Parallel PI Voltage-H∞ Current Control of a Neutral Leg
As explained in Chapter 11, in order to maintain a stable and balanced neutral point, the current flowing through the split DC capacitors should be close to zero. This fact is not fully reflected in the controllers designed in the previous two chapters. Here, this is taken into account and a parallel voltage-current control strategy is designed to improve the performance of the independently-controlled neutral leg. The current controller is designed by using the H∞ control theory to minimise the current iC flowing through the split DC capacitors. Moreover, an extra voltage controller is added to bring the voltage shift caused by the mismatch of capacitors and/or the non-linearity of the switches back to normal. Since the voltage controller mainly deals with DC signals and the current controller mainly deals with the harmonic current components, these two controllers are decoupled in the frequency domain and can be arranged in a parallel control structure instead of a cascaded control structure. This improves the stability of the system. The control strategy leads to very small deviations of the neutral point from the mid-point of the DC source, in spite of the possibly very large neutral current. Experimental results are presented to demonstrate the excellent performance of the control strategy.
13.1 Description of the Neutral Leg
Again, the neutral leg under consideration is the independently-controlled neutral leg shown ...