UNIVERSITI TENAGA NASIONAL

COLLEGE OF ENGINEERING

BACHELOR IN ELECTRICAL AND ELECTRONICS ENGINEERING (BEEE)

2015

`

UMAH GHANTEN A/L REVI CHANDER

EE089909

"Modeling and Analysis of Modular Multilevel Converter Based STATCOM for Smart Grid"

Supervisor: Toh Chuen Ling, Dr.

Date of Submission: 1/12/2015

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CONTENTS

Description Page number

1. Introduction 3-4

2. MMC model topology build up 5-9

3. Simulation results and analysis 10-11

4. Reference 12

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1.0 Introduction

The framework of the progress report 3 is to discuss LS PWM method and control technique used for MMC topology for this project . The procedure also involves devising the optimal circuit parameters. Then, the design is verified by means of detailed MATLAB SIMULINK simulations. Several power ratings are considered to provide a thorough performance evaluation of the designed system. Carrier based PWM techniques for MMC are widely studied [1]-[3]. There are several studies that consider carrier based PWM methods for MMC switching. One of the earliest studies [3], introduces LS PWM methods and compares their performances. Carrier based PWM methods comparisons for specific ma (amplitude modulation index) and mf (carrier to fundamental frequency ratio) values are given in [4] and [5]. Another work [1], also rates sinusoidal PWM methods, for specific ma and mf values in terms of harmonics performances. The switching methods for modular multilevel converters are illustrated in Figure 1.1.

Figure 1.1 : Switching methods for modular multilevel converters

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1.1 Level-shift (LS) (Sub-Harmonic) Methods

These methods require N identical triangular carriers being displaced contiguously in the whole dc-link voltage; Vdc. In order to provide a balanced exploitation of circuit elements that create different voltage levels, peak-to-peak amplitudes of the carriers are set equal to each other, Vdc/N, which is a necessary but not a sufficient condition. They have frequency of fc. Carriers do not cross. Depending on the phase-shift of carriers with respect to each other, level-shift methods branch into three different sub-methods: phase disposition (PD), phase opposition disposition (POD) and alternative phase opposition disposition (APOD)[6].

1.2 Phase Disposition (PD) Method

The MMC topology in this project uses phase disposition method. All the carriers are in phase. For an MMC having 3 submodules per phase arm (N=3), carriers of PD method are displaced in the Vdc band as illustrated in Figure 1.2 was taken from MATLAB SIMULINK simulation result.

Figure 1.2: Result of PD method gained from equation 1/3[(u(t)+0),(u(t)+1/3),(u(t)+2/3)].

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2.0 MMC model topology build up

MMC block was constructed in MATLAB SIMULINK. The construction of the model started from one Sub-module unit then went on to the final construction of a full working simulation model. The process of build up is shown below.

The model was developed by using a 7-level shifting PWM method and it consists of 6 Sub-Module units as it follows the formula of (N sub-module * 2 +1) to enable a efficient working model and the model use parameters that was compared and taken from several conference papers to get a accurate results[6]. The real hardware model of a MMC model’s sub-module unit will have a capacitor but it was later replaced by a voltage source as shown in figure 2. The purpose of the replacement is in a real hardware the capacitor will be precharged but in a simulation model it cant be recharged, under circumstance a replacement with voltage source will get the model to give a better result. Figure 2.1 shows a subsytem selection that will create a mask for the sub-module unit(SM unit), this modification will make the circuit block more neat. This is a 7-level shifting PWM based MMC model, hence it have 3 SM unit in its upper arm and 3 SM unit at lower arm that totals up to 6 SM unit that is shown in figure 2.2 and figure 2.3. Figure 2.4 is a single phase MMC model topology and it was modified to a three phase MMC model topology as shown in figure 2.5[7].

Figure 2 : One Sub-Module unit that was modified 10V2 voltage source

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Figure 2.1 : Subsystem block that creates a mask for the Sub-module unit at figure 2.

Figure 2.2 : Upper arm which consist of 3 SM units.

