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ISSN 1978 - 2365 Ketenagalistrikan Dan Energi Terbarukan, Vol. 9 No. 1 Juni 2010 : 1 - 13

START-UP CONTROL USING DC POWER SUPPLY FOR ISOLATED MODE OPERATION OF 100 kW WIND POWER PLANT

Estiko Rijanto Research Center for Electrical Power and Mechatronics (Puslit TELIMEK), Indonesian Institute of Sciences (LIPI) Jl.Cisitu No.21/154D, Tel.022-2503055, Bandung 40135 [email protected]; [email protected]

ABSTRACT This paper reports design of a start-up controller for a 100 kW wind electrical power plant. The plant is designed to be able to work either in isolated or grid-connected circumstances. This paper aims at designing a controller for start-up and operation of the plant in isolated mode. The controller has been designed to provide excitation voltage during the start-up and to maximize energy conversion during operation as well as to maintain the DC link voltage be at a desired value. Controller design uses vector control and hysteresis PWM approaches. From computer simulation results it is concluded that the plant can start-up at cut in wind speed of 2.5 m/sec by maintaining DC link voltage at 96 V, and it changes over from motoring mode to generating mode in 15.5 seconds giving start-up electrical power of 761 Watt. Moreover, the controller can make the generator produce electrical power of 23.48 kW and 111.58 kW for average wind speed of 7 m/s and rated wind speed of 12 m/s, respectively. Keywords: start-up, control, isolated mode, wind, electrical power. ABSTRAK Makalah ini melaporkan perancangan kontroler start-up untuk pembangkit listrik tenaga bayu (PLT Bayu) skala 100 kW. PLT Bayu ini dirancang agar dapat bekerja secara terisolasi (mandiri) dan tersambung ke jala-jala (on grid). Makalah ini bertujuan merancang kontroler PLTBayu tersebut untuk start-up dan operasi pada mode terisolasi. Kontroler dirancang agar mampu memberikan tegangan eksitasi saat start-up dan memaksimalkan konversi energi saat operasi serta berfungsi menjaga tegangan DC link agar tetap sesuai keinginan. Perancangan kontroler memakai pendekatan kendali vektor dan PWM histerisis. Dari hasil simulasi komputer disimpulkan bahwa pembangkit listrik tersebut dapat start-up pada kecepatan angin cut in 2.5 m/detik dengan mempertahankan tegangan DC link 96V dan berubah dari mode motor ke mode generator dalam 15.5 detik lalu memproduksi daya listrik 761 Watt. Lebih lanjut, kontroler mampu mengatur generator menghasilkan daya listrik 23.48kW dan 111.58kW masing-masing untuk kecepatan angin rata-rata 7 m/detik dan kecepatan angin nominal 12 m/detik. Kata kunci: start-up, kontrol, mode terisolasi, angin, daya listrik.

INTRODUCTION

Measurement of wind speed and direction at a selected location at west coast of

Java has been conducted for one year by the Research and Development Center for Electricity Technology and New/Renewable

Naskah diterima: 22 Pebruari 2010, revisi kesatu: 14 Juni 2010, revisi kedua: 21 Juni 2010, revisi terakhir: 28 Juni 2010

Ketenagalistrikan Dan Energi Terbarukan, Vol. 9 No. 1 Juni 2010 : 1 13

Energy of the Ministry of Energy and Mineral Resources (Kementrian ESDM)

[1]

by the load power factor

[4]

. The problem is

.

The

further aggravated by the uncertainty of the machine to re-excite after a short circuit, unless some charge is provided [5]. When the excitation comes from the DC side capacitor of the inverter/converter, varying the reactive component of the current varies the flux in the generator. The switching of the inverter/converter makes the DC line capacitor acts like a three-phase capacitor. When the fundamental switching is frequency varied the of the inverter/converter reactive

anemometer set at 30 meter height measured the wind speed at interval of 190 minutes in one year long. From statistical analysis of this measurement result it has been confirmed that it is feasible to build a wind electrical power plant with rated power of 100kW. The National Institute of Aeronautics and Space (LAPAN) has been conducting research on wind electrical power generation where its present status is on the power level of 10kW using permanent magnet generator

[2]

