Voltage H Repetitive Control with a Frequency-adaptive Mechanism - Control of Power Inverters in Renewable Energy and Smart Grid Integration - page 105

5
Voltage H ∞ Repetitive Control
with a Frequency-adaptive
Mechanism
In the previous chapter, an H ∞ repetitive controller is designed with the feedback of the output
voltage and the output current. In this chapter, this is further developed after simplifying the
model of the inverter and taking the voltage only as the feedback for the repetitive controller.
The feedback of the inductor current is adopted to form an inner-loop controller. The control
plant for the repetitive controller is then reduced to a single-input-single-output (SISO) one
and the complexity of the control design is considerably reduced. The stability evaluation of
the system becomes easier as well. Moreover, a frequency-adaptive mechanism is embedded
into the internal model and, hence, the controller is able to cope with grid frequency variations
in the grid-connected mode. This mechanism allows the controller to maintain very good
tracking performance over a wide range of grid frequencies. Extensive experimental results
are included to demonstrate the control strategy.
5.1 System Description
The general idea of the control strategy is to adopt an individual controller for each phase
in the natural frame, under the assumption that the system is implemented with a neutral
point controller (Zhong et al. 2006). The overall control structure of the system is shown in
Figure 5.1. It consists of three cascaded controllers: a power controller to generate the voltage
reference u ref while regulating the power exchanged with the grid, a voltage controller to track
u ref , and a current controller to generate the PWM signals to drive the power switches while
tracking the current reference generated by the voltage controller. The current controller is
chosen as a simple proportional controller K c with the inductor current as feedback. It can be
equipped with current protection as well. It does not affect the tracking accuracy of the outer
voltage loop (Li et al. 2006; Vilathgamuwa et al. 2006). Actually, according to Chapter 7, this