Vishnu. K - Academic Scienceacademicscience.co.in/admin/resources/project/paper/f201706221498148301.pdfVishnu K R,Nisha.G.Poothullil International Journal of Electronics, Electrical

Vishnu K R,Nisha.G.Poothullil

International Journal of Electronics, Electrical and Computational System

IJEECS

ISSN 2348-117X

Volume 6, Issue 6

June 2017

Abstract—This paper presents control strategies to control the generator sides and the grid sides of a variable speed wind energy conversion system. At the generator side, rotor speed is controlled for maximum power extraction. At the grid side, the DC link voltage is regulated for maximum power transfer from generator side to grid side. Rotor speed and DC link voltage are controlled using sliding mode controller and PI Controller. Sliding mode controller provides robustness to external wind speed variations compared to the conventional PI Controller. Simulation results proves the effectiveness of the control scheme.

Index Terms—Rotor speed, PI Controller, Sliding mode control, Wind Energy Conversion system.

I. INTRODUCTION

The renewable energy sources are one of the biggest

concerns of our times[3]-[7].High prices of oil and global

warming make the fossil fuels less and less attractive

solutions. Wind power is a very important renewable

energy source. It is free and not a polluter unlike the

traditional fossil energy. It obtains clean energy from the

kinetic energy of the wind by means of the wind turbine.

The wind turbine transforms the kinetic wind energy into

mechanical energy through the drive train and then into

electrical energy by means of the generator.

Although the principles of wind turbines are simple,

there are still big challenges regarding the efficiency,

control and costs of production and maintenance. Wind

power is growing and most of the wind turbine

manufactures are developing new larger wind turbines.

The power rating of the wind turbines built in 1980 was

of 50 kW and it’s rotor diameter was 15 m long. In 2003

the power was of 5MW and the rotor diameter was 124

m.

There are different wind turbine configurations. They

can be with or without gearbox, the generator can be

synchronous or asynchronous and finally the connection

with the grid can be through a power converter or it can

be directly connected. Different modes of operation can

be used depending on the wind turbine configuration.

They are classified as variable speed and fixed-speed

turbines. For fixed-speed operation, the system is very

simple and thus the cost is usually low .As a drawback,

the conversion efficiency is far from optimal.

For the variable-speed wind turbine, the system is

controlled to maximize the power extracted from the

wind. Normally they are connected to the grid by means

of a power converter. It increases the cost of the whole

system but provides full controllability of the system. Among all these configurations, the trend is to use

variable speed wind turbines because they offer more

efficiency and control flexibility which is becoming very

important to comply with the grid requirements.

Permanent Magnet Synchronous Generator (PMSG) is

an interesting solution which is based on variable-speed

operation. Since the speed of wind turbine is variable, the

generator is controlled by power electronic devices. With

permanent magnets there is no need for a DC excitation

system. With a multi pole synchronous generator it is

possible to operate at low speeds and without gearbox.

Therefore the losses and maintenance of the gearbox are

avoided.

The generator is directly connected to the grid through

a full scale back-to-back power converter. The power

converter decouples the generator from the grid. With a

full scale power converter, there are more losses which

may be a drawback but it allows a full controllability of

the system. With the use of the power converter it is

possible to comply with the grid connection

requirements.

The full scale back-to-back converter can be divided

into two parts: the generator side converter and the grid

side converter. The generator side converter is mainly

used to control the speed of the generator in order to

maximize the output power at low wind speeds. The grid

side converter is mainly used to keep the voltage in the

DC-link capacitor constant and also to control the

reactive power delivered to the grid. Nowadays different

techniques are used to control the grid side converters.

ANALYSIS OF CONTROL STRATEGIES IN WIND ENERGY

CONVERSION SYSTEM

Vishnu. K .R Nisha. G. poothullil

P G scholar Assistant professor

Department of Electrical and Electronics Engineering Department of Electrical and Electronics Engineering Govt. Engineering College Thrissur Govt. Engineering College Thrissur



IJEECS

ISSN 2348-117X

Volume 6, Issue 6

June 2017

Accurate Control in the variable speed wind energy

conversion system is required for maximising power

coefficient over a wide range of wind speeds. It is done

by tracking the rotor speeds corresponding to the

maximum power coefficient, despite the sudden changes

in wind gusts.