. To

capacitance of the DC line capacitor will be varied as seen from the induction machine side. Over all the DC line capacitor provides all the reactive current or the VAR required by the induction generator [6][7][8]. In an isolated-mode of wind turbine electrical power plant, there should be a system that regulates the output voltage. When using DC link capacitor the output voltage is the DC voltage and the system is to keep the DC voltage at a desired level. The previous works by other researchers proposed a control method to maintain the DC voltage level by manipulating q-component current using feedback control law which processes error signal between the measured DC voltage and the desired DC voltage level this control method

[9][10]

support realization of national energy mix target, as described in the energy white book [3], an initiative has been launched in the beginning of 2009 to build 100 kW wind electrical power generation by the Research and Development Center for Electricity Technology and New/Renewable Energy in cooperation with the Research Center for Electrical Power and Mechatronics of Indonesian Institute of Sciences (LIPI). The power plant is designed in order to be able to work either in isolated mode or grid-connected mode. When there is no electrical power from the grid, the power plant should start-up and operate in an isolated mode. In isolated applications a three phase induction generator may be operated in selfexcited mode by connecting three capacitors or using an inverter/converter which has a DC link capacitor. In such a capacitor excited induction generator the value of capacitance should be varied in order to maintain the terminal voltage constant at different generator output power. It is also confirmed that the value of the capacitance is influenced by the load as well as

. As a result, the energy

limits

conversion value from the wind turbine kinetics energy to generator electrical energy, stated other wise, this control method does not maximize energy conversion.

ISSN 1978 - 2365 Ketenagalistrikan Dan Energi Terbarukan, Vol. 9 No. 1 Juni 2010 : 1 - 13

Objective The objective of this paper is to design a controller for start-up and operation of the power plant in isolated mode. The controller design specifications are being able to:

Table 1. Wind speed specification. Specification Cut in speed Average speed Rated speed Value (m/sec) 2,5 7 12

1) control start-up of the system, 2) maximize energy conversion, 3) maintain the DC voltage at certain value.

The controller is designed based on the wind speed specification listed in Table 1.

METHODOLOGY

Figure 1 shows an illustration of the wind energy conversion system (WECS) design proposed in this paper. The system is consisted by 6 main elements: (1) horizontal axis wind turbine, (2) three phase squirrel cage induction generator (IG), (3) inverter/converter, (4) DC link, (5) inverter/converter controller, and (6) DC link voltage controller.

Figure 1. Ilustration of the proposed 100kW WECS The wind turbine produces rotor kinetics power of 100kW at rated speed and it produces larger power for higher wind speed. A gear box with gear ratio of 25 couples the turbine and generator axles. The generator is a 3-phase squirrel cage induction generator which has 4 poles with the following specification: (a) nominal power 130kW, (b) nominal terminal voltage 380 V, and (c) nominal frequency 50 Hz

[11]

excitation power to the generator from the DC power supply during start-up and absorbs electrical power produced by the generator during operation. The inverter/converter controller utilizes vector control approach to maximize power conversion from wind kinetic power to generator electrical power. Switching of the power electronics components in the inverter/converter is conducted using hysterics PWM approach. The DC voltage controller is designed to maintain voltage of the DC link in a desired range. The operation procedure is as follows:

.

The

inverter/converter

functions

in

bidirectional manner i.e. inverts DC voltage from the DC link to three phase AC voltage at the generator terminal during motoring mode and converts AC voltage from the generator terminal to DC voltage in the DC link during generating mode. The DC link supplies

1) Starting-up. The start-up procedure is

conducted by exciting the generator using electrical current supplied by the DC

Naskah diterima: 22 Pebruari 2010, revisi kesatu: 14 Juni 2010, revisi kedua: 21 Juni 2010, revisi terakhir: 28 Juni 2010

Ketenagalistrikan Dan Energi Terbarukan, Vol. 9 No. 1 Juni 2010 : 1 13

power supply. When the rotational speed of the turbine reaches a certain value, the slip turns from positive to negative and the machine changes over from motoring mode to generating mode.

Field

Oriented

Control

approach

(FOC), speed control law, and DC voltage control law. Control System Structure Figure 2 shows the structure of control system proposed in this paper. Basically the control system consists of two controllers those are: (1) inverter/converter controller, and (2) dc voltage rotational controller, controller. speed angle The inverter/converter field oriented matrix controller is realized by flux controller, controller, estimator,

2) Maximizing energy conversion. Energy

conversion from wind energy is maximized by controlling the rotational speed of the turbine so that the turbine works at its maximum power coefficient.

