Implementation of Modular Multilevel Inverter for ...

Implementation of Modular Multilevel Inverter for Extraction of Wind Energy

1K.Rachananjali, 2R.Srinu Naik, 3K.Bala Krishna

Abstract

Extraction of energy by renewable sources particularly solar and wind are moving at a faster pace. For extraction of energy from wind, conversion system has been introduced. It is extracted via wind turbines and is converted to utilized form with the help of power converters. This paper utilizes this concept of capturing wind energy via wind turbines and converts the captured energy into highly efficient, reliable energy with the usage of modular multilevel inverter. The proposal is being discussed and presented in this paper. The proposed system can be used at wide range for extracting wind energy. The efficacy of the system proposed is being simulated by using MATLAB Simulink and realized in hardware. Keywords: Wind energy conversion, Grid connected, Pulse width modulated rectifier, Total Harmonic Distortion, Modular Multilevel Inverter. 1 Introduction

The world is developing at a faster pace and in order to match up with the pace all the countries require large amount of energy. To achieve this, energy generation needs to be increased. Energy generation can be increased in many folds without taking a toll on pollution levels. Without comprising on pollution levels energy generation can be increased through non-conventional sources. Solar and wind serve a crucial role in energy generation because of its continuous availability. In order to capture the energy stored in solar and wind, there arises the need for interfacing such as photovoltaic cell in case of solar and turbines with respect to wind. These interfaces

451-461, Alpha Publishers

This is an Open Access publication. © 2019 the Author(s). All rights reserved.

Journal of Green Engineering (JGE)

Volume-9, Issue-3, October 2019

Journal of Green Engineering, Vol. 9_3,

1Assistant Professor, Department of Electrical and Electronics Engineering, Vignans Foundation for Science, Technology & Research, India. E-mail: [email protected] 2 Assistant Professor, Department of Electrical Engineering, AU College of Engineerin g (A), India. 3 Assistant Professor, Department of Electrical and Electronics Engineering, Vignans Foundation for Science, Technology & Research, India.

452 K.Rachananjali et. al.

capture and convert into electrical energy. Converter converts the electrical energy to the required specifications. Without the usage of converter, the efficiency of the conversion system would be less. The energy harnessed via wind turbine is AC and is fed to the Pulse width modulation (PWM) rectifier which converts AC to DC [1]. This DC acts as common DC bus and is fed to the power converter. Conventionally two-level inverters were developed for conversion of DC to AC but they were disregarded due to higher order harmonics [3]. To improvise the output, research studies introduced multilevel inverters. Multilevel inverter connects the conventional inverters in different connections so as to generate multilevel output [2,7-10]. The multilevel inverters employ different modulation schemes and accordingly controls the different inverters available which generates the output in steps. The conventional two-level inverter generates the square wave output whereas multilevel inverter generates the output in steps which closely resembles to that of sinusoidal shape. On increasing the number of levels, the resemblance refers completely to sinusoidal wave which thereby decreases the Total Harmonic Distortion (THD). Due to multilevel output it produces output with low THD, switches are strained less when compared to conventional inverter so which in turn decreases switching losses and lower switch rating. Because of its features they are being preferred for high and medium power applications. The disadvantage with multilevel inverters is they require isolation transformer which increases losses, size and complexity. The creation of circulation current is one of the parameters to disregard cascaded multilevel converter. Research studies introduced Modular Multilevel Converters (MMC) [1,3] to overcome the disadvantages associated with conventional inverters. Modular multilevel converter (MMC) or in particular modular multilevel inverter (MMI) employs a submodule which is nothing but a half bridge or full bridge connected in cascade. Using the literature studies this paper utilizes the PWM rectifier to convert the energy harnessed from Wind turbine and feeds it to the MMI. The energy obtained from MMI is fed to the grid. The proposal presented in the paper of capturing wind energy and connecting to the grid is shown in Fig 1.

Generator RectifierDC-DC

Converter

Wind Turbine

Modular Multilevel

InverterGrid

Figure 1. Proposal of WECS with MMI

2 Related Works The interface used to capture wind energy is wind turbine. Wind turbines are

manufactured in vertical and horizontal axis. Majority of the conversion systems utilizes the principle of back to back conversion system [4]. Wind turbine usually coupled with permanent magnet synchronous generators (PMSG). This configuration has gained popularity due to its conversion efficiency, controllability and reliability. This configuration eliminates the usage of gear box which reduces the system failure rate [4,5]. The kinetic power is expressed in Eq (1)

=

(1)


where ρ represents the density of air mass, A is the area and Vm/sec. Mechanical power developed is expressed in terms of kinetic power of wind and various mechanical parameters and is shown in Eq (2)

=

Where (, ) is known as power performance coefficient. Blade tip speed ris a dimensionless quantity and is shown in Eq (3)

=

The variation of power performance coefficient Fig 2.

