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Fig.1 Single Phase Cascaded H-Bridge 5 level MLI

Advantages: 1. The amount of possible output current levels is much more than two occasions the amount of electricity sources (m = 2s + 1).

2. The quantity of H-bridges creates modularized layout and packaging. This might permit the manufacturing method to be practiced more rapidly and cheaply.

Disadvantages: Separate electricity sources are crucial for the H-bridges. This might limit its application to items that presently have multiple SDCSs easily available

Principle of operation: Let’s consider names within the switches of each bridge as S11, S12, S13, and S14 for first bridge, S21, S22, S23, S24 for second bridge switches.

Think about the 3 current sources are equal as V1=V2=V.

*NC=No Connection(OFF), C=Connection(ON)

Fig. 2 Switching Pattern for Existing Topology

Fig .3 Output Current Waveform of Existing Topology

III. CASCADED H-BRIDGE MULTILEVEL INVERTER EMPLOYING TRANSFORMER AND SINGLE Electricity INPUT Power Source (Suggested TOPOLOGY-I)

This method includes a remote cascaded multilevel inverter employing low-frequency three-phase transformers along with a single electricity input power source. This circuit configuration reduces numerous transformers instead of traditional three-phase multilevel inverters using single-phase transformers. It controls switching phase angles to obtain a perfect switching pattern identified while using the fundamental frequency within the output current. For this reason control strategy, harmonic areas of the output current and switching losses may be reduced significantly.

H-bridge modules attach to the identical electricity input source in parallel, and every secondary within the transformer is connected in series. During this configuration, the output current becomes the sum terminal voltages of each H-bridge module. The amplitude within the output current is determined by the input current and switch ratio within the transformer. Usually, a typical cascaded H-bridge ripping tools employs a multi pulse isolation transformer to get the input electricity source.

Once the

traditional cascaded H-bridge ripping tools must isolate inside the ac output, it takes a 3-phase transformer relating to the inverter along with the ac outputs. However, the suggested inverter has the advantage of galvanic isolation relating to the source along with the output voltages, which will originate from being together with transformers. However, once the circuit, proven in Fig. 4, must modify its configuration to be used in three-phase applications, there’s a drawback, the reliance upon more transformers, with the same amount of transformers ought to be present in each phase.

Fig.4 Cicuit Diagram of Suggested Topology-I

IV. HYBRID MULTILEVEL INVERTER WITH SINGLE Electricity SOURCE (Suggested TOPOLOGY-II)

Hybrid multilevel inverter features a standard 3-leg inverter (one leg for achieve phase) and H-bridge in series with every single inverter leg. The inverter may be used in hybrid electric vehicles and electric vehicles .Also, you are able to enhance the power density since the inverter removes the bulky inductor in our Electricity-Electricity boost ripping tools. Figure Below shows the suggested technique.

Principal of Operation:

A simplified single-phase topology is proven in Fig. 5 The underside can be a leg in the standard 3-leg inverter obtaining a electricity power source. The very best is unquestionably an H-bridge in series with every single standard inverter leg. H-bridge can use another electricity power source or maybe a capacitor because the electricity power source The output current V1 in the leg (based on the ground) is either +Vdc/2 (S5 closed) or −Vdc/2 (S6 closed). This leg is connected in series obtaining a complete H-bridge which will receive a capacitor current. When the capacitor is stored billed to Vdc/2, your output current within the H-bridge usually takes across the values +Vdc/2 (S1, S4 closed), (S1, S2 closed or S3, S4 closed), or −Vdc/2 (S2, S3 closed). Once the output current V = V1 + V2 is needed to get zero, you may either set V1 = +Vdc/2 and V2 = −Vdc/2 or V1= −Vdc/2 and V2 = +Vdc/2. It’s mainly the versatility when choosing making that output current zero that’s exploited to handle capacitor current. Here only a electricity power source can be utilized within the inverter, that’s, h-bridge uses capacitor because the electricity power source.

