[ieee 2014 international conference on embedded systems (ices) - coimbatore, india...

Dynamic behaviour of UPF rectifier for PMSG based Wind Energy Conversion System

Sreekesh Kesava Pillai Electrical Engineering Department

MNNIT Allahabad, India

[email protected]

Dr. Paulson Samuel Electrical Engineering Department

MNNIT Allahabad, India

[email protected]

Abstract— This paper addresses dynamic behaviour of Unity Power Factor (UPF) rectifier for permanent magnet Synchronous Generator (PMSG) based Wind Energy Conversion System (WECS). Furthermore SPWM based five level diode clamped inverter also is introduced to drive three phase balanced R-L load. The performance of the rectifier is compared to that of a traditional uncontrolled diode bridge rectifier. The features of the front end rectifier are low cost, small size and high efficiency. Three bidirectional switches are used in the configuration which is switched at only twice the generated frequency. The THD and power factor of the input current of the front end rectifier are the parameters of significance in this analysis. The performance is analysed for both variation in wind speed and also the dynamics of the load. The whole model is simulated using MATLAB, Simulink software platform.

Keywords—PMSG, Wind Energy conversion system (WECS), Unity power factor (UPF) Rectifier,

I. INTRODUCTION With high increase in energy demand by every passing day,

the importance of non conventional energy sources has become more evident. These sources are abundant and are nature friendly unlike the conventional sources like coal, diesel etc. Wind energy, which is a form of renewable energy is widely available, is a clean energy source and also does not produce any greenhouse gases. In 1996 the total power generated from wind was 6,100 MW [1] and it became 3,18,137 MW by 2013 [2]. Statistics also show that the global wind power could contribute about 12% of global electrical power generation by 2020. This figure is expected to increase to 20% by 2030 [1]. Also wind energy does not make use of any fresh water resource unlike most conventional power plants. As the requirement of power grows day by day, so does the need for clean and renewable energy.

Power generation and transmission over long distances results in transmission losses. Power production near to the load requirement reduces these losses and hence standalone WECS has its advantages. The idea of the paper is to design a control strategy which will enable the operation of the used rectifier satisfactorily for both dynamics in wind and for dynamics of the load such that the THD and power factor are optimal.

II. STANDALONE WIND ENERGY CONVERSION SYSTEM (WECS)

A. Turbine Modelling Wind energy is extracted with the help of wind turbines

which convert the linear wind energy into rotational form. This kinetic energy is then converted in electric power. The power extracted by the wind turbine is given by the following equation

1 3 (1)2 UP Av v waρ=

Where, a is the density of air in kgm3, Av is the area swept by the wind turbine in m2 and Uw is the wind velocity in m/s. The output of the wind turbine depends on its power coefficient Cp and is given by (2).

( ). (2)P Pw vpC λ, β=

As can be seen Cp is a function of tip speed ratio λ and blade pitch angle β . The tip speed ratio is given by

.

(3)r mUwωλ =

Where ‘r’ is the wind turbine blade length and ‘ m’ is the rotor angular velocity. Blade pitch angle β is defined as the angle at which the wind hits the blade surface. The function Cp is approximately defined by the following equation [3]

52( ) . . . + . (4)63 41

C

i

CC e CC Cp iC −

λλ, β = − β− λλ

Where C1=0.5176, C2=116, C3=0.4, C4=5, C5=21 and

C6=0.0068

International Conference on Embedded Systems - (ICES 2014) 200

978-1-4799-5026-3/14/$31.00 © 2014 IEEE

Fig. 1. Poposed generalised block diagram

1 1 0.035= (5)3 1i

−λ+0.08.βλ +β

For low speeds β can be assumed to be zero [3].

B. Selection of Generator Generator types like, PMSG, DFIG and squirrel cage

induction generator find their use in WECS. Among these DFIG and PMSG are the most commonly used. PMSG is used as the wind turbine generator in this paper as it has several advantages when compared to DFIG. PMSG is light, small in size, is highly reliable and has high efficiency in comparison to DFIG [4] [5]. Also the gear box is not required and thus allowing the turbine to be directly attached to the generator.

