Low Cost PWM Converter for Utility Interface of Variable Speed Wind Turbine Generators

Ali M. El-Tamaly* H. H. El-Tamaly** E. Cengelci * P. N. Enjeti* E. Muljadi***

*Power Quality Laboratory **Electrical Engineering Dept. ***National Renewable Energy Electrical Engineering Dept. Faculty of Engineering Laboratory Texas A&M University Elminia University, 16 17 Cole Boulevard Collage Station, TX. 77843-3 128 Elminia, Egypt Golden, Colorado 80401 Te1.(409)845-7466 Tel. 2-086-322083 Tel: (303) 384-6904 Fax: (409) 845-6259 Fax: 2-086-342601 Fax: (303) 384-6901 E-mail: enjeti@tamu.edu E-mail: rumenia@,rusys.eg.net E-mail: eduard muliadi@nrel.aov

Abstract - In this paper a low cost PWM converter for a variable speed Wind Turbine Generator (WTG) is descussed. A six-switch PWM converter is utilized to convert a three-phase power from a WTG to a single- phase electric utility. The proposed converter consists of two stages. The first stage is a variable frequency PWM converter with 4-IGBT's to control the output power of the generator. The second stage employs 2-IGBT's to return the power to the electric utility. These two stages have common DC-link. The utility output current is sinusoidal with a good quality. Suitable PWM control via Space Vector Modulation is designed to control the generator. And current regulated PWM is used to control the utility side. Results from the simulation of the power converter are presented in this paper. The proposed converter is suitable for utility interface of WTGs up to 20 kW ratings

I-Introduction Wind energy conversion is a very mature technology after

many years of development. Its applications are widely accepted all over the world. The wind power has a huge potential to supply electrical power without generating pollution. It has the potential to provide many countries with one-fourth or more of their electricity demand. There are two major types of operation to generate the electricity using WTGs. The first one is called constant-speed constant- frequency (CSCF) operation and second one is variable-speed constant-frequency (VSCF) operation. Most of the new WTGs are designed for VSCF operation. The VSCF operation has many potential benefits over CSCF operation such as

(1) It generates more energy by operating at higher

(2) The cost per k W h is lower. (3) It reduces stresses in the drive train due to flywheel

(4) It minimizes the audible noise when operating in light

( 5 ) It simplifies the mechanical design.

turbine efficiency.

effect of the rotor.

winds.

To interface a VSCF WTGs to electric utility, a power converter is required. The power converters must be controlled to give a constant voltage, constant frequency output at the utility side. In addition, it should be low cost and reliable to minimize the cost of energy. The aerodynamic power from the wind is converted to mechanical power. The power from the wind turbine is transferred to AC generator via gearbox to increase the shaft speed. Some turbines use direct-drive concept where the turbine is directly connected to a generator. The output of AC generator is connected to electric utility via power conditioner (power converter). The output power of the generator (which is in the form of variable frequency AC) must be transferred to the utility at constant frequency, constant voltage and good power quality

I ,

Fig. 1 The bonventional method to interface three phase WTG bith single phase electric utility

11-Proposed System The existing configuration for the converter as shown in

Fig.1, uses six switches (Six switch Topology, SST) at the generator side and four switches at the utility side, a total of ten. Fig.2.shows the proposed six-switch P W M converter with four switches (Four Switch Topology, FST) at generator side and two switches at the utility side. Then, four switches in SST are replaced with only one capacitor at DC link. It is apparent that the cost and reliability are two major advantages of the proposed converter. The cost reduction can be accomplished by reducing the number of switches and by reducing the complexity of control system. Space Vector Modulation (SVM) technique is chosen as the algorithm to control the generator. At the utility interface, the current

0-7803-5160-6/99/$10.00 0 1999 IEEE. 889

regulated PWM converter gives a good power quality (it can easily achieve the requirements of IEEE 519-1992 [I]). The power can flow in both directions and the output power at the utility can be adjusted to produce or to absorb reactive power (leading or lagging power factor). Thus the generator can function as a motor to start the wind turbine and the utility side can function as a reactive power compensator at light load.