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Figure 2.3 : Lower arm which consist of 3 SM units

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Figure 2.4 : Single phase MMC model topology without load

Figure 2.5 : Three phase MMC model topology without load

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3.0 Simulation results and analysis

3.1 Level-shift Pulse Width Modulation(LS PWM) with Phase Disposition(PD) Method

PD method has been implemented with two different carrier sets for upper and lower arms in order to construct the phase voltage in N+1 level. The sine wave was generated inside a discrete PWM generated and it acts as reference signal. Two sine waves were injected to the circuit , to the upper arm and the other sine wave was inverted by adding a gain for the lower arm to make a succesful modulation. The sine waves were modified by 0.5*(u(1)+1) equation to ensure the sine waves fits perfectly in between the carrier waves and this will give an effective PWM cycle period. The carrier wave is a triangle wave which was level-shifted by the equations of 1/3[(u(t)+0),(u(t)+1/3),(u(t)+2/3)] which represents 7-level shift. The modulation was taken successfully from the MATLAB SIMULINK simulation and it is shown in figure 3.1[7].

Figure 3.1 : Shows a successful modulation occured at upper arm and lower arm and the two sine waves sits perfectly between the carrier waves. The three different colours represents the 3 different level positioning of one carrier wave.

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3.2 The simulation result and parameters of MMC model single phase topology

The simulation results shown in figure 3.2 justifies the MMC model single phase is working in MATLAB SIMULINK simulation platform. The first graph of sine wave

shows that the DC source is converted to AC source. The second graph shows voltage wave that have levels, this shows that the circuit using a level-shifted PWM by phase disposition method[7]. Table 1 shows the parameters used to get the simulation results.

Figure 3.2 : sine wave AC source and voltage wave that have 7 level

Table 1 : Parameters of the MMC setup[7]

Rated power 238WNumber of SMs per arm 3 unitsPer SM voltage 30 VArm inductance 1.3mhDC link voltage 90 VRL load R= 5Ω , L= 8mh

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Number of output volatge levels 7Switching frequency 2Khz

4.0 Reference

1. G. S. Konstantinou, and V. G. Agelidis, "Performance evaluation of half-bridge cascaded multilevel converters operated with multicarrier sinusoidal PWM techniques," Industrial Electronics and Applications, 2009. ICIEA 2009. 4th IEEE Conference on, pp.3399-3404, 25-27 May 2009.

2. A. Hassanpoor, S. Norrga, H. Nee, and L. Angquist, "Evaluation of different carrier based PWM methods for modular multilevel converters for HVDC application," IECON 2012 - 38th Annual Conference on IEEE Industrial Electronics Society, pp.388-393, 25-28 Oct. 2012.

3. G. Carrara, S. Gardella, M. Marchesoni, R. Salutari, and G. Sciutto, "A new multilevel PWM method: a theoretical analysis," Power Electronics, IEEE Transactions on, vol.7, no.3, pp.497-505, Jul 1992.

4. B. P. McGrath, and D. G. Holmes, "A comparison of multicarrier PWM strategies for cascaded and neutral point clamped multilevel inverters," Power Electronics Specialists Conference, 2000. PESC 00. 2000 IEEE 31st Annual, vol.2, pp.674-679, vol.2, 2000.

5. B. P. McGrath, and D. G. Holmes, "Multicarrier PWM strategies for multilevel inverters," Industrial Electronics, IEEE Transactions on, vol.49, no.4, pp.858-867, Aug 2002.

6. S. Debnath, Q. Jiangchao, B. Bahrani, M. Saeedifard, and P. Barbosa, "Operation, Control, and Applications of the Modular Multilevel Converter: A Review," Power Electronics, IEEE Transactions on, vol.30, no.1, pp.37- 53, Jan. 2015.

7. C. L. Toh and L. E. Norum, “Implementation of redundancy control by degrading voltage level in modular multilevel converter,” 2014 IEEE Conference on Energy Conversion (CENCON).

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