C. Need for unity Power Factor and low Harmonics In most of the traditional WECS makes use of diode bridge

rectifier for rectification. But it is a well know fact that the diode bridge rectifier injects large amount of harmonics into the supply as it is a non linear load. Also the power factor it offers to the supply is poor. For these reason the power drawn from the wind generator in not maximum and a part of the energy is present as reactive power, hence reducing the maximum available useful power. This demands the requirement of a high power factor rectifier which can provide good power factor and also inject less harmonics into the supply. In this work the author has compared the performance of the rectifier used in this paper to a conventional diode bridge rectifier for analysis. The parameters of interest in this study are the input power factor and the harmonics. Low power factor and presence of harmonic components cause heating due to copper and iron losses (at harmonic

frequencies), audible noise from the machine, and reduced overall efficiency of machine[7][8][9]. This emphasizes on the need for a converter which is efficient and is capable of imitating resistive loads and elimination of non linearity of load.

III. THE FRONT END RECTIFIER The rectifier topology used here was first introduced in [10]. Here the selection of the line inductance is of great importance and plays critical role in maintaining unity power factor and reducing input current harmonics. The line inductance La, Lb and Lc is the net inductance as seen by the front end rectifier on the source side. It is the sum of the transformer leakage inductance or generator stator winding inductance (as in case of WECS) and the source inductance [11][12]. There are three bidirectional switches, one for each of the input phase, that are acting as a bypass across the diodes and are connected to a balanced central node at centre of two capacitors in series connected across the dc link. The configuration of the bi-directional switch is as shown in fig.2. When the switch on any one of the phases is turned on, the current rises in the corresponding phase. When it is off, the corresponding diodes in the bridge conducts. Thus the control of the bidirectional switches helps to control the power factor. This is because in a conversional diode bridge rectifier the current starts conducting only 30 degrees after the voltage has started rising from zero in the corresponding phase. The bidirectional switch employed is constructed using a MOSFET and 4 diodes, as shown in fig.2. Compared to the single boost switch configuration [13] the voltage stress is lesser as the switch is connected to half the DC link voltage.

It was observed that if the switch conducted for 1/6th of the total time period a significant improvement in THD and harmonics were observed [10]. So a DC link current based


control strategy is devised in this paper which generates the pulse required for power factor control for both varying loads and also for changes in input wind speed.

Fig. 2. Control structure of gate signal generation

DA DB

DC DD

S

Fig. 3. Configuration of bidirectional switch

A. Converter Configuration As discussed earlier the diode bridge rectifier is typically

used for most traditional wind systems. These converters inject harmonics into the source current and results in large losses and reduction of efficiency of the PMSG. As a result of this the power drawn from the PMSG is lesser than the rated power of the machine. Also the power factor being poor results in reactive power generation and thus again reducing the available useful power. In this paper hence a near unity power factor rectifier is analysed with the help of a new control strategy.

The rectifier configuration is connected to a diode clamped multilevel inverter connected to load. The performance of the system is studied for both variation in wind speeds and also for the variation in load. A ramp signal is generated in such a way that its frequency is twice that of the voltage generated from PMSG. This is shown in fig 7. The ramp signal generated is used to then compare with the value of α given by equation 6 to generate the required pulse for the bidirectional switches.

2= 0.885. -3.576. +29.53-(50-f )/5 (6)I I dcdc sα

Where Idc is the DC link current and fs is the frequency of the source voltage. This control equation was arrived at using

curve fitting method by thorough study of the circuit behaviour.

0.9 1 1.1 1.2 1.3-300

-200

-100

0

100

200

300

Time (sec)

Phase VoltagePhase Current

(a)

0.9 0.95 1 1.05 1.1 1.15 1.2 1.25 1.30

50

100

150

200

250

300

350

Time (sec)

DC

Lin

k V

olt

age,

Vd

c (V

olt

s)

(b)

0 100 200 300 400 500 600 700 800 900 10000

10

20

30

40

50

Frequency (Hz)

Fundamental (50Hz) = 3.508 , THD= 8.10%

Mag

(% o

f Fun

dam

enta

l)

(c)

0 100 200 300 400 500 600 700 800 900 10000

10

20

30

40

50

Frequency (Hz)

Fundamental (50Hz) = 5.473 , THD= 5.05%

Mag

(% o

f Fun

dam

enta

l)

(d)