Vd = 2 VL-L I moi (1)

Vap SI S3 VI 0 0

Constant I IG ,Variable frequency PWM Frequency PWM

W G Converlcr I ~ c ~ i n k I Converter I

va VP Vap=Va+jVp .2n -- vd vd J-

- e 2 f i .2 f i J z

Wind

I I 1 1 I

Fig2 Proposed converter 1

The main shortcoming with this topology (FST, Fig.2) is that the DC link voltage must be at least twice the maximum line to line input voltage as shown in (1). The topology in Fig,.2 is suitable for lower utility voltage (230V). Another Variation of the converter is shown in Fig.3. This topology is suitable for higher utility voltage (460V). However, the induction motodgenerator needs to be lower voltage (230V). Since standard induction motodgenerator are available in dual voltage (460/230V) the induction generator connection can be altered and topology in Fig.3 can be employed.

Constant ~ ~ ~ ...

IG V able frequency P W DC Link Frequency PWM

a n v c n s r I I Converter I

I I I 1 I

Fig.3 Proposed converter 2

111-System Analysis In this study Space Vector Modulation, (SVM) for Fig.2 is

discussed. In Fig.2 let us assume the conduction state of power switches be associated to the binary variables SI to s6. The binary 1 indicates a closed switch and 0 an open one. The switches (SI, S2), (S3, S4) and ( S S , s6) are complementary and as consequence, &=I- S2, S3=1- S4 and S5=l- s6. The points A, B and C correspond to the terminal of generated

voltage from AC generator and A, B and C are corresponding to the terminal voltage of AC generator.

The line voltages at the terminals of AC generator (A, B and C) expressed in terms of variables S1 and S2 as follows

(2) V d V d V d 2 2 2 VIE = s, - - s, - = (2s1 - 1)- And,

(3) vel, = SI v. - s, VJ = (2s1 - 1)- V d 2 2 2 From equation (2) and (3), it can be found that VeaA, = v,,,, - v,.,,

= ( S I - S 1 ) v d The SVM is better understood if the line quantities are

transferred into the a p form. By taking mid point of DC bus as a reference, let

Where Vmp = A VAoBT# (4)

r

Applying (2), (3) into (4), we get

( 5 )

(6) And, Vp = $( s3 - i) vd The combination of states of switches (S1-S4) produce four

different vectors in the a p plane as shown in Table (1). These vectors are shifted of d 2 from each other as shown in Fig.4. From this figure, we can see that, the vectors V, and V, are opposite directions and they have equal amplitude. The amplitude is 6 times the amplitude of 5 and V3 . Similarly, vectors VI and VI are opposite direction and equal in magnitude.

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vb t

V

If t1-t3>0, then tl= tl-t3

If t1-t3

V . S , = 0 . 5 + &fl ; S i n ( w , t + 3 0 )

S , = 0 . 5 - f lp L S i n ( w i t + 3 0 )

S , = 0 . 5 + f l f l I S i n ( w i t + 9 0 )

S , = 0 . 5 - f l f l ; S i n ( w , t + 9 0 )

V d

V d

V d

V d

V .

V .

V .

Then the shift angle for switching signal of leg A is 30" and for leg C is 90". Then,

ccc" = mal+ - 150 (27) From (1) the DC voltage must be at lease twice the

maximum of input line to line voltage to avoid the input current distortion. Vd can be maintained constant by adjusting mai at changing of V L - ~ (output of AC generator). The value of v d is function in power flow across the converter. It is required that the net-power-flow through the capacitor to be zero. If the power output fiom WTG is greater than the power transferred to the electric utility. Then, the voltage across the capacitors will be increased gradually. On the other hand, if the power output from WTG is less than the power transferred to the electric utility. Then, the voltage across the capacitors will be decreased gradually. The control system must ensure the power balance between the input and the output power.

