DC bus control of variable speed wind turbine using a buck boost converter

Abstract— For most of the peak power extraction methods in wind turbine generation system described in the current literature, it is necessary to know the wind turbine’s maximum power

Abstract— For most of the peak power extraction methods in

wind turbine generation system described in the current

literature, it is necessary to know the wind turbine’s maximum

power curve and the wind speed measurement These methods

used the maximum power curve obtained via simulations or tests

for individual wind turbines This makes these methods difficult

and expensive to implement in practice In addition, the use of

wind speed sensor to measure the wind speed adds to a system a

cost and presents some difficulties in practical implementation

This paper describes the design of a buck-boost converter circuit

used to achieve the maximum power control of wind turbine

driven permanent magnet synchronous generator (PMSG) The

PMSG is suitably controlled according to the generator speed

and thus the power from a wind turbine settles down on the

maximum power point by the proposed MPPT control method,

where the wind turbine’s maximum power curve and the

information on wind velocity are not required

Index Terms— Wind turbine, Variable-Speed, synchronous

generator, Maximum power, Buck-boost converter

I INTRODUCTION

Wind energy is a clean energy which had a fast

development for the two last decades Wind energy seems to

be an interesting alternative that makes possible to control the

energy injected into the network and to produce not polluting

fuel usable in the buildings, and thus to diversify the energy

markets To be used on wide applications and to satisfy the

economic constraints, the conversion chain of this energy must

be robust and reliable It must also present a better efficiency

and be realized at low cost For that, it is necessary to extract

the maximum power from wind turbine The operating power

of wind turbine depends on the wind speed intensity and

especially the turbine speed [1] If the transfer of power

between wind turbine and the load is not optimal, the total

efficiency of wind energy system will be largely affected [2]

The wind turbine can be operated at the maximum power

operating point for various wind speeds by adjusting the

turbine speed optimally [3],[4] In previously published works,

most of the MPPT methods described require the knowledge

of the wind turbine’s maximum power curve and the wind

speed measurement [5-7] The maximum power curve needs

to be obtained via simulations or tests for individual wind turbines, which makes these methods difficult and expensive

to implement in practice [8]-[9] In addition, the use of wind speed sensor to measure the wind speed adds to a system a cost and presents some difficulties in practical implementation

These MPPT methods described in the current literature are too expensive compared with generator whose rated capacity

is small [10]

This paper describes the design of a buck-boost converter circuit used to achieve the maximum power control of wind turbine driven PMSG The PMSG is suitably controlled according to the generator speed and thus the power from a wind turbine settles down on the maximum power point by the proposed MPPT control method, where the wind turbine’s maximum power curve and the information on wind velocity are not required

II WIND TURBINE CHARACTERISTICS

The conversion from wind speed to mechanical power (Pm) can be described in steady state by [11]

( ) * 0.647 * *

Pm=Cp λ A v (1) Where

r = radius of the rotor [m]

ρ = air density [Kgm-3]

A = wind turbine rotor swept area [m2]

v = wind speed [m/s]

The power coefficient expresses the conversion efficiency of the turbine as a function of the tip-speed ratio (λ) given by [12]:

r. m

v

λ= Ω (2) Where Ωm is mechanical angular velocity of the generator (rad/s)

A typical relationship between Cp and λ is shown in Fig.1 For constant-speed operation, the turbine speed (Ωm) is forced to remain constant Thus, as the wind speed changes, the tip speed ratio and the power coefficient will vary Since the Cp characteristic has a single maximum at a specific value

of it is apparent that, when operating at a constant speed, the power coefficient will be maximum at only one wind speed

Hydrogen Research Institute Université du Québec à Trois-Rivières, C.P 500, Trois-Rivières, (QC), G9A 5H7, Canada

1-4244-0493-2/06/$20.00 ©2006 IEEE.

