Format Template Vol. 4, No. 1 | Januay - June 2021 SJET | P-ISSN: 2616-7069 |E-ISSN: 2617-3115 | Vol. 4 No. 1 January – June 2021 67 Analysis of Harmonic Distortion Reduction through Modular Multi-Level Inverter using Nearest Level Modulation (NLM) Control Strategy Syed M. Baqar Shah1, Mazhar Hussain Baloch1, Amir Mahmood Soomro2 Shafqat Hussain Memon1, Dur Muhammad Soomro3 Abstract: The research paper presents the novel control strategy of Modular Multilevel Converter (MMC) to reduce Total Harmonic Distortion (THD) losses by the implementation of Nearest Level Modulation (NLM) control technique. However, the MMC strategy reduces the THD in comparison with the conventional converters which results in improvement of power quality. In addition to this, the MMC has been designed with higher scalability with high voltage and power capacity. Sub-module is an integral part of MMC which is built up as an identical and controllable part of it. This type of converter is also known as a controllable voltage source (CVS) converter. Many researchers have conducted a detailed review regarding control methods and necessary operations applied to MMC-based systems which can be applied for High Voltage Direct Current (HVDC), particularly focusing to reduce the THD. Power converters use many modulation techniques, but the existing techniques contribute to major switching losses. However, in this paper authors discussed in detail about MMC up to 49 levels, by implementing the NLM technique, it is showed that the MMC offers simplicity and good controllability in the THD reductio. In this work, the reduction of THD by increasing the voltage levels in MMC is comprehensively analyzed. The simulation results obtained through Matlab/Simulink are used to analyze the effectiveness of the proposed control strategy for stable operation of MMC used in HVDC applications. Keywords: MMC; NLM; THD; Power Quality Analysis; HVDC; MATLAB/Simulink 1Department of Electrical Engineering, MUET SZAB Campus Khairpur Mir’s, Sindh, Pakistan 2Department of Electrical Engineering, MUET main Campus Jamshoro, Sindh, Pakistan 3Department of Electrical Engineering, UTHM, Malaysia. Corresponding Author: mazharhussain@muetkhp.edu.pk _________________________________ 1. Introduction The Modular Multilevel Converter (MMC) belongs to the multilevel converters’ family [1- 3]. It has attracted substantial interest owing to its various unique features including multiple output voltage levels, higher flexibility, and modular design scalability, capacitor-less dc- link as well as transformer-less operations [3- 4]. The MMC has become the futuristic and preferred choice for HVDC applications due to having high redundancy, least cost, lower size of the filter, and low power semiconductor losses. The circuit topology of MMC is shown in Fig. 1. The inverter is one of the types of power electronic converters which converts Direct Current (DC) power supply to Alternating Current (AC) waveform. Inverters are usually used in solar panels and other sorts of devices to use DC power into powering mailto:mazharhussain@muetkhp.edu.pk Syed M. Baqar Shah (et al.), Analysis of Harmonic Distortion Reduction through Modular Multi-Level Inverter using Nearest Level Modulation (NLM) Control Strategy (pp. 67-79) Sukkur IBA Journal of Emerging Technologies - SJET | Vol. 4 No. 1 January – June 2021 68 some useful AC loads. The output of the inverter depends upon its type which is being used. It can be a square wave, quasi-square- wave, or sine-wave inverter. Even if a sine- wave inverter is being used, its output is not purely sinusoidal as it contains some harmonics. These harmonics are the components of a periodic wave that are multiple of the fundamental frequency and produce distortion in the output [4-5]. Fig. 1: Circuit Topology These harmonic distortions can be minimized by using a suitable modulation technique or by using the suitable converter topology. Modular Multilevel Converters involves large number of levels with higher efficiency and lesser harmonic distortions [4] [5]. They can manage high voltage operations without involvement series-connecting switching devices. They provide lower common-mode voltages and higher power quality. They offer various advantages i.e. high modularity & scalability, transformer-less operation, lesser switching losses, and lower filtering cost [6]. 