A singlestage ACAC converter has been designed for a wind energy conversion system (WECS) that eliminates multistage operation and DClink filter elements, thus resolving size, weight, and reliability issues. A simple switching strategy is used to control the switches that changes the variablefrequency AC output of an electrical generator to a constantfrequency supply to feed into a distributed electrical load/grid. In addition, a modified random sinusoidal pulse width modulation (RSPWM) technique has been developed for the designed converter to make the overall system more efficient by increasing generating power capacity and reducing the effects of interharmonics and subharmonics generated in the WECS. The technique uses carrier and reference waves of variable switching frequency to calculate the firing angles of the switches of the converter so that the threephase output voltage of the converter is very close to a sine wave with reduced THD. A comparison of the performance of the proposed RSPWM technique with the conventional SPWM demonstrated that the power generated by a turbine in the proposed approximately increased by 5% to 10% and THD reduces by 40% both in voltage and current with respect to conventional SPWM.
I. INTRODUCTION
Environmental contributions such as solar, wind, ocean, and biomass energy affect the production of electricity. For a small wind generator, a permanent magnet synchronous generator (PMSG) is preferred because of its reliability and high efficiency. In a wind energy conversion system (WECS), load specification is achieved using a power electronic converter. Power electronic converters are used to extract the maximum power and control electrical energy at constant frequency and voltage
[1]
. Various types of converters such as twolevel PWM converters, matrix converters, and multilevel converters are described in different literature
[2]
,
[3]
. These converters are designed to control the output voltage, current, and power for distributed load operation. Two different configurations in literature are reported to convert wind energy into electrical energy. In first configuration (
Fig. 1
(a)), PMSG is driven by a turbine and controlled by a controlled rectifier, followed by the DC bus capacitor
[4]
. In the second configuration (
Fig. 1
(b)), the PMSG’s power is controlled by a diode bridge and a chopper is used to control the output voltage following the wind pattern
[5]
. In these configurations, the PMSG effort and converter power capability is lower because of the multistage operation
[6]
.
Different configurations of WECS. (a) Controlled Rectifier ACDCAC converter. (b) Diode rectifier and chopper controlled ACDCAC converter. (c) ACAC converterbased wind energy system.
The multistage operation is solved
[7]
with the application of a matrix converter consisting of nine bidirectional switches. The advantages of a matrix converter are wellknown for gridconnected or distributed load systems. In a distributed load system, the load may be threephase or singlephase
[8]
. The power requirement of each singlephase or threephase load causes unbalance in the threewire power system. An isolated neutral wire is required to independently control the phase supply voltage to balance a distributed system. In a fourwire load system, a neutral wire is used to allow the zerosequence current to minimize the unbalanced load effect. Hence the topology of a 3×3 Matrix converter may be replaced with a fourwire single stage ACAC converter wherein 12 bidirectional switches are required to connect the generator to the four wire outputs
[9]

[11]
.
The improvement in the conversion process is achieved by fixedtime switching patterns based on modulation algorithms such as space vector PWM or carrierbased PWM
[12]
,
[13]
. The problem associated with these algorithms is that these are used under normal input voltage conditions at constant frequency. The duty cycles of the power switches are precalculated and tabulated to obtain a desired frequency
[14]
,
[15]
. Under distorted input voltage conditions at variable frequencies, a fixed time strategy is not appropriate because the disturbance on the input side of the converter reflects on the output of the converter
[16]
. The speed (frequency) variation that occurs in a variable speed generator affects the duty cycles of time switching patterns and creates a complex controller for controlling constraint
[17]
. Therefore, it is necessary to calculate the duty cycles of switching patterns instantaneously by measuring the output voltages at each sampling period.
During power conversion, the ACAC converter distorts the output of generator system but is improved using PWM techniques
[18]
. Until now, several carrierbased PWM strategies are used to improve the THD of the converter. In the existing PWM control switching scheme, the converter switches operation at a higher frequency than the AC line frequency for the LC filter to easily remove the switching. The AC line current waveform can be more sinusoidal at the expense of switching losses
[19]
. In a variable speed generator system, these PWM techniques are not feasible because of the variation in speed. If input supply varies, the harmonic spectrum, third, fifth, seventh, and inter and subharmonics of the input supply frequency also vary
[20]
. In a carrierbased modulation technique, the carrier frequency is not considered a rational integer multiple of either the input frequency or the output frequency for harmonics mitigation. Hence proposing a random harmonic elimination technique as an alternative to conventional PWM techniques for variation in the input frequency for an ACAC converter is challenging
[21]
.