Fig. 5 Single Phase Suggested Topology-I

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If S1, S4 are closed (to make certain that V2 = +Vdc/2) together with S6 closed (to make certain that V1 = −Vdc/2), your capacitor is discharging and V = V1 + V2 = .However, if S2, S3 are closed (to make certain that V2 = −Vdc/2) and S5 can also be closed (to make certain that V1 = +Vdc/2), your capacitor is charging and V = V1+V2 = . The procedure includes monitoring the capacitor current to make certain that in occasions of zero current output, either the switches S1, S4, and S6 are closed or possibly the switches S2, S3, S5 are closed based on whether you need to charge or discharge the capacitor.

Fig.6 Switching Pattern for Suggested Topology -I

V. SIMULATION STUDIES

Fig 7. Simulation of Cascaded H-Bridge MLI(Existing Topology)

Fig 8. Output Phase Voltages of Existing Topology

Fig. 9 FFT Analysis of Existing Topology for Phase Voltages

Fig 10. Output Line Voltages of Existing Topology

Fig 11. FFT Analysis of Existing Topology for Line Voltages

Fig 12. Simulation of Suggested Topology-I

Fig13. Output Phase Voltages of Suggested Topology-I

Fig14. FFT Analysis of Suggested Topology-1 for Phase Voltages

Fig 15. Output Line Voltages of Suggested Topology-I

Fig16. FFT Analysis of Suggested Topology-I for Line Voltages

Fig 17. Simulation of Suggested Topology-II

Fig 18. Output Phase Voltages of Suggested Topology-II

Fig 19 FFT Analysis of Suggested Topology-II for Phase Voltages

Fig20. Output Line Voltages of Suggested Topology-II

Fig 21 FFT Analysis of Suggested Topology-II for Line Voltages

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Printed: 23, March 2015

Multilevel converters are generally broadly-accustomed to synthesis a regular single- or three-phase current waveform. Probably the most well-loved multi-staircase output current is acquired by mixing several electricity current sources. Solar panel systems, fuel cells, batteries and ultracapacitors are the commonest independent sources used. An important usage of multilevel converters is founded on medium- and-power conversion. Nowadays, you’ll find three commercial topologies of multilevel current-source inverters: neutral point clamped (NPC), cascaded H-bridge (CHB), and flying capacitors (FCs). Of individuals inverter topologies, cascaded multilevel inverter reaches the greater output current and power levels (13.8 kV, 30 MVA) along with the greater reliability because of its modular topology.

Diode-clamped multilevel converters operate in conventional high-power ac motor drive applications like conveyors, pumps, fans, and mills. They’re also contained in oil, gas, metals, power, mining, water, marine, and chemical industries. There is also been considered as present in a back-to-back configuration for regenerative applications. Flying capacitor multilevel converters are really present in high-bandwidth high-switching frequency applications for example medium-current traction drives. Finally, cascaded H-bridge multilevel converters are really applied where high power and power quality are important, for instance, static synchronous compensators active filter and reactive power compensation applications, photovoltaic power conversion, uninterruptible power supplies, and magnetic resonance imaging. Furthermore, among the growing applications for multilevel motor drives is electric and hybrid power trains.

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For growing current levels the amount of switches can also increase in number. Therefore, the current stresses and switching losses increases along with the circuit will becomes complex. While using the suggested topology amount of switches will reduce considerably and so the efficiency will improve.

In high power applications, the harmonic content within the output waveforms should be reduced whenever feasible to prevent distortion within the grid and to achieve the most energy-efficiency. The task connected in a way is to find the analytical solutions within the non-straight line transcendental equations which have trigonometric terms which naturally exhibit multiple categories of solutions. The lower order harmonics are causing more effects than the greater order harmonics. It’s big challenge for virtually any investigator to get rid of the following order harmonics using simple techniques. For almost any motor load its effects are high. This paper proposes approach to eliminate lower order harmonics.

During this paper Selective Harmonics Elimination strategy is used. Third and fifth harmonics are eliminated making use of this technique. The transcendental non-straight line equations are solved when using the record technique known as Newton Raphson method. Traditional two and three level inverters are investigated while using the harmonic analysis and cascaded H-bridge seven level inverter is modelled and harmonic analysis is transported out. Finally the suggested topology is provided the implementation of SHE. The THD values for the Traditional, Conventional and Suggested inverters are compared and analysed.