2. Modular Multilevel Converter It includes arms consisting of the upper and lower arm. Each arm has submodules (SM), and each is linked in the series [6] [7]. The upper arms and lower arms are connected to an inductor and a resistor to control fault current due to the potential difference that appeared in upper arms or lower arms [8]. The capacitor is placed in each submodule while each submodule consists of a half-bridge circuit to get a particular desired voltage level [9]. Also, each switch involves a diode in parallel to it intending to control the current flow. The working principle and generation of output levels of MMC can be understood by applying KVL to the basic structure of the circuit as shown in Fig. 2. Fig. 2: Basic Structure of MMC Where, Vdc is a supplied DC voltage, Vm [1-4] is a voltage across submodules and Vo is the output voltage. By neglecting arm inductance and assuming the capacitors as DC sources, KVL is applied on upper and lower arms rspectively. For the upper arm: 𝑉𝑜 = 𝑉𝑑𝑐 2 − 𝑉𝑚1 − 𝑉𝑚2 (1) For the lower arm: 𝑉𝑜 = − 𝑉𝑑𝑐 2 + 𝑉𝑚3 + 𝑉𝑚4 (2) The voltage Vo = Vdc/2 can be calculated by bypassing two modules at the upper top and linking both modules in the lower loop. The potential of Vo = -Vdc/2 can be achieved by connecting both upper modules and bypassing two lower modules [10]. Similarly, to achieve zero levels, bypass one module from both the upper and lower loops. This can be achieved in four different ways. In this way, the output Syed M. Baqar Shah (et al.), Analysis of Harmonic Distortion Reduction through Modular Multi-Level Inverter using Nearest Level Modulation (NLM) Control Strategy (pp. 67-79) Sukkur IBA Journal of Emerging Technologies - SJET | Vol. 4 No. 1 January – June 2021 69 voltage levels are generated corresponding to each loop. The submodule operation has three states; each is described in Table I. TABLE I: Summary of the Operation of SMs 2.1. Cell Structure The literature proposes different types of submodule configurations. Following are the most well-known cell configurations: Half-bridge submodule cell is the basic and the most common cell configuration in all SMs configurations due to its only two switching devices. If the upper switch S1 is operating its conduction, SM is in the “ON” state. Interchangeably, if the lower switch S2 is operating its conduction, SM remains in an “OFF” state. The output voltage of a cell is equal to SM capacitor voltage that depends on the switching operation of S1 and S2 as shown in Fig. 3. Fig. 3: Half-bridge SM Cell Full-bridge submodule (FBSM) cell configuration contains four switching devices, and its output voltage is equal to the voltage of the capacitor, which depends on the switching operation of switching devices, as shown in Fig. 4. Fig. 4: Full-bridge SM Cell Clamp-double SM (CDSM) configuration has two half-bridge SMs, one extra transistor with connected diodes, and additional diodes, as shown in Fig. 5. The switch S5 remains always in the “ON” state, and this configuration operates like two series-connected half-bridge SMs. Fig. 5: Clamp-double SM Cell Clamp-single SM (CSSM) consists of three switching devices with its parallel- connected diodes, a capacitor, and an extra diode, as shown in Fig. 6. States SM 1 SM 2 D1 D2 Capacito r Blockin g State OFF OFF ON OF F OF F ON Inserted Not Inserted Cut-in State ON OFF - - Inserted Cut-off State OFF ON - - Not Inserted Syed M. Baqar Shah (et al.), Analysis of Harmonic Distortion Reduction through Modular Multi-Level Inverter using Nearest Level Modulation (NLM) Control Strategy (pp. 67-79) Sukkur IBA Journal of Emerging Technologies - SJET | Vol. 4 No. 1 January – June 2021 70 Fig. 6: Clamp-Single SM Cell Apart from these SM topologies, many topologies of SMs are also present in literature [9] & [12]. In this paper, the most common and basic building block, Half-bridge configuration, is discussed in the rest of the section. 