based on the maximum power point

tracking (MPPT) and limited power point tracking (LPPT)

algorithms is used to improve the system operation When

necessary to limit the power injected to the grid, due to

system operator demands, a control unit is implementing to

switch between two regimes of wind turbine operation: at

maximum power and at limited power The MPP tracker is

simple perturb and observation (P&O) controller in

combination with two optimum wind turbines power/torque

versus speed characteristics Two control loops: inner feed

forward current control loop and outer voltage control

closed loop are applied for the non-inverting buck-boost

converter The performance of the dynamic models and the

control loops is tested under various wind conditions The

simulation results are shown The results prove the strategy

and models reliability

Keywords—wind energy, converter control, modeling,

simulation

I INTRODUCTION The permanent magnet synchronous generator (PMSG)

based variable speed wind turbines (VSWTs) is widely

used in the wind industry because of his advantages such

as compact size and weight, dense flux, etc To operate in

VSWT system, this generator should be connected either with controlled rectifier or diode rectifier Thus, two configurations are possible: full size back-to-back configuration with two voltage source converters as rectifier and inverter and DC capacitor and configuration with DC-DC converter The Boost converter is most used

DC chopper for such configuration, possessing important advantages in islanding systems (micro hydro turbines) or

in hybrid operation systems [1] However, the boost converter has a major drawback limiting the range of the voltages and respectively the generator speed For the present study, novel configuration with diode rectifier and non-inverting buck-boost DC chopper is investigated The non-inverting buck-boost converter is interesting for his ability to produce higher or lower DC voltage than the source voltage [2] This specific feature of the converter may be very useful in the cases of high wind speeds and when the network system operator demands less power to

be transferred into the grid [3] Combined with appropriate control strategy, the advantages of this particular DC-DC converter can ensure the VSWT with more flexibility and operation time The proposed new control strategy is combination of hybrid MPPT algorithm [4] for the normal operation of the VSWT and Limited Power Point Tracking (LPPT) logic algorithm for the limit power operation and represent an electrical mode to limit the aero dynamical power instead of using mechanical pitch system The advantages of the faster electrical system response and his relatively simple control can make the VSWT very attractive for standard networks application, as well for smart grid systems

Wind

profile

Wind turbine

Drive

Diode rectifier

Boost converter LOAD

P elect

VSWT Aero dynamical / Mechanical unit

MPPT LPPT

Operator

Controller

Control VSWT Control unit

VSWT Electrical unit

14th International Power Electronics and Motion Control Conference, EPE-PEMC 2010

II SYSTEM MODELING The considered configuration is presented in Fig 1 In

order to investigate the different modes of operations,

every element of the conversion system is modelled and

simulations are performed The models are developed in

the MATLAB/Simulink® software environment

A Wind turbine model

The wind turbine extracts the wind aero dynamical

power The model of the wind turbine uses several inputs

to estimate precisely the mechanical torque and power,

such as: the wind speed, the blade pitch angle and the

rotor speed The wind speed is provided by a wind model

Detailed wind models for complex aero dynamical

calculation can be found in [5]and [6] For the electrical

simulations in this study, simplifying model is used

Typical wind profile is shown on Fig 2

5

10

15

20

Time [s]

Fig 2 Typical wind profile

The aero dynamical power is described by (1)

( )λ θ ν

ρ

2

p

Where ρ is the air density equal to 1.225 kg/m3, A is the

turbine blade surface, v is the wind speed and Cp is turbine

power coefficient which depends on the tip speed ratio (2)

and θ is the pitch angle The power coefficient is different

for every particular turbine and a convenient way to

reproduce the power curve is found in [7], using (2) and

(3)

