Applications in Single-phase to Three-phase Conversion - Control of Power Inverters in Renewable Energy and Smart Grid Integration - page 228

When the loads are linear and balanced, there are

I a +

1

I s = (3 I a cos θ ) 2 − 6 I a cos 2 θ = + 3 cos 2 θ I a ,

and i r = i , which means

I r = I = 3 I a cos θ.

Hence,

I s ≤ 2 I a and I r ≤ 3 I a .

These can be applied to determine the current ratings of the switches of the rectifier leg and
the neutral leg.

14.4 Controller Design
As mentioned before, the controller of the four legs can be decoupled and designed indepen-
dently. The main function of the controller for the neutral leg is to make the voltage of the
common point of the split capacitors with respect to the mid-point of the DC link stable, and
close to zero and to provide a return current path for the rectifier leg and the phase legs. As
a result, the common point of the split capacitors can be used as the neutral point N that is
common to the supply and the two phases generated. This is vital to the operation of the system.
The main function of the controller for the rectifier leg is to draw a clean sinusoidal current that
is in phase with the supply voltage and to maintain a constant DC link, which automatically
transfers power to the two phases. This is quite similar to the function of a combined active
power filter and reactive power compensator. The main function of the controller for the phase
legs is to convert the DC link voltage into two phase voltages with low harmonics to form
balanced three phase voltages together with the single-phase supply.

14.4.1 Synchronisation Unit
In order to make sure that the current drawn from the supply is in phase with the supply
voltage and also that the generated phase voltages are able to form balanced three-phase
voltages together with the supply voltage, a synchronisation unit is needed to provide the
phase information of the supply. This can be obtained in many ways, e.g. with phase-locked
loops (PLL) (da Silva et al . 2010; Rodriguez et al . 2007b) or sinusoidal tracking algorithms
(STA) (Ziarani and Konrad 2004). In this chapter, a STA is adopted to form a synchronisation
unit, as shown in Figure 14.6(a). Once the phase information of the supply

ω t is obtained, the

2 3 to it. The unit
also provides the RMS value of the supply voltage to be used as the reference RMS value for
the generated phase voltages.

phase information of the generated voltages can be obtained via adding ±