3. Modulation Technique Modulation technique plays a vital role in the proper working, performance and efficiency f the converter. By using different PWM techniques, the levels of output can vary as per the desired requirement and application [11] [12]. The techniques of sinusoidal PWM are very common and used as control techniques, and they are additionally categorized into Level-Shifted and Phase- Shifted PWM techniques. Other variations of the level-shifted PWM techniques include In- Phase Deposition (IPD), Phase-Opposite Deposition (POD), and Alternate-Phase Opposite Deposition (APOD). These techniques use sine wave and different triangular waves, these are then compared to generate different levels of output voltages [13]. As there are many waves compared so this contributes to more switching losses. The Nearest Level Modulation (NLM) technique is an alternative method for carrier- based modulation techniques that utilizes more waves [14]. It has the advantage of being simple for implementation. It was introduced mainly for large-drive multilevel systems, but it can also be used for MMC-based HVDC applications as it is more flexible and provides easy digital implementation when the converter is operating at a higher number of levels. It is used to avoid the triangular-carrier waves and directly computes the states of switching and duty cycles [15] [16]. The main idea behind this is to sample the reference at frequency FS and then approximate it to the nearest level which results in the natural fundamental switching frequency with fewer switching losses. For an MMC of N sub-modules per arm, NLM produces an N+1 output voltage level. If the converter operates at a higher frequency it will generate a better reference signal in terms of approximation. Now the reference waveform becomes a staircase, and the lower levels are used longer than the higher ones and hence this leads to the unbalance of capacitor voltage [17]. Therefore, NLM is not suitable for directly assigning to the sub-modules, instead, it requires a sorting algorithm before it is fed to the converter to ensure submodule energy balance. Fig. 7 shows the block diagram and the working of NLM. The block diagram shows that at first the reference voltage is normalized with the capacitor voltage of the submodule in the gain block then the round-function generates the closest integer number of the submodules to insert to approximate the reference voltage with the nearest voltage level [17] [18]. Fig. 7: Block Diagram & working Principle of NLM. Syed M. Baqar Shah (et al.), Analysis of Harmonic Distortion Reduction through Modular Multi-Level Inverter using Nearest Level Modulation (NLM) Control Strategy (pp. 67-79) Sukkur IBA Journal of Emerging Technologies - SJET | Vol. 4 No. 1 January – June 2021 71 4. 49 Levels (H-Bridge) Modular Multilevel Converter The MATLAB model of a three-phase circuit of MMC is shown in Fig. 8. RL load is connected at the grid side (AC side). The number of levels is varied to analyze the impact on the Total Harmonic Distortion (THD) and on the smoothness of output waveform. THD loss can be mitigated by inserting submodules in the converter [19]. Submodules are increased up to 49 output voltage levels. Hence, THD loss mitigation mainly contributes in power quality improvement [20]. The results are presented in the comparative analysis section for different levels including 22, 28, 33, 41, and 49 levels with RL load. 5. Results And Discussion Comparative result analysis of multiple levels for (H-Bridge) MMC is done by using MATLAB/Simulink software (2018b). The output voltages and currents are compared concerning Total Harmonic Distortion for 22, 28, 31, 41, and 49 levels. 