13th International Conference on DEVELOPMENT AND APPLICATION SYSTEMS, Suceava, Romania, May 19-21, 2016

978-1-5090-1993-9/16/$31.00 ©2016 IEEE

Small Power Wind Systems Automatic Control

Ciprian Vlad, Silviu Epure, Gelu Gurguiatu, Ciprian Daniel Bălănută, Toader Munteanu Automation and Electrical Engineering Department

“Dunarea de Jos” University of Galati, Galati, Romania

Abstract— Given the increasing interest in small wind systems, the authors propose three applicable control strategies, confirmed by Matlab/ SimPowerSystem simulation software in terms of energy conversion optimization and stable operation. The analyzed small power wind energy conversion system has a Permanent Magnet Synchronous Generator - PMSG, a diode rectifier, a DC-DC converter, a battery and DC consumers. In order to highlight the performances of the small power wind system three control solutions were investigated in order to obtain maximum power conversion and stable operation. For the first case a shaft rotational speed loop with reference depending on the wind speed was used and for the second, a power loop with reference depending on shaft rotational speed cube and reaction after the electrical power. The third case had a power loop and a shaft rotational speed loop. In all the cases the chopper that feeds the battery uses an on-off controller.

Keywords— maximum power point tracking, automatic control, renewable energy, wind energy component; formatting; style; styling; insert (key words)

I. INTRODUCTION

Small power wind systems based on Permanent-Magnet Synchronous Generator were investigated in recent researches, but most of them are intended for grid connected systems [1], [2], [3]. A simple control strategy for an optimal energy conversion from grid connected PMSG-based variable speed wind system with a rectifier and a DC to DC boost converter is presented in [4]. In [5] and [6] performances of a PMSG based wind system with back-to-back converters for grid interconnection are illustrated. PMSGs in wind energy conversion systems have advantages such as reliability, low maintenance, high efficiency, direct drive etc [1], [2], [3].

The analyzed small power wind energy conversion system structure is represented in Fig. 1. The wind turbine directly drives the PMSG that feeds the loads (lead-acid battery and resistive devices) through a DC-DC converter. The wind system can be controlled directly by the chopper duty cycle, x, in order to modify its operating point.

Fig. 2 presents a solution for unitary control in small wind systems which involves a shaft speed control loop, whose set point is given by the wind speed [7], [8], [9], as in (1):

optref v

R

λ Ω = ⋅

(1)

where Ω is the shaft rotational speed, λopt is the optimal value of the tip speed ratio, R – wind turbine blade length and v is the wind speed value.

The mechanical power, Pm, is calculated by relation (2) and the wind torque by relation (3).

30.5m pP A v Cρ= ⋅ ⋅ ⋅ ⋅ (2)

2 3/ 0,5 ( ) /w m pT P v R Cπ ρ λ λ= Ω = ⋅ ⋅ ⋅ ⋅ ⋅(3)

where ρ – air density; A=πR2 – sectional area of the wind turbine, v – wind speed and Cp – power coefficient, which give the energy efficiency conversion and depends on tip speed ratio, λ (relation (4)).

R

vλ ⋅Ω= (4)

PMSG3 ~

Rectifier Wind speed wT

DC-DC converter/Chopper Turbine Load

rV

rI

x

dcI

dcV

Ω

v

loadR

Fig. 1. Structure of the considered small power autonomous wind system

In the full load region (Fig. 2, CD curve) or region for which the mechanical power of the wind turbine is limited to the nominal value at wind speeds from nominal to maximum (Fig. 2, region 3), the „stall speed regulation” strategy is applied which limit the mechanical power by bringing the turbine into stall mode, based on the reduction of the shaft rotational speed. The linear region 2a, (Fig. 2, AB line) correspond to the optimum control regime (MPPT regime obtained if λ=λopt or Cp=Cpmax), according to relation (1). In this regime maximum power is obtained for every wind speed values between the cut-in value and vm2a. Region 2b (BC line) corresponds to shaft rotational speed limitation to the maximum value Ωmax. AB and BC regions correspond to the partial loads of the turbine and CD curve corresponds to the

