ABSTRACT:
This paper describes