Robust Droop Control with Improved Voltage Quality - Control of Power Inverters in Renewable Energy and Smart Grid Integration - page 321

20

Robust Droop Control with
Improved Voltage Quality

In Chapter 19, a robust droop controller is presented to achieve accurate sharing of both real
power and reactive power for different types of inverters while maintaining excellent capability
of voltage regulation. Apart from C-inverters, the THD of the output voltage is high when the
load is non-linear. In this chapter, the strategy described in Chapter 8 is combined with the
robust droop controller to improve the voltage quality. As a result, all the three major problems
associated with the parallel operation of inverters, that is, sharing accuracy, voltage regulation
and voltage THD, are solved with a compact controller. Experimental results are provided to
verify the analysis and design. In this chapter, R-inverters are taken as an example but the
concept can be extended to L- and C-inverters.

20.1 Control Strategy
For a single-phase inverter depicted in Figure 20.1, the controller presented in Chapter 8,
as shown in Figure 20.2(a), is able to bypass the harmonic components in the load current
and hence the output voltage THD is low even when the load is non-linear. It consists of a
proportional feedback of the inductor current (Zhong 2012c), which is also widely referred
to as a virtual resistor (Dahono 2003, 2004; Dahono et al . 2001; Li 2009) to dampen the
resonance of the LC filter. The output impedance of the inverter is forced to be predominantly
resistive with a large enough K i . In addition to the current feedback through the virtual resistor
K i , the voltage error

v r through a block K R ( s ), which is able to reduce the
output impedance of the inverter at harmonic frequencies. As a result, the high voltage THD
caused by a large K i when nonlinear loads are present is reduced.
The block K R ( s ) in Figure 20.2(a) can be chosen as the resonant harmonic compensators
(Castilla et al . 2009; Shen et al . 2010)

v r − v o is added to

2 ξ h ω s

K R ( s ) =

) 2 ×

K h , (20.1)

s 2 + 2 ξ h ω s + ( h ω

h = 3 , 5 , ···