Synchronverters: Grid-friendly Inverters that Mimic Synchronous Generators - Control of Power Inverters in Renewable Energy and Smart Grid Integration - page 282

6 6

i A i A −i a

4 4

2 2

0 0

−2 −2

−4 −4

−6 −6

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08

Time [s] Time [s]

(a) Inverter current (b) Phase load current

30 6

u A i a

4

15

2

0 0

−2

−15

−4

−30 −6

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08

Time [s] Time [s]

(c) Local load voltage (d) Grid output current

10

5

THD of i a =12.27%

THD of u A =3.02%

4

3

5

2

1

0 0

0 5 10 15 20 25 30

0 5 10 15 20 25 30

Harmonics order Harmonics order

(e) Spectra of the local load voltage (f) Spectra of the grid output current

Figure 18.12 Loading performance in the grid-connected mode with a non-linear local load

current i a are shown in Figures 18.12(e) and 18.12(f) respectively. The quality of the local
load voltage remained very good in spite of the heavily non-linear local load. The current
sent to the grid was distorted. The recorded local load voltage THD was 3

. 02% while the grid

voltage THD was 1 27%. Again, the local load

voltage was controlled very well but the current exchanged with the grid was not. There were
significant 5th and 7th harmonic components.

. 08% and the grid output current THD was 12 .

18.5.3.3 With an Unbalanced Load
The local load voltages, the local load currents, the inverter output currents and the grid
currents are shown in Figure 18.13. Although the local load currents were not balanced, the
output voltages were balanced. The inverter output currents were nearly balanced, which made
the grid currents unbalanced. The spectra of output voltage u A and the grid output current i a
are shown in Figures 18.13(e) and 18.13(f) respectively. The output voltage was of very good
quality but the current sent to the grid was distorted, so was the inverter output current. The
recorded local load voltage THD was 1 . 78% while the grid voltage THD was 1 . 15% and the

grid output current THD was 5 54%. The quality of the local load voltage and the quality of

the current sent to the grid were similar to results with balanced resistive load.

.