HYBRID CASCADED MULTILEVEL CONVERTER WITH REDUCED TOTAL HARMONIC DISTORTION

HYBRID CASCADED MULTILEVEL CONVERTER WITH REDUCED TOTAL HARMONIC DISTORTION

RAJASEKHAR V

13l31D4208

2nd year M.Tech, EEE (P&ID)

UNDER THE ESTEEMED GUIDANCE OF

Adari .G V . CHIRANJEEVI

Asistente .Profesor

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Department of Electrical and Electronics Engineering

VIGNAN’S INSTITUTE OF INFORMATION TECHNOLOGY

Prepared by

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CONTENTS

• INTRODUCTION

• BLOCK DIAGRAM

• MULTY LEVEL INVERTERS TOPOLOGY

• CASCADED H-BRIDGE MULTILEVEL INVERTER

• SIMULATION RESULTS

• CONCLUSION

• REFERENCES

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INTRODUCTION

• In a traditional method, all the battery cells are directly connected in series and are

charged or discharged by the same current.

• A voltage equalization circuit is often needed in practical applications to protect the

battery cells from over charging or over discharging.

• The equalization circuit is composed of a group of inductances or transformers and

converters, which can realize energy transfer between battery cells.

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• In EV energy storage systems, a large number of battery cells are usually connected in

series to enhance the output voltage for motor driving.

• These vehicles have battery storage with large capacity and these batteries are

required to be charged continuously.

• The ac output of the HCMC is multilevel voltage, while the number of voltage levels is

proportional to the number of cascaded battery cells.

• So the HCMC used in the applications of EV with a larger number of battery cells, the

output ac voltage is approximately ideal sine waves.4

Multilevel Voltage Source Inverter

One phase leg n-level inverter

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• The voltage balance is realized by energy exchange between cells.

• To simplify the circuit, multilevel converters are widely used in medium or high

voltage motor drives.

• If their flying capacitors or isolated dc sources are replaced by the battery cells, the

battery cells can be cascaded in series combining with the converters instead of

connection in series directly.

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A + B +

A - B -

V a

V b

V load = VA - VB

BLOCK DIAGRAM

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MULTY LEVEL INVERTERS TOPOLOGY

• The most common multilevel converter topologies are:

• Diode clamped (neutral-point clamped)

• Flying capacitor (capacitor-clamped)

• Cascaded topology.

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Corresponding values of Vab •A+ closed and B– closed, Vab = Vdc •A+ closed and B+ closed, Vab = 0 •B+ closed and A– closed, Vab = –Vdc •B– closed and A– closed, Vab = 0

• The free wheeling diodes permit current to flow even if all switches are open

• These diodes also permit lagging currents to flow in inductive loads

Vdc

Load

A+ B+

A– B–

Va Vb

H BRIDGE INVERTER

ABBAload VVVV

+ Vdc −

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Vdc

Load

A+ B+

A– B–

Va Vb

H BRIDGE INVERTER

ABBAload VVVV

+ 0 −

10

11




Vdc

Load

A+ B+

A– B–

Va Vb

H BRIDGE INVERTER

ABBAload VVVV

− Vdc +

11

12




Vdc

Load

A+ B+

A– B–

Va Vb

H BRIDGE INVERTER

ABBAload VVVV

+ 0 −

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• The cascaded multilevel inverter consists of a series of H-bridge inverter units.

• The cascaded H-bridge converters are used for the voltage balance of the battery cells.

• The converter can also realize the charge and discharge control of the battery cells.

• The ac output of the converter is multilevel voltage, while the number of voltage levels

is proportional to the number of cascaded battery cells.

• So in the applications of Electric Vehicles with a larger number of battery cells, the

output ac voltage is approximately ideal sine waves.

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Hybrid Cascaded Multilevel Converter14

• It includes two parts, the cascaded half-bridges with battery cells shown on the left

and the H-bridge inverters shown on the right.

• The output of the cascaded half-bridges is the dc bus which is also connected to the dc

input of the H-bridge.

• Each half-bridge can make the battery cell to be involved into the voltage producing

or to be bypassed.

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• By control of the cascaded half-bridges, the number of battery cells connected in the

circuit will be changed, that leads to a variable voltage to be produced at the dc bus.

• The H-bridge is just used to alternate the direction of the dc voltage to produce ac

waveforms.

• Hence, the switching frequency of devices in the H-bridge equals to the base

frequency of the desired ac voltage.

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• For the cascade half-bridge converter, define the switching state as follows:

Sx = 1, upper switch is conducted, lower switch is OFF

= 0, lower switch is conducted, upper switch is OFF.

• When Sx = 1, the battery is connected in the circuit and is discharged or charged which

is determined by the direction of the external current.

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• The other is the higher voltage devices used in the H-bridges which worked just in base

frequency.

• So the high voltage large capacity devices such as GTO or IGCT can be used in the

H-bridges.

• The number of battery cells in each phase is n, then the devices used in one phase

cascaded half-bridges is 2 n.∗

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19

V1

V2

V 11

H-B 1

H-B 1

H-B 1

A

B

Proposed 23 level hybrid Cascaded Multi Level converter19

Three-phase hybrid cascaded multilevel converter. 20

• It means that not all the battery cells are needed to supply the load at the same time.

• As the output current is the same for all cells connected in the circuit, the charged or

discharged energy of each cell is determined by the period of this cell connected into the

circuit, which can be used for the voltage or energy equalization.