In this paper, a singlestage threephase to threephase ACAC converter with a fourwire system has been proposed for a variable speed wind turbine driving a PMSG that converts variablefrequency AC output of electrical generator into a constantfrequency supply that can be fed into a distributed load, as shown in
Fig. 1
(c). A single stage ACAC variable frequency to constant frequency power electronic converter used in WECS has several merits as compared with twostage converters. In this converter, only three switches are conducted a time whereas a twostage converter, a minimum of four to six switches conduct in rectification and inversion at a time. The conduction of more switches at a time increases the total loss in the two stages compared with the single stage converter. This improves the efficiency of the proposed converter and resolves size and reliability issues. The direct carrierbased modulation technique is replaced by a modified random sinusoidal pulse width modulation (RSPWM) technique with a variable switching frequency that makes the overall system more efficient by increasing the generated power capacity and reducing the effects of inter and subharmonic generated in WECS.
II. MODELING OF ACAC CONVERTER
Fig. 2
(a) shows the block diagram of a WECS with a variable frequency to constant frequency threephase to threephase AC to AC converter. The side1 of the converter is connected to the threephase output of PMSG at frequency fi and voltage magnitude V
_{i}
, whereas a threephase load is connected on side2. This converter converts the threephase balanced sinusoidal voltage on side1 to a threephase balanced sinusoidal system on side2 with new electrical characteristics, frequency f
_{o}
and voltage magnitude V
_{o}
. The power circuit of a threephase to threephase AC to AC converter, connected at the threephase output of PMSG is depicted in
Fig. 2
(b). Each of the three converters (PA, PB and PC) functions for one of the three phases. In a steadystate operation, all the three converters are identically operated. Hence it is sufficient to analyze one converter operation because the other converters are symmetrical with a phase difference of 120 and 240. Converter PA is redrawn in
Fig. 2
(c) for a better visualization of its operation.
Wind energy converter topology. (a) Block diagram of 3Phase ACAC Converter for WECS. (b) Power circuit of 3Phase 4 wire ACAC Converter. (c) Power circuit of single unit converter PA.
 A. Operation of Converter for Variable Frequency Input Supply
The trigger pulses required for the different switches of converter PA are illustrated in
Fig. 3
(a). The input signal V
_{iA}
that is directly proportional to the wind speed has a variable frequency f
_{i}
that is, in fact, the output voltage of PMSG.
Switching strategy for converter. (a) Gate pulses for variable frequency to constant frequency for ACAC converter. (b) Converter operation for positive output waveform. (c) Converter operation for negative output waveform.
The signal V
_{oA}
is at the output frequency f
_{o}
of the grid at 50 Hz. These two signals are used to trigger the converter according to the logical switching pattern. Signals G
_{1A}
, G
_{2A}
, G
_{3A}
, and G
_{4A}
are identified as the trigger signals for the four pairs of switches (T
_{1A}
, T
_{4A}
), (T
_{2A}
, T
_{3A}
), (T
_{4A}
, T
_{1A}
), and (T
_{3A}
, T
_{2A}
), respectively.
Table 1
shows the truth table for triggering the switches of the converter in
Fig. 2
(c). In the table, positive output is considered as logic ‘1’, whereas negative output is presented by logic ‘0’. When both input and output are positive, it is represented by logic ‘11’, and when both are negative it shown as ‘00’. Thus the switching state will change according to the wind speed and frequency of the output voltage.
TRUTH TABLE FOR TRIGGERING THE SWITCHES
TRUTH TABLE FOR TRIGGERING THE SWITCHES
 B. Switching Strategy for Positive Output Waveform
The converter will produce a positive output when switches T
_{1A}
and T
_{4A}
conduct a positive input cycle, whereas switches T
_{2A}
and T
_{3A}
conduct a negative input cycle, as shown in
Fig. 3
(b).