This paper is organised as follows: The wind turbine

system is explained in section II, Control strategies in

section III and section IV and the Simulation results in

section V.

Fig. 1. Block Diagram of PMSG based wind turbine

II. WIND TURBINE SYSTEM

The power extracted by a wind turbine is related to the

available wind power and the power curve of the

machine as expressed by the formula [1]

𝑃𝑚= 0.5ρ𝐶𝑝(λ)𝜋𝑟2𝑣𝑤3 (1)

Where ρ is the air density, r is the radius of turbine

blades, vw

is the wind speed and Cp is the power coefficient of the

wind

turbine as a function of the tip-speed ratio.

λ=𝑟𝜔𝑟

𝑣𝑤 (2)

where ωr is the turbine rotor speed. From the tip-speed

ratio

expression (2), any change in the wind speed while

keeping the rotor speed constant will modify the tip-

speed ratio which leads to the change of the power

coefficient Cp, as well as the generated power from the

wind turbine. Therefore, if the rotor

speed is adapted relative to the wind speed variation, the

tip speed ratio can be preserved at an optimum point λopt

which could yield to maximum power extraction by

operating the turbine at the speed reference.

III. PI CONTROLLER

The control system is an important feature for the wind

turbine performance. It maximizes the extracted power

from the wind through all the components and also

ensures that the delivered power to the grid complies

with the interconnection requirements. The control

strategy is applied to the converters.

The PMSG is driven by advanced power electronics. A

back- to-back Converter is used to connect the generator

to the grid and it provides the full controllability of the

system. It can be divided in two parts: the generator side

and the grid side. The first one controls the speed of the

rotor so that the power is maximized. The second one

controls the voltage across the DC-link and also the

reactive power delivered to the grid.

Fig. 2. Generator side control using PI controller

A. GENERATOR SIDE CONTROL

The generator side control consists of an outer loop PI-based Controller to control the rotational speed for maximum power extraction applications, and an inner loop PI-based current control to regulate the d-q components of the currents and provide the pulses for the IGBT-converter [2].

The speed controller is developed from the equation



IJEECS

ISSN 2348-117X

Volume 6, Issue 6

June 2017

𝑑𝜔𝑟

𝑑𝑡=

−𝐵

𝐽ωr +

1

𝐽𝑇𝑒 -

𝑇𝑡

𝐽 (3)

where Te is the electric torque, Te is expressed by PI controller as

𝑇𝑒 = 𝐾𝑝(𝜔𝑟 − 𝜔𝑟𝑒𝑓) + 𝐾𝑖 ∫(𝜔𝑟 − 𝜔𝑟𝑒𝑓) (4)

q component of the current is expressed as

𝑖𝑞𝑟𝑒𝑓 =𝐽

𝜌ɸ𝑇𝑒 (5)

d axis current is set to zero .The expression for the current controller model is given as follows:

𝑑𝑖𝑑

𝑑𝑡=

−𝑅

𝐿𝑑𝑖𝑑 +

1

𝐿𝑑[𝑝𝐿𝑞𝜔𝑟𝑖𝑞 + 𝑣𝑠𝑑] (6)

𝑑𝑖𝑞

𝑑𝑡=

−𝑅

𝐿𝑞𝑖𝑞 +

1

𝐿𝑞[𝑝𝐿𝑑𝜔𝑟𝑖𝑑 − ɸ𝑝𝜔𝑟 + 𝑣𝑠𝑞] (7)

The current control laws are given as follows :

𝑣𝑠𝑑 = 𝐾𝑝(𝑖𝑑 − 𝑖𝑑𝑟𝑒𝑓) + 𝐾𝑖 ∫(𝑖𝑑 − 𝑖𝑑𝑟𝑒𝑓)dt-

𝑝𝐿𝑞𝜔𝑟𝑖𝑞 (8)

