Suraj Punj Journal For Multidisciplinary Research ISSN NO ...

COMPARISON STUDY OF DIFFERENT

CARRIER BASED PWM TECHNIQUES IN

THE MODULAR MULTILEVEL CONVERTER Dr.R.Gandhi1, R.Yuvraj 2,A.Venkatesh3

Professor1, Assistant professor2,3 1,2,3Department of Electrical and Electronics Engineering, Gnanamani College of Technology,Namakkal

[email protected] [email protected]

[email protected]

Abstract—This paper reports a comparison study on Modular Multilevel Converter based PWM techniques. Different PWM

techniques has been considered in this study, namely Phase Opposition Disposition (POD) PWM, Carrier Overlapping PWM

(COPWM), Phase Shift PWM (PSPWM), In-Phase Disposition (PD) PWM, Alternative Phase Opposition Disposition (APOD) PWM,

Variable Frequency PWM (VFPWM). The importance of circulating currents and control methods has been considered under study.

Test results and harmonics data has been presented to understand the benefits of the different modulation strategies.

Keywords— Modular Multilevel Converter, Multicarrier PWM, In Phase disposition (PD), Phase Opposition Disposition (POD),

Alternate Phase Opposition Disposition (APOD), Carrier Overlapping PWM (CO-PWM), Variable Frequency PWM (VF-PWM),

Cascaded Multilevel Inverter, Total harmonic Distortion (THD)

I. INTRODUCTION

The modular multilevel inverter is one of the best topologies for high voltage generation and renewable

energy systems. The MMC has n-number of H-bridge switches which in turn generate positive and

negative voltage outputs. The power modules can be stacked in series for higher voltage levels. Each

power module has an independent power source which supplies the active and reactive power necessary

for the power converter output. The importance in using MMC is having balanced power source or

capacitor due to n levels. This can be achieved by deploying voltage balancing techniques. However, each

MMC can be a half bridge or full bridge-based circuit with a voltage source capacitor at the input. Each

MMC has circulating current associated with H-bridge leg and it needs control to avoid power devices

abnormal operation. MMC topology has found practical insights with up to more than 200 submodules per

phase and up to more than 400 submodules. The number of submodules to be opted depends on the

voltage step levels and power electronic application.

Suraj Punj Journal For Multidisciplinary Research

Volume 11, Issue 4, 2021

ISSN NO: 2394-2886

Page No: 518

Asus

Textbox

Therefore, variety of modulation techniques followed in MMC. The commonly used ones are CO-PWM,

PS-PWM, VF-PWM, PD-PWM, POD, APOD-PWM. These different PWM techniques has advantages in

terms of harmonics achieving better power quality and offering high efficiency.

Sinusoidal PWM can be classified according to number of carriers involved and modulating signal. The

PWM is created based on overlying sine wave with a triangular wave or sawtooth wave in-order to

generate firing pulses for the gate of the IGBTs. Some of the PWM generation methods has been detailed

belowIn-Phase Disposition (PD-PWM): The carrier waves are in identical phase with each other.Phase

Shift PWM (PS-PWM): The carrier waves has identical magnitude and frequency but they are phase

shifted by 90 degrees to each other.Phase Opposition Disposition (POD-PWM): The carrier waves above

the zero reference are out of phase by 180° from those carrier waves under the zero reference.Alternative

Phase Opposition Disposition (APOD-PWM): Individual carrier band is shifted from adjacent bands by

180°.Carrier Overlapping PWM (CO-PWM): The carrier waves will have identical frequency and equal

peak to peak magnitude and are inclined in such a way that the bands they dwell in will overlay with each

other.Variable Frequency PWM (VF-PWM): The carrier wave will have variable frequencies and identical

magnitude with each other.

(a) Phase Opposition Disposition (POD) PWM

The carrier waves are organized in-phase opposition and 180º out of phase with carrier waves under zero reference. Here the

converter is switched to + Vdc/2 when the sinewave overlaps with carrier1, + Vdc/4 when the sinewave overlaps with

carrier2, -Vdc/4 when the sinewave overlaps with carrier3, - Vdc/2 when the sinewave overlaps with carrier4.