5.1. Case 1: 22 Levels In this case, 22 level MMC, the output voltage & THD are shown in Fig. 9 & 10 respectively, and the output current & THD are shown in Fig. 11 & 12 respectively. At this stage, output voltage and current waveforms are not sinusoidal, they contain harmonics and the value of voltage harmonics is 2.58% with a fundamental frequency of 50 Hz. And the current THD is 0.94%. 5.2. Case 2: 49 Levels In this case, MMC is increased up to 49, the output voltage and current waveforms become very smooth and closer to sinusoidal. The total harmonics distortion is reduced to a certain value. The output voltage and THD are shown in Fig. 13 & 14 respectively, and the output current & THD are shown in Fig. 15 & 16 respectively. The drastic reduction in voltage and current THD can be seen in the waveforms. Almost 56% of THD is reduced by an increasing number of levels. This also fulfils IEEE 519 standard about harmonics Current and Voltage limits. The comparative result analysis has showed that 49 levels of MMC is having lesser THD in the output as compared with other MMC levels such as 22, 28, 31, and 41. As the number of levels increases, it means more submodules are being inserted and the converter produces a highly sinusoidal waveform at its output side. More levels contribute to decreasing instantaneous error, the converter generates the desired waveform, which is very close to the sinusoidal waveform. The summarized results of the comparative analysis are given in Table II and Fig 1. Syed M. Baqar Shah (et al.), Analysis of Harmonic Distortion Reduction through Modular Multi-Level Inverter using Nearest Level Modulation (NLM) Control Strategy (pp. 67-79) Sukkur IBA Journal of Emerging Technologies - SJET | Vol. 4 No. 1 January – June 2021 72 Fig. 8: Three-Phase MMC based Circuit Model Fig. 9: Three-Phase Output Voltage for 22-Level MMC with RL load (2.58% THD) Syed M. Baqar Shah (et al.), Analysis of Harmonic Distortion Reduction through Modular Multi-Level Inverter using Nearest Level Modulation (NLM) Control Strategy (pp. 67-79) Sukkur IBA Journal of Emerging Technologies - SJET | Vol. 4 No. 1 January – June 2021 73 Fig. 10: Three-Phase Output Voltage THD for 22-Level MMC with RL load Fig. 11: Three-Phase Output Current for 22-Level MMC with RL load (0.94% THD) Syed M. Baqar Shah (et al.), Analysis of Harmonic Distortion Reduction through Modular Multi-Level Inverter using Nearest Level Modulation (NLM) Control Strategy (pp. 67-79) Sukkur IBA Journal of Emerging Technologies - SJET | Vol. 4 No. 1 January – June 2021 74 Fig. 12: Three-Phase Output Current THD for 22-Level MMC with RL load Fig. 13: Three-Phase Output Voltage for 49-Level MMC with RL load (1.13% THD) Syed M. Baqar Shah (et al.), Analysis of Harmonic Distortion Reduction through Modular Multi-Level Inverter using Nearest Level Modulation (NLM) Control Strategy (pp. 67-79) Sukkur IBA Journal of Emerging Technologies - SJET | Vol. 4 No. 1 January – June 2021 75 Fig. 14: Three-Phase Output Voltage THD for 49-Level MMC with RL load Fig. 15: Three-Phase Output Current for 49-Level MMC with RL load (0.27% THD) Syed M. Baqar Shah (et al.), Analysis of Harmonic Distortion Reduction through Modular Multi-Level Inverter using Nearest Level Modulation (NLM) Control Strategy (pp. 67-79) Sukkur IBA Journal of Emerging Technologies - SJET | Vol. 4 No. 1 January – June 2021 76 Fig. 16: Three-Phase Output Current THD for 49-Level MMC with RL load TABLE II. Summarized results of the analysis of THD Submodules (Upper/lower arm) Levels Voltage THD Current THD 7 8 8.11% 5.36% 21 22 2.58% 0.94% 27 28 2% 0.64% 32 33 1.69% 0.49% 40 41 1.35% 0.34% 48 49 1.13% 0.27% Syed M. Baqar Shah (et al.), Analysis of Harmonic Distortion Reduction through Modular Multi-Level Inverter using Nearest Level Modulation (NLM) Control Strategy (pp. 67-79) Sukkur IBA Journal of Emerging Technologies - SJET | Vol. 4 No. 1 January – June 2021 77 GRAPH I: Summarized results of the analysis of THD 6. Conclusion It is concluded for obtained simulation results that Modular Multilevel Converter with H-bridge SM configuration produces less Total Harmonics Distortion (THD) when