A single-phase simplified DC-AC converter using DC-link capacitors and an H-bridge

Sai Divya Sindhura Nunna, Akhilesh Ketha, Srivastav Sai Goud Padamat, K. Rambabu, Ujwala Anil Kshirsagar, Abhilash Tirupathi Department of Electrical and Electronics Engineering, Aditya Engineering College, India Department of E&TC, Symbiosis Institute of Technology, Symbiosis International (Deemed University), (SIU), Pune, Maharashtra, India Department of Electrical Engineering, Accendere Knowledge Management Services, CL Educate Ltd., India


INTRODUCTION
Single-phase DC-AC converters are predominant in several industrial and household applications like lathe machines, centrifugal pumps, uninterrupted power supplies, etc. Multilevel single-phase DC-AC converters are highly attractive than two-level inverters due to the advantages of higher power rating, improved power quality and higher reliability. In this context, cascaded converters are highly flexible and modular in the family of multilevel inverters. In this group, "cascaded H-bridge (CHB)" converters [1]- [4] are the classical and traditional types. CHB converters have the advantages of equal voltage stress in symmetrical configurations, easy to add/remove the H-bridges to increase/decrease the voltage levels in the output. MLI technology is spreading to several areas such as AC drives, static reactive compensators, microgrid systems and renewable energy sources [5]- [8]. The "Flying capacitors clamped (FCC)", "neutral point clamped (NPC)" and "CHB converters" [9]- [11] are established as normal topologies in the MLI family. In this configurations, the device count increases exponentially in reference to the increased in the voltage levels of output. The requirement of unequal voltage ratings of the clamping diodes, unequal capacitor size and a greater number of dc sources puts limitations on these topologies. Several new MLI configurations with the intention of avoiding the drawbacks in the standard topologies were proposed in the literature for several applications [12]- [16]. In recent times, cascaded converters are attracting attention from industries as well as  [20]. In this configuration, the converter has fewer components and lower blocking voltage across the switching devices, resulting in lower costs. The remain papers are organised as; section 2 describes the operating models, section 3 describes the modulation principles that is used to produced the essential "output voltage waveforms", section 4 describes the simulation results that were obtained for different kinds of modulation indices that were used to validate the proposed converter, and section 5 concludes the paper.

PROPOSED INVERTER
The recommended converter is depicted in Figure 1. It is used in single-phase alternating current output voltage. It is referred to as an inverter. The converter makes use of eight bidirectional switching devices, one direct current source, and three direct current link condensers. Despite the fact that the switching devices are bidirectional, they only block voltage in single route due to the existence of anti-parallel diode in the circuit. The proposed architecture can make use of three different dc sources, which can be attained from battery banks, photovoltaic systems, or rectifier circuit, respectively. Figures 2 (a) and 2 (b) depict the switchings state at negative and positive zero's output voltage crossing, correspondingly. V0=0 + known as positive zero-crossing output voltage. V0=0represents an output voltage with a negative zero-crossing voltage. All switches are subjected to the same amount of switching states in order to maintain a increased temperature.   Figure 3 represents the process of the projected converters to produce a positive levels voltage transversely to the output's terminal. Figure 3 (a) generated V0=Vdc/3, in this working modal, the control semiconducting material used in devices for IGBTs S2, S4, S5 and S8 conduct. Figure 3 (b) produces V0=2Vdc/3, in these operating modes, the IGBTs S2, S3, S5 and S8 conduct. Figure 3 (c) shown the peak's voltage of V0=Vdc, in these operating modes, the IGBTs S1, S5 and S8 manner. The working methods of the converters used to croped the adverse output voltages levels are shown in Figure 4. Figure 4 (a) generated V0=-Vdc/3, in these operating models, the IGBTs S2, S4, S6 and S7 turn on. Figure 4 (b) provides the output voltage V0=-2Vdc/3, during this period the IGBTs S2, S3, S6 and S7 turn on. Figure 4 (c) stretches the output voltages V0=-3Vdc, throughout these periods the IGBTs S1, S6 and S7 turn on.

MODULATION TECHNIQUE
In order to make it easier to describe Table 1 shows the switch states at numerous output voltage in the proposed converter, while showing the switch states at several output voltage in the proposed converter. The digits 1 and 0 are used to signify the on and off states of the IGBTs in Figure 1, as shown in the accompanying table: Table 1 the switches are all correctly positioned. S5, S6, S7 and S8 are better due to their lower switching frequencies. Figure 5 shows the modulation technique [21]- [23]. The concept will use it to create a gate pulse for IGBTs in the suggested system. Six triangular waveforms are used to produce a pure sinusoidal waveform. Both the sinusoidal wave and the triangular wave are known as reference and carrier waves, respectively. The reference waves interact with each carrier wave at specific time intervals, 1, 2, 3, 1', 2', and 3'. This explains why the subsequent pulses are P1-P3 and N1-N3. The output voltage is generated by logical gate circuits that efficiently use these pulses. The total number of output levels is based on the modulation index (M.I.), that is described as: (1) Figure 5. Sine-triangle comparison method

SIMULATION RESULTS
The reproduction parameters [24], [25] are meant to result in an output voltage of 230 V and a frequency of 50 Hz, respectively. Table 2 contains a list of the extra parameters that will be considered in the simulation. The waveforms of the inverter current and output voltage for an M.I. of 0.9 and 0.6 are shown in Figures 6 (a) and 7 (a), respectively, for the inverter current and output voltage. For example, Figures 6 (a) and 7 (a) show the respective outputs voltage waveforms with currents, with the former representing seven levels and the latter representing five levels, respectively. As a result, when the M.I. is reduced, the peak voltage of the output will decrease as well. (V0peak).   Figures 6 (c) and 7 (c) represents each harmonics spectrum of the present output waveforms. At 0.9, the values of I0 peak is 4.9 A, whereas the total harmonic distortion (THD) is roughly 0.4%. The value of I0peak decreases in response to the voltage reductions in M.I. The amount of I0peak is disgunished as 3.2 A and the THD is around 0.6%. Figure 8 depicts the performance of the shown in configuration to withstand the sudden changes in load. It can be observed in Figure 8 that the projected techniques is effective. The circuit can be produced the undistorted output w.r.t changes in the load.

CONCLUSION
This article features the simplified DC-AC MLI configuration. When it comes to creating the requisite number of output voltage levels for the inverter while employing the fewest number of components possible, a tri-tier circuit and an H-bridge are the most efficient methods available. The analyses and operating modes that were used to achieve zero, positive, and negative levels were discussed in depth in order to achieve these results. Without regard to the fundamental frequency, H-bridge switches in the projected network topology have a "lower switching frequency" when compared to other switches. With the suggested architecture, switching losses are more controlled, and as a result, total efficiency is increased. A sinusoidal PWM approach generates the firing pulses required by employing the most efficient technique available at the time. The findings of the MATLAB/SIMULINK system have been confirmed for MI of 0.9 and 0.6, correspondingly. As can be observed from the FFT output spectrum, the THD in the waveforms of the system meet with the IEEE 1547 standard.