Cascaded Current-Voltage H Repetitive Control - Control of Power Inverters in Renewable Energy and Smart Grid Integration - page 124

2008; Li and Kao 2009; Mohamed and El-Saadany 2008; Xiarnay et al. 2008). Traditionally,
the inverters used in microgrids behave like current sources when they are connected to the
grid and as voltage sources when they work autonomously (Chen et al. 2010). This involves
the change of the controller when the operational mode is changed from stand-alone to grid-
connected or vice versa (Yao et al. 2010). It is advantageous to operate inverters as voltage
sources because there is no need to change the controller when the operation mode is changed.
A parallel control structure consisting of an output voltage controller and a grid current
controller was proposed in (Yao et al. 2010) to achieve seamless transfer via changing the
references to the controller without changing the controller.
As mentioned before, since non-linear and/or unbalanced loads can represent a high pro-
portion of the total load in small-scale systems, the problem with power quality is a particular
concern in microgrids (Prodanovic and Green 2006). The THD of the inverter local load volt-
age and the current exchanged with the grid (referred to as the grid current) needs to be kept
low according to industrial regulations. It has been known that it is not a problem to obtain
low THD for either the inverter local load voltage (Hornik and Zhong 2010b; Weiss et al.
2004) or for the grid current (Hornik and Zhong 2009, 2011). However, it is a challenge to
obtain low THD for both the inverter local load voltage and the grid current simultaneously.
This may even have been believed impossible because there may be non-linear local loads.
In this chapter, a cascaded control structure consisting of an inner-loop voltage controller and
an outer-loop current controller is presented to achieve this, after recognising that the inverter
LCL filter can be split into two separate parts. The LC part can be used to design the voltage
controller and the grid interface inductor can be used to design the current controller. The
voltage controller is responsible for the power quality of the inverter local load voltage, power
distribution and synchronisation with the grid, and the current controller is responsible for
the power quality of the grid current, the power exchanged with the grid and over-current
protection. With the help of the H ∞ repetitive control (Hornik and Zhong 2009, 2010b, 2011),
the control strategy is able to maintain low THD in both the inverter local load voltage and
the grid current at the same time. When the inverter is connected to the grid, both controllers
are active; when the inverter is not connected to the grid, the current controller works with
zero current reference. Hence, no extra effort is needed when changing the operation mode
of the inverter, which considerably facilitates the seamless mode transfer for grid-connected
inverters. For three-phase inverters, the same individual controller can be used for each phase
in the natural frame when the system is implemented with a neutral-point controller, e.g. the
one described in (Zhong et al. 2006) and Part II, to cope with unbalanced utility grid voltages
and utility voltage sags, etc, which are the two most common utility voltage quality problems
(Li et al. 2006; Vilathgamuwa et al. 2006). As a result, the inverter can cope with unbalanced
local loads as well as three-phase applications. In other words, the harmonic currents and
unbalanced local load currents are all contained locally and do not affect the grid.
It is worth stressing that the cascaded current-voltage control structure improves the quality
of both the inverter local load voltage and the grid current at the same time. Moreover, it is
able to achieve the seamless transfer of the operation mode. The outer-loop current controller
is to provide a reference for the inner-loop voltage controller, which is the key to allow
the simultaneous improvement of the THD in the grid current and the inverter local load
voltage and to achieve the seamless transfer of operation mode. This is different from the
conventional voltage-current control scheme (Li et al. 2006), where the (inner) current loop is
to regulate the filter inductor current of the inverter (not the grid current), so it is impossible
to achieve simultaneous improvement of the THD in the grid current and the inverter local