II. TRADITIONAL INVERTERS

Generally inverters may be divided by 50 % major groups: single-phase inverters and three-phase inverters. The easiest inverter structure is half bridge single-phase inverter which generates 2-level square waveform, whereas output waveform in the full-bridge single-phase inverter is 3-level square waveform.

Three Level Inverter

Within the three level inverter zero level is added with two level inverter. The output current waveform resembles the 2 level inverter.

The ability circuit of three level inverter includes four power switches. Exactly the same switches in same leg shouldn’t be began as much as avoid rapid circuit with electricity source. In three level inverter when (S1,S2) are stored on and (S3,S4) are off, load current is equivalent to +Vdc whereas, within the situation of (S1,S2) are off and (S3,S4) are stored on, -Vdc is noted on load. To utilize zero current on load, (S1,S4) must be on and (S2,S3) must be off or the opposite way round. The switching request 3 level inverter is proven within the table 2.

The issue 1 describes whenever S1 and S3 are switched across the source current is provided for that load. Within the condition there’s no connection between source and cargo hence output current is zero. Whereas in condition -1 the building blocks relates to load within the reverse direction as individuals of condition 1. Therefore, the current reaches reverse direction. This can be frequently described using output current waveform proven within the figure 2.

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The harmonic spectrum analysis is transported out for the output current waveform within the three level inverter. Inside the figure 3 THD value acquired for the three level output current is 26.36%.When searching for that two level and three level harmonic spectrum analysis. Three level inverter will get the greater quality of output.

Fig 1. Three level Inverter

Table 1. The switching request 3 level inverter

## Switching Condition

## S1

## S2

Fig 2. Output current waveform of three level Inverter

But overall the performance within the traditional inverters isn’t meeting the commercial needs. Hence multilevel inverters are emerged.

Fig 3. Harmonic Spectrum for Output current of three level inverter

III. H-BRIDGE MULTILEVEL INVERTER

The traditional 2 or 3 levels inverter doesn’t completely get rid of the undesirable harmonics within the output waveform. Therefore, when using the multilevel inverter as opposed to traditional PWM inverters is investigated.

During this topology the amount of phase current levels inside the ripping tools terminals is 2N+1, where N is the amount of cells or electricity link voltages. During this topology, each cell has separate electricity link capacitor along with the current inside the capacitor might differ one of the cells. So, each power circuit needs only one electricity current source. The amount of electricity link capacitors is proportional to the amount of phase current levels .Each H-bridge cell might have positive, negative or zero current. Final output current may be the sum all H-bridge cell voltages that is symmetric regarding neutral point, so the amount of current levels is odd.

Cascaded H-bridge multilevel inverters typically use IGBT switches. These switches have low block current and switching frequency.

Think about the seven level inverter it takes 12 IGBT switches and three electricity sources. The ability circuit of inverter is proven within the figure 4. A cascaded H-bridges multilevel inverter is simply a series connection of multiple H-bridge inverters. Each H-bridge inverter will get exactly the same configuration as being a typical single-phase full-bridge inverter.

The cascaded H-bridges multilevel inverter introduces the thought of using Separate Electricity Sources (SDCSs) to create an AC current waveform. Each H-bridge inverter relates to a distinctive Electricity source Vdc. By cascading the AC outputs of each H-bridge inverter, an AC current waveform is created.

Fig 4. Cascaded H-bridge 7-level Inverter

By closing the very best switches, each H-bridge inverter can establish three different voltages: +Vdc, and -Vdc.

Fig 5. Output Current of cascaded H-bridge seven level inverter

It’s also easy to be modularized circuit layout and packaging because each level will get exactly the same structure, and you will find no extra clamping diodes or current balancing capacitors. The amount of switches is reduced when using the new topology.

This circuit is simulated when using the MATLAB software. The outcome have been verified within the later sections in greater detail.

## IV.Suggested TOPOLOGY

The primary objective should be to increase the risk for quality output current within the multilevel inverter with reduced amount of switches. An essential issue in multilevel inverter design may be the current waveform is near sinusoidal along with the lower order harmonics are eliminated. An essential concern within the fundamental switching plan’s to uncover the switching angles to be able to increase the risk for fundamental current without generating specific lower order harmonics.

