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Amey Patil(BT11EEE05)

Amey Khot(BT11EEE06)

Charudatt Awaghate(BT11EEE18)

Srikant Pillai (BT11EEE51)

Purushotam Kumar (BT11EEE53)

Introduction

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• To reduce the ripple content in the output and to improve the power quality inverters need very high switching frequency and various PWM strategies.

• In 2-level inverter output voltage waveform is produced by using PWM with two voltage levels.

• This has limitations at high voltage. This causes the output voltage and current to be distorted and the THD of the voltage is poor.

• Hence multi-level inverters are used.

Classification of High Power Converters

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Multilevel ConvertersADVANTAGE

• They are able to generate high output voltages with very low distortion and lower dv/dt.

• They are able to bring in input current with very low input distortion.

• Well suited for reactive power compensation.

• They can be functioned with a much lower switching frequency.

DISADVANTAGE

• it required a huge amount of semiconductors switches.

• It required isolated voltage sources or a bank of series capacitors for voltage balancing.

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• APPLICATIONS

1. DC power source utilization

2. Uninterruptible power supplies

3. HVDC power transmission

4. Variable-frequency drives

5. Electric vehicle drives

6. Air conditioning

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Comparison of conventional two level inverters and multilevel inverters

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Concept• Multi-level inverters are the modification of basic

bridge inverters. They are normally connected in series to form stacks of level.

The topological structure of multilevel inverter must cope with the following points-

1. It should have less switching devices as far as possible.

2. It should be capable of enduring very high input voltage such as HVDC transmission for high power applications.

3. Each switching device should have lower switching frequency owing to multilevel approach. 8

TYPES OF MULTILEVEL INVERTERS

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Diode Clamped Multi-Level Inverter• Advantages

1. When the number of levels is high enough, harmonic content will be low enough to avoid the need for filters.

2. Efficiency is high due to all devices which are being switched at the fundamental frequency.

3. We are able to control the reactive power flow.

4. The control method is easy for a back to back intertie system

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Disadvantages1.Excessive clamping diodes are being required when the number of levels get high.2.It is hard to do a real power flow control for individual converter.

SWITCHING SEQUENCE for 5 level

output s1 s2 s3 s4 s1’ s2’ s3’ s4’

Vdc 1 1 1 1 0 0 0 0

3Vdc /4 0 1 1 1 1 0 0 0

Vdc/2 0 0 1 1 1 1 0 0

Vdc/4 0 0 0 1 1 1 1 0

0 0 0 0 0 1 1 1 1

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Flying capacitor multi-level inverter

• Advantages1. Huge amount of storage capacitors

will provide additional ride through capabilities during power rage.

2. Switch combination redundancy are provided for balancing different voltage levels.

3. When the number of levels is high enough, the harmonic content will be low enough not to use the filter.

4. We are able to control both the real and reactive power flow, and making a possible voltage source converter candidate for high voltage dc transmission

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Disadvantages1.When the number of converter levels get high, a huge amount of storage capacitor is required. Those high level systems are more difficult to package and those bulky capacitors are expensive.2.The switching frequency and switching losses will sore high for real power transmission and the converter control will get very complicated.

SWITCHING SEQUENCE for 5 level output s1 s2 s3 s4 s1’ s2’ s3’ s4’

Vdc 1 1 1 1 0 0 0 0

3Vdc/4 1 1 1 0 0 0 0 1

Vdc/2 1 1 0 0 0 0 1 1

Vdc/4 1 0 0 0 0 1 1 1

0 0 0 0 0 1 1 1 113

Cascaded Multi-level Inverter• Advantages:1. The modularized structure

allows easy packaging and storage.

2. The quantity of possible voltage levels is more than DC and FC type.

• Disadvantages:1. Separated DC sources or

capacitor are required for each module.

2. A More complex controller is required due to the amount of capacitors, which need to be balanced.

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Phase waveform for 5-Level CHB Inverter

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COMPARISON AMONG MULTILEVEL INVERTERS INAPPLICATION ASPECTS

ADVANTAGE

• Compared with other multi level inverter configurations cascaded type requires least number of components to similar voltage level

• Since each structure is similar packaging and circuit layout can be optimized effectively.

