Evaluation of Level-Shifted and Phase-Shifted PWM Schemes for Seven Level Single-Phase Packed U Cell Inverter

ABSTRACT:

 An evaluation of level shifted and phase shifted triangular and saw tooth carrier modulation schemes for a seven level packed U cell (PUC) inverter is presented in this paper. The investigated PUC is the recently introduced topology for multilevel inverter having reduced switch count in comparison to the conventional topologies of multilevel inverters. The PUC inverter has six switches for 7 level inverter which is very less in comparison to the conventional topologies. In this paper, the level-shifted pulse width modulation (LS-PWM) and phase-shifted PWM (PS-PWM) for triangular and saw tooth carrier are presented and compared. A comparative harmonic analysis for all the cases is performed and results are presented in the paper. The difference in harmonics of the two modulation methods given by the theoretical approach for both the carrier is validated by the experimental results. DC voltage controller and load current controller of the PUC inverter are also designed and presented. The investigated PUC topology is tested in dynamic and steady state conditions and results obtained are presented. The analysis is done and validated using simulation in MATLAB® Simulink environment and experimental approaches using FPGA platform.

 KEYWORDS:

1.      Level shift

2.      Multilevel inverter

3.      Modulation

4.       Phase shift

5.       PI controller

6.      PUC inverter

SOFTWARE: MATLAB/SIMULINK

CONCLUSION

The paper has presented the comparison of different PWM schemes which can be applied to the PUC inverter. Investigating the suitable modulation schemes is very essential with respect to local grid integration, as the power quality is directly dependent on THD. Triangular carrier based PWM schemes is exhibiting the better result than the saw tooth carrier based PWM schemes as the triangular level shifted carrier PWM scheme is better as compared to saw tooth level shifted carrier because in triangular level shifted carrier both edges (falling and rising) of pulses are modulated which improves the harmonic spectrum. However, in the saw tooth level shifted carrier only rising edges are modulated. Hence triangular level shifted carrier PWM scheme can be applied for integrating the PUC inverter with PV and local grid systems. Triangular level shifted carrier PWM scheme for PUC inverter has been suggested based on observing the THD in voltage and current which are respectively just 17.92% and 2.43%. The whole system i.e. solar panel, boost converter with PUC inverter will be very cost effective, besides having good reliability and power quality as it has the minimum number of power electronics devices compared to previously introduced multilevel inverter topologies. With reduced number of capacitors and power switches seven levels of voltages have been achieved for PUC inverter.

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