𝑣𝑠𝑞 = 𝐾𝑝(𝑖𝑞 − 𝑖𝑞𝑟𝑒𝑓) + 𝐾𝑖 ∫(𝑖𝑞 − 𝑖𝑞𝑟𝑒𝑓)dt-p𝐿𝑑𝜔𝑟𝑖𝑑 + 𝑝𝜔𝑟ɸ (9)

B.GRID SIDE CONTROL

The grid side converter connected to a three-phase power supply, through an RL filter [2], is vector controlled in grid voltage reference frame. In voltage vector (d, q) reference frame, the dynamic model of the grid voltage is given by:

𝑣𝑑 = 𝑣𝑖𝑑 − 𝑅𝑖𝑑 − 𝐿𝑑𝑖𝑑

𝑑𝑡+ 𝜔𝐿𝑖𝑞 (10)

𝑣𝑞 = 𝑣𝑖𝑞 − 𝑅𝑖𝑞 − 𝐿𝑑𝑖𝑞

𝑑𝑡− 𝜔𝐿𝑖𝑑 (11)

𝑒𝑑 = 𝑣𝑑 + 𝐿𝜔𝑖𝑞 (12)

𝑒𝑞 = 𝑣𝑞 − 𝐿𝜔id (13)

Fig. 3. Grid side control using PI controller

where L and R are the filter inductance and resistance respectively, vid and viq are the inverter voltage components.

The grid side control system consists of two control loops.The outer DC link voltage control loop is used to set the d-axis current reference, while the inner control loop assures that the d and q- components of the current track the corresponding references as shown in Figure 3. The current controllers will provide a voltage reference for the grid side inverter that is compensated by adding rotational emf compensation terms.

IV. SLIDING MODE CONTROLLER

In simplest terms, the Sliding Mode control is a kind of non linear control which has been developed primarily for the control of variable structure systems. Technically, it consists of a time-varying state-feedback discontinuous control law that switches at a high frequency from one continuous structure to another according to the present position of the state variables in the state space, the objective being to force the dynamics of the system under control to follow exactly what is desired and pre-determined. The main advantage of a system with Sliding mode control characteristics is that it has guaranteed stability and robustness against parameter uncertainties.



IJEECS

ISSN 2348-117X

Volume 6, Issue 6

June 2017

Moreover, being a control method that has a high degree of flexibility in its design choices,the Sliding mode control method is relatively easy to implement as compared to other non linear control methods. Such properties make Sliding mode control highly suitable for applications in non linear systems, accounting for their wide utilization in industrial applications, e.g., electrical drivers, automotive control, furnace control, etc.

Sliding mode control is divided into two phases: In

first phase (Reaching phase) regardless the initial position

of the system, the Sliding mode control will force the

trajectory towards the sliding manifold. This is possible

by hitting condition.When the trajectory touches the

sliding manifold, the system enters the second phase

(known as sliding phase) of the control process and is also

said to be in Sliding mode operation.

A. GENERATOR SIDE CONTROL

The rotor dynamic system is governed by the following equation [1]:

𝑑𝜔𝑟

𝑑𝑡= −𝐴𝜔𝑟 − 𝐵𝑇𝑔 + 𝑑 (14)

Where Tg is the generator torque or the input used,

𝐴 =𝐾

𝐽and 𝐵 =

1

𝐽 are the rotor parameters, d is considered

as an unbounded disturbance. The function of the speed

controller is to track the reference speed.

𝑒 = 𝜔𝑟 − 𝜔𝑟𝑒𝑓 = 0 (15)

The dynamics of the speed tracking error is given as follows

𝑑𝑒

𝑑𝑡= ὠ𝑟 − ὠ𝑟𝑒𝑓

(16)

= -Ae+u+d

where the control input u is given as follows;

𝑈 = −𝐵𝑇𝑔 − 𝐴𝜔𝑟𝑒𝑓 −𝑑𝜔𝑟𝑒𝑓

𝑑𝑡 (17)

The sliding mode control strategy used for the speed

control is given as follows:

𝑈 = −𝑘1√𝑒𝑠𝑔𝑛(𝑒) + 𝜔

𝑑𝜔𝑟

𝑑𝑡= −𝑘2𝑠𝑔𝑛(𝑒) (18)

where k1 and k2 are positive constants From the above equations generator input torque is given as follows:

𝑇𝑔 =−1

𝐵[𝐴𝜔𝑟𝑒𝑓 + ὠ𝑟𝑒𝑓 − 𝑘1√𝑒𝑠𝑔𝑛(𝑒) + 𝜔

𝑑𝜔

𝑑𝑡= −𝑘2𝑠𝑔𝑛(𝑒) (19)

The wind turbine is coupled with a PMSG,Therefore

electromagnetic torque has the following expression:

𝑇𝑔 =3

2 𝜌ɸ𝑣𝑖𝑠𝑞 (20)

where isq is the q component of stator current, ɸ is the

permanent magnet flux linkage, p is the no: of pole pairs. Vector control strategy is used to regulate the d-q components of generator axis stator current which are expressed as follows:

𝑖𝑠𝑞𝑟𝑒𝑓=2

3𝜌ɸ𝑣𝑇𝑔 (21)

isdref is kept to zero.

Fig. 4. Generator side control using sliding mode controller



IJEECS

ISSN 2348-117X

Volume 6, Issue 6

June 2017

B.GRID SIDE CONTROL

For proper transfer of power from the generator side to the grid side , DC link voltage should be maintained a constant. In order to maintain the DC link voltage constant we use the sliding mode control approach and provide the required reference current required for the vector control strategy.

Grid dynamics with reference frames(d,q) are modelled as follows[1]:

𝑣𝑑 = 𝑣𝑖𝑑 − 𝑅𝑖𝑑 − 𝐿𝑑𝑖𝑑

𝑑𝑡 +ωL𝑖𝑞 (22)

𝑣𝑞 = 𝑣𝑖𝑞 − 𝑅𝑖𝑞 − 𝐿𝑑𝑖𝑞

𝑑𝑡 -ωL𝑖𝑑 (23)

vid and viq are the are the grid side inverter voltage components, vd and vq are the grid voltages id and iq are the gridcurrents, L and R filter inductance and resistance respectively. The reference frame is taken as follows:

𝑣 = 𝑣𝑑 + 𝑗0 (24)

The active and reactive power can be determined by:

𝑃 =3

2𝑣𝑑𝑖𝑑 (25)

𝑄 =3

2𝑣𝑑𝑖𝑞 (26)

The DC link voltage equation is carried out as follows:

𝐶𝑑𝑣𝑑𝑐

𝑑𝑡= 𝑖𝑔 − 𝑖𝑠 (27)

where C is the DC link capacitor, ig is the current between grid and DC link,is is the current between grid and stator side.

For an ideal inverter the power transffered is expressed

as follows:

𝑉𝑑𝑐𝑖𝑔 = 3

2𝑣𝑑𝑖𝑑 (28)

The DC link voltage dynamics can be expressed as:

𝑑𝑉𝑑𝑐

𝑑𝑡=

3𝑣𝑑

𝐶2𝑉𝑑𝑐𝑖𝑑 −

1

𝐶𝑖𝑠 (29)

The above equation can be modified as:

𝑑𝑉𝑑𝑐

𝑑𝑡= (𝐵𝑣 + 𝛥𝐵𝑉)𝑖𝑑 −

1

𝐶𝑖𝑠 (30)

Where

𝐵𝑣 =1

𝑐

3

2

𝑣𝑑

𝑣𝐷𝐶𝑖𝑑

VDC is the DC link voltage reference and 𝛥Bv is the

variation between actual voltage and reference voltage.