On assuming the speed of wind as steady the characteristics of wind turbine is

explained in Fig 3. The characteristics is being plotted with power vs steady wind speed.

Figure

Depending on speed of wind whatever the power is generated from PMSG is fed to conversion system. PWM rectifier converts the captured AC voltage into DC for meeting the required voltage. As the motive igrid constant voltage should be maintained. In this regard DCcontinuously maintains the constant DC output voltage which is fed to MMI which in turn feeds to the grid.


where ρ represents the density of air mass, A is the area and Vω is the wind speed in

Mechanical power developed is expressed in terms of kinetic power of wind and s mechanical parameters and is shown in Eq (2)

(, )

is known as power performance coefficient. Blade tip speed ris a dimensionless quantity and is shown in Eq (3)

The variation of power performance coefficient (, ) w.r.to β and λ is shown in

Figure 2. Variation of Cp with respect to β and λ

On assuming the speed of wind as steady the characteristics of wind turbine is explained in Fig 3. The characteristics is being plotted with power vs steady wind

ure 3. Wind turbine characteristics with steady wind speed

Depending on speed of wind whatever the power is generated from PMSG is fed to conversion system. PWM rectifier converts the captured AC voltage into DC for meeting the required voltage. As the motive is to fed the captured energy to the grid constant voltage should be maintained. In this regard DC-continuously maintains the constant DC output voltage which is fed to MMI which in

Implementation of Modular Multilevel Inverter for Extraction of Wind Energy 453

is the wind speed in

Mechanical power developed is expressed in terms of kinetic power of wind and

(2)

is known as power performance coefficient. Blade tip speed ratio (λ)

(3)

w.r.to β and λ is shown in

On assuming the speed of wind as steady the characteristics of wind turbine is explained in Fig 3. The characteristics is being plotted with power vs steady wind

Depending on speed of wind whatever the power is generated from PMSG is fed to conversion system. PWM rectifier converts the captured AC voltage into DC

s to fed the captured energy to the -DC Converter

continuously maintains the constant DC output voltage which is fed to MMI which in


3 Related Works Bora Novakovic, Adel Nasiri (2017) presented a medium voltage wind energy

conversion system which has implemented power electronic converter using modular multilevel topology along with integrated storage. The procedure for sizing was discussed in detail and the converter showed that the power can be controlled for both ac and dc. The proposed structure would be beneficial for wind energy applications in medium voltage range. To eliminate the problems linked with long cables the output should be sinusoidal.

Toshiki Nakanishi et al. (2014) discussed Micro grid connected MMC designed for extraction of wind energy. This paper converts the voltage obtained at generator of 3.3 KV in to D.C. voltage of 340 V. MMC is implemented using H bridge cells for conversion. The system can also be operated as step down rectifier. The converter implemented along with a controller which continuously measures the capacitor voltage and arm current and accordingly controls each arm.

Wang Tiezhu et al (2017) proposed a modified modulation strategy to control modular multilevel converter integrated with wind farm. This paper highlights about the defects with the traditional nearest level modulation strategy and proposed an improved modulation strategy using the capacitor voltage. This paper verified the modulation strategy integrated for wind farm by electromagnetic transient model and simulation analysis.

Mian Wang et al, (2016) presented the application of MMC for medium voltage high power permanent magnet synchronous generator. The difficulties associated with converter for wind energy conversion system were discussed and solutions are proposed.

Suman Debnath et al (2013) designed a hybrid MMC for extraction of wind energy. This paper proposed 3 H bridge modules in conjunction with 3 level MMC. It also discussed the internal dynamics comprising circulating currents and capacitor voltage ripples.

4 Modular Multilevel Inverter

For effective conversion of renewable energy, power converters are employed. One section of converters employs midway DC link and another category directly converts fixed AC to variable AC without any midway DC link. Voltage source converters utilizes midway DC link. Depending on the output voltage, voltage source converters have been classified as two-level inverters and multilevel inverters. As multilevel inverters have better efficiency when compared to two level inverters they are preferred. The various topologies in multilevel converters are H bridge, flying capacitor and neutral point clamped converter. The disadvantage associated with these topologies are presence of circulating current, requirement of isolation transformer. These two features reduce its importance in medium and high-power applications. Hence an advanced version of multilevel converters came into picture known as modular multilevel inverters (MMI) [6,11-15]. Power converters are employed to enhance the efficiency of the conversion. In order to convert DC to AC inverters are proposed. Literature survey suggest to employ multilevel inverter but due to circulating current they were disregarded and modular multilevel inverter have been proposed. The system employed MMI to convert the extracted wind energy to AC. Cascaded Converter topologies feature is being employed in MMI which eliminates the need for isolated DC sources and phase shifting transformer. MMI consists of submodules which are connected in series. The various submodule configurations are i) Half bridge ii) Full bridge iii) Flying capacitor iv) Cascaded half bridge v) Double


