IJSRD - International Journal for Scientific Research & Development| Vol. 6, Issue 03, 2018 | ISSN (online): 2321-0613 All rights reserved by www.ijsrd.com 145 THD Comparison for 180, 120 & 150 Degree Conduction Mode of Three Phase Inverter Abhishek M. Patel Assistant Professor Department of Electrical Engineering Vadodara Institute of Engineering, Vadodara, India Abstract— Device that converts dc input voltage to ac output voltage of desired magnitude and frequency is called inverter. Inverter can be classified into single phase inverter & three phase inverter. Inverters are widely used in industrial application such as uninterrupted power supply (UPS), flexible ac transmission system (FACTS) devices, variable frequency drives (VFD), active power filters, high voltage direct current transmission system, etc. So, the improvement in the output voltage & reduction in harmonic distortion is very important factor to be considered. A variable output voltage can be obtained by varying the gain of the inverter, which is normally accomplished by pulse width modulation control within the inverter. Key words: Three Phase VSI, THD Comparison, MATLAB Simulation I. INTRODUCTION Basic construction of three phase voltage source inverter is shown in figure. They are normally used for high power applications. In order to get three phase output, three single phase inverters can be connected in parallel. The gating signal of the three single phase inverter should be advanced voltage. Three phase voltage source inverter contains six switches. Switch may be IGBT, MOSFET, GTO, etc. depending upon the application, the diodes are connected across the s1 to s6 switches are the feedback diodes. These diodes will return back the stored energy from the inductive load to the dc supply. If the proper gate signals are given to the switches. This capacitor also suppressed the harmonic fed back to the dc source. II. DESIGN OF AN INVERTER SWITCHING TOPOLOGY IN MATLAB In the all industrial application variable frequency & voltage require for different application. Three phase full bridge inverter is more popular than any other inverter topology. There are six switches connected in a bridge form which provider three legs. Each leg generate single phase. Upper switch provide positive voltage & lower switch provide negative voltage. The output of inverter is six pulse because conduction of switches change in a six different steps. Fig. 1: Simulation Circuit of Three Phase Inverter for R- Load Fig. 2: Simulation Circuit of Three Phase Inverter for RL- Load In 180 degree conduction mode of inverter, each switch conducts for 180 degree. They are turned on at regular interval of 60 degree in the sequence ofS1, S2, S3, S4, S5 and S6.only three switches are in conduction state at any instant. The conduction of different switches are listed in the table below. The output terminal of this bridge is connected to the star connected load. In 120 degree conduction mode of inverter, each switch conducts for 120 degree. It is preferable for a delta connected load because it provides a six steps waveform across any phase. Only two switches are in conduction state at any instant. The conduction of different switches are listed in the table below. Sr no. Conduction of states Conduction of switches 1 0 to 60 S5 S6 S1 2 60 to 120 S6 S1 S2 3 120 to 180 S1 S2 S3 4 180 to 240 S2 S3 S4 5 240 to 300 S3 S4 S5 6 300 to 360 S4 S5 S6 Table 1: Conduction of Different States & Switches in 180 Degree Conduction Mode of Inverter Sr no. Conduction of states Conduction of switches 1 0 to 60 S1 S6 2 60 to 120 S1 S2 3 120 to 180 S2 S3 4 180 to 240 S3 S4 5 240 to 300 S4 S5 6 300 to 360 S5 S6 Table 2: Conduction of Different States & Switches in 120 Degree Conduction Mode of Inverter In 150 degree conduction mode of inverter, each switch conducts for 150 degree. It has twelve steps, each of 30 degree duration for completing one cycle of the output ac voltage. The conduction of different switches are listed in the table below.