Neutral Point Voltage Ripple Suppression For A Three-Phase Four-Wire Inverter With An Independently Controlled Neutral Module

An independently controlled neutral module (ICNM) can be adopted in a three-phase four-wire inverter to form the neutral line and provide the path for zero-sequence current when the loads are unbalanced. The original control objective of the ICNM is to stabilize the neutral point voltage. Since there is a -180 degrees phase jump at the resonant frequency of the neutral inductor and split dc-link capacitors in the system loop gain, system instability may occur. In order to ensure system stability, the inductor current feedback scheme (ICFS) can be incorporated, which is equivalent to introducing a resistor in series with the inductor branch. However, it will lead to increased zero-sequence current flowing through the dc-link capacitors, thus increasing the neutral point voltage ripple. In this paper, three improved control schemes, namely, notch-filter-incorporated inductor current feedback scheme (NF-ICFS), proportional resonant cascaded regulator plus inductor current feedback scheme (PR + ICFS), and neutral current feed-forward plus inductor current feedback scheme (NCFF + ICFS), are proposed, which not only ensure system stability, but also maintain a small neutral point voltage ripple. A 10-kW prototype of the three-phase four-wire inverter with ICNM is built and tested in the laboratory, and the experimental results verify the effectiveness of the proposed control schemes.