During time period T
_{sp1}
, T
_{1A}
and T
_{4A}
must conduct while the other switches must be turned off. Similarly, during time period T
_{sp2}
, T
_{2A}
and T
_{3A}
must conduct while other switches must be turned off. Thus the output voltage for one positive half output cycle is given by Equation (1),
where
T
_{sp1}
and T
_{sp2}
are the time periods for positive switching period,
N1 and N2 is maximum number of switching pulses obtained for triggering switches T
_{1A}
& T
_{4A}
and T
_{2A}
& T
_{3A}
in one output half cycle.
T
_{s}
is the time period for output waveform, and
t
_{n1}
and t
_{n2}
is the duration of the trigger pulses required for triggering switches T
_{1A}
and T
_{4A}
, and T
_{2A}
and T
_{3A}
in one output half cycle, respectively.
 C. Switching Strategy for Negative Output Waveform
The negative half output of the converter is obtained by conducting pair switches (T
_{4A}
and T
_{1A}
), and (T
_{3A}
and T
_{2A}
), as shown in
Fig. 3
(c). During time period T
_{sn1}
, switches T
_{3A}
and T
_{2A}
must conduct while the other switches must be turned off; during time period T
_{sn2}
, T
_{4A}
and T
_{1A}
will conduct and the other switches must be turned off. The output voltage for one half negative output cycles is given by Equation (4),
where T
_{sn1}
, and T
_{sn2}
are the time periods for a negative switching period, and N3 and N4 are the maximum number of switching pulses obtained for triggering the pair switches (T
_{3A}
and T
_{2A}
), and (T
_{4A}
and T
_{1A}
) in one output half cycle, respectively.
In
Fig. 3
(a: ix), the actual output voltage of the converter is distorted and if it is fed to the distributed load system directly, the performance downgrades. To optimize the harmonics in the input and output waveforms, the gate pulses to the different switches are modulated and filtered so that output approaches almost sinusoidal and THD reduces to a minimum value.
The PWM techniques that are used in WECS utilize a carrier wave with a constant switching frequency. If the switching frequency of the carrier frequency is high, the THD and filter size reduce considerably. In this paper, a new approach is adopted where the frequency of the carrier signal is not considered constant but varies according to the wind speed. The intersection of varying frequency carrier waves and reference waves modulate the switching angles of the devices used in the converter to improve the output of the converter is improved and increase the power generated by the turbine. In the next section, a new modified random SPWM approach is discussed in detail.
III. PROPOSED RSPWMTECHNIQUE
In the proposed technique, a random sinusoidal reference signal
V_{iA}
with variable frequency
f_{i}
is compared with a high frequency triangular carrier wave of fixed amplitude
V_{c}
with variable frequency, as illustrated in
Fig. 4
. The carrier frequency
f_{c}
, of the triangular carrier signal is related with the input supply frequency
f_{i}
such that
f_{c}
=
f_{s}
±
k_{f}
, where,
k_{f}
is a frequency multiplier and it takes an integer multiple value of the input frequency. In this application,
f_{s}
is the fixed switching frequency. Thus
Switching patterns of RSPWM for phase A. (a) Proposed random sinusoidal PWM technique. (b) Switching state according to random carrier triangular and reference sinusoidal waveform.
The average number of pulses, P, obtained in one cycle by comparing the reference signal and carrier signal is calculated as
Where P is an odd number
[23]
that meets the following requirements:
1) It is taken in multiples of three to cancel even harmonics.
2) It is normally more than ten.
3)
k_{1}
is always less than 2P.
Thus taking the maximum number of pulses in one cycle, say, equal to 45, the value of k
_{1}
comes out to 30 for a switching frequency of 3 kHz. The pulses of the proposed scheme yield the hybrid characteristics of the random SPWM and random carrier frequency PWM, as shown in
Fig. 4
(a). This topology is used to reduce the interharmonics and subharmonics of the converter that are present in the output because of the variation of the input frequency of the turbinegenerator system.
The equations for a sinusoidal reference wave with modulation index, M
_{a}
and for triangular carrier wave are given by Equations (9) and (10), respectively:
where,

Ma= modulation index (Vr/Vc),

Vr= magnitude of reference wave,

Vc= magnitude of carrier wave,

Mf= frequency ratio (fc/fi),

T=2×π(time period for source),

(slope of triangular wave)
The condition for switching angles is given in Equations (1112). The equations describing the natural sampled switching instants are transcendental and have the general distinct solutions for odd and even meeting points.