• The cell with higher voltage or SOC can be discharged more or to be charged less in

using, then the energy utilization ratio can be improved while the overcharge and over

discharged can be avoided.

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• The hybrid cascaded modular multilevel converter proposed here is shown.

• It includes two parts, the cascaded half-bridges with battery cells shown on the left and the H-bridge inverters shown on the right.

• The output of the cascaded half-bridges(CHB) is dc bus which is connected to the dc input of the H-bridge.

• Each half-bridge can make the battery cell to be involved into the voltage producing or to be bypassed.

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• By control of the cascaded half-bridges, the number of battery cells connected in the circuit will be changed, that leads to a variable voltage to be produced at the dc bus.

• The H-bridge is used to alternate the direction of the dc voltage to produce ac waveform.

• Hence, the switching frequency of devices in the H-bridge equals to the base frequency of the desired ac voltage.

• There are two kinds of power electronics devices in the proposed circuit.

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• One is the low voltage devices used in the cascaded half-bridges these devices

work in higher switching frequency to reduce harmonics.

• MOSFETs with low on-resistance are used in these circuits for switching

action.

• The Switches used in H-Bridge should withstand high voltages and operate

low frequency usually at grid frequency.

• Devices such as IGBT, GTO or IGCT can be used.24

• All the half-bridges are controlled individually, a staircase shape half-sinusoidal-wave voltage is produced on the dc bus of MMC.

• As a result multilevel ac voltage can be formed at the output side of H-Bridge.

• The number of ac voltage levels in any phase is equal to 2 n–1, where n is the ∗number of cascaded half-bridges in each phase.

• The more of the cascaded cells, the more voltage levels at the output side, and the output voltage is closer to the ideal sinusoidal.

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Magnitudes of Voltages with respect to Switching states of a MMC

S.No S1 S2 S3 . . S10 S11 Vout

1 0 0 0 - - 0 1 V1

2 0 0 0 - - 1 1 2V1

3 0 0 0 - - 1 1 3V1

4 0 0 0 - - 1 1 4V1

5 0 0 0 - - 1 1 5V1

6 0 0 0 - - 1 1 6V1

7 0 0 0 - - 1 1 7V1

8 0 0 0 - - 1 1 8V1

9 0 0 1 - - 1 1 9V1

10 0 1 1 - - 1 1 10V1

11 1 1 1 - - 1 1 11V1

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Modulating and Carrier Signals used to control Half – Bridges of MC

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Methodology Adopted for identifying optimum number of Stages

• Cascaded HCMC circuit proposed in [1] taken into consideration.

• Cascaded Half Bridge modules of required number are taken for

production of different levels in the output voltage across the terminals

of a H-Bridge.

• The output levels that are generated are 3,5,7,9,11,13,15,17,19,21 and 23.

• Peak amplitudes of Phase voltage, % THD are estimated.

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Results

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Simulink circuit 1630

Outputs of Voltage Across H-Bridge While Producing 23 Levels (Phase Voltages)

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Outputs of Voltage Across H-Bridge While Producing 23 Level MLI

(Line Voltages)

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Output Load Currents of 23 level MLI (sinusoidal waveform)

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Harmonic Spectrum of 23- level Phase Voltage

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CONCLUSION

• The levels in output voltage increases and the high power switches are switched

at low frequency as a result the switching loss decreases.

• The HCMC converter has the ability of producing the required number of levels

in the output voltage; this makes it suitable for variable voltage

applications.

• The Converter offers a reasonably good THD in the load voltages as results

the cost of filters will get reduced.

• As the number of levels increases beyond 19 there in no considerable change

in THD values measured for different levels. 35

• So it is better to restrict the number of levels to a value between 17 or 23

for which the THD value is lies between 2.13 and 2.16.

• Depending upon the load power requirements the numbers of levels

required in output are opted.

• For example for low power applications 17 levels in output may

chosen for high power applications 23 levels and beyond may be opted.

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References

1. Zedong Zheng, Kui Wang, Lie Xu, Yongdong Li, “A Hybrid Cascaded Multilevel Converter for Battery Energy Management Applied in Electric Vehicles”, IEEE TRANSACTIONS ON POWER ELECTRONICS, vol. 29, no. 7, pp. 3537 – 3546, july 2014.

2. S. M. Lukic, J. Cao, R. C. Bansal, F. Rodriguez, and A. Emadi, “Energy storage systems for automotive applications,” IEEE Trans. Ind. Electron., vol. 55, no. 6, pp. 2258–2267, Jul. 2008.

3. H. M. Zhang and S. P. Ding, “Application of synergic electric power supply in HEV,” in Proc. 8th World Congr. Intelligent Control Autom., 2010, pp. 4097–4100.

4. A. Emadi, Y. J. Lee, and K. Rajashekara, “Power electronics and motor drives in electric, hybrid electric, and plug-in hybrid electric vehicles,” IEEE Trans. Ind. Electron., vol. 55, no. 6, pp. 2237–2245, Jun. 2008.

5. K. Jonghoon, S. Jongwon, C. Changyoon, and B. H. Cho, “Stable configuration of a Li-Ion series battery pack based on a screening process for improved voltage/SOC balancing,” IEEE Trans. Power Electron., vol. 27, no. 1, pp. 411–424, Jan. 2012.

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HYBRID CASCADED MULTILEVEL CONVERTER WITH REDUCED TOTAL HARMONIC DISTORTION

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