11

Classical Control of a Neutral Leg

In this chapter, a controller is developed for the independently-controlled neutral leg by using classical control techniques to provide a neutral line for a 3-phase 4-wire inverter. The neutral point can be steadily regulated at the mid-point of the DC link by a simple controller, involved in voltage and current feedback, even when the three-phase converter system (mostly, the load) is extremely unbalanced. The achievable performance is analysed quantitatively and the parameters of the neutral leg are determined accordingly.

11.1 Mathematical Modelling

Here, the linear model of the neutral leg is developed in a slightly different way from that in (Zhong et al. 2006) in order to gain some insightful understanding about the neutral leg, by ignoring the resistance of the inductor and assuming that the capacitors are the same.

Assume that the voltages across the capacitors CN with respect to the neutral point N are V+ and V−, respectively. Then the DC link voltage is

and the shift of the neutral point is1

According to Kirchhoff’s laws, the following equations are satisfied:

Assume that the average of the firing pulse during a switching period ...

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