Fig. 4. Performance at wind speed of 10m/s with 50% load increase at time t=1sec. (a) Input current and Voltage, with load variation at 1 sec at wind speed of 14m/s. (b) DC Link voltage variation with load variation at 1 sec at wind speed of 14m/s (c) harmonic spectrum of input current for load before time t=1sec (d) harmonic spectrum of input current for load after time t=1sec


0.9 1 1.1 1.2 1.3-200

-100

0

100

200

Time (sec)

Line VoltageLine Current

(a)

0.9 1 1.1 1.2 1.30

50

100

150

200

250

Time (sec)

DC

Lin

k V

olta

ge, V

dc (V

)

(b)

0 100 200 300 400 500 600 700 800 900 10000

10

20

30

40

50

Frequency (Hz)

Fundamental (42.857Hz) = 2.864 , THD= 10.02%

Mag

(% o

f Fun

dam

enta

l)

(c)

0 100 200 300 400 500 600 700 800 900 10000

10

20

30

40

50

Frequency (Hz)


Mag

(% o

f Fun

dam

enta

l)

(d)


0.9 1 1.1 1.2 1.3-200

-100

0

100

200

Time (sec)

Phase VoltagePhase Current

(a)

0.9 1 1.1 1.2 1.30

50

100

150

200

Time (sec)

DC

Lin

k V

olta

ge, V

dc (V

olts

)

(b)

0 100 200 300 400 500 600 700 800 900 10000

5

10

Frequency (Hz)


Mag

(% o

f Fun

dam

enta

l)

(c)

0 100 200 300 400 500 600 700 800 900 10000

10

20

30

40

50

Frequency (Hz)


Mag

(% o

f Fun

dam

enta

l)

(d)



1.9 1.95 2 2.05 2.1-200

0

200

Am

plitu

de

1.9 1.95 2 2.05 2.10

0.01

0.02

Time (sec)

Am

plitu

de

Fig. 7. Ramp wave generated from the input voltage template

TABLE I SIMULATION SETUP PARAMETERS

Wind Turbine Capacity 10kVA

Input line inductance 32mH DC link capacitance 1000uF x 2

SPWM switching frequency 1000

IV. SIMULATION RESULTS The fig.4(a) shows the input phase voltage and the

corresponding current waveform for a wind speed of 14m/s. The load is doubled at time t=1 sec. The fig. 4(b) shows the corresponding variation in DC link voltage. The load to the system is R=20ohm and L=20mH before time t=1 sec. The load is doubled at t=1sec. The corresponding harmonic spectrum before and after the load variation are shown in fig. 4(c) and 4(d). The fundamental component here is at 50Hz. The major components are the 5th, 7th, and 11th, all of which are less than 5% of the primary component.

TABLE II

PERFORMANCE PARAMETERS FOR VARIOUS WIND AND LOAD Wind Speed Input phase

voltage Input Phase

Current

Power Factor %THD

14m/s

127.3 5.473 0.9995 5.04 127.3 4.91 0.9991 5.62 127.3 3.887 0.995 7.22 127.3 2.323 0.993 8.10

12m/s

109.1 4.479 0.994 6.34 109.1 3.544 0.993 7.81 109.1 3.244 0.993 8.69 109.1 2.864 0.991 10.02

10m/s

90.9 3.64 0.992 9.00 90.9 3.177 0.992 10.72 90.9 2.578 0.991 12.50 90.9 2.323 0.991 14.60

The fig.5(a) shows the input phase voltage and the

corresponding phase current waveform for a wind speed of 12m/s. The load is doubled at time t=1 sec. The fig. 5(b) shows the variation in DC link voltage due to load change. The load to the system is R=20ohm and L=20mH before time t=1 sec. The load is doubled at t=1sec. The harmonic spectrum before and after the load variation are shown in fig. 5(c) and 5(d). The fundamental component here is at approximately 43Hz. The major components are the 5th, 7th, 11th and 13th, all of which are less than 5% of the primary component.

Fig. 6(a) shows the input phase voltage and the corresponding current waveform for a wind speed 10m/s. The load is doubled at time t=1 sec. The fig. 6(b) shows the corresponding variation in DC link voltage. The load to the system is R=20ohm and L=20mH before time t=1 sec. The load is doubled at t=1sec. The corresponding harmonic spectrum before and after the load variation are shown in fig. 6(c) and 6(d). The fundamental component here is at approximately 36Hz. The major harmonic components are the 5th, 7th, 11th, 13th and 17th. The 11th and 13th harmonics were found to be more than 5% of the fundamental component.

