Current H Repetitive Control - Control of Power Inverters in Renewable Energy and Smart Grid Integration - page 89

5

i ref i a

2.5

0

−2.5

−5

1.5

e i

1

0.5

0

−0.5

−1

−1.5

−0.04 0 0.04 0.08 0.12 0.16 0.2 0.24

Time [s]

Figure 3.9 Transient response of the current H ∞ repetitive controller in the grid-connected mode
without a local load

output current reference I d was set at 2 A (after connecting the inverter to the grid) and the reac-
tive power was set at 0 Var ( I q

0). The output current i a , the reference current i ref , the current
tracking error e i and the spectrum of the grid output current i a are shown in the right column
of Figure 3.8. The tracking error e i remained almost unchanged. Although the load is heavily
non-linear, the current fed to the grid is quite clean, which means that the harmonic currents
of the non-linear loads were contained inside the microgrid and was not injected into the grid.

=

3.4.3 Transient Response (without a Load)
In order to evaluate the transient performance of the controller, a step change in the current
reference I d from 2 A to 3 A was applied (while keeping I q 0). Since there was no local

load connected to the system in this experiment, all the generated active power was injected
into the grid via a step-up transformer. The output current i a , reference current i ref and the
corresponding current tracking error e i are shown in Figure 3.9. It took about 5 cycles to settle
down. This reflects the inherent property of repetitive control, i.e. the compromise between
the response speed and low THD.

=

3.5 Summary
Based on (Hornik and Zhong 2009, 2011), a current H ∞ repetitive controller is designed,
implemented and evaluated with experiments in this chapter. The experimental results show
that the controller offers significant improvement in terms of THD, even in the presence of
non-linear local loads. Because of the feed-forwarded grid voltage to the inverter controller,
the synchronisation process is quite straightforward. The main drawback is the compromise
between slow dynamics and low THD.
Note that the processing delay W d taken into account in Chapter 5 is not considered here
but no visible degradation of performance is shown.