Intersection (positive slope intersection),
Where,
Intersection (negative slope intersection),
Where,
The width of the pulse can be obtained by subtracting one odd meeting point from the immediate even successor, as represented by Equation (13). Thus the switching instants and resultant pulse widths for SPWM are analytically represented in Equation (14):
Width of the i
^{th}
pulse,
The switching points may be generalized as (T
_{sp1}
, T
_{sp2}
, T
_{sn1}
, and T
_{sn2}
) for positive and negative sequences. These modulating points are ANDed with the switching instant in Equation (15),
Fig. 4
(b) shows the switch patterns during the n
^{th}
time sampling period. The output voltage waveform may follow this path according to operative patterns.
The trigger pulses for the two other converters are generated in a similar manner at a phase difference of 120° and 240
^{0}
.
IV. IMPLEMENTATION OF TRIGGERING STRATEGY
The functional diagram for the implementation of the triggering strategy to generate the required constant frequency output with RSPWM is shown in
Fig. 5
(a). The input signal with variable frequency
f_{i}
, is stepped down from a stepdown transformer and converted into a square signal using a zero crossing detector (ZCD). The variable frequency signal is generated using Agilent 33220A20 MHz power supply. The output of ZCD and a reference signal with an output frequency
f_{o}
are fed to a demultiplexer circuit. A zero crossing detector is used to observe the input supply waveform being converted to a digital ONOFF signal. It consists of a comparator, LM741, that gives a TTL compatible output. The demultiplexer is implemented by a 2×4 decoder. Each channel is dedicated to a particular switch pair. A particular output channel is selected by taking 2 bit word A
_{1}
A
_{0}
and decoding it by a 2×4 decoder according to the switching states, as shown in
Table I
. Depending on the desired output frequency, the multiplexer address is selected. IC 74VHC139 is used as a demultiplexer that is a highspeed dual 2to4 decoder that accepts two binary weighted inputs (A
_{0}
–A
_{1}
) and provides four mutually exclusive activeLOW outputs (S
_{1}
–S
_{4}
). Each output of the demultiplexer has an activelow enable (E). When E is high, all outputs also become high. The enable can be used as the data input for a 4output demultiplexer application.
Functional diagram for the implementation of a triggering strategy. (a) Proposed frequency controller block diagram for phase A. (b) Detailed circuit diagram of PLL used as a Random Triangular Wave Generator.
A triangular wave of variable frequency is generated using PLL (LM566) that is used as a random frequency carrier wave generator. The PLL consists of a phase comparator, amplifier, low pass filter, and VoltageControlled Oscillator (VCO), as shown in
Fig. 5
(b). The phase comparator compares an average control voltage, V
_{so}
, with the random stepped down sine wave signal V
_{i}
... The resulting total control voltage, V
_{s}
, is used as the input of the voltagecontrolled oscillator to yield a random triangular wave with randomly varied frequencies, as shown in
Fig. 4
. The voltagecontrolled oscillator (VCO) is a circuit that provides a varying output signal whose frequency can be adjusted over a range, controlled by a DC voltage, V
_{dc}
and set by an external resistor and capacitor to generate a triangular wave. The frequency of carrier wave is given by Equation (17).
The four trigger signals (S
_{1A}
–S
_{4A}
) generated at the demultiplexer’s output are ANDed by IC74HC08 with modulated output V
_{RSPWM}
(
Fig. 4
(a)). The pulses produced at the AND gates are usually at a low power level and boosted to high power level by a driver circuit. A transistor (CL100) in the driver circuit operates in the active region. The amplified pulses are isolated using optocoupler 4N35 and fed to the gate of respective IGBTs. The same switching patterns are adapted to operate the converter PB and PC at variable frequency patterns.
V. SIMULATION RESULTS
The MATLAB/SPS software has been used to simulate the single stage AC–AC converterbased WECS with SPWM and RSPWM. For long durations, the wind behavior may be considered as a combination of ramp, gust, and step and noise behavior. In this manuscript, the wind behavior is listed in
Table II
.
