Harmonic Evaluation of Multicarrier Pwm Techniques for Cascaded Multilevel Inverter

Multilevel voltage source inverter has been recognized as an important alternative to the normal two level voltage source inverter, especially in high power application. Using multilevel technique, the output voltage amplitude is increased, switching devices stress is reduced and the overall harmonics profile is improved. Three presentable topologies can b considered for multilevel inverter (MLI): diode –clamped, flying capacitor and cascaded Hbridge cells with separate dc sources. This paper focuses on cascaded multilevel inverter. For the cascaded multilevel inverter variety of modulation strategies have been reported, with the most popular being carrier – based and space vector modulation (SVM). This paper investigates and analyses different multicarrier PWM methods for a five – level cascaded inverter using PSIM simulation. Multicarrier PWM methods can be categorized into two groups: Carrier Disposition methods (CD) and Phaseshifted (PS) PWM methods. Also, hybrid method which is a combination of the above two methods is discussed. In addition, a new PWM scheme known as trapezoidal pulse –width modulation for the cascaded inverter is proposed as it provides higher fundamental output voltage compared to sinusoidal pulse –width modulation. This paper gives a comparative study of different PWM methods for the five – level inverter and the investigation is made in terms of THD, switching frequency and harmonic rms voltage and current.