113

full load regime, region 3 [8], [9]. In the most part of region 3, the gradient of wind torque characteristics is positive. In these circumstances, if the chopper fed by the PMSG is part of a current loop with PI controller, then the turbine-generator assembly forms an unstable system [8].

v vmax vn vm2a vcut_i

Ωmin

Ωma

Ωref

A

B C

D region 2a

region 2b

region 3

full load region

Fig. 2. The set point of shaft rotational speed based on wind speed

There are several solutions to achieve a stable operating system in region 3. One of these is the use of a torque wind estimator. In this case the output is injected into the rotational speed control loop in order to reach loop stabilization [7]. Another possibility is to limit the chopper input current gradient with a 1st order passive filter. In this case, the electromagnetic torque characteristic shape is virtually identical to the natural electromagnetic torque characteristic of PMSG. With low power equipment, such as those investigated in this paper, the above mentioned solution is acceptable if it provides specific protection features for power electronics.

The torque family characteristics of the wind turbine ( )w v constT =Ω is illustrated in Fig. 3 in solid lines, and the

family of the PMSG electromagnetic torque characteristics ( )em x constT =Ω for various duty cycle “x” of the chopper is

represented with dotted line.

Fig. 3. Wind turbine characteristics (solid line) for different values of the wind speed and PMSG electromagnetic torque (dotted line) for differents loads (duty cycle values, x)

The equivalent circuit of the PMSG - rectifier assembly is shown in Fig. 4 [10], [11], [12], [13], [14], [15], [16].

Based on this circuit and knowing that the voltage equation is [16]:

Ω Ωdc e x dcU K K I= − (5)

πmax

e3 2 p

KΦ

= (6)

3

πxpL

K = (7)

are coefficients depending on magnetic flux and pole pairs number, Idc – current from rectifier output, p – pole pairs number, Φmax – phase amplitude of the flow induced by permanent magnets and L – phase self inductance.

The electrical power provided by the PMSG is [10], [16]:

2

2 e dcdc dc dc e dc x dc dc

x x

K UP U I K I K I U

K K

= = Ω − Ω = − Ω (8)

where Ω is the shaft rotation speed. Neglecting losses, it will be considered in first approximation, -Pw=Pdc and, consequently, it can be deduced the approximate expression of the torque load to the generator shaft [10], [16]:

2

22w dc e dc

em e dc x dc dcx x

P P K UT K I K I U

K K

= = = − = − Ω Ω Ω Ω

(9)

If a chopper is added to the scheme in Fig. 4, the average voltage at the output chopper becomes (10), where x is the duty cycle.

ch dcU x U= ⋅ (10)

Fig. 4. Equivalent scheme of PMSG-rectifier assembly

As presented in Fig. 3, the stable operation of the system is possible if the next equation is satisfied:

em wT T∂ ∂>

∂Ω ∂Ω (11)

taking into account that the time constant of the PMSG is

em w

JT

T T=

∂ ∂−

∂Ω ∂Ω

(12)

where J is the inertia of the system.

114

II. SIMULATION IN SIMPOWERSYSTEM OF LOW POWER

WIND SYSTEMS

For the wind turbine model simulation in Simulink, a radius of 2.5 m was considered, which allows obtaining a power of up to 4-5 kW.

The simulation scheme of the wind turbine-PMSG assembly is given in Fig. 5 and it will be found in all the schemes in this paper. The block that performs simulation of wind turbine is marked in gray while the associated elements to the PMSG, the measuring system and three-phase rectifier are marked in blue. The generator feeds a resistive load.

Fig. 6 to 11 presents the response of the system for step variations of wind speed (Fig. 6), as follows: shaft rotational speed (Fig. 7); mechanical power (Fig. 8); electrical power (Fig. 9); unfiltered voltage on three-phase rectifier output (Fig. 10) and load current variation (measured and filtered current) (Fig. 11).

