04-10-2016, 02:25 PM
1457686378-AnalysisofMultilevelInverter.docx (Size: 212.63 KB / Downloads: 3)
ABSTRACT
This study deals with the diode clamped multilevel inverter. The main advantage of using this type of inverter is, to reduce the switching loss and harmonics when compared to the flying capacitor and H-bridge inverter. In this proposed scheme the conventional 2-level inverter is replaced by 3 and 5 diode clamped inverter. A comparative study on total harmonics distortion between 3 and 5 diode clamped inverter is carried out in this paper.
INTRODUCTION
The demand for high-voltage high-power inverters is increasing, and it is impossible to connect a power semiconductor switch to a high-voltage network directly. Therefore, multilevel inverters had been introduced and are being developed now. With an increasing number of dc voltage sources in the input side, a sinusoidal like waveform can be generated at the output. As a result, the total harmonic distortion (THD) decreases, and the output waveform quality increases, which are the two main advantages of multilevel inverters. In addition, lower switching losses, lower voltage stress of dv/dt on switches, and better electromagnetic interference are the other most important advantages of multilevel inverters. These kinds of inverters are generally divided into three main categories, i.e., diode clamped multilevel inverters, flying capacitor multilevel inverters, and cascaded multilevel inverters. When compared to other two, diode clamped multilevel inverter reduce the switching loss and harmonics.
The diode clamped inverter provides multiple voltages levels through connection of the phases to a series bank of capacitors. According to the original invention the concept can be extended to any number of levels by increasing the number of capacitors. Early descriptions of this topology were limited to the three levels where two capacitors are connected across the dc bus resulting in one additional level. The additional level was the neutral point of the ac bus, so the terminology neutral point clamped inverter was introduced. However, with an even number of voltage levels, the neutral point is not accessible, and the term multiple point clamped(MPC) is sometimes applied Due to capacitor voltage balancing issues, the diode clamped inverter implementation has been mostly limited to the three level. Because of industrial developmet over the past several years, the three level inverter is now used extensively in industry applications.
PROPOSED SCHEME
In order to increase the number of generated output levels by using a lower number of power electronic devices, a basic unit is proposed in this project. By a series connection of several proposed basic units, a diode clamped multilevel inverter is proposed. Then, the conventional 2-level inverter is replaced by 3 and 5 diode clamped inverter. A comparative study on total harmonics distortion between 3 and 5 diode clamped inverter is carried out in this paper.
MULTILEVEL INVERTER
There are three major multilevel inverter structures in industrial applications: diode clamped, capacitor-clamped and cascaded H-bridge inverter with separate dc sources. The neutral point converter (NPC) proposed by Nabae et al. in 1981 is the simplest diode-clamped inverter. An m- level diode-clamped inverter has an m-level output phase voltage and a (2m-1)-level output line voltage. The multilevel diode-clamped inverter can be applied as an interface between a high-voltage dc transmission line and an ac transmission line, as a variable speed drive for high-power medium-voltage motors, and for static var compensation. The structure of the capacitor-clamped inverter is similar to that of the diode-clamped inverter except that instead of using clamping diodes, the inverter uses capacitors in their place, which is introduced by Meynard et al. in 1992.
Multilevel inverters have drawn tremendous interest in the power industry. They present a new set of features that are well suited for use in reactive power compensation. Multilevel inverters will significantly reduce the magnitude of harmonics and increase the output voltage and power without the use of step-up transformer. Multilevel inverters are preferred over traditional inverters as single devices can’t handle the V and I because for that the device voltage rating required 8-10KV is generally not available so voltage handling capability problem arises also poor power quality due to harmonic distortions and high switching losses.
3 Level Diode Clamped Inverter
Fig 1 represents the simulation diagram of 3 level diode clamped inverter. In this voltage source is given as input and gate pulse is provided for each thyristor which is used to generate a pulse. Three phase system is analyzed in this level. As the output of voltage, current and output load is measured. When compared to 2 level diode clamped, the harmonics gets reduced in 3 level diode clamped. The total harmonics distortion (THD) attains in this 3 level is 40.17%.
level Diode Clamped Inverter
Fig 2 represents the simulation diagram of 5 level diode clamped inverter. In this voltage source is given as input and gate pulse is provided for each thyristor which is used to generate a pulse. Three phase system is analyzed in this level. As the output of voltage, current and output load is measured. When compared to 2 level diode clamped and 3 level diode clamped the harmonics gets reduced in 5 level diode clamped. The total harmonics distortion (THD) attain in this 5 level is 19.39%
CONCLUSION
Thus the three phase waveform of voltage, current and output load for 3-level diode clamped inverter and 5 level diode clamped inverter was obtained through stimulation/MAT lab. And also their both total harmonics distortion (THD) is calculated. For 3 level diode clamped is 40.17% and 5 level diode clamped is 19.39%. When compared to 2, 3 level the harmonics are improved in 5 level diode clamped inverter. If the level of the inverter goes on increasing then the harmonics and THD is reduced. By comparing the 3 and 5 level diode clamped the 5 level diode clamped inverter is the best because THD is less than 3 level diode clamped inverter.