PARAMETERS FOR THE CONVERTER
PARAMETERS FOR THE CONVERTER
For short durations, only the step operation is considered. The emphasis is on how the controller works for this condition. However, the controller is also expected to work for other wind patterns. The overall performance of the converter is obtained in both the input supply side and generator side and output load side using the parameters shown in
Table II
.
 A. Input Supply Side Performance
Fig. 6
shows the input voltage and input current as well as the THD performance of the converter for the SPWM technique, where a fixed switching frequency of 3 kHz is used to operate the converter for varying input frequencies of 39–46 Hz. The harmonics in the input currents are lesser when the input frequency f
_{1}
is 46 Hz, whereas when the transition of the input frequency from 46–39 Hz, the harmonics initially increase and subside to an almost negligible value after few seconds. The THD obtained a value of 2.54% and 9.96% for input voltage and input current, respectively, with the SPWM modulation technique along with an input filter, as shown in
Figs. 6
(a) and
6
(b).
Input performances of converter with SPWM. (a) Input Voltage waveform and harmonic spectrum. (b) Input current waveform and harmonic spectrum.
When RSPWM technique with an input filter is applied, the harmonics are reduced for both voltage and current. A reduction in THD has been observed at 1.52% and 5.08% for the input voltage and input current, respectively, as shown in
Figs. 7
(a) and
7
(b).
Fig. 8
shows the comparative performance of converter for SPWM and RSPWM technique where the switching frequency of 3 kHz is used to operate the converter. Initially, the wind speed is kept at 12 m/s that make the generator/turbine rotate at 200 rpm, as shown in
Fig. 8
(a). At time t = 0.10 sec, the wind speed decreases to 8 m/s and makes PMSG rotate at 150 rpm. The wind velocity decrement dramatically decreases the input power (
Fig. 8
(b)) because of the decrease in turbine mechanical power. The active power supplied from the converter reduces from 2.5 pu to 0.8 pu, as shown in
Fig. 8
(c). The responses of the load active power and the input power are shown for the modified RSPWM technique, as seen in
Figs. 8
(b)(c), respectively. The output power increased compared with the conventional SPWM technique. In the RSPWM, the average output power increased to approximately 5% to 10 % compared with the SPWM technique.
Input Performances of converter with RSPWM. (a) Input voltage waveform and harmonic spectrum. (b) Input current waveform and harmonic spectrum.
Performance of ACAC converter.
Further reduction in the THD for input current and input voltage can be obtained by increasing the frequency of the carrier wave, as shown in
Table III
that illustrates the impact of switching frequency on the THD of system. For a fixed input filter size, the THD of the current and voltage reduces when the switching frequency is increased.
IMPACT OF SWITCHING FREQUENCIES ON THE THD OF THE SYSYEM
IMPACT OF SWITCHING FREQUENCIES ON THE THD OF THE SYSYEM
 B. Load Side Performance
The presence of the harmonic components in the voltage and current affect the power quality and efficiency of the converter because of the nonlinearity of the power switches (IGBTs). The output voltage of the converter can be improved further by the addition of an LC filter at a cutoff frequency equal to 850 Hz that is greater than 10 to 20 times the fundamental frequency and lower than 10 to 15 times the switching frequency
[23]
. A 20 μF capacitor and 7.95 mH inductor are used to mitigate the higher order harmonics of the system.
Fig. 9
(a) shows the output performance of the converter with SPWM without using any filter.
Fig. 9
(b) illustrates the converter performance with a filter and the SPWM technique. The modulation technique reduces the lower order harmonic magnitude; whereas the higher order harmonics are compensated with the help of LC filter. Without using any filter, the output of THD is very high with a value of approximately 70.4% and is reduced to 18.4% after inserting an LC filter in the output power circuit.
Output performance of the converter with the SPWM. (a) Output voltage and harmonic spectrum with SPWM. (b) Output voltage and harmonic spectrum with SPWM and LC filter. (c) Output current waveform and harmonic spectrum with SPWM and LC filter.
Fig. 9
(c) depicts that the output current of the converter tends to be sinusoidal. The output performance of the converter with RSPWM is illustrated in
Fig. 10
. Without using any filter, the THD is 54.4%, and is less than the value obtained with the SPWM. With a filter, the THD is reduced by approximately 50%, as shown in
Figs. 10
(b) and
10
(c).
