ANALYSIS AND PERFORMANCE EVALUATION OF SPACE VECTOR PWM CONTROLLED LOW SWITCHING FREQUEN.CY VOLTAGE SOURCE INVERTER

Abstract

In high-power drives, the switching frequency of the inverter is quite low due to the high losses in the power devices. This project is aimed to develop Real-time PWM strategies, which result in reduced harmonic distortion under low switching frequencies and have maximum possible DC bus utilisation. The space vector approach is taken up for the generation of synchronised PWM waveforms with 3-Phase Symmetry, Half Wave Symmetry and Quarter Wave Symmetry, required for high-power drives. By applying the rules of synchronization and above mentioned symmetries are applied to the conventional and modified forms of space vector modulation, leading to six strategies namely synchronised conventional space vector strategy and the Basic Bus Clamping Strategy-I respectively and four synchronised, bus-clamping PWM strategies, namely Asymmetric Zero-Changing Strategy, Boundary Sampling Strategy-I, Basic Bus Clamping Strategy-Il and Boundary Sampling Strategy-I!, are developed .A modified two-zone approach to overmodulation is used to increase the DC bus utilization. This is applied to the Six synchronized PWM strategies, dealt with in the present work, to extend the operation of These strategies up to the six-step mode. Linearity is ensured between the magnitude of the Reference and the fundamental voltage generated in the whole range of modulation up to the Six-step mode. By using six strategies a Hybrid PWM strategy is designed this strategies leads to a significant reduction in the harmonic distortion of the drive at medium and high speed ranges over the conventional space vector strategy. In vector controlled induction drive when operated with low switching frequency inverter the controller performance is effected by the lower order harmonics percent in the current wave form to overcome this drawback Fundamental Component Extraction technique is used to estimate the fundamental component of current so that the current controllers do not respond for the low frequency switching harmonics. Thus, allowing higher current controller bandwidth even during square wave mode of operation. These synchronized strategies, overmodulation and fundamental extraction strategy are simulated and the results are presented for the v/f and vector control algorithms.