Figure 1: Waveform of Phase Opposition Disposition (POD) PWM

(b) Alternative Phase Opposition Disposition (APOD) PWM

Alternative Phase Opposition Disposition method involves each of the (n-1) carrier waves for an n-

level output to be phase displaced from other alternatively by 180 degree.



ISSN NO: 2394-2886

Page No: 519

Figure 2: Waveform of Alternative Phase Opposition Disposition (APOD) PWM

(c) Phase Disposition (PD) PWM

Phase Disposition PWM method involves each of the (n-1) carrier waves for an n-level output to be in-phase.

Figure 3: Waveform of Phase Disposition (PD) PWM

(d) Phase Shift PWM (PS) PWM

The carrier waves in total of which are suitably phase shifted as shown in Figure 4.

Figure 4: Waveform of Phase Shift (PS) PWM

II. MMC OPERATION

The MMC system is computed of n-level of identical sub-modules and its output are connected in series.

The sub-modules are symmetrical in both upper and lower arms of the MMC.



ISSN NO: 2394-2886

Page No: 520

Figure 5: Configuration of MMC Phase Leg

Figure 6: Internal architecture of MMC

The MMC consists of series connection of sub modules and buffer inductors. The sub-modules have

floating capacitors which can serve as independent power source to the sub-modules and it is powered by



ISSN NO: 2394-2886

Page No: 521

AC to DC Converters. The voltage output of the sub-modules produces both positive, negative and zero

outputs as mentioned in the table below

Table 1 : MMC sub-module output for different switching modes

Mode S1 S2 S3 S4 Vo

1 1 0 0 1 Va

2 1 1 0 0 0

3 0 1 1 0 -Va

4 0 0 1 1 0

The voltage output of each sub-modules equals to Va if S1=1, S2=0, S3=0, S4=1 and –Va if S1=1, S2=1,

S3=0, S4=0 and zero if S1=1, S2=1, S3=0, S4=0; S1=0 S2=0, S3=1, S4=1.

The current through solid state switch flow through IGBT and its body diode depending on the polarity of

the sub module. The number of sub-modules depend on the voltage levels required of the application

purpose.

Figure 7: Conduction path of MMC

The sub-module can be operated to output Vdc or Zero volts. There are two operating modes namely

Enabled and Bypassed. During Enabled condition the output of sub module will output +Vdc/-Vdc and



ISSN NO: 2394-2886

Page No: 522

zero at bypass. The bypass mode will pass the voltage generated by other submodules connected serially.

The bypass mode can be configured either by activating upper side or lower side switches.

Figure 8: a) Full Bridge MMC circuit diagram, (b) discrete model and (c) continuous model. (d) submodule half-bridge topology, (c)

circulating current control block diagram, (e) series repetitive controller (RC) block diagram, (f) parallel repetitive controller (RC) block

diagram



ISSN NO: 2394-2886

Page No: 523

Consider MMC has to generate multilevel output in H-bridge cascaded multilevel converter

Vout =∑(i=1..N) Vcell

Where,

Vout = Converter output

Vcell = Output voltage of cell

Let assume N number of modules in each leg of the converter. There are (2N+1) Voltage levels. If N=1, 3level converter, N=2,

5level converter, N=3, 7level converter.

This paper addresses the circuit analysis, circulating current and modulation techniques.

Circulating current control:

It can be noted from figure above that common mode and different mode currents affects the performance of the MMC.

The circulating currents tend to circulate across the sub modules and not noticed at the output due to the potential difference

across the sub modules. It can be nullified by applying differential voltage.

Figure 9: Circulating Current control Strategy for MMC

III. COMPARISON OF THD SIMULATION RESULTS FOR VARIOUS PWM TECHNIQUE

There are various PWM study has been carried out in the field of power electronics to overcome the drawbacks of existing

modulation topology and enhance the switching methods w.r.t load and power quality of the outputs. For comparison study the

test results of different PWM technique has been analyzed and referred from various research work carried out in the recent

publications. Table 2 show the comparison study of three PWM topology namely PD, POD and APOD technique. The study

has been published in journal for Electrical & Computer Engineering: An International Journal (ECIJ) Volume 4, Number 3,

September 2015 and it is taken for assessment [15].