clamp [11-13]. Fig 4 represents three phase MMI consisting of three legs. Fig 5 shows one leg of proposed MMI which has half bridge inverter as the submodule. The submodule consists of two switches which are to be operated in complementary manner. The switches have the freewheeling diode connected in parallel. When one switch is ON either the switch or the freewheeling diode will conduct. To estimate the dynamic behavior the mathematical description of any system is required. Each arm is represented with the electromotive force and voltage drop across series connected impedance.

Larm

Larm

Larm

Larm

Larm

Larm

SM1

SMn

SM1 SM1

SMn SMn

SMnSMnSMn

SM1 SM1 SM1

idc

ia ib ic

Vdc

Figure 4. Circuit diagram of three phase MMC

On utilizing four sub modules the arm voltage generates five voltage levels as 0, Vcap, 2Vcap, 3Vcap, 4Vcap. As for to generate 4Vcap, the 4 switches need to be turned on whereas for 3Vcap, 3 switches need to be turned on. The voltage obtained from the arm is given by Eq (4) Vxy=Vh1+Vh2+Vh3+Vh4 (4)

Eq (4) represents the voltage Vxy is the aggregate of submodule voltages.

Vcap

Vcap

Vcap

Vcap

S1

S2

S3

S4

S4

S3

S2

S1

X

Y

vh1

vh2

vh3

vh4

Figure 5. One leg of proposed MMI


5 Simulation Results The system has been designed in MATLAB with wind turbine, generator,

PWM rectifier, converter and MMI. The simulation parameters of wind turbine together with MMI are displayed in Table 1-2. Both converters are operated on PWM switching strategies.

Table 1. Wind turbine Simulation parameters

Quantity Value

Nominal Vdc 1150 V Nominal Pmechanical 1.5 MW

ω 11 Maximum β 27

ωci 6 m/s ωco 30 m/s

Table 2. MMI Simulation Parameters

Quantity Value Sub module capacitance 20 μF Switch resistance 0.1 Ω No of submodules in each arm 5 PWM Carrier frequency 1 KHz

Fig 4 represents the MATLAB model of the wind turbine along with WECS

Figure 4. MATLAB model of Wind turbine along with WECS

Fig 5 shows WECS implemented in Simulink.

Figure 5. WECS implemented in Simulink

MATLAB model of WECS and DC-DC is represented in Fig 6.


Fig 7 shows the single phase MMI implemented in Simulink.

For the simulation three phase MMI can be realized. The output obtained from PWM rectifier i.e. input to DC

The output attained by DC


Figure 6. Simulink model of WECS and DC-DC

Fig 7 shows the single phase MMI implemented in Simulink.

Figure 7. Simulink model of single phase MMI

For the simulation three phase MMI can be realized. The output obtained from PWM rectifier i.e. input to DC-DC Converter is represented in Fig 8.

Figure 8. Voltage output from PWM rectifier

The output attained by DC-DC is represented in Fig 9.


For the simulation three phase MMI can be realized. The output obtained from PWM


In simulation single and three phase MMI are realized. The output line voltage from

MMI is shown in Fig 10.

6 Experimental VerificationThe proposed system has been implemented in hardware by using a wind

energy training system along with a DCConverter and MMI is realized by IRPF 150N MOSFET devices. The realization of converter and inverter along with its

Figure 11. Experimental set up for DC

Fig 12 shows the result for the single phase 11 level MMI implemented.

et. al.

Figure 9. Voltage output from DC-DC

In simulation single and three phase MMI are realized. The output line voltage from MMI is shown in Fig 10.

Figure 10. Line voltages obtained from MMI

Experimental Verification The proposed system has been implemented in hardware by using a wind

energy training system along with a DC-DC Converter and MMI. The DCConverter and MMI is realized by IRPF 150N MOSFET devices. The realization of converter and inverter along with its pulses is shown in Fig 11.

Experimental set up for DC-DC Converter, Single phase MMI and pulses to MMI


In simulation single and three phase MMI are realized. The output line voltage from

The proposed system has been implemented in hardware by using a wind DC Converter and MMI. The DC-DC

Converter and MMI is realized by IRPF 150N MOSFET devices. The realization of

DC Converter, Single phase MMI and pulses to MMI



The system proposed provides modularity and has increased the efficiency of

the wind conversion system rather than conventional already existing systems. The lower the THD the efficient the system is. THD depends on the number of levels which depends on number of sub modules.