it is used with Nearest Level Modulation (NLM) control technique. It is further, analyzed that (MMC) through the NLM Modulation technique is found better in achieving very minimum THD as compared to the conventional inverters/ converters. However, in this work, the THD is reduced to 0.02% approximately with increased levels up to 49, which results in mitigation of harmonics distortion and improved voltage and current waveforms at the output side. MMC leads to various applications where the proposed MMC-based HVDC structure can be used to replace conventional power networks, particularly for HVDC systems. AUTHOR CONTRIBUTION All authors contributed equally to the work. DATA AVAILABILITY STATEMENT CONFLICT OF INTEREST The authors declare no conflict of interest. FUNDING Not applicable. ACKNOWLEDGMENT Not applicable. REFERENCES [1] Beheshti, N., Rezanejad, M., & Mehrasa, M. “Linearized control technique with Lyapunov function-based compensators for MMC-based HVDC system under load variation and fault condition”, International Journal of Electrical Power Syed M. Baqar Shah (et al.), Analysis of Harmonic Distortion Reduction through Modular Multi-Level Inverter using Nearest Level Modulation (NLM) Control Strategy (pp. 67-79) Sukkur IBA Journal of Emerging Technologies - SJET | Vol. 4 No. 1 January – June 2021 78 & Energy Systems, Vol. 124, 106333. doi:10.1016/j.ijepes.2021. 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IOP Publishing, 2018. https://www.researchgate.net/profile/Mahendar_Kumar5/publication/326449978_An_Overview_of_Uninterruptible_Power_Supply_System_with_Total_Harmonic_Analysis_Mitigation_An_Experimental_Investigation_for_Renewable_Energy_Applications/links/5b4e374545851507a7a99294/An-Overview-of-Uninterruptible-Power-Supply-System-with-Total-Harmonic-Analysis-Mitigation-An-Experimental-Investigation-for-Renewable-Energy-Applications.pdf https://www.researchgate.net/profile/Mahendar_Kumar5/publication/326449978_An_Overview_of_Uninterruptible_Power_Supply_System_with_Total_Harmonic_Analysis_Mitigation_An_Experimental_Investigation_for_Renewable_Energy_Applications/links/5b4e374545851507a7a99294/An-Overview-of-Uninterruptible-Power-Supply-System-with-Total-Harmonic-Analysis-Mitigation-An-Experimental-Investigation-for-Renewable-Energy-Applications.pdf https://www.researchgate.net/profile/Mahendar_Kumar5/publication/326449978_An_Overview_of_Uninterruptible_Power_Supply_System_with_Total_Harmonic_Analysis_Mitigation_An_Experimental_Investigation_for_Renewable_Energy_Applications/links/5b4e374545851507a7a99294/An-Overview-of-Uninterruptible-Power-Supply-System-with-Total-Harmonic-Analysis-Mitigation-An-Experimental-Investigation-for-Renewable-Energy-Applications.pdf https://www.researchgate.net/profile/Mahendar_Kumar5/publication/326449978_An_Overview_of_Uninterruptible_Power_Supply_System_with_Total_Harmonic_Analysis_Mitigation_An_Experimental_Investigation_for_Renewable_Energy_Applications/links/5b4e374545851507a7a99294/An-Overview-of-Uninterruptible-Power-Supply-System-with-Total-Harmonic-Analysis-Mitigation-An-Experimental-Investigation-for-Renewable-Energy-Applications.pdf https://www.researchgate.net/profile/Mahendar_Kumar5/publication/326449978_An_Overview_of_Uninterruptible_Power_Supply_System_with_Total_Harmonic_Analysis_Mitigation_An_Experimental_Investigation_for_Renewable_Energy_Applications/links/5b4e374545851507a7a99294/An-Overview-of-Uninterruptible-Power-Supply-System-with-Total-Harmonic-Analysis-Mitigation-An-Experimental-Investigation-for-Renewable-Energy-Applications.pdf https://www.researchgate.net/profile/Mahendar_Kumar5/publication/326449978_An_Overview_of_Uninterruptible_Power_Supply_System_with_Total_Harmonic_Analysis_Mitigation_An_Experimental_Investigation_for_Renewable_Energy_Applications/links/5b4e374545851507a7a99294/An-Overview-of-Uninterruptible-Power-Supply-System-with-Total-Harmonic-Analysis-Mitigation-An-Experimental-Investigation-for-Renewable-Energy-Applications.pdf Syed M. 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