Current H Repetitive Control - Control of Power Inverters in Renewable Energy and Smart Grid Integration - page 79

3

Current H ∞ Repetitive Control

In this chapter, a current controller is designed for grid-connected inverters based on the H ∞
and repetitive control techniques. As a result, the inverter is able to inject clean currents into
the grid. Moreover, a simple and effective mechanism for the inverter to quickly synchronise
with the grid is also discussed. Experimental results show that the controller is able to achieve
very low current THD.

3.1 System Description
The control system, as shown in Figure 3.1, adopts an individual controller for each phase in
the abc frame and the system is equipped with a neutral point controller discussed in (Zhong
et al. 2006) and Part II. It has a current loop including a repetitive controller so that the
current injected into the grid could track the reference current i ref , which is generated from
the dq -current references I d and I q abc transformation. A PLL is used to

provide the phase information of the grid voltage needed to generate i ref . The real power and
reactive power exchanged with the grid are determined by I d and I q . The inverter is powered
by a constant DC power source so no controller is needed to regulate the DC-link voltage.
Otherwise, a controller can be introduced to regulate the DC-link voltage and to generate I d
accordingly.
When the references I d and I q are all equal to 0, the generated voltage should be equal to
the grid voltage, i.e., the inverter should be synchronised with the grid and the circuit breaker
could be closed at any time if needed. In order to achieve this, the grid voltages ( u ga , u gb
and u gc ) are feed-forwarded and added to the output of the repetitive current controller via a
phase-lead low-pass filter

using the dq →

33(0 . 05 s + 1)

F ( s ) =

1) ,

(3.1)

( s + 300)(0 . 002 s +