Voltage H Repetitive Control with a Frequency-adaptive Mechanism - Control of Power Inverters in Renewable Energy and Smart Grid Integration - page 115

30 5

u A u B u C THD of u A =1.18%

4

15

3

0

2

−15

1

−30 0

4 5

i A i B i C THD of i A =1.17%

4

2

3

0

2

−2

1

−4 0

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0 5 10 15 20 25 30

Time [s] Harmonics order

(a) Voltages at the PCC and load currents (b) Spectra of voltage u A and current i A

Figure 5.8 Stand-alone mode: with an unbalanced load

Although the utility grid voltage was used as the reference without any filtering, the quality of
the local voltage is actually better than that of the grid voltage, with the THD of 1 . 29%.

5.4.1.2 With a Non-linear Load

The results are shown in the right column of Figure 5.7. Again, the reference voltage was
tracked very well without noticeable fundamental components in the tracking error. The
recorded local load voltage THD was 3

. 34%, while the grid voltage THD was 1 . 33% and the

local load current THD was 75 . 68%. The voltage controller performed very well with this

highly non-linear load.

5.4.1.3 With an Unbalanced Load

The results are shown in Figure 5.8. The Phase- B current was zero as there was no load
connected. The recorded local load voltage THD was 1 . 18% while the grid voltage THD

was 1 17%. Since the control structure adopted

a separate controller for each phase, the load imbalance had no influence on the controller
performance and the local load voltages remained well balanced.

. 25% and the local load current THD was 1 .

5.4.2 Steady-state Performance in the Grid-connected Mode
In addition to the three experiments carried out in the stand-alone mode, the case without a
local load was tested in the grid-connected mode as well. Moreover, two other experiments
were carried out to demonstrate the transient responses to a step change in the grid output
current I d reference and to the variations of the grid frequency.
In the grid-connected mode, the current reference of the grid output current I d was set at
2A(1 . 41 A RMS), after connecting the inverter to the grid. The reactive power was set at 0

Va r ( I q 0). The instantaneous grid current reference i ref could be obtained from I d and I q =