The table II shows the various performance parameters listed for various wind speeds with variation in load. The load is varied and the corresponding values of input phase voltage and corresponding input phase current are also mentioned along with the power factor and %THD.

20 30 40 50 60 70 80 90 100 1100.988

0.99

0.992

0.994

0.996

0.998

1

Percentage Load

Pow

er F

acto

r

Fig. 8. Variation of power factor with loading for wind speed of 14m/s

At 14m/s the phase voltage is higher than in other two cases. As a result we have higher current. It can be observed that the THD is lower when the power drawn is high. This is because as the current is large the effect of inductance comes into picture. The power factor remains more than 0.993 and is max of 0.9995.

At 12m/s the phase voltage is lower than that at 14m/s. The THD is higher when compared to 14m/s. This is because the current drawn for the same load is lower. The power factor again is above 0.991 and maximum being 0.994.

At 10m/s the phase voltage is lower than that at 14m/s and 12m/s. The current dawn is lower and thus the THD is higher. Power factor is quite good with it being above 0.991.

The variation of power factor with loading is shown in fig.8 for wind speed of 14m/s. It can be observed that the power increases with loading.

V. CONCLUSION A unity power factor for a Wind Energy conversion system (WECS) was explored in this paper. Based on the control strategy devised the bidirectional switches are controlled to achieve near unity power factor with low THD. The performance was analysed for three wind speeds and for dynamics in the load. The performance of the system with the proposed control was found to quite satisfactory. The THD was found to be quite satisfactory for loads near rated for each wind speed and much better at all loads when compared to the conventional diode bridge rectifier. This result of proposed


system shows better performance in dynamic changes in load and wind velocity conditions.

VI. REFERENCES [1] Online. Available: http://www.gwec.net/wp-

content/uploads/2012/11/GWEO_2012_lowRes.pdf [2] Online. Available: http://www.gwec.net/global-figures/wind-energy-

global-status/ [3] V. Sheeja, P. Jayaprakash, B. Singh, and R. Uma, “Stand alone wind

power generating system employing permanent magnet synchronous generator,” inProc. IEEE ICSET, 2008, pp. 616–621

[4] E. Muljadi,; C.P. Butterfield, Yih-Huie Wan, "Axial-flux modular permanent-magnet generator with a toroidal winding for wind-turbine applications", IEEE Transactions on Industry Applications, vol.35, no.4, pp.831-836, Jul/Aug 1999.

[5] Tze-Fun Chan, Loi Lei Lai, "Permanent-Magnet Machines for Distributed Power Generation: A Review", IEEE Power Engineering Society General Meeting, vol., no., pp.1-6, June 2007.

[6] IEEE Std. 519-1992, IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems.

[7] IEEE Std. 519-1992, IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems.

[8] K. Tan and S. Islam, “Optimum control strategies in energy conversion of PMSG wind turbine system without mechanical sensors,”IEEE Trans. Energy Convers., vol. 19, no. 2, pp. 392–399, Jun. 2004.

[9] D. S. Oliveira, M. M. Reis, C. Silva, L. Colado Barreto, F. Antunes, and B. L. Soares, “A three-phase high-frequency semi-controlled rectifier for PM WECS,”IEEE Trans. Power Electron., vol. 25, no. 3, pp. 677–685, Mar. 2010.

[10] E. L. M. Mehl and I. Barbi, “An improved high-power factor and low-cost three-phase rectifier,”IEEE Trans. Ind. Appl., vol. 33, no. 2, pp. 485–492, Mar./Apr. 1997.

[11] A. I Maswood and L. Keng Song, “Design aspects of planar and conventional SMPS transformer: A cost benefit analysis,”IEEE Trans. Ind. Electron., vol. 50, no. 3, pp. 571–577, Jun. 2003.

[12] A. I. Maswood and Z. Yoong, “Design aspects of a switch-mode transformer under wide input voltage variation,” IEEE Trans. Ind. Electron., vol. 53, no. 3, pp. 752–758, Jun. 2006.

[13] A. R. Prasad, P. D. Ziogas, and S. Manias, “An active power factor correction technique for three-phase diode rectifiers,” in IEEE/PESC Conf. Rec., pp. 58–66,1989.


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