3) Maintaining DC link voltage. The DC link

voltage is maintained by controlling electrical current which flows out from the DC link capacitor to the load as well as electrical current which flows in to the DC link capacitor from the DC power supply.

transformation, and switching using hysteresis PWM. The inverter controller requires a current sensor to measure phase current, and a rotational speed sensor to measure generator rotational speed . It produces pulses which drive power electronics elements in the IGBTs bridge. The dc voltage controller reads the DC link voltage and its reference signal . It manages the status of switches and as well as the current flowing out to load. The hysteresis PWM approach consists in direct forcing of line current flow according to the current reference signals. When the instantaneous value of the line phase current exceeds its reference value than the respective phase is instantly connected to the negative node of the DC-link voltage. Otherwise the phase is switched to the positive node in the DC-link. phases [12]. This process is carried out

CONTROL SYSTEM DESIGN

Controller Design A wind turbine performs non-linear characteristics in its power versus rotational speed curve. Given a certain value of wind velocity it can produce different kinetics rotor power depending on its rotor rotational speed. Therefore, a controller should be designed to regulate turbine rotor speed in order to maximize power conversion. Such a controller is realized as inverter/converter controller. The methodology of controller design is according to the following procedure:

1) Determining control system structure. 2) Modeling of the system. 3) Determining

control laws. Three control laws are proposed in this paper: generator torque control law based on

simultaneously and independently for two other

ISSN 1978 - 2365 Ketenagalistrikan Dan Energi Terbarukan, Vol. 9 No. 1 Juni 2010 : 1 - 13

Figure 2.The proposed control system structure Modeling of the System In deriving model of the system, in this paper core loss of the generator is neglected. Given base value or rated value of angular frequency (= (electrical radians per second)), voltage equations of a three phase symmetrical induction machine in terms of flux linkage per second (= (volt or per unit)) and reactance where: (=(Ohm or per unit) in a synchronously rotating reference frame are given below [13]. (1) (2) The variables, and denote voltage, current and resistance respectively. The subscripts , and represent that the variable is on the axis, axis and axis. The subscripts and indicate that the variable belongs to the stator circuit and the rotor circuit, respectively. The superscript indicates that the variable is expressed in the synchronously rotating reference frame . Generator Torque Control Law In this paper,

[14]

and the rotor winding leakage reactance on the stator side. The fluxes, can be calculated as follows. (4) where: Mechanical dynamic of the induction generator is given as follows. (5) (6) represents the inertia constant, is electromechanical torque developed by the machine, is the mechanical torque produced by the wind turbine, and is the damping torque in the direction opposite to rotation. The electromechanical torque developed by the machine (Nm) is given by the following equation. (7) denotes the number of magnetic poles.

denotes the speed of the synchronously rotating reference frame which equals to the angular speed of the stator magneto motive force in electrical radians per sec, while denotes the speed of the rotating rotor magneto motive force. The primed "" rotor quantities denote values are referred to the stator side. The flux linkage equations are given as follows. (3) represents the stator winding leakage reactance, the magnetizing reactance on the stator side,

a

generator

torque

controller is designed using field oriented control approach . When the synchronously rotating reference frame is selected so that its axis is aligned with the rotor field, the component of the rotor field in the chosen

reference frame would be zero. As a result, from the flux linkages equations in (3) and from the torque equation in (7) the following equations are obtained. (A) (Nm) (8) (9)

Naskah diterima: 22 Pebruari 2010, revisi kesatu: 14 Juni 2010, revisi kedua: 21 Juni 2010, revisi terakhir: 28 Juni 2010

Ketenagalistrikan Dan Energi Terbarukan, Vol. 9 No. 1 Juni 2010 : 1 13

Substituting (3.9) into (3.8) yields (Nm) (10) This shows that if the rotor flux linkage is not disturbed, the torque can be controlled by the stator component current. Another consequence of the assumption that the component of the rotor field be zero (=0) and by recalling that the rotor voltages are zero can be derived by substituting and into axis voltage equation of the rotor winding in equation (2) to get the following relationship. Elect. rad./sec (11) On the other hand if the rotor flux linkage in axis is not disturbed and by recalling the component of the rotor field be zero in the axis (=0), from the axis rotor voltage equation in (3.2) and the axis rotor flux equation in (3.3) the following is obtained. (12) Substituting (12) and (8) into (11) the following can be derived. (13) From component rotor flux linkage equation it is clear that. Substituting this into axis rotor voltage equation yields (14) Introducing Laplace transform variable above equation reduces to (15) where is the rotor circuit time constant. Given a desired value of rotor flux the desired value of can be calculated using equation (12). On the other hand, given a desired value of torque at the given value of rotor flux, the desired value of can be obtained. The rotor field orientation angle estimated using the following equation. is the signal, gain. where: and The rotor angle rotational speed sensor.