Table 2 : MMC PWM Technique - THD Comparison Chart



ISSN NO: 2394-2886

Page No: 524

S.No PWM Technique No of

Level

THD %

1 Alternate Phase

opposition

Disposition

3 Level 54.17%

2 Phase Opposition

Disposition

3 Level 54.17%

3 Phase Disposition 3 Level 52.06%

4 Alternate Phase

opposition

Disposition

5 Level 37.13%

5 Phase Opposition

Disposition

5 Level 26.96%


7 Alternate Phase

opposition

Disposition

7 Level 25.20%

8 Phase Opposition

Disposition

7 Level 22.48%


10 Alternate Phase

opposition

Disposition

9 Level 17.10%

11 Phase Opposition

Disposition

9 Level 17.08%




ISSN NO: 2394-2886

Page No: 525

An IEEE conference paper published on 2015 [16] has presented a study on THD parameters for 5level Modular Multilevel

converter topology inverter. This research was carried on using constant D.C voltage source for comparison of PWM

techniques: PS-PWM, PD-PWM, POD-PWM and APOD-PWM techniques, the assessment is basically targeted on total

harmonics distortion and shown in figure 10-12. The harmonic level is less than 21% for the filtered output voltages.

Figure 10: FFT of 5-Level MMC using APODPWM

(THD = 20.41%)

Figure 11: FFT output of 5-Level MMC using APODPWM (THD = 15.24%)



ISSN NO: 2394-2886

Page No: 526

Figure 12: FFT output of 5-Level MMC using APODPWM (THD = 16.84%)

An IEEE International Power Electronics conference paper published on 2014 [17] has presented a comparison study on Phase

shifted and level shifted PWM technique. The principle behind PD-PWM is that the number of carriers is equal to the number

of sub modules per arm and the carriers ae in phase to each other. Interleaving the lower and upper arm can be achieved by

phase shifting the carrier by 180 degree. PS-PWM also need N-carriers per arm. Therefore, the carriers conceal the entire

modulation index with consecutive carrier phase shifted by 2π/N. Despite of these the interleaving carrier operation provides

better performance compared to the non-interleaved PWN techniques. Whereas the interleaved PD-PWM demonstrates better

line-line common mode voltage as compared to PS-PWM in terms of total harmonic distortion.

Table 3 : MMC Simulation results for interleaving and Non-interleaving technique

Modulation technique Fc (Hz) %THD Fsw (Hz)

PD-PWM (Non- Inter.) 3000 5.98 345

PS-PWM (Non-Inter.) 300 8.77 343

PS-PWM+S(Non-Inter.) 300 8.76 340

PD-PWM (Interleaving) 3000 4.33 345

PS-PWM (Interleaving) 300 4.38 350

PS-PWM+S(Interleaving) 300 4.37 350



ISSN NO: 2394-2886

Page No: 527

Figure 13: FFT spectrum of line to line common voltage interleaving and non-interleaving cases

IV. CONCLUSION

In this paper different carrier based PWM techniques has been considered and operation of MMC is discussed. PWM

technique study involves Phase Opposition Disposition (POD) PWM, Carrier Overlapping PWM (COPWM), Phase Shift PWM

(PSPWM), In-Phase Disposition (PD) PWM, Alternative Phase Opposition Disposition (APOD) PWM, Variable Frequency

PWM (VFPWM). Few research paper data results have been presented to understand the harmonics of different converters

levels to be used and it is illustrated that Phase Disposition and interleaving technique has reduced harmonic content comparing

to other PWM techniques presented in this study.