The circuit and phasor diagrams at the rectifier side are shown in Fig.6 and Fig.7 respectively. Where Vi is the no load phase voltage (internal EMF) of AC generator and, Vconv,i is the phase voltage at the rectifier legs. The real and reactive powers out of WTG are: -

* V , , , * Sin Si p , = 3

xs (29)

Fig.7 The vector diagram of the rectifier section

At the utility side, V,,,,,, represents the converter output voltage. The maximum output voltage V,o,v,o is given as

The angle of V C ~ ~ ~ , ~ ~ is 6, with respect to utility grid voltage this angle controls the output power to electric utility as shown in (32). The equivalent circuit and vector diagram of inverter stage are shown in Fig.8 and Fig.9 respectively

The real and reactive power output from WTG to electric utility are

* UG (33)

v,,,, *VUG *COS6, - ViG X U G

eo = Where VOutl is the fundamental component of the inverter

output voltage.

I

I I Fig8 The circuit diagram of inverter section

Fig.6 The circuit diagram of rectifier section

Io Fig.9 The vector diagram of the inverter section

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IV-Design Example for the Computer Simulation For a 10 kW Induction Generator, IG, VLL=460V (for

SWC) and 230 V. Its reactance is 0.8mH per phase. The electric utility voltage is 230 V and its inductance is 0.3 mH. If the power factor required for IG is 0.9 leading (IG needs reactive power for voltage build up). Then the input current is 27.891-2. From Fig.6 tii =2.66'. And from (29) VcOnv =I36 V for m~ = 1, Vd=667 V (Compared with 1300V with SWC). For 0.96 output power factor by using (32) and (33) tio= 1 .O 15' and V,,v,0=23 1.23. The output modulation index can be calculated using (31), mao =0.982. The output current is IoU,1=45.29A.

V-Simulation Results The DC link capacitors are charged initially to twice of

peak line to line voltage, then Vd(O)=667V. In practice, this voltage is initiated by the output of the rectifier. The switching frequency is chosen as 6kHz at variable frequency converter and 9kHz at constant frequency converter. Fig. 10 and Fig. 1 1 show the waveform of DC voltage, Vd and Fourier analysis for this voltage. Fig.12 and Fig.13 show the three- phase input currents and Fourier analysis for these currents. Fig.14 and Fig.15 show the waveform of the output voltage and Fourier analysis for this voltage. Fig. 16 and Fig. 17 show the waveform of the output current and Fourier analysis of this current. The Total Harmonic Distortion (THD) of input current to the converter is a function in the modulation index. The THD of input currents is about 11% at mai=l. The THD of output current is about 3.2% with a modulation index, m,,=0.982. THD of output voltage is 50%, this harmonic componets occures at high frequency (arround the switching frequency) which can be eleminated by using very simple low pass filter. The THD can be reduced to about 5% if the connection done with three phase electric utility and using the output modulation frequency multible of three. This connection is not much differ than Fig.2 where four switches used in variable frequency converter and also four switches in the constant frequency converter.

6Q#l~ , , , , , , , , , , , , , , j o m on OB ox om on 117s om om on om oa om on m r of l

1(*)

Fig.10 The steady state waveform of DC voltage

"I1

...-. Fig.13 Fourier analysis of the three phase input current

I 8 s 8ZU 0254 IZ5S QZd LET 8 X 4 82R 8- 811 #tn

w Fig.14 Inverter output voltage and electric utility voltage

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Fig.15 Fourier analysis of output voltage from the inverter v.,.

*ut na .

. -.

Jon.

a1 .

01.

-en-

-nn- n i r rk o n oh ob QJ oi t i o n oh

Fig.16 The steady state output current waveform WI

(InJon, 1m1

I

mn In zn 11: UR 5n CR 7n 8~ SR 10.: i i n izn IIR IUR I rimi

Fig.17 The Fourier Analysis for the output current

VI-The control System The control system used in WES must achieves the

(1) Constant frequency constant voltage at the point of

(2) Adjustable reactive power control, (3) Good tracking capability for the torque speed curve

specified to maximize WTGs

following objectives,

common coupling (PCC),

(4) Good stability and reliability over wide speed range (sub-synchronous and super-synchronous).

