Design of a Controller Algorithm for Cascaded H- Bridge Multilevel Inverter System for Reduced THD and Improved Performance

Abstract

Multi-level inverter have huge advantages than the conventional inverter. This paper proposes 17 level cascaded multi-level inverter with minimum number of semiconductor switches and DC sources. A new controller is designed to trigger the switches of semiconductor devices to obtained the required output. To control the switching sequence of the metal oxide semiconductor field effect transistor (MOSFET) an embedded code is developed. 3- level, 5 -level ,17 -level and 27 -level cascaded H Bridge multi-level inverters are modelled on MATLAB/SIMULINK and Total Harmonic distortion (THD) of each level are compared to show the improved performance.

Introduction

I. INTRODUCTION

Multi-level inverter has various applications in the area of power industry due to its ability to reach higher voltage with producing lower harmonic content. The concept of multilevel inverter came into existence because of the demand for medium voltage and high power in many applications. There are different topologies in multi-level inverter namely Diode clamp, flying capacitor and cascaded H bridge multi-level inverter compared to all the above choosing cascaded multi-level inverter is better because of its arrangement containing a smaller number of semiconductor switches and also less harmonic distortion in the output. The major disadvantage of cascaded H bridge multi-level inverter (CHBMLI) is it requires a separate DC source for an individual H bridge cell which in turn increases the number of semiconductor switches that leads to complexity in the circuit, higher cost and increase in size. To overcome this issue asymmetric arrangement of CHBMLI is used. If DC source in the H bridge cell is of same magnitude, then the arrangement is called symmetrical CHBMLI. If each DC source magnitude is different in each H bridge cell it is called Asymmetric CHBMLI and this arrangement helps to increase the number of steps in the output voltage without adding more semiconductor devices. A new control algorithm is used in multilevel inverter for controlling the gating signals for various switching topologies employing embedded coding. According to the control algorithm CHBMLI for 3- level, 5-level ,17-level and 27- level is modelled in the MATLAB/SIMULINK. Total harmonic distortion in output current of various simulated multilevel inverter system were compared in this paper under RL load

II. SYSTEM CONFIGURATION AND OPERATING PRINCIPLE

M-level multilevel inverter using H bridge topology consists of (M - 1)/2 number of single-phase H-bridge cells, each having individual DC source. Total amplitude of voltage in the output is equal to the sum of synthesized voltage of each H bridge cell individually. Each H bridge cell produces three output voltages +Vdc, 0, and -Vdc. It can synthesize output voltage waveform that approaches sinusoidal shape. Fig. 1 shows the schematic diagram of three-level cascaded H bridge multi-level inverter. It consists of single H bridge cell that has four power switching devices (s1, s2, s3 and s4) and a single DC-source.

For the output-voltage v0 ? ?Vdc switches s1 and s4 should be turned ON. Either switch (s1, s2) or (s3, s4) should be turned ON for output-voltage v0 ? 0. For output-voltage v0 ? ?Vdc s2 and s3 should be turned ON. The output- voltage generated consists of three-levels. Table I shows the switching of devices for 3-level inverter.

Table I: Switching Sequence For Three-Level Inverter

The schematic diagram of single phase 5-level cascaded H bridge multilevel inverter is shown in Fig. 2. It consists of 2 H bridge cell that has 8 switching devices (s1, s2, s3, s4, s5, s6, s7, and s8) and two DC-sources of equal magnitude. This inverter system generates five levels in the output voltage (+2Vdc, +Vdc, 0, -Vdc, -2Vdc,). Switching states to produce the five steps in the output voltage is explained in Table II. These states are fed to the gate terminal of the switching devices.

Similarly, on increasing the number of H bridge cells in the inverter system number of steps in the output AC voltage can be increased. According to this topology higher level such as seventeen-level uses eight H-bridge cells, i.e., it requires large number of DC sources and power devices. To reduce the components used in higher levels cascaded H- bridge topology can be modified. DC-source of each H- bridge cells should be of different magnitude. This modified topology reduces the part counts on increasing the steps in the output AC voltage. Fig. 3 shows the asymmetric arrangement of DC- sources producing seventeen-level in the output-voltage. It consists of 12 switching devices (S1 to S12) and 3 DC-sources of different magnitude (Vdc, 3Vdc, and 9Vdc) which is much lesser than symmetrical arrangement of seventeen-level inverter system. Switching sequences is explained in Table III.

According to the switching states seventeen-level can be obtained in output voltage.

III. CONTROL ALGORITHM FOR MULTILEVEL INVERTER

This section presents the proposed scheme for controlling the gating signals fed to gate terminals of the power semiconductor devices used in multilevel inverter to get the desired output voltage. The block diagram of the proposed scheme is shown in Fig. 4. This block diagram includes a gating signal generator which is the main controlling unit. Pulses fed to different switches are generated through this unit using an embedded code. Multilevel inverter generates the desired output in response to the gating signal received. This unit consists any of the multilevel inverter structure mentioned above. And the output waveforms can be seen through any of the displaying unit. This embedded code can directly load to any microcontroller or DSP controller for real time implementation.