Fig 6 Suggested Power circuit for 7-level output

You will find three modes of operation for the 7-level multilevel inverter. These modes are known as below.

Powering Mode This happens when both load current and current have similar polarity. Within the positive half cycle, once the output current is Vdc, the present pass comprises the lower supply, D6, Q1, load, Q4, also to the lower supply. Once the output current is 2Vdc, current pass may be the lower source, Q5, the top of source, Q1, load, Q4, also to the lower source. Within the negative half cycle, Q1 and Q4 are substituted with Q2 and Q3 correspondingly.

Free-Wheeling Mode Free-wheeling modes exist when one of the greatest switches is switched-off since the load current must continue its pass because of load inductance. This is achieved while using anti-parallel diodes within the switches, along with the load circuit is disconnected inside the source terminals. During this mode, the positive half cycle current pass comprises Q1, load, and D2 or Q4, load, and D3, although inside the negative half cycle the present pass includes Q3, load, and D4 or Q2, load, and D1.

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Regenerating Mode During this mode, area of the energy stored inside the burden inductance is came back for the origin. This occurs with the times when the burden current is negative with the positive half cycle and vice-versa, in which the output current is zero. The positive current pass comprises load, D2, Q6, the lower source, and D3, since the negative current pass comprises load, D1, Q6, the lower source, and D4.

Inside the figure 7 switching pattern for the various switches are described. During this paper fundamental frequency switching plan’s utilized which cuts lower round the switching losses. Since the switching frequency is less during this method than the other methods. Switching losses are directly proportional for that switching frequency.

Fig 7 Waveforms within the suggested seven level inverter

V.SELECTIVE HARMONICS ELIMINATION

The Selective Harmonic Elimination Walked-Waveform (SHESW) strategy is very appropriate for just about any multilevel inverter circuit. With this particular technique coupled with multilevel topology, the low Total Harmonic Distortion THD output waveform with no filter circuit is possible.

A.Fourier series and Harmonics Elimination theory

After applying Fourier theory for that output current waveform of multilevel converters, that’s odd quarter-wave symmetric, we’re able to identify the Fourier expression within the multilevel output current as (1). When the Electricity voltages are equal within the multilevel ripping tools, the equation for the fundamental frequency switching control method may be expressed as:

Inside the equation, it may be observed the output current doesn’t have harmonics since the output current waveform is odd quarter-wave symmetric. It is also seen from (2) the peak values of people odd harmonics are expressed based on the switching angles 1 ,2, €¦ and s. Furthermore, the harmonic equations created from (2) are transcendental equations.

While using harmonic elimination theory, if someone wants to get rid of the nth harmonic, then

Intending to pick numerous switching angles allowing the requirement of the nth harmonic be zero. Therefore, a formula with s switching angles will know about control the s different harmonic values. Generally, a formula with s switching angles enables you to look for the fundamental frequency value, and to eliminate s-1 low order harmonics.

By having an equation with three switching angles, (2) becomes

B. Transcendental Equations to resolve

During this paper we derived harmonic equations for eliminating the next and fifth order harmonics. The resulting harmonic equations are:

To simplify the expression, (4) may be written as

These harmonic equations (4)-(6) are transcendental equations. They’re hard to solve without requiring some form of record iterative technique. Here Newton Raphson technique is useful for solving these equations.

C.Solving the harmonic equations using Newton Raphson method

To resolve the harmonic equations by resultant theory, they must be altered into polynomials. First, modify the variables,

Also, make use of the following trigonometric identities:

Then, apply individuals towards the transcendental harmonic equations above, along with the following polynomial harmonic equations are available.

For the fundamental frequency harmonic:

For the 3rd harmonic:

For the fifth harmonic:

The polynomial equations may be solved while using the Newton Raphson method. Listed here are steps for solving the equations. Substitute the initial guesses for variables. Then in the jacobian matrix with newton’s formula. Repeat exactly the same steps before the resolution converge. Thus the solutions acquired receive below

The MATLAB simulation circuit was created for the conventional seven level and suggested inverter with SHE implementation.

