An Improved Carrier Based PWM Technique to Reduce Neutral Point Voltage Oscillation and CMV of Three Level NPC Inverter

The power electronic converters, specially the three level neutral point clamped inverters (3LNPCI), which interfaces the renewable power generation to the grid or load, have two main problems, 1) balance of neutral point voltage (NPV) on the DC side 2) the common mode voltage (CMV). It is required to maintain the NPV in a 3LNPCI to minimize the voltage stress on the converter switches and reduce the output voltage harmonics, at the same time the reduction of CMV is also necessary to eliminate the leakage current, otherwise it causes safety issues. There are basically two modulation techniques to operate the power electronic converters, 1) carrier based approach of pulse width modulation (PWM) 2) space vector based approach of PWM. This paper proposes a modified carrier based PWM strategy which is based on the inclusion of a zero sequence voltage and is able to reduce the NPV oscillation and the CMV. The attractive features of the proposed PWM scheme compared to space vector based PWM are, 1) it is easy to calculate and implement on a digital platform 2) reduced switching loss 3) able to reduce the NPV oscillation 4) reduced CMV. A detailed analysis of the proposed modified carrier based PWM scheme is presented. Simulation comparison based results are presented to verify the effectiveness of the proposed PWM strategy.