Output Performance of Converter with RSPWM. (a) Output voltage waveform and harmonic spectrum with RSPWM. (b) Output voltage waveform and harmonic spectrum with RSPWM and LC filter. (c) Output current waveform and harmonic spectrum with RSPWM and LC filter.
A prototype of the proposed ACAC converter is developed in the lab to verify the results.
Fig. 11
shows the photo of the experimental setup that includes an input interfacing circuit consisting of stepdown transformer and zero crossing detector (ZCD), trigger circuit, driver circuit, isolation circuit, and power circuit made up of IGBTs (BUP 314D) with an ultrafast soft recovery diode. The 220 V rms variable frequency supply is used with a low pass LC filter, 5.5 mH inductor, and 20 μF capacitor. The input transformer rating is given by V
_{rms}
× I, where V
_{rms}
and I are the rated voltage and current of the transformer that are 220 V and 5A, respectively. The input transformer rating is equal to 1100 VA. If the power factor is assumed to be 0.9, the output power of transformer P
_{i}
is equal to 1100 × 0.9 = 990 W. Thus the maximum power of 990 W may be transferred to grid in the experiment.
Experimental Setup.
VI. EXPERIMENTAL RESULTS
The basic trigger signal for IGBTs (T
_{1A}
, T
_{4A}
), and (T
_{2A}
, T
_{3A}
) are generated at the demultiplexer’s output as shown in
Fig. 12
(a).
Pulse generation for ACAC converter. (a) Pulses for T_{1A}T_{4A} (upper trace) and T_{2A}T_{3A} (lower trace) (5V/div, 5 ms/div). (b) Triangular waveform (upper trace: 5 V/div, 12 ms/div) and input waveform (lower trace: 5 V/div, 12 ms/div). (c) Modulated pulses for (lower trace: 5 V/div, 1 ms/div)
Fig. 12
(b) exhibits the random carrier signal and random reference pulse. These two signals are compared in a comparator to obtain the required time resolution pulses as shown in
Fig. 12
(c). The basic signals are ANDed with modulated output and fed to the gate of respective IGBTs through driver circuits. The combined trace of input and output voltage waveforms of the module is shown in
Fig. 13
. The highly distorted, unmodulated output of the converter is depicted in
Fig. 13
(a).
Fig. 13
(b) exhibits the output with the conventional SPWM, whereas
Fig. 13
(c) shows the converter output with RSPWM. Frequent turning ON and OFF of the IGBTs is needed because the number of pulses per half cycle is larger in
Fig. 13
(c) than in
Fig. 13
(b). The output with the RSPWM and LC filter is depicted in
Fig. 13
(d). A constant output frequency of 50 Hz is achieved in almost all cases.
Fig. 14
shows the output voltage and grid current for SPWM and RSPWM. The current waveform tends to be more sinusoidal in
Fig. 14
(a) with RSPWM as compared to
Fig. 14
(b) with SPWM.
Input Voltage of Varying frequency and output voltage for constant frequency of 50 Hz. (a) Input Voltage (upper trace: 100V/div, 20 ms/div) & output voltage (lower trace: 50V/div, 20 ms/div) without modulation. (b) Input voltage V_{i} (upper trace: 100V/div, 10 ms/div) & output voltage V_{o} (lower trace: 50V/div, 10 ms/div) with SPWM. (c) Input voltage (upper trace: 100V/div, 10 ms/div) & output voltage (lower trace: 50V/div, 10 ms/div) with RSPWM. (d) Input voltage (upper trace: 100V/div, 20 ms/div) & output voltage (lower trace: 50V/div, 10 ms/div) with LC filter.
Output Voltage and Grid Current. (a) Output Current I_{o} (upper trace: 1 A/div, 20ms/div) & Output voltage V_{o} (lower trace: 50V/div, 20 ms/div) with RSPWM. (b) Output Current I_{o} (upper trace: 1 A/div, 20ms/div) & Output voltage V_{o} (lower trace: 50V/div, 20 ms/div) with SPWM.