• Soft switching techniques can be used to reduce switching losses and device stresses.

DISADVANTAGE

• It needs separate DC power sources there by limiting its utility and applications

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Comparison of different multilevel inverter topologies

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Multi-level Inverters TopologiesCHB Inverter with Equal dc Voltage

As the name suggests, the cascaded H-bridge multilevel inverter uses multiple units of H-bridge power cells connected in a series chain to produce high ac voltages. A typical configuration of a five-level CHB inverter is shown in Fig. below, where each phase leg consists of two H-bridge cells powered by two isolated dc supplies of equal voltage E.

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3Phase 5 level CHB Inverter

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Unipolar PWM with two phase-shifted modulating waves (mf = 15, ma = 0.8, fm = 50 Hz, and fcr = 750 Hz).

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Switching table for 5 level CHB inverter

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2 cell 7 level inverter CHBUnequal Dc Voltages

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Carrier based PWM schemes

The carrier-based modulation schemes for multilevel inverters can be generally classified into two categories: phase-shifted and level-shifted modulations. Both modulation schemes can be applied to the CHB inverters.

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Phase shifted carrier scheme

In general, a multilevel inverter with m voltage levels requires

• (m – 1) triangular carriers

• In the phase-shifted multicarrier modulation, all the triangular carriers have the same frequency and the same peak-to-peak amplitude, but there is a phase shift between any two adjacent carrier waves, given by

• Phcr = 360°/(m – 1).

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5 Level inverter phase shifted carrier wave

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Level-Shifted Multicarrier Modulation

• It requires (m-1) triangular carriers, all having the same frequency and amplitude.

• The frequency modulation index is given by mf = fcr/fm.

• The switching frequency of the inverter using the level-shifted modulation is equal to the carrier frequency, that is, fsw,inv = fcr

• Average device switching frequency is fsw,dev = fcr/(m – 1)

• The conduction time of the devices is not evenly distributed either.

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where ˆVm is the peak amplitude of the modulating wave vmand ˆVcr is the peak amplitude of each carrier wave.

There are three schemes for the level-shifted multicarrier modulation:

(a) In-Phase Disposition (IPD), where all carriers are in phase;

(b) Alternative Phase Opposite Disposition (APOD), where all carriers are alternatively in opposite disposition; and

(c) Phase Opposite Disposition (POD), where all carriers above the zero reference are in phase but in opposition with those below the zero reference.

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Level-shifted multicarrier modulation for five-level inverters28

• vAN is composed of 5 voltage levels while the line to-line voltage vAB has 9 voltage levels.

• The dominant harmonics in vAN and vABappear as sidebands centered around mf.

• The inverter phase voltage contains triplenharmonics, such as mf and mf ± 6, with mf being a dominant harmonic.

• Since these harmonics do not appear in the line-to-line voltage, the THD of vAB is less than vAN.

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Phase-Shift Sine PWMLine & Phase Voltage (Ma=0.8, mf=3)

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IPD RMSMa=0.9 Mf=20

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POD RMSMf=20 Ma=0.8

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APOD RMSMf=20 Ma=0.8

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Phase-Shift Sine PWMLine & Phase voltage (Ma=0.6, Mf=15)

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Line & Phase Voltage (Ma=0.8, mf=15)

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Line & Phase Voltage (Ma=1, mf=15)

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FFT analysis of Phase Shifted CHB Inverter at Ma=.8 and Mf=5

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FFT analysis of Phase Shifted CHB Inverter at Ma=.8 and Mf=10

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FFT analysis of Phase Shifted CHB Inverter at Ma=.8 and Mf=15

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FFT analysis of Phase Shifted CHB Inverter at Ma=.8 and Mf=20

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Unbalanced (2 cell) 7 level CHB Inverter

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Unbalanced 7 level inverter FFT

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Unbalanced 7 level inverter THD

