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

Three-phase inverters are usually required to supply unbalanced loads or be connected to an unbalanced grid in distributed generation system.The independently-controlled neutral module(ICNM)can be adopted into a three-phase inverter to form the neutral line to provide the path for zero-sequence current.The common-mode current from the neutral point to the ground is eliminated,and the control of the neutral point voltage is independent of the control of the three-phase output voltages.This thesis is dedidated to the voltage ripple suppression of the neutral line in the digital controlled three-phase four-wire inverter with ICNM.The primary control objective of the ICNM is to stabilize the neutral point voltage at half of the input voltage.However,when digital control is employed,the system instability may occur.In order to ensure system stability,the inductor current feedback scheme(ICFS)can be incorporated to increase the phase of the loop gain at the frequencies higher than fr.A step-by-step method of designing the inductor current feedback coefficient and voltage regulator parameters is proposed according to the requirements of the neutral point voltage ripple and system phase margin.With this design method,the complete satisfactory regions of the inductor current feedback coefficient and the system cross-over frequency can be obtained,and then the controller parameters can be easily picked out.This parameters design method is very straightforward,and the trial-and-error procedures are avoided.The ICFS is equivalent to adding a virtual resistor in series with the neutral inductor,leading to increased neutral point voltage ripple.When supplying three-phase unbalanced linear load,most part of the current flowing through the neutral line is at the fundamental frequency.Considering this condition,two control schemes,namely,notch-filter incorporated inductor current feedback scheme(NF-ICFS)and proportional resonant cascaded regulator plus inductor current feedback scheme(PR+ICFS)are proposed,which reduces the close-loop impedance of the neutral inductor branch at the fundamental frequency,and thus reduces the neutral point voltage ripple.Furthermore,the parameters design of notch-filter and PR regulator are given.When the inverter powers unbalanced non-linear loads or the loads are step changed,the neutral current contains not only the fundamental component,but also rich harmonics.In order to eliminate the effect of the neutral current on the neutral point voltage,the neutral current feed-forward plus inductor current feedback scheme(NCFF+ICFS)is proposed.The feed-forward function of the neutral current consists of a proportional component and a derivative component.Since most parts ofthe neutral current are at low hormonic frequencies,where the derivative component is quite small compared to the proportional component.Thus,only the proportional component is required,and the feed-forward path of neutral current and the feedback path of neutral inductor current can be combined and replaced by the feedback of the current of equivalent capacitor.Then,the complexity of the control can be reduced.A 10 kVA three-phase four-wire inverter has been built and tested in the lab.The experimental results are given to confirm the effectiveness of the proposed NF-ICFS,PR+ICFS and NCFF+ICFS and the correctness of the proposed controller parameters design method.