A.Simulation within the conventional seven level inverter

This circuit includes 12 IGBT switches with 3 equal electricity sources. The gate pulses derive from when using the pulse generator. Really the only phase capacitor-start induction motor can be utilized as being a load.

Fig 8 Simulation Model for conventional seven level Inverter

Fig 9. Harmonic spectrum of Output current of seven level H-bridge inverter

Inside the harmonic analysis of seven level output current within the 12 switch H-bridge inverter, the THD value is acquired as 34.43%.

B.Simulation within the Suggested Inverter Topology

The Simulink model diagram for the suggested circuit is proven in figure 10.It is only seven switches and same load can be utilized. Gate pulses derive from this mixture of XOR gates and pulse generators.

Inside the figure 11 it’s apparent that seven level or three walked waveform is acquired. Then harmonic analysis is transported by helping cover their the induction motor.

Fig 10 Simulation model for Suggested Inverter

Fig 11 Seven level output current of suggested Inverter

Fig 12. Harmonic spectrum of output current within the suggested inverter

Inside the table 3 different inverter THD values are compared. The suggested inverter THD value is acquired as 11.16%, the very best of. This means that company’s seven level inverter continues to be enhanced.

Table 3 THD comparison

Type of the MLI

7 Level Conventional MLI

7 Level Suggested MLI with motor load

In comparison to typical PWM switching schemes, multilevel fundamental switching can result in lower switching losses. Consequently, when using the multilevel fundamental frequency switching plan can result in elevated efficiency.

This paper is presented a process to selectively eliminate certain harmonics within the multilevel inverter while using the multilevel fundamental frequency switching plan. Record techniques, Newton Raphson technique is presented during this paper require a preliminary guess to discover a solution.

For almost any cascaded H-bridges multilevel inverter utilizing three equal electricity sources, usually the switching angles may be selected therefore the output current THD is under 12%. Thus while using the suggested plan THD and switching losses are reduced.

Ayoub Kavousi, Behrooz Vahidi, Reza Salehi, Mohammad azem Bakhshizadeh, Naeem Farokhnia and S.Hamid Fathi,(april 2012) Technique Bee Formula for Selective Harmonic Elimination Strategy in Multilevel Inverters, IEEE Transactions on power electronics, vol. 27, no. 4, pp1689-1696.

Damoun Ahmadi, KeZou, Cong Li, Yi Huang and Jin Wang,(october 2011) A Universal Selective Harmonic Elimination Approach to High-Power Inverters, IEEE Transactions on power electronics, vol. 26, no. 10,pp2743-2752,.

FaeteFilho, Leon M. Tolbert, Yue Cao and BurakOzpineci,(september/October 2011) Real-Time Selective Harmonic Minimization for Multilevel Inverters Connected with Solar Power Systems Using Artificial Neural Network Position Generation,IEEE Transactions on industry applications, vol. 47, no. 5, pp2117-2124.

Hossein Sepahvand, Jingsheng Liao and Mehdi Ferdowsi,(november 2011) Analysis on Capacitor Current Regulation in Cascaded H-Bridge Multilevel Converters With Fundamental Frequency Switching, IEEE Transactions on industrial electronics, vol. 58, no. 11,pp5102-5111.

Jason R. Wells, XinGengPatrick L. Chapman Philip T. Kreinand Brett M. Nee,(the month from the month of the month of january 2007) Modulation-Based Harmonic Elimination, IEEE Transactions on power electronics, vol. 22, no. 1,pp336-340.

J. Napoles, A. J. Watson, J. J. Padilla, J. I. Leon, L. G. Franquelo, P. W. Wheeler and M. A. Aguirre,(june 2012)Selective Harmonic Minimization Approach to Cascaded H-Bridge Converters with Non-Equal Electricity Link Voltages, IEEE Transactions on power electronics,pp1-9.

John N. Chiasson, Leon M. Tolbert, Keith J. McKenzieand Zhong Du,(march 2004) A Unified Method of Solving the Harmonic Elimination Equations in Multilevel Converters,IEEE Transactions on power electronics, vol. 19, no. 2, pp478-500.

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