THD Analysis of Cascaded Multi-level Inverters using different PWM Techniques

Multi-level inverters (MLI) are used in places or industries that require very high voltage and huge current. Multi-level inverter topologies have various advantages over single level converters. These advantages are like: rise in output voltage, reduction in the total harmonic distortion (THD), reduction in electro-magnetic interference (EMI) generation etc. Key attribute of a multi-level inverter is the reduction in the voltage stress on each power device. This reduction is due to the use of several levels available on the DC bus. The dawn of multi-level inverter


INTRODUCTION
Normally, the inverter is used to convert the dc supply into ac supply but during the conversion of this supply there are too many losses present in the converted output and the waveform of the generated ac supply is not purely sinusoidal in its entire time period. It is quite well known that, the distorted voltages and currents waveforms in the ac supply, produce harmonic distortions, high frequency noise and power losses that can affect not only the power load but also the distribution system. All these undesirable things affect the machine and other equipment like controllers and auxiliary systems. Thus, to overcome the losses and increase the performance of the machine, multi-level inverters came into existence. They are used to achieve the high-performance applications. Moreover, the benefits of using multi-level inverters are improved power quality, reduction in switching losses of the system and improved voltage capability. The MLIs generate common mode voltage (CMV). CMV reduces the voltage stress of the motor without damaging it. MLIs can draw input current with very low distortion. MLIs can operate at very low or very high switching frequencies, compared to the fundamental frequency. Lower switching frequency basically means lower switching losses and vice versa. This ability of operation results in the higher efficiency of the operation. The combination of Selective Harmonic Elimination Technique and the multi-level topologies reduces the THD in the output. This reduces the requirement of filter circuit.

II.
MULTI-LEVEL INVERTER (MLI) An n-level inverter will generate an output voltage with n levels w.r.t. the negative terminal of the Capacitor. Basically, the term multi-level begins with three levels, if we increase the number of levels in the inverter; the output voltage waveform will be like a staircase waveform having different intermediate levels, which results in reduction in THD.
The requirement of an MLI is to generate a high-power output from medium voltage sources like batteries, supercapacitors, solar panels, low power wind turbines etc. MLIs have different topologies by arranging diodes and switches in different manner. In MLI, the arrangement of switches' angles is of utmost importance. Basically, the multi-level inverter is used to achieve the perfect sinusoidal waveform of the output and thereby reducing losses and thus increasing output. There are three different topologies of multi-level inverters

Flying Capacitor
It is made up of a series connection of capacitor-clamped switching-cells. These capacitors convert only a limited amount of voltage to provide to the electrical devices. This topology also has the switching redundancy within phase, in order to balance the flying capacitors. Flying Capacitor topology has the ability to control the active as well as the reactive power flow, but due to high frequency switching, switching losses take place.

Diode Clamp
The key working notion of Diode Clamped inverter is to use diodes in order to provide multiple voltage levels through different phases to the capacitor banks, which are further connected in series. The DC voltage that you need to apply at the input level needs to be twice the maximum output voltage that you desire. The problem is resolved by increasing the number of switches, diodes and capacitors used. There exists a capacitor balancing issue, due to which these are limited only to three level multi-level inverters. Diode Clamp MLI provides improved performance and efficiency due to the use of fundamental frequency for all the switching devices. Moreover, it is a simple method for back to back power transfer systems.

III. SINGLE-PHASE 5-LEVEL CHB MULTI-LEVEL INVERTER
The cascaded inverter shown in the figure has five voltage levels. The output voltage that will be obtained can be 0, ±E, ±2E. It is observed that different voltage levels can obtain when two or more switches are in switching state. This MLI provides huge pliability for switching pattern design, mainly for SVM schemes. Generally, if N is the number of singlephase H-bridges per phase, the total number of levels of the inverter is: where, N is the number of H-bridge cells per phase leg and for CHB inverter the value of m should be odd. The Cascade H-Bridge MLI showed above, can be extended to any number of voltage levels. Let Ns be the total number of active switches (IGBTs, MOSFETs) used in the CHB inverters can be determined by, Ns = 6(m -1) where, Ns = the total number of switches used m= the voltage level.

IV. SINGLE-PHASE 7-LEVEL CHB MULTI-LEVEL INVERTER
The per-phase diagram of seven-level inverter is depicted in Figure 2, where seven-level inverter has three H-bridge cells in cascade as shown in the figure. The 7-level CHB Multi Level Inverter has improved efficiency and better THD handling. It has reduced Harmonic Distortion than five-level MLI. This model is by far the best model which we made. It gave the best simulation results and least THD in the output.

V.
LEVEL SHIFTED MODULATION SCHEME: Multiple carrier pulse modulation A number of carriers are used in multi-level SPWM. For 'n' level inverter 'n-1' carriers are used. These are following three types

IN-PHASE DIPOSITION (IPD)
In this all the carries are in phase with respect to each other.

PHASE OPPOSITIONDISPOSITION (POD)
In POD technique, the carrier waveforms that are above the zero are all out of phase with those below the zero by 180º.

VI.
SIMULATION RESULTS In this section we will be showing the simulation result of both the simulations. The output we get at the load side and the FFT analysis of both the simulation when we used APOD technique. CONCLUSION The Simulation results shows the performance and effectiveness of 1-Ф 5-level cascaded H-bridge multi-level inverter and 7-level cascaded H-bridge multi-level inverter of the proposed circuit for R-load, the result obtained is having 5-level and 7-level of voltage and current .form by using all the three PWM techniques like as:-IPD,POD,APOD. After the comparison of the THD values obtained from all the techniques we have concluded that APOD is much better than other two techniques for the practical approach.