Two aspects should be highlighted as they will be present in all of the following schemes:

1) notable difference between the mechanical power produced by wind energy conversion and electric power. This is the consequence of the low efficiency in small multipolar PMSG. In Simulink diagrams, this important issue is not highlighted rigorously, therefore SimPowerSystem analysis provides more realistic performance on wind energy conversion systems;

2) in order to remove the pulsations which affect all the system variables, a filter with transfer function was used in the measurements "circuits":

4 2

1( )

10 0.014 1H s

s s−=+ +

(13)

Fig. 6. Wind speed, v Fig. 7. Shaft speed, Ω

Fig. 8. Mechanical power, Pm Fig. 9. Electrical power, Pe

Fig. 10. Three-phase rectifier output Fig. 11. Load current (filtered)

Continuous

pow ergui

1

den(s)

fi ltru1

1

den(s)

fi l tru

v+-

semnale1.mat

To File1

semnale.mat

To File

omega

cuplu eol

putere mec

v v ant

lambda

Subsystem1

Scope9

Scope8

Scope7

Scope6

Scope5

Scope4

Scope3

Scope2

Scope17

Scope13

Scope10

Scope1

A

B

C

+

-

Redresor

Product1m

A

B

C

Tm

Permanent Magnet

Synchronous Machine

m

is_abcis_qd

v s_qdwm

thetamTe

[lam]

Goto9

[I] [If]

[Pe]

Goto6

[U]

Goto5

[Uf]

Goto4

[wm]

Goto3

[Me]

Goto2

[i_abc]

Goto10

[v]

Goto1

[Pm]

-1

[lam]

[I]

From8

[If]

From7

[U]

From6

[U1f]

From5

[Pe]

From4

[Me]

From3

[Pm]

From2

[i_abc]

From10

[wm]

From1

[v]

From

i+ -

5ohm

2ohm

Fig. 5. SimPowerSystem scheme of the assembly wind turbine – PMSG

115

For the low power system presented in Fig. 5, three control solutions are investigated in order to show the performance on SimPowerSystems software simulation.

A. The wind system which feeds a battery, controlled on optimum characteristic by a rotational speed loop

SimPowerSystem scheme of the wind system in this variant of the automatic control is shown in Fig. 12. PMSG-turbine assembly in Fig. 5 has been supplemented as follows:

1) the wind load system is a 12 V battery which was modeled by a DC source in series with a huge capacity (8500 F) and a very low resistance (0.01 Ω). In parallel with the battery , a resistive load of 5 Ω is provided. The items listed are shown in yellow;

2) the chopper is achieved through GTO thyristor, inductance L1, diode D1 and current bi-positional (on-off) controller that performs the PWM modulation. These items are marked in red;

3) the shaft rotational speed control loop (marked in orange), which ensures the optimum feature operation. The loop reference is:

7

2.5optref

opt v vR

λΩ = = (14)

The simulation results are given in Fig. 13-22 and include wind speed (Fig. 13), the mechanical power (Fig. 14), electrical power (Fig. 15), shaft rotational speed (Fig. 16), electromagnetic torque (Fig. 17), voltage at the chopper input (Fig. 18), bi-positional (on-off) controller reference and the chopper current (Fig. 19), tip speed ratio “λ” (Fig. 20), power coefficient - Cp, (Fig. 21) and the error from the rotational speed loop (Fig. 22). It is noted that the tip speed ratio is kept to the optimal value (λopt=7) with negligible errors.

Fig. 13. Wind speed, v Fig. 14. Mechanical power, Pm

Fig. 15. Electrical power, Pe Fig. 16. Shaft rotational speed, Ω

Fig. 17. Electromagnetic torque, Tem Fig. 18. Chopper input voltage, Udc

Fig. 12. Wind system scheme with shaft rotational speed control loop

116

Fig. 19. Setpoint on bi-positional

(on-off) controller (blue) and the current through the chopper (red)

Fig. 20. Tip speed ratio, λ

Fig. 21. Power coefficient, Cp Fig. 22. Error of the shaft rotational

speed control loop

B. Wind system with power loop, having reaction after the electrical power and with on-off controller

Small power wind system SimPowerSystem circuit, optimized by a power controller, is shown in Fig. 23.