REFERENCES [1] R. Marquardt, "Modular multilevel converter: An universal concept for HVDC-networks and extended dc-bus-applications." in

Proc. IEEE 1PKC. 2010, pp. 502-507. [2] R. Marquardt. "Modular multilevel converter topologies with DC-short circuit current limitation." in Proc. IEEE ICPE 2011,

pp. 1425-1431. [3] W. Jun. R. Burgos, and D. Boroyevich, "A survey on the modular multilevel converters - modeling, modulation and controls." in

Proc. IKKE ECCE.2013, pp. 3984-3991. [4] M. Vasilopitas and A. Rufer. "Analysis and control of modular multilevel converters with integrated battery energy storage" IEEE

Trans. Power Electron., early access. 2014, doi: 10.1109/TPEL.2014.2303297. [5] J. W. Moon. C. S. Kim. J. W. Park, D. W. Kang. and J. M. Kim. "Circulating current control in MMC under the unbalanced

voltage," IEEE Trans. Power Del. vol. 28, pp. 1952-1959, Oct. 2013. [6] D. G. Holmes and T. A. Lipo. Pulse width modulation for power converters; principles and practice. IEEE press series on power

engineering. 2003. [7] S. Fan. K. Zhang, J. Xiong, and Y. Xue, "An improved control sys-temfor modular multilevel converters with new

modulation strategy and voltage balancing control." IEEE Trans. Power Electron., vol. doi: 10.1109/TPEL.2014.2304969, 2014. [8] C.Nagarajan and M.Madheswaran - ‘Experimental Study and steady state stability analysis of CLL-T Series Parallel

Resonant Converter with Fuzzy controller using State Space Analysis’ - Iranian Journal of Electrical and Electronic Engineering, Vol.8 (3), pp.259-267, September 2012.

[9] R- Dams, J. Pou, G. Konstantinou, S. Ceballos, and V. G. Agelidis. "Circulating current control and evaluation of carrier dispositions in modular multilevel converters." in Proc. IEEE ECCE Asia. Jun. 2013, pp. 332-338.



ISSN NO: 2394-2886

Page No: 528

[10] C.Nagarajan and M.Madheswaran - ‘Performance Analysis of LCL-T Resonant Converter with Fuzzy/PID Using State Space Analysis’- Springer, Electrical Engineering, Vol.93 (3), pp.167-178, September 2011.

[11] G. Konstantinou and V. G. Agelidis, "Performance evaluation of half-bridge cascaded multilevel converters operated with multicarrier sinusoidal PWM techniques." in Proc. IEEE ICIEA, 2009. pp. 3399-3404.

[12] G. Konstantinou, M. Ciobotaru. and V. G. Agelidis, "Analysis of multi -carrier PWM methods for back-to-back HVDC

systems based on modular multilevel converters," in Proc. IEEE IECON. 2011, pp. 4391-4396. [13] M. Hagiwara, and H. Akagi, "Control and experiment of pulse width-modulated modular multilevel converters," IEEE

Trans. Power Electron. vol. 24. pp. 1737-1746. Jul. 2009. [14] G. Konstantinou, M. Ciobotaru. V. G. Agelidis, "Selective harmonic elimination pulse width modulation of the modular

multilevel converter," IET Power Electronics, Vol. 6. No. 1. pp., Jan. 2013. [15] Sourabh Rathore, Mukesh Kumar Kirar and S. K Bhardwaj “Electrical & Computer Engineering: An International Journal

(ECIJ) Volume 4, Number 3, September 2015”. [16] C.Nagarajan and M.Madheswaran - ‘Stability Analysis of Series Parallel Resonant Converter with Fuzzy Logic Controller

Using State Space Techniques’- Taylor & Francis, .Electric Power Components and Systems, Vol.39 (8), pp.780-793, May 2011

[17] Rosheila Darns, Georgios Konstantinou, Josep Pou, Salvador Ceballos and Vassilios G. Agelidis “Comparison of Phas e-Shifted and Level-Shifted PWM in the Modular Multilevel Converter” 2014 International Power Electronics Conference .



ISSN NO: 2394-2886

Page No: 529

Suraj Punj Journal For Multidisciplinary Research ISSN NO ...

Documents

Transcript of Suraj Punj Journal For Multidisciplinary Research ISSN NO ...