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Unbalanced 9 level inverter

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Unbalanced 9 level inverter FFT

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Unbalanced 9 level inverter THD

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IPD RMSMa=0.9 Mf=20

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IPD FFTMa=0.9 Mf=20

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IPD THD

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POD RMSMf=20 Ma=0.8

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POD FFT

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POD THD

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APOD RMSMf=20 Ma=0.8

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APOD FFT

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APOD THD

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Phase Sifted CHBCurrent through S11

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Phase Sifted CHBCurrent through One Leg

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Level Shift CHBCurrent through S11

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Level Shift CHBCurrent through S21

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Level Shift CHBCurrent through one leg

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Level Shift CHBCurrent through S21 and S22

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Comparison Between Phase- and Level-Shifted PWM Schemes

Assumption

• The average switching frequency of the solid-state devices is the same for both schemes.

• All the devices operate at the same switching frequency and conduction time.

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Comparison Between the Phase- and Level-Shifted PWM Schemes

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RMS Voltage Variation with Ma at Mf=15

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Ma Phase voltage RMS Line voltage RMS

0.6 92 151

0.8 120 204

1 146 252

THD(%) Analysis of Line Voltages of CHB Inverter based on Different Schemes

at Mf=20 Vdc=100

Ma PHASE SHIFTED IPOD 2 CELL 7 LEVEL 2 CELL 9 LEVEL

0.2 68.6 25.59 153.55 167

0.4 68 25.57 79.67 91.625

0.6 27 25.5 40.72 53.46

0.8 29 21.73 36.01 42.56

1 25 16.9 27.65 30

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THD profile of vAB produced by the five-level CHB inverter with phase and level-shifted modulation schemes.

0.2 0.4 0.6 0.8 1

Phase Shifted PWM 68.6 68 48 35 27 26 25 25

IPD 25.59 25.57 25.55 25.52 25.5 23 21.73 16.9

0

10

20

30

40

50

60

70

80

THD

MA

Fsw,dev=1000HzFive Level Inverter

Phase Shifted PWM IPD

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SUMMARYFeatures of CHB INVERTERModular structure: Multiple units of identical H-

bridge power cells ,leads to a reduction in manufacturing cost.

Lower voltage THD and dv/dt: Due to Output voltage is formed by several voltage levels with small voltage steps leads to much lower THD and dv/dt.

High-voltage operation without switching devices in series: The H-bridge power cells are connected in cascade to produce high ac voltageseliminates the problems of equal voltage sharing for series-connected devices.

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Drawbacks

Large number of isolated dc supplies. The dc supplies for the CHB inverter are usually obtained from a multi pulse diode rectifier employing an expensive phase shifting transformer.

High component count. The CHB inverter uses a large number of IGBT modules. A nine-level CHB inverter requires 64 IGBTs with the same number of gate drivers.

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Proposed Work in next phase II

In this phase we have addressed the different type of controlling method, We did simulation and we are obtaining good response by using these topologies.so in next phase we will work on Hardware part of it.

In next phase we will address the Hardware part

Of Five level CHB using Unipolar Phase-Shifted carrier based sine PWM topology.

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REFERENCES1. W. A. Hill and C. D. Harbourt, Performance of Medium Voltage

Multilevel Inverters,IEEE Industry Applications Society (IAS) Conference, Vol. 2, pp. 1186–1192, 1999

2. P. W. Hammond, A New Approach to Enhance Power Quality for Medium Voltage AC Drives, IEEE Transactions on Industry Applications, Vol. 33, No. 1, pp. 202–208, 1997.

3. P.S.BHIMBRA, POWER ELECTRONICS.4. M.RASHID, POWER ELECTRONICS.5. N. Mohan, T. M. Undeland, et al., Power Electronics—

Converters, Applications and Design, 3rd edition, John Wiley & Sons, New York, 2003.

6. Web link-https://www.academia.edu/6742849/Multilevel_Inverter_3-level_topologies_Diode_and_capacitor_clamped_and_controlscheme_spwm_

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