Applications in Single-phase to Three-phase Conversion - Control of Power Inverters in Renewable Energy and Smart Grid Integration - page 235

When started, the supply current was not in phase with the supply voltage. After the rectifier
leg and the neutral leg were turned on at 0.04 s, the supply current became in phase with the
supply voltage, as shown in Figure 14.9(a). Moreover, the amplitude of the grid current was
reduced because only the real power (of Phase a ) was drawn from the supply. The DC-link
voltage was regulated to be around 800 V and the neutral point was kept stable. The current
i C was kept small, without a visible fundamental component.
After the two phase legs were turned on, the supply current increased but was still clean
and in phase with the supply voltage. The currents flowing through the rectifier leg and the
neutral-leg inductor all increased because of the power consumed by the loads connected to
Phase b and Phase c . The DC-link voltage dropped but recovered in about 5 cycles. The ripples
in the DC link voltage became larger because the power exchanged became higher. The shift
V a v e of the neutral point became larger as well because of the increased current i L but was still
within

±

3 V. The current i C was still kept small. The three-phase output voltages, as shown
in Figure 14.9(g), settled down in about two cycles and the THD of the generated Phase b and
Phase c voltages, as shown in Figure 14.9(i), was less than 2%.

14.5.2 With Three-phase Non-linear Unbalanced Loads
In this simulation, the three-phase loads were non-linear and unbalanced. Two single-phase
rectifier loads shown in Figure 14.8 with different parameters were connected to Phase a and
Phase c , respectively, and a linear load was connected to Phase b . The parameters of the loads
are given in Table 14.2. The simulation was started at 0 s with the rectifier leg and the neutral
leg turned on at 0.04 s and the phase legs turned on at 0.12 s, respectively. The results are
shown in Figure 14.10.
Before the rectifier leg and the neutral leg were turned on, the supply current contained a
significant amount of harmonic components with THD

40% and was not in phase with the
supply voltage either, as shown in Figure 14.10(a). After the rectifier leg and the neutral leg
were turned on, the supply current became clean with a very low THD and was in phase with
the supply voltage. The DC-link voltage was regulated well around 800 V and the neutral point
was also maintained very well. The current i C was kept small, without a visible fundamental
component.
After the two phase legs were turned on at 0.12 s, the supply current increased and was
still clean and in phase with the supply voltage, although the Phase c load was non-linear.
The THD of the supply current was kept below 1%. Because of the sudden connection of the
loads on Phase b and Phase c and also the start of the two phase legs, the DC-link voltage
dropped but recovered within 7 cycles. The ripples in the DC-link voltage became higher and
the variation of the neutral point became bigger as well (but was still within

>

± 2 V), because

Table 14.2 Parameters of the three-phase loads

Phase Parameters of the load

a R = 3 , L = 1 mH, R r = 10 , C r = 800 μ F

b linear, with 5 in series with an inductor of 10 mH

c R = 10 , L = 5 mH, R r = 15 , C r = 1000 μ F