(16) is measured using a rotor

Rotational Speed Control Law Based on the above vector control approach the following control law for generator rotational speed is used. (17) where: represents rotor electrical rotational speed reference signal, is proportional gain and is integral gain.

DC Link Voltage Control Law Dynamics of the DC link is governed by the following differential equation. (18) denote DC link voltage and is current which capacitance, respectively.

flows from the DC link to the load, and is current flows from the generator to the DC link. In this paper the following control law is proposed for DC link voltage control. (19) where: represents DC link voltage reference is proportional gain and is integral

RESULTS AND ANALYSIS

The following three cases of computer simulation have been done to validate the control method proposed in this paper:

1) the first simulation is the procedure of

start-up at cut in speed of 2,5 m/sec,

ISSN 1978 - 2365 Ketenagalistrikan Dan Energi Terbarukan, Vol. 9 No. 1 Juni 2010 : 1 - 13

2) the second simulation is operation at

wind speed of 7 m/sec,

successfully

control

rotational

speed

to

maximize energy conversion during start-up. The upper curve in Figure 4 shows the slip during start-up at cut in speed, while the lower curve in Figure 4 shows the electrical power produced by the generator. Initially the wind turbine is stationary. When wind blows at speed of 2,5 m/sec the controller first rotates the generator in motoring mode by exciting the motor using electrical current supplied from the DC power supply at the DC link side. Therefore the slip is initially 1, and it decreases to zero when the rotational speed increases from zero to the reference speed. At time 15,5 seconds the generator then changes over from motoring mode to generating mode denoting the slip goes from zero to negative value. After the rotational speed settles at the reference speed the slip value settles at -0.0265. During motoring mode the generator electrical power consumption increases gradually from zero at stationary to its maximal value of 6.2 kW at the time of rotational speed cross over. Later, the power consumption decreases sharply in a short period of time towards zero value, and it changes to negative value indicating generating mode.

3) the third simulation is operation at wind

speed of 12 m/sec. Simulation data is measured and plotted in figure 3 through figure 11. Analysis of each data is given for each figure. Figures 3, 4, 5, and 6 show the simulation results during the start-up procedure. Figure 3 shows the rotational speed of the generator rotor during the start-up procedure. From wind turbine power characteristics, rotational speed of the turbine rotor which gives maximum power conversion for each wind speed has already been analyzed. At cut in wind speed of 2,5 m/sec the rotational speed which maximizes power conversion is 325 rpm. Therefore, rpm.

350 300

during

start-up

the

reference

rotational speed for speed control is set 325

Rotational speed (rpm)

250

200

150

100

50

0 0

2

4

6

8

10 12 Time (sec.)

14

16

18

20

22

After an overshoot, the generated electrical power settles at -761 Watt. This implies that at steady state condition, when the rotor generator rotates at 325 rpm it produces 761 Watt.

Figure 3. Rotational speed during start-up From figure 3 it can be seen that the rotational speed experiences acceleration, overshoot, under shoot and settlement period. The rotational speed settles at the reference speed after 18 seconds with steady state error of 0.01 rpm. This means that the inverter/converter controller designed in this paper can

Ketenagalistrikan Dan Energi Terbarukan, Vol. 9 No. 1 Juni 2010 : 1 13

Figure 6 shows DC link current where

0.3 0.2 Slip 0.1 0 -0.1 0 2 4 6 8 10 12 Time (sec.) 14 16 18 20 22

the solid black line denotes the current flows from the DC link to the generator while the broken red line denotes the current flows from the DC link to load. During motoring mode, the current flows from the DC power supply through the DC link to the generator. During

0 2 4 6 8 10 12 Time (sec.) 14 16 18 20 22

Generator electrical power (kW)