Compared to the previous scheme, the shaft rotational speed loop was replaced by a power control loop, operating in tracking mode. Reference of this loop, indicated in green in Fig. 23, depends on the cubed shaft rotational speed.

The results are illustrated in Fig. 24-29 and include variations of the following parameters: wind speed (Fig. 24), mechanical power (Fig. 25), electrical power (Fig. 26), chopper current (Fig. 27), tip speed ratio (Fig. 28) and the control loop error (Fig. 29).

Compared to the results from Fig. 12 scheme, there is an increase in the efficiency of the electromechanical equipment, from about 63.5% to 70%. This can be explained by increasing the battery voltage from 12 V to 24 V, which reduces the current flow in the synchronous generator, hence losses in electrical circuits.

PMSM

5ohm

Fig. 23. Small power wind system SimPowerSystem scheme, when a power controller is used

117

Fig. 24. Wind speed, v Fig. 25. Mechanical power, Pm

Fig. 26. Electrical power, Pe Fig. 27. Chopper current

Fig. 28. Tip speed ratio, λ Fig. 29. Shaft speed loop error

C. Wind system with power loop, with reaction on electrical power, with shaft rotational speed loop and chopper bi-positional (on-off) controller

It is known [2] that the related control systems of small wind power equipment use a loop to limit the current variations in D.C. - D.C converter. If this current loop has a PI controller, it changes substantially the allure of

electromagnetic torque characteristic, ( , )refem emT T I= Ω [8].

The assembly formed by this characteristic and the wind torque characteristic usually leads to the appearance of an unstable regime operating system. In this situation, the classical solution to avoid the instability involves the use of an intermediate loop control angular speed, which is inserted between the power loop and the current loop. One of the roles of the shaft rotational speed loop is to ensure the stability of the entire system.

In the schemes analyzed in SimPowerSystem for the small wind power system investigated, was used a bi-positional (on-off) controller for current that provides PWM modulation in the chopper. As shown by the results obtained with the scheme in Fig. 23, the control of the power in the partial-load regime operates in stable conditions. However, it has been considered appropriate to insert an intermediate loop for the angular speed, as in the case of the D.C. - D.C converter scheme, included in a current loop with PI controller. In this case the SimPowerSystem scheme of the wind system is presented in Fig. 30. The system was analyzed in deterministic variations as step type, and the results are shown in Figs. 31-37, illustrating the stable operation of the system.

Fig. 30. Wind system with power loop, having reaction on electrical power, with shaft rotational speed loop and chopper with bi-positional (on-off) controller

118

Fig. 31. Wind speed, v Fig. 32. Mechanical power, Pm

Fig. 33. Electrical power, Pe Fig. 34. Shaft rotational speed, Ω

Fig. 35. Power loop error Fig. 36. Shaft speed loop error

Fig. 37. Reference variation in current control loop with on-off controller (blue) and the current through the chopper (red)

III. CONCLUSIONS

Three control solutions were investigated in Matlab SimPowerSistem in order to highlight the performance of the small power wind system. The investigations were made for power optimization and stable operation. The results indicate the following:

• In wind system scheme which supplies a battery, driven by the optimum characteristic and shaft rotational speed control loop, it is obtained a negligible error of the tip speed ratio compared to the optimal value (λopt=7);

• In the wind power system scheme with power loop, with electrical power as reaction and with chopper

achieved with on-off controller, an yield increase of the electromechanical equipment by around 3.5% is achieved as compared to the previous version control;

• In wind power system scheme with power loop, with electrical power as reaction, shaft rotational speed loop and chopper achieved with bi-positional (on-off) controller, it was achieved a stable operation of the system due to the insertion of an intermediate shaft rotational speed control loop.

The results obtained through simulations can be helpful in deciding which type of control has to be used to achieve the best results in terms of energy optimization and stable operation.

ACKNOWLEDGMENT

„This work was supported by a grant of the Romanian National Authority for Scientific Research and Innovation, CNCS – UEFISCDI, project number PN-II-RU-TE-2014-4-1761”.