7 Conclusion

This paper introduced a conversion system which extracts energy from wind. To extract the energy, the interface used is Wind turbine. The combination of wind turbine and generator extracts the energy and supplies in the fobetter controlling and effective realization, it is being converted to DC. For maintaining the constant voltage from MMI, DCsupplies constant voltage to MMI with the help of controller. The Modular MultileInverter is being realized by half bridge sub modules. Two stage conversion is being preferred so as to increase the efficiency and control over the system. Due to converter features such as modularity and scalability it is being favored for high and mpower applications. Performance of the overall conversion system has been evaluated under MATLAB environment and prototype implemented in hardware.

Acknowledgement

This publication is an outcome of the R&the Visvesvaraya PhD Scheme of implemented by Digital India Corporation. References [1] Suman Debnath,

Converter for Grid Connection Sustainable Energy

[2] Nuntawat Thitichaiworakorn, Voltage Large Wind Turbine GeneraMultilevel CascadeIn Power Electronics

[3] Rachananjali K,


Figure 12. Voltage output of 11 level MMI

The system proposed provides modularity and has increased the efficiency of the wind conversion system rather than conventional already existing systems. The lower the THD the efficient the system is. THD depends on the number of levels

mber of sub modules.

This paper introduced a conversion system which extracts energy from wind. To extract the energy, the interface used is Wind turbine. The combination of wind turbine and generator extracts the energy and supplies in the form of AC supply. For better controlling and effective realization, it is being converted to DC. For maintaining the constant voltage from MMI, DC-DC converter is placed which supplies constant voltage to MMI with the help of controller. The Modular MultileInverter is being realized by half bridge sub modules. Two stage conversion is being preferred so as to increase the efficiency and control over the system. Due to converter features such as modularity and scalability it is being favored for high and mpower applications. Performance of the overall conversion system has been evaluated under MATLAB environment and prototype implemented in hardware.

Acknowledgement

This publication is an outcome of the R&D work undertaken project under the Visvesvaraya PhD Scheme of MEITY, Government of India, being implemented by Digital India Corporation.

Maryam Saeedifard,” A New Hybrid Modular Multilevel rid Connection of Large Wind Turbines”, IEEE Trans

Sustainable Energy, Vol 4, No 4, Oct 2013, pp 1051-1064. Nuntawat Thitichaiworakorn, Makoto Hagiwara, Hirofumi Akagi

Large Wind Turbine Generation System Using an AC/AC Modularscade Converter”, IEEE Journal Of Emerging And Selected Topics

Power Electronics, Vol 4, No 2, June 2016, pp 534-546. Srinu Naik R, “Design of Modular Multilevel Converter for


The system proposed provides modularity and has increased the efficiency of the wind conversion system rather than conventional already existing systems. The lower the THD the efficient the system is. THD depends on the number of levels

This paper introduced a conversion system which extracts energy from wind. To extract the energy, the interface used is Wind turbine. The combination of wind

rm of AC supply. For better controlling and effective realization, it is being converted to DC. For

DC converter is placed which supplies constant voltage to MMI with the help of controller. The Modular Multilevel Inverter is being realized by half bridge sub modules. Two stage conversion is being preferred so as to increase the efficiency and control over the system. Due to converter features such as modularity and scalability it is being favored for high and medium power applications. Performance of the overall conversion system has been evaluated

D work undertaken project under , Government of India, being

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Biographies

Rachananjali K completed her B.Tech from Vignan’s Engineering College, Vadlamudi in 2011 and M.Tech from VNIT, Nagpur in 2013. Pursuing PhD under Visvesvaraya PhD Scheme in AU College of Engineering (A), Visakhapatnam. Presently working as Asst Prof in the Department of EEE at VFSTR, Vadlamudi. Her research interest includes modular multilevel converter to renewable energy sources.

Srinu Naik R completed his B.E from Bapatla Engineering College and M.E and Ph.D from AUCE(A), Visakhapatnam. Presently working as Asst Prof in the Department of EE, AUCE (A), Visakhapatnam. His research interest includes converters, application of power electronics to renewable sources.

Bala Krishna K completed B.Tech from Vignan’s Engineering College, Vadlamudi in 2009 and M.Tech from Veermata Jijabhai Technological Institute, Mumbai in 2012. Pursuing his PhD in VFSTR, Vadlamudi. Presently working as Asst Prof in the Department of EEE at VFSTR, Vadlamudi. His research interest includes renewable energy sources, placement of phasor measurement units.

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