6 4 2 0 -2

generating mode, the current produced by the generator is forwarded to the load. From this figure, the following two results are obtained:

Figure 4. Slip and generated electrical power [During start-up at cut in speed] Figure 5 shows DC link voltage during start-up. When the generator accelerates from stationary to 325 rpm, the DC link voltage is maintained constant at the desired value. During this acceleration period the DC link voltage controller maintains the DC link voltage by controlling electrical current flowing from the DC power supply to compensate current required to rotate the generator. The DC link voltage then experiences over-charge, sharp discharge, and settlement. This result proves that the DC link voltage controller designed in this paper can successfully regulate the DC link voltage at around 96 Volt.

98 97 96

(1) Maximum current needed to rotate the

generator during motoring mode is 83.5 A. Thus the DC power supply should have power capacity at least 8 kW.

(2) After the rotational speed has settled to 325

rpm, the current flowing to the load is 3.28 A. This corresponds to power of 315.18 Watt. The converter efficiency becomes 41.4%. It is important to note that after settlement, the steady state frequency is 10.7 Hz.

80

60 DC link current (A)

40

20

0

-20 95 DC link voltage (V) 0 94 93 92 91 90 89 88 2 4 6 8 10 12 Time (sec.) 14 16 18 20 22

Figure 6. DC link current. Figures 7, 8, and 9 show simulation

0 2 4 6 8 10 12 Time (sec.) 14 16 18 20 22

results when wind flows with its average speed of 7 m/sec. Figure 4.5 demonstrates current of phase A of the generator showing its frequency is 36.4 Hz.

Figure 5. DC link voltage [Reference voltage is 96 Volt].

ISSN 1978 - 2365 Ketenagalistrikan Dan Energi Terbarukan, Vol. 9 No. 1 Juni 2010 : 1 - 13

250 200 150 100 Current of phase A (A) 50 0 -50 -100 -150 -200 -250 21.5

Figure 9. DC link current at wind speed of 7 m/sec. Figure 9 shows electrical current of the DC link. The black solid line indicates the current which flows from the generator to the capacitor of the DC link with its mean value is

21.55 21.6 21.65 21.7 21.75 21.8 Time (sec.) 21.85 21.9 21.95 22

-51.1A. The red broken line shows the current which flows from the DC link to the load with its mean value is 51.08A. Thus the average electrical power flows to the load is 17.29 kW. The inverter/converter efficiency becomes

Figure 7. Current of phase A. Figure 4.6 shows slip and generated electrical power at average wind speed of 7 m/sec. At steady state condition, the slip varies from -0.08 to -0.06 with its average of -0.069, while the generated electrical power varies from -22 to -25 kW with its average of -23.48kW.

-0.05 -0.06 Slip -0.07 -0.08 -0.09 21.5 Generator electrical power (kW)

73.63%. Figure 10 and figure 11 show simulation results when wind speed is 12 m/sec. In this case the DC link voltage is maintained at 608 V and the electrical frequency is 50.29 Hz. Figure 4.8 shows slip and generated electrical power. At steady state condition, the slip varies from -0.11 to -0.097 with its average of -0.104, while the generated electrical power varies from -109 to -115 kW with its average value is -111.58kW.

-0.09

21.55

21.6

21.65

21.7 21.75 21.8 Time (sec.)

21.85

21.9

21.95

22

-20

-22

-24

-0.1

-26 21.5 21.55 21.6 21.65 21.7 21.75 21.8 Time (sec.) 21.85 21.9 21.95 22

Slip -0.11 -0.12 23.5 Generator electrical power (kW)

Figure 8. Slip and generated electrical power [at average wind speed of 7 m/sec].

60

23.55

23.6

23.65

23.7 23.75 23.8 Time (sec.)

23.85

23.9

23.95

24

-108 -110 -112 -114 -116 23.5

40

23.55

23.6

23.65

20 DC link current (A)

23.7 23.75 23.8 Time (sec.)

23.85

23.9

23.95

24

0

Figure 10. Slip and generator power.

-20

-40

-60 21.5

21.55

21.6

21.65

21.7 21.75 21.8 Time (sec.)