REFERENCES [1] N.A. Orlando, M. Liserre, R.A. Mastromauro, and A. Dell'Aquila, “A

Survey of Control Issues in PMSG-Based Small Wind-Turbine Systems”, IEEE Transactions on Industrial Informatics, vol. 9 (3), pp. 1211 – 1221, ISSN 1551-3203, 2013.

[2] F. Khater, A. Omar, “A Review of Direct Driven PMSG for Wind Energy Systems”, Journal of Energy and Power Engineering, vol. 7, pp. 1592-1603, 2013.

[3] M.G. Simoes, F.A. Farret, and F. Blaabjerg, “Small Wind Energy Systems”, Electric Power Components and Systems, vol. 43(12), pp.1388–1405, 2015.

[4] M. Kesraoui, N. Korichi, and A. Belkadi, “Maximum power point tracker of wind energy conversion system”, Renewable energy, vol. 36(10), pp. 2655-2662, 2011.

[5] M. Singh, V. Khadkikar, and A. Chandra, “Grid synchronisation with harmonics and reactive power compensation capability of permanent magnet synchronous generator-based variable speed wind energy conversion system”, IET Power Electronics, vol. 4(1), pp. 122-130, 2011.

[6] L.G. González, E. Figueres, G. Garcerá, and O. Carranza, “Maximum-power-point tracking with reduced mechanical stress applied to wind-energy-conversion-systems”. Applied Energy, vol. 87(7), pp. 2304-2312, 2010.

[7] M. Yundong, W. Junqi, Y. Hong, and H. Zurong, “Research and Design of Fixed-Pitch Non-Grid-Connected Wind Power System”, International Conference on Renewable Energies and Power Quality, (ICREPQ’11), Las Palmas de Gran Canaria (Spain), 13-15 April, 2011.

[8] A. Burlibasa (Scarlat), I. Munteanu, and A.I. Bratcu, “Unitary power control strategy for low-power wind energy conversion system using active speed stall control for full-load regime”, IET Renewable Power Generation, Volume 8, Issue 6, p. 696 –706, 2013.

[9] I. Munteanu, A.I. Bratcu, N.A. Cutululis, and E. Ceangă., Optimal Control of Wind Energy Systems. Toward a Global Approach, Springer, London, 2008.

[10] C. Vlad, Automatic control of autonomous wind systems, Galati University Press – GUP, ISBN 978-606-8348-42-1, Galati, 2012.

[11] C. Vlad, T. Munteanu, G. Murariu., and L. Georgescu, “Low power autonomous wind system simulation in Matlab/Simulink”, Annals of “Dunarea de Jos” University of Galati, fascicle II, Mathematics, Physics, Theoretical Mechanics, ISSN 2067 – 2071, 2014.

119

[12] I. Boldea, Variable speed generators, CRC Press, Taylor & Francis Group, 2006.

[13] A.G. Kladas, M.P. Papadopoulos, and M. P. Tegopoulos, “Multipole permanent magnet generator design for gearless wind power applications”, Proceedings ICEM’98, Istanbul, pp. 2055-9, 1998.

[14] G. Bălută, A. Graur, R. Pentiuc, C. Diaconescu, and C. Popa, “Virtual laboratory for DC servomotor using the internet”, Advances in Electrical and Computer Engineering, vol. 14, no. 1, 2014, ISSN 1582-7445, e-ISSN 1844-7600, pp. 103-108, 2014.

[15] C. Koutroulis, A. Kladas, and A. Mamalis., “Advanced Permanent Magnet Machine Design and Construction for Wind Power Generation and Traction Applications”, International Journal on Applied Electromagnetics and Mechanics, Volume 13, Numbers 1-4/2001/2002, pp. 285-290, 2002.

[16] A. Mirecki, Comparative study of energy conversion channels dedicated to a small wind turbine/ Etude comparative de chaînes de conversion d’énergie dédiées à une éolienne de petite puissance, PhD. thesis, Toulouse, 2005.

120