The output from WTGs is variable in voltage and frequency and to interface it to UG Vd must be constant value. The control of Vd can be done by controlling mai as shown in (1). The value of Vd also is function in power flow where Vd increases when the input power greater than the output power or visa versa. Then the feed forward compensation from the power measurement can be added to the controller in the output to improve the performance of controller where the input power must be equal the output power plus the switching losses. The control circuit to regulate Vd is shown in Fig. 18. The value of Vd is regulated at its reference value V i by using PI controller. The

amplified error between Vd and V i is multiplied with signal proportional to the input voltage Vi waveform to produce the reference current signal ii. Then we use a fixed frequency control as a current controller to obtain the controller voltage, which compared with a fixed frequency (switching frequency fs) triangular waveform. Then, from (32) we can control the output power by controlling the angle 6 ,

As shown in Fig.2 the Ac generator and the load are not completely de-coupled because phase B of AC generator is connected directly to the electric utility. This makes the control system must insure the DC bus is constant, and without changing with sudden change of input voltage. To enhance the performance of the control system a battery set is used in parallel with the two-capacitor [6] as shown in Fig. 19. The battery deliver or absorb energy from the system if the power required by the loads plus losses is larger or smaller than the power delivered by AC generator respectively.

The pole changing technique of IG can modify the performance of this topology and it can extend the speed range of output power by using higher number of poles at low speed and lower number of poles at high speed. The application of pole changing technique in wind energy schemes has many advantages 1- Extend the speed range of output power without

increasing the IG ratings 2- Reduce the maximum torque difference at low and high

speeds 3- Constant Vlf can be easily achieved for wide speed

range 4- It reduces the reactive power required for IG work. 5- No special induction machine needed but pole changing

can done with a very simple commercial machine

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Vi I

PI Regulator

Feed Forward

- A -b Switchmode

Comparator --+ __* -

WTG

constant Frequency

DC p W Converteq Variable frequency I IG I converter I Link I Single Phase U

I

I I

I I I I I Fig.19 Three phase to single phase converter using two batteries shunt with DC link

VJI-Conclusions This paper introduces a low cost system to deliver power

from a three-phase WTGs to a single phase utility grid. The system employs only six switches. The controller uses Space Vector Modulation technique to control the generator at the rectifier side and a current regulated PWM converter at the utility side. The vector analysis is presented. The maximum circular locus (referance vector) of this converter is 0.866 compared than conventional six- switch converter, howerver, for a small wind turbine up to 20 kW this system will be an excellent alternative. The results of the simulation are shown to prove the concept and to verify the stability of the system. The total harmonic distortion is calculated and presented. The main advantages of this topology is the minimum number of switches which leads to a lower cost and a higher reliability design. The control algorithm is simple and stable. Using two battaries in shunt with the capacitors in DC link will enhance the stability of the control strategy. The output at the utility side has a good power quality with adjustable power factor. With bidirectional power capability, the system can be used as a motor or as a generator. Pole changing technique can modify the operating performance for this system.

G

V

References: [l] ANSIlIEEE Standard 519 1992 IEEE Guide for harmonic control and

reactive compensation of static power converters [2] A. M. Lima, C. B. Jacoba, E. R. C. Dasilva and R. L. A. Rribeiro Vector

and scalar control of a four switch three phase inverter IEEE,1995 [3] Gi-Taekkim and Thomas A. Lip0 VSI-PWM rectifiedinverter system with

a reduced switch count, IEEE199 [4] S. Freysson, H. H. hansen and S. Hansen Design of a low cost single phase

to three phase converter with power factor control Master Thesis, Institute of Energy technology, Aalborg university, Aalborg, Denmark, 1994

[SI H. W. van der Broeck,Analysis of the harmonics in voltage feed converter drives caused by PWM schemes with discontinuouse switch operation, in conference rec. 1991 EPE., pp.3:261-3:266

[6] C. B. Jacobina and, E. R. C. da Silva, A. M. N. Lima, and R. L. A. Ribeiro Induction generator static systems with a reduced number of components, IEEE 1996

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