The harmonic spectrum of the input voltage for SPWM at a 3kHz fixed frequency is shown in
Fig. 15
(a). The most dominant harmonics of voltage is placed around the switching frequency (3 kHz), whereas the residuary harmonics are spread around the switching frequency. In RSPWM, the most dominant harmonics of voltage is placed around the switching frequency; the residuary harmonics are minimized as shown in
Fig. 15
(b). The harmonic spectrum of the output voltage for SPWM and RSPWM is shown in
Fig. 16
. Comparisons for
Fig. 6
and
Fig. 7
with
Fig. 15
and
Fig. 9
, and
Fig. 10
with
Fig. 16
, the harmonic spectrum has been found to be much better in the RSPWM compared with the SPWM. Simulation and experimental results verify these findings.
FFT of input voltage. (a) Voltage harmonics spectrum for SPWM. (b) Voltage harmonics spectrum for RSPWM.
FFT of output voltage of converter. (a) Voltage harmonics spectrum for SPWM. (b) Voltage harmonics spectrum for RSPWM.
VII. CONCLUSIONS
A single stage ACAC converter has been designed for WECS that changes the variable input frequency of PMSG to a constant output frequency fed to the grid/load. A new modulation technique, RSPWM, has been developed where the frequency of the carrier wave is not considered constant but varies randomly in proportion to the wind speed. The frequency of the reference wave also varies. The intersection of the reference waveform and carrier waveform modulates the firing angles of the switches to increase the output power generated by the turbine by approximately 5%–10% compared with the conventional SPWM technique. The THD of both input current and input voltage of converter is also reduced by 40% compared with the conventional SPWM technique. Load voltage improves when this technique is applied with a constant LC filter. The simulated results are verified by experimental results, and are consistent with each other.
BIO
Navdeep Singh was born in Pratapgarh, India on March 22, 1987. He graduated from the Uttar Pradesh Technical University, Lucknow, India with B.Tech degree in Electrical Engineering in 2009 and completed his M.Tech in Power Electronics and ASIC Design at MNNIT, Allahabad in July 2011. He is working as Research Scholar in Electrical Engineering Department, NIT Allahabad, and Uttar Pradesh, India. His research interests include wind energy, power electronic devices, converters, and AC to AC converter.
Vineeta Agarwal graduated from Allahabad University, Allahabad, India, in 1980, and received her Master’s degree in 1984, from the same university. She joined the Electrical Engineering Department at the MOTILAL Nehru Regional Engineering College, Allahabad, India as lecturer in 1982. While teaching there, she obtained her Ph.D. in Power Electronics. At present she is a Professor in the Department of Electrical Engineering at the MOTILAL Nehru National Institute of Technology, Allahabad, India. She has taught numerous courses in Electrical Engineering and Electronics. Her research interests are in single phase to threephase conversion and AC drives. She has a number of publications in journals and conferences in her field. She has attended and presented papers at both national and international conferences.
Singh N.
,
Agarwal V.
2013
“A review on power quality enhanced converter of permanent magnet synchronous wind generator,”
International Review of Electrical Engineering (IREE)
8
(6)
1681 
1693
Melicio R.
,
Mendes V. M. F.
,
Catalao J. P. S.
2010
“Power converter topologies for wind energy conversion systems: Integrated modelling, control strategy and performance simulation,”
Renewable Energy
35
(10)
2165 
2174
DOI : 10.1016/j.renene.2010.03.009
Santiago A. V.
,
Maria I. V.
2012
“Direct connection of WECS system to the MV grid with multilevel converters,”
Renewable Energy
41
336 
344
DOI : 10.1016/j.renene.2011.11.022
Orlando N. A.
,
Liserre M.
,
Monopoli V. G.
,
Mastromauro R. A.
,
DellAquila A.
“Comparison of power converter topologies for permanent magnet small wind turbine system,”
Industrial Electronics, 2008. ISIE 2008. IEEE International Symposium on
2008
2359 
2364
Ahmedy T.
,
Nishida K.
,
Nakaoka M.
2010
“Wind power grid integration of an IPMSG using a diode rectifier and a simple MPPT control for gridside inverters,”
Journal of Power Electronics
10
(5)
548 
554
DOI : 10.6113/JPE.2010.10.5.548
Friedli T.
,
Kolar J. W.
,
Rodriguez J.
,
Wheeler P. W.
2012
“Comparative evaluation of threephase AC–AC Matrix converter and voltage DClink backtoback converter systems,”
IEEE Trans. Ind. Electron.
59
(12)
4487 
4510
DOI : 10.1109/TIE.2011.2179278
Cha H. J.
