Parallel PI Voltage-H Current Control of a Neutral Leg - Control of Power Inverters in Renewable Energy and Smart Grid Integration - page 209

13

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 i C flowing through the split DC capacitors. More-
over, 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 con-
troller 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 neu-
tral 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
in Figure 13.1, which is the combination of a split DC link and a conventional neutral leg
(Zhong et al . 2005a, 2006).
The total DC-link voltage is the difference between the two capacitor voltages measured
with respect to the neutral point, i.e.

V DC = V + − V − .