21.85

21.9

21.95

22

Ketenagalistrikan Dan Energi Terbarukan, Vol. 9 No. 1 Juni 2010 : 1 13

2. When wind speed is at its average speed of 7

100

m/sec, at steady state condition the generator produces electrical power of 23.48kW with frequency of 36.4 Hz. The inverter/converter efficiency is 73.63%.

50 DC link current (A)

0

-50

3. When wind speed is at its rated speed of 12

m/sec and at steady state condition the

23.55 23.6 23.65 23.7 23.75 23.8 Time (sec.) 23.85 23.9 23.95 24

-100

23.5

generator produces electrical power of 111.58kW with frequency of 50.29Hz. The inverter/converter 60.24%. efficiency becomes

Figure 11. DC link current Figure 11 shows electrical current of the DC link. The black solid line indicates the current which flows from the generator to the capacitor of the DC link with its mean value is -110.6 A. The red broken line shows the current which flows from the DC link to the load with its mean value is 110.55 A. Thus the average power flows to the load is 67.21kW. The inverter/converter efficiency becomes 60.24%.

Recommendation Following the results reported in this paper, presently research on control system for grid connected mode is being undertaken. In order to enhance inverter/converter efficiency it is recommended to consider other PWM switching methods.

CONCLUSION

From the simulation results in this paper the following conclusion can be drawn:

ACKNOWLEGMENT

The author would like to deliver his gratitude to all members of the research and development join team of 100kW Wind Electrical Conversion System between the Research Center for Electrical Power and Mechatronics of the Indonesian Institute of Sciences and the Research and Development Center for Electricity Technology and New/Renewable Energy of the Ministry of Energy and Mineral Resources, Republic of Indonesia.

1. Both the inverter/converter controller and

the DC link voltage controller designed in this paper function well during start-up at cut in wind speed of 2.5 m/sec. After experiencing acceleration in motoring mode, the system turns into generating mode at time 15.5 seconds. At steady state condition, the generator rotates at 325 rpm producing electrical power of 761 Watt with frequency of 10.7 Hz, and the DC link voltage is regulated at 96 V. The inverter/converter efficiency is 41.4%.

REFERENCE

ISSN 1978 - 2365 Ketenagalistrikan Dan Energi Terbarukan, Vol. 9 No. 1 Juni 2010 : 1 - 13

[1] Verina

J.W.,

"Term

Of

Reference

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[2] Sri Rahayu, "Wind Energy Technology in

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[8] Cardenas, R., Pena, R., Asher, G., and

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[3] The State Ministry for Science and

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[9]

Dawit Seyoum, "The Dynamic Analysis and Control of a Self-Excited Induction Generator Driven by a Wind Turbine", Doctoral Thesis, The University of New South Wales, March 2003.

[4] Malik, N.H., and Al-Bahrani, A.H.,

"Influence of the Terminal Capacitor on the Performance Characteristics of a SelfExcited Induction Generator", IEE Proc. C., Vol.137, No.2, March, 1990, pp.168173.

[10] Seyoum, D., Rahman, F., and Grantham,

C., "Terminal Voltage Control of a Wind Turbine Driven Isolated Induction Generator Using Stator Oriented Field Control", IEEE-Applied Power Electronics Conference and Exposition, Miami Beach, Florida, USA, February 9-13, 2003, pp.846-852.

[5] Shridhar, L., Singh, B., and Jha, C.C.,

"Transient Regulated Performance Short Shunt of Self the Self Excited

Induction Generator", IEEE Transactions on Energy Conversion, Vol.10, No.2, June 1995, pp.261-267.

[11] PT. PINDAD, "The 130 kW Induction

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[6]

Bhadra,

S.N.,

Ratnam,

K.V.,

and

Manjunath, A, "Study on Voltage Build up in a Self-Excited Variable Speed Induction Generator/Static Inverter System with DC Side Capacitor", International Conference on Power Electronics, Drives and Energy System, Vol.2, 1996, pp.964-970.

[12] Michal

Knapczyk

and

Krzysztof

Pienkowski, "Analysis of Pulse Width Modulation Techniques for AC/DC LineSide Converters", Studia i Materialy, Nr.26, 2006.

[13] Paul C. Krause, et.al., Analysis of Electric

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Ketenagalistrikan Dan Energi Terbarukan, Vol. 9 No. 1 Juni 2010 : 1 13

Edition, IEEE Press and John Wiley & Sons Inc., USA, 2002.

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