A simple coding has been done to produce 3-level, 5- level and 17-level inverter with the help of an embedded function on MATLAB. Fig. 5 shows the flow chart describing the coding sequence for the three-level cascaded H bridge multilevel inverter.

Embedded   coding   has   been   done   on   the   basis   of the switching sequence explained in section II for different levels in the output voltage. At starting different variables (switches and sample number) are initialized. Total number of samples in one period is 400. So, for 3-level inverter there will be 4 intervals. Each interval will have 100   samples. For five-level there are 8 intervals and each will have 50 samples. For seventeen-level there are 32 intervals and each interval will have 12.5 samples in each interval. According to these intervals switching states are generated. Now, the first condition is checked if sample is greater than or equal to 400. If the condition is not found true than it will check second condition if it is found true then switches s1 and s4 have to be turned ON and the sample number is incremented generating the output pulses, else it will check the third condition. In this way when all the conditions are checked and the sample number reaches 400, then first condition will hold true that will make the sample number again to 1 and the process is repeated for the next cycle. To implement 5-level and 17-level cascaded H bridge multilevel inverter coding can be done in the similar manner as explained for the three-level inverter.

IV. RESULTS AND DISCUSSION

The MATALB/Simulink models of different multilevel inverter system using the proposed embedded controller are presented in this section and their results are also discussed in this section. Fig. 6(a) shows MATLAB model of single- phase three-level cascaded H-bridge multilevel inverter. DC-source voltage is equal to 250 V for three-level inverter and it is simulated under RL load. Embedded MATLAB function block is used to write the coding for generating the gate pulses.

For 3-level inverter there will be 4 output gate signals from this block. Fig. 6(b) shows MATLAB model of single phase five-level cascaded H bridge multilevel inverter. Each DC source voltage is equal to 115 V for 5-level inverter and it is simulated under RL load. There are 8 output gate signals from embedded function block for 5- level inverter system. Fig. 6(c) shows MATLAB model of single phase 17-level cascaded H bridge multilevel inverter. DC-source voltage for each level is equal to

28.75 V for seventeen-level inverter and it is simulated under RL- load. For seventeen-level there are 12 output gate signals     from embedded  function block. the output-voltage waveform of developed 3-level inverter system is shown Fig.   7(a), whereas   Fig. 7(b) and 7(c) demonstrate the output-voltage waveforms of 5- level and 17-level inverter system respectively.

It can be seen from Figs. 7(a) to 7(c) that the waveform of the AC output-voltage for the developed inverter system is improved significantly from 3-level to 17-level. The waveform becomes more sinusoidal as we increase the steps in the output voltage. Fig. 7(d) shows the THD in output- voltage of the 3-level cascaded H bridge multilevel inverter system is 46.41%. The THD in output voltage further reduces to 27.36% when the level in the output voltage reaches 5. Fig. 7(e) shows the harmonic spectrum of output- voltage of 5-level cascaded H bridge multilevel inverter system and the harmonic spectrum of output-voltage of 17- level inverter system that indicate 13.18% THD in output- voltage is shown in Fig. 7(f). It shows that on increasing the levels in the output-voltage decreases the THD in the output-voltage which improves the power quality. The fundamental voltages of three, 5 and 17-level are found to be 225.9 V, 233.4 V and 227.2 V respectively which is near to ideal AC voltage 230 V.

Fig. 7(g) shows the output-current waveform of cascaded H bridge 3-level inverter system, whereas Fig. 7(h) and 7(i) shows the output-current waveforms of 5-level and 17-level inverter system respectively. It can be seen from Figs. 7(g) to 7(i) that the current waveform of the developed inverter system becomes more sinusoidal from 3-level to 17-level as the level in the output-voltage is increased. Fig. 7(j) shows the THD in output-current of the 3-level cascaded H bridge multilevel inverter system is 22.71%. The THD in output- current further reduces to 10.52% and 7.30% when the level in the output voltage is increased to five and seventeen steps respectively as shown in Fig. 7(k) and Fig. 7(l). The fundamental current is almost equal to 40 A.

Conclusion

The comparison of simulation results of different levels of CHBMLI obtained on MATLAB/SIMULINK platform and the hardware implementation shows that maximizing the levels makes the output waveform more sinusoidal. It is concluded that the proposed controller helps to minimize the total semiconductor devices which reduces cost and complexity of the circuit. From the graphs we can conclude that Total Harmonic Distortion decreases with the increase in the levels. Finally, we can conclude that the execution of the designed controller increases the overall performance.

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Copyright © 2022 Nagashree. S, Mrs. G. Shashikala. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.