,
Enjeti P. N.
“A threephase AC/AC highfrequency link matrix converter for VSCF applications,”
Power Electronics Specialist Conference
2003
Vol. 4
1971 
1976
Marwali M. N.
,
Min D.
,
Keyhani A.
2006
“Robust stability analysis of voltage and current control for distributed generation systems,”
IEEE Trans. Energy Convers.
21
(2)
516 
526
DOI : 10.1109/TEC.2005.860406
Vechiu I.
,
Curea O.
,
Camblong H.
2010
“Transient operation of a fourleg inverter for autonomous applications with unbalanced Load,”
IEEE Trans. Power Electron.
25
(2)
399 
407
DOI : 10.1109/TPEL.2009.2025275
Singh M.
,
Khadkikar V.
,
Chandra A.
,
Varma R. K.
2011
“Grid interconnection of renewable energy sources at the distribution level with powerquality improvement features,”
IEEE Trans. Power Del.
26
(1)
307 
315
DOI : 10.1109/TPWRD.2010.2081384
Cardenas R.
,
Pena R.
,
Wheeler P.
,
Clare J.
,
Juri C.
2012
“Control of a matrix converter for the operation of autonomous systems,”
Renewable Energy
43
343 
353
DOI : 10.1016/j.renene.2011.11.052
Rodriguez J.
,
Rivera M.
,
Kolar J. W.
,
Wheeler P. W.
2012
“A review of control and modulation methods for matrix converters,”
IEEE Trans. Ind. Electron.
59
(1)
58 
70
DOI : 10.1109/TIE.2011.2165310
Senjyu T.
,
Tamaki S.
,
Muhando E.
,
Urasaki N.
,
Kinjo H.
,
Funabashi T.
2006
“Wind velocity and rotor position sensor less maximum power point tracking control for wind generation system,”
Renewable Energy
31
(11)
1764 
75
DOI : 10.1016/j.renene.2005.09.020
Xia C.
,
Zhao J.
,
Yan Y.
,
Shi T.
2014
“A novel direct torque control of matrix converterfed PMSM drives using duty cycle control for torque ripple reduction,”
IEEE Trans. Ind. Electron.
61
(6)
2700 
2713
DOI : 10.1109/TIE.2013.2276039
Kumar R. B.
,
Kumar A. N.
2012
“Performance analysis of wind turbinedriven permanent magnet generator with matrix converter,”
Turkish Journal Electrical Engineering & Computer Science
20
(3)
299 
317
Ponmani C.
,
Rajaram M.
2013
“Compensation strategy of matrix converter fed induction motor drive under input voltage and load disturbances using internal model control,”
International Journal of Electrical Power & Energy Systems
44
(1)
43 
51
DOI : 10.1016/j.ijepes.2012.07.031
Yang K.
,
Bollen M. H. J.
,
Larsson E. O. A.
,
Wahlberg M.
2014
“Measurement of harmonic emission versus active power from wind turbines,”
Electric Power Systems Research
108
304 
314
DOI : 10.1016/j.epsr.2013.11.025
Tentzerakis S. T.
,
Papathanassiou S. A.
2007
“An investigation of the harmonic emissions of wind turbines,”
IEEE Trans. Energy Convers.
22
(1)
150 
158
DOI : 10.1109/TEC.2006.889607
Wang B.
,
Sherif E.
2013
“Spectral analysis of matrix converters based on 3D fourier integral,”
IEEE Trans. Power Electron.
28
(1)
19 
25
DOI : 10.1109/TPEL.2012.2206118
Haque M. E.
,
Negnevitsky M.
,
Muttaqi K. M.
2010
“Novel control strategy for a variablespeed wind turbine with a permanentmagnet synchronous generator,”
IEEE Trans. Ind. Appl.
46
(1)
331 
339
DOI : 10.1109/TIA.2009.2036550
Agarwal A.
,
Agarwal V.
2012
“FPGA based variable frequency AC to AC power conversion,”
Electric Power Systems Research
90
67 
78
DOI : 10.1016/j.epsr.2012.04.003
Cha H.
,
Vu T.K.
“Comparative analysis of lowpass output filter for singlephase gridconnected Photovoltaic inverter,”
in Proc. Applied Power Electronics Conference and Exposition (APEC)
2010
1659 
1665