i

a

i

−

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

− θ

− λ

=

5

4 3 2

⎟⎟

⎞

⎜⎜

⎛

+ θ

− θ + λ

= λ

1

1

3 2 1

b b

The coefficients ai and bi are chosen to fit very small

power wind turbine (approximately 2 kW) The pitch

angle is θ and λ is the tip speed ratio (TSR) The power

coefficient of the turbine is shown in Fig 3

Fig 3 Power coefficient curves for 2kW wind turbine model

The turbine rotor swing is described by a standard one-mass model

B PMSG and diode rectifier models

The models of the generator and the diode rectifier (DR) are developed using the SimPowerSystem® library

in the Matlab/Simulink® The DR is universal bridge 3 arms diode rectifier

C Non-inverting buck-boost converter model

The non-inverting buck-boost converter has two switches, driven synchronously by PWM modulator as it

is shown in Fig 4

Fig 4 Non-inverting buck-boost converter

This model is similar to the model of the buck-boost converter, but the output voltage polarity is not inverted

The voltage ratio is defined as:

u V

V

DCin

Where V DCin is the supply voltage, V DCout is the output

voltage and u is the duty cycle ratio The state-space

equation of the converter (5) can be used in average Simulink models

DCin

V L

u x x C C

u x

x

⎥

⎥

⎦

⎤

⎢

⎢

⎣

⎡ +

⎥

⎦

⎤

⎢

⎣

⎡

⎥

⎥

⎥

⎦

⎤

⎢

⎢

⎢

⎣

⎡

−

−

−

−

=

⎥

⎦

⎤

⎢

⎣

⎡

0 1

1

1 0

2 1 2

1





(5)

Depending on the control strategy, the converter is driven in buck or in boost mode The non-inverting buck-boost converter model is implemented in the study simulations using the electrical ports of Matlab SimPowerSystems® library, reproducing the electrical circuit in Fig 4

D Converter control

The model of the converter control is based on linear PI

current regulator and pulse width modulator, generating

the drive signal The regulator compares the current

trough the boost inductance signal with current reference

set point from the power point tracker and the error is

process to the PWM generator The converter drive

diagram can be seen in Fig 5

Power point

tracker

Current

sense

i*

i

Fig 5 DC-DC converter controller

III NEW POWER CONTROL STRATEGY

In order to extract the maximum power available at

various wind speeds, two different control algorithms are

normally applied to the VSWT: pitch angle control and

speed (torque) control

The pitch control algorithm is used to change the pitch

angle and the blades turn out slightly of the wind stream

as so limiting the aero dynamical power in above rated

wind speeds This control acts directly upon an additional

hydraulic system and is very important to protect the

blades from breaking

The speed control algorithm serves to keep the

generator rotor speed in certain boundaries As the wind

fluctuates, the rotor will accelerate or decelerate in order

to maintain that TSR, which give the maximum power coefficient The control governs the generator by power converters electrical means The grid synchronization is also accomplished by the converters (not considered in this work) As the foreseen power converter configuration consists of passive diode rectifier and of DC-DC converter, the speed control is achieved by the non-inverting buck-boost converter

The reference signal for the converter drive is derived from the power point tracker The tracker is ruled by new control strategy The strategy is focused on the possibility

to control the VSWT in the right-side of power coefficient curve, only using the wind turbine generator and the power converters, instead of the pitch mechanism This strategy is based on hybrid MPP tracker [8] with simple perturb and observation (P&O) algorithm [9], limited power point tracker, switch logic algorithm and predefined power/torque versus speed wind turbine characteristics Two specific regime of operation are distinguished: normal turbine operation and restricted turbine operation When the conditions are normal, i.e all produced electric power is authorised to be grid injected, the control strategy rely on the MPPT algorithm to keep the turbine converting the maximum available aero dynamical power When conditions change, i.e the network system operator enforces the turbine to limit the power transferred to the grid below of the maximum available, the switch logic algorithm enables the LPPT algorithm The algorithm keeps the turbine at one of the limited power point curves, as it can be seen on Fig 6, depending on the restricted conditions

0 2 4 6 8 10

12

14

16

18x 10

5

Mechanical speed [rad/s]