The DC link voltage dynamics is given below:

𝑑𝑉𝐷𝐶

𝑑𝑡= 𝐵𝑣𝑖𝑑 −

1

𝐶𝑖𝑠 + 𝑑𝑣 (31)

where dV represents the uncertainties in voltage The

voltage error dynamics is given as follows:

𝑑𝑒

𝑑𝑡=

𝑑𝑣𝑑𝑐

𝑑𝑡−

𝑑𝑣𝐷𝐶

𝑑𝑡

= 𝐵𝑉𝑖𝑑 −1

𝑐𝑖𝑠 − 𝑉𝐷𝐶𝑟𝑒𝑓 + 𝑑𝑣

= uv + dv (32)

Now uv is the control input given by

𝑢𝑣 = 𝐵𝑉𝑖𝑑 −1

𝑐𝑖𝑠 − 𝑉𝐷𝐶 (33)

The sliding mode control strategy applied for the DC

link voltage is given as follows:

𝑢 = −𝑘1√𝑒𝑠𝑔𝑛(𝑒) + 𝜔

𝑑𝜔

𝑑𝑡= −𝑘2𝑠𝑔𝑛(𝑒) (34)

So id is given by

𝑖𝑑 =1

𝐵𝑉(−𝑘1√𝑒𝑠𝑔𝑛(𝑒) + 𝜔 +

1

𝐶𝑠+ 𝑣𝐷𝐶)

𝑑𝜔

𝑑𝑡= −𝑘2𝑠𝑔𝑛(𝑒) (35)



IJEECS

ISSN 2348-117X

Volume 6, Issue 6

June 2017

V. SIMULATION RESULTS

The proposed wind energy conversion system and the

comparison of two controllers were carried out in

MATLAB software. Here a wind turbine system is

connected to the grid through a PMSG and a back to back

converter. The simulation of the wind energy conversion

system with both proportional integral control and

sliding mode control is observed. It can be seen that for

constant wind speed the speed control response almost

remains same as shown in Fig 6 and Fig 9. The DC link

voltage is set to be controlled at 55 V as shown in Fig 7

and Fig 10. The difference occurs when there is a

variable wind speed, For variable wind speed sliding

mode control exhibit robustness, where as for PI control

where we need to tune the control parameters for

different speeds. This can be observed from Fig 8 and

Fig 11.

TABLE I

PARAMETERS OF WIND TURBINE

Number of blades

3

Air density(kg/m^2)

1.225

Diameter(m) 1.15

Pulley ratio 24:12

Moment of inertia(kg -m2) 0.028

TABLE II

PARAMETERS OF GRID SIDE

DC Link Voltage(V)

55

DC link capacitor(mF) 1.8

Filter resistance(ῼ) 0.5

Filter inductance(mH)

25

TABLE III

PARAMETERS OF THE PMSG

Rated current(A)

3

stator resistance(ῼ)

1.3

stator d axis

inductance(mH)

1.5

stator d axis

inductance(mH)

1.5

Flux linkage(wb)

0.027

Number of pole pairs

3

Moment of inertia(kg-

m2)

1.7*10^6

Coefficient of

friction(Nm-s/rad)

0.314*10^6

Fig. 6. Rotor speed control using PI controller



IJEECS

ISSN 2348-117X

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June 2017

Fig. 7. DC link voltage regulation using PI controller

Fig. 8. Variable speed control using PI controller

Fig. 9. Rotor speed control using Sliding mode controller

Fig. 10. DC link voltage regulation using sliding mode

controller

Fig. 11. Variable speed control using sliding mode controller

VII. CONCLUSION

The Controllers namely PI Controller and sliding mode

controller is investigated to deal with problems of

simultaneous control of the rotational speed and the DC-

link voltage to operate a variable speed wind energy

conversion system. By comparing the results of sliding

mode control strategy to the PI based control strategy, It



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ISSN 2348-117X

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June 2017

can be observed that sliding mode control strategy is

robust against parametric variations and unknown

disturbances. The proportional and integral gains of the

PI controller are chosen by trail and error method and

need to be re-evaluated in case of new perturbation

scenario. The results shows the effectiveness and

robustness of the sliding mode control strategy.

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Vishnu. K - Academic Scienceacademicscience.co.in/admin/resources/project/paper/f201706221498148301.pdfVishnu K R,Nisha.G.Poothullil International Journal of Electronics, Electrical