Unlike constant-speed control, a variable-speed control can

adjust the speed of the turbine as the wind speed changes, so

as to operate at the peak of the Cp curve This will maximize

the power generated for a particular wind speed, as it shown

on Fig.2 In this figure, curves for the power generated at

various wind speeds are given The curve connecting the

peaks of these curves will generate the maximum power for a

given wind speed and follow the path for maximum Cp

operation

III CONVERTER DESIGN

The figure 3 gives the schematic diagram of the stand-alone

wind energy system under consideration A three-phase

PMSG is connected to a DC battery bank via a rectifier The

DC/DC converter is connected with rectifier circuits like fig.3

It is assumed that the power generated from the generator is

converted into DC power through diode bridge rectifier

circuits with a unity power factor and the load current is

continuous [13]

P t = 3VI =V I d c dc (3)

Where, V dc , I dc are DC side voltage and current, respectively

The mean value of DC voltage is shown like the following

6

3

co s

π

where VLLmax is the maximum value of line-to-line voltage

max

3

π

= (5)

From this, the relationship between V dc, and line to line

voltage V LL and phase voltage V, is shown as following,

V d c 3 2V L L

π

= (6)

V d c 3 6V

π

= (7)

From (3) and (7), the equation of I dc and I, concisely

expressed is obtained

6

d c

I = π I (8)

The PMSG is connected with rectifier circuits like fig.4 It is assumed that the AC power generated from the generator is converted into DC power through diode bridge rectifier circuits In continuous conduction mode, the buck-boost converter assumes two states per switching cycle The ON State is when IGBT is ON and diode is OFF The OFF State is when IGBT is OFF and diode is ON A simple linear circuit can represent each of the two states where the switches in the circuit are replaced by their equivalent circuit during each state The circuit diagram for each of the two states are shown

in Fig.4-a and Fig.4-b If the output load current is reduced below the critical current level, the inductor current will be zero for a portion of the switching cycle In a buck-boost power stage, if the inductor current attempts to fall below zero, it just stops at zero and remains there until the beginning

of the next switching cycle This operating mode is called discontinuous conduction mode (see Fig.4-c) A power stage operating in discontinuous conduction mode has three unique states during each switching cycle as opposed to two states for continuous conduction mode The inductor current condition where the power stage is at the boundary between continuous and discontinuous mode is shown in Figure 4 This is where the inductor current just falls to zero and the next switching

cycle begins immediately after the current reaches zero

In continuous conduction mode, the ratio between the output and input voltages turns out to be:

1

b a t

d c

V V

α α

=

− (10) Where α is the duty cycle and Vbat is the battery voltage

Fig 2 Typical wind power versus turbine rotor speed

Fig.3 : Connection diode rectifier circuits to the generator

1

α

−

⎝ ⎠ (12) From (7),(8) and (11), the following equation is obtained

d c 1 82

d c

d c

R

I = π I = (13)

Resistance value Rg, per phase of rectifier circuits is shown

like the following by generator terminal voltage V, and line

current I,

R g V

I

= (14)

The following equation is obtained, when (14) is substituted in

(13)

2 18

R =π R (15)

From (21) and (24), the following equation is obtained

2 2

1 18

α

−

⎝ ⎠ (16)

When the reactive impedance of the PMSG will be high and

the impedance of the battery load will be low (case of high

wind speeds), the poor impedance matching will limit power

transfer to the load In this case, the duty ratio of the

buck-boost converter must be controlled in order to effectively take

out the electric power

side of the MPP where dp/dΩ < 0 and the second on the left side of the MPP where dp/dΩ > 0

Fig 6 gives the algorithm of the proposed MPPT control method, where the information on wind velocity is not required For searching the maximum wind power operating point and tracking this point in order to reduce the error

Fig.5 Probable displacement of the operating point

between the operating power and the maximum power, in the event of change of the wind speed, the control of the buck-boost converter perturbs periodically the operating point of the wind turbine By acquiring the output voltage and current of PMSG, the control used this information to increase or decrease the duty cycle of the buck-boost converter to change the operating point of the wind turbine After the perturbation, there is a displacement of the operating point from (k-1) to (k), Four cases of perturbation from operating point are distinguished:

If P(k)>P(k-1) and Ω (k)> Ω (k-1), The power increases after perturbation This indicates that the MPP research is oriented to the good direction So, the search of the MPP continues in the same direction and reaches the operating point (k+1) by increasing the duty cycle by ∆α

If P(k)<P(k-1) and Ω (k) < Ω (k-1), The power decreases after perturbation This indicates that the MPP search is oriented to the bad direction The MPP search direction must be changed and the duty cycle is increased by two ∆α to reach the operating point (k+1)

If P(k)>P(k-1) and Ω (k)< Ω (k-1), The power increases after perturbation This indicates that the MPP search is oriented to the good direction Then, the MPP search direction must be maintained and the duty cycle is decreased by one ∆αd to reach the operating point (k+1)

If P(k)<P(k-1) and Ω (k)> Ω (k-1), The power decreased This indicates that the MPP search is oriented to bad direction The MPP search direction must be changed and the duty cycle is increased by two ∆αd to reach the operating point (k+1) Search rules of the various cases of operation are summarized

in the table I

Fig.4 Operation modes of buck boost converter

TABLE 1

SUMMARY OF CONTROL ACTION FOR VARIOUS OPERATING

POINTS

Fig 7 gives the wind turbine output power without MPPT

control method at low wind speed In this case, one notices

that the turbine does not produce energy, because the induced

voltage in the PMSG will not be high enough to overcome the

reverse bias in the diode bridge Fig 8 gives the wind turbine

output power with MPPT control method at low wind speed

The use of the MPPT converter imposes a low DC bus voltage

to recover the wind energy at the low winds speeds

PWM : d ‹— d 0

Measure I, V

N yes

First measure

P i+ 1 = V.I

d ‹— d + ∆d

P i ‹ ― P i+1

d ‹— d - ∆d

d ‹— d + ∆d

yes

Measure I, V

N

( ∆p/∆d) i >0

&

( ∆p/∆d) i+3 >0

d opt ‹— d i+2

N yes

yes ( ∆p/∆d) i+1 <0

&

( ∆p/∆d) i+2 <0

( ∆p/∆d) i <0

&

( ∆p/∆d) i+3 <0

N yes ( ∆p/∆d) i+1 >0

&

( ∆p/∆d) i+2 >0

N yes

Start

Fig.6 : Algorithm of the proposed MPPT control method

Fig.7: Wind turbine output power without MPPT control method for low wind speed

Fig.8: Wind turbine output power with MPPT control method for low wind speed

Fig 9 and 10 give the wind turbine output power without and with MPPT control method for high wind speed The impact

of the proposed MPPT control method used to generate power

at high wind speed can be clearly seen in Fig 13 The energy efficiency is improved on average by 24% The MPPT converter is designed to be efficient in high and low wind speeds

Fig.9 : Wind turbine output power without MPPT control method for high wind speed

Fig.10 : Wind turbine output power with MPPT control method for high wind speed

∆ P > 0 < 0 > 0 < 0

∆P/∆α > 0 < 0 < 0 > 0

∆Ω - – + +

a wind turbine settles down on the maximum power point.

There is a need to control the duty cycle of the buck-boost

converter to implement maximum power tracking in wind

turbine application Therefore, we have proposed an optimal

control method, where the information on wind velocity is not

required The use of buck-boost converter to control the dc bus

voltage shows that it is possible to operate at high efficiency in

the high and low wind speed region The energy efficiency is

improved on average by 24%

VI ACKNOWLEDGEMENTS

This work has been supported by the LTE Hydro-Québec,

Natural Resources Canada and the Natural Sciences and

Engineering Research Council of Canada

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