Parallel multi level three phase voltage source inverters using three limb coupled inductors

This paper presents Parallel Voltage Sourced Inverters to improve the efficiency in high current applications. Generally when two inverters are paralleled, they have unmatching currents flowing between them and to reduce that, we go for inductors. These inductors reduces the circulating currents flowing between the inverters and these currents are produced by common mode voltage differences between the output terminals of the two inverters. Standard Discontinuous PWM techniques when applied to parallel connected inverters with three separate inductors naturally produce high frequency common mode voltage differences. So an Interleaved Discontinuous PWM scheme along with sinusoidal reference is proposed to eliminate these voltage differences in parallel connected inverters. By using this technique, a three phase parallel inverter can be operated using a single three limb coupled inductor instead of three separate inductors. And this significantly reduces the high frequency differential current ripple and improves the efficiency of the system. Analysis are done for common mode voltage differences and for the common mode circulating currents using the above mentioned two techniques and it is shown that the common mode circulating currents (I_cm) and the common mode voltage differences (V_cm) are reduced by using Interleaved Discontinuous PWM technique. This is fed to an Induction motor load and the output currents are sinusoidal with less current ripples. Computer simulations are done using PSIM for Standard Discontinuous PWM and for Interleaved Discontinuous PWM and their respective waveforms are compared and it is shown that Interleaved Discontinuous PWM is efficient.