6m/s 10m/s 12m/s 14m/s MPP curve LPP curve at 90%

LPP curve at 70%

LPP curve at 50%

Region of LPPT control Region of MPPT control

Fig 6 Regions of MPP and LPP control

Over again, the switch logic algorithm recalls the

hybrid MPPT algorithm, when restricted conditions are

cancelled Normally, the MPP tracker count on the P&O

algorithm to find the maximum point The algorithm is

implemented and is shown in Fig 7 and Fig 8

ΔP = P(k) - P(k-1);

Δω = ω(k) - ω(k-1); Discretization

Kt = Δω/ΔP;

|Δω*(k)| = |ΔP(k).Kt| Initialization

If (Δω*(k-1) = = 0)

S = Sign [ΔP];

else

S = Sign [ΔP].Sign[Δω*(k-1)];

Δω*(k) = S.|ΔP(k).Kt|

If (|ΔP(k)| < = P band)

ω*(k) = ω*(k-1);

else ω*(k) = ω*(k-1) + Δω*(k);

Update;

End;

ω*(k)

Computation

Output

Fig 7 Perturb and observe (P&O) algorithm

1/Z

1/Z

w

P

W*

Kt

Fig 8 Simplified Simulink model of the P&O controller

Nevertheless, when sudden large change in the wind

speed occurs, the MPP tracker use predefined turbine

characteristics of the MPPT curve as it shown in Fig 9

0 50 100 150 200 250

0 10 20 30 40 50 60 70

Speed (rad/s)

MPP curve LPP curve at 70% LPP curve at 40%

Fig 9 Predefined torque curves

IV SIMULATION RESULTS The simulations with models of the above mentioned configuration in MATLAB/Simulink® were performed The non-inverting buck-boost converter is controlled in boost mode, which is suitable for variable speed wind turbine operation The input and output converter voltages are shown in Fig 11, following lightly the changes of the wind profile at Fig 10 The inductor current and the reference current show adequate convergence and fast controller response in Fig 12

5.5 6 6.5 7

Time [s]

Fig 10 Wind speed

480 500 520 540 560 580 600 620 640 660

Time [s]

Non-inverting buck-boost DC voltage Diode rectifier DC voltage

Fig 11 DC voltages at real wind conditions

0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

1

1.2

1.4

1.6

1.8

2

2.2

2.4

2.6

2.8

3

Time [s]

Converter current Reference current

Fig 12 Reference and converter currents with MPPT control

600

800

1000

1200

1400

1600

1800

2000

2200

Time [s]

P curve MAX

P curve DC converter with LPPT at 100%

P curve DC converter with LPPT at 70%

Restricted condition of 70%

Fig 13 Maximum and converter power curve

with MPP and LPP tracking

The converter power curve is close to the turbine

maximum power curve for about 10 m/s wind speed of the

modeled turbine, as it shown on Fig 13(upper curves) and

the turbine convert almost all available power The

converter power curve is below the imposed restricted

limit, in Fig 13 (bottom curve), even thought the turbine

continues to operate and to transfer electric power to the

network

V CONCLUSION The simulation results display satisfying convergence

of the converter power curve with the turbine maximum

power curve and good performance of the LPP tracker

The non-inverting buck-boost converter operates

satisfactorily with the new control strategy and the

combination of the non-inverting chopper and this new

controller seems to be promising Nevertheless, investigation in more complex voltage control strategy for the non-inverting buck-boost chopper is foreseen to improve the advantages of this type of converter in high wind speed region and increased rotor speed The advantages of the LPP control strategy can be also applied

in hybrid systems and autonomous grids, along with non-inverting buck-boost converter or other power converter configuration as boost DC-DC converter

ACKNOWLEDGMENT The authors would like to thank the Bulgarian National Research Fund for the financial support (contract

EE 106/07) and the Technical University-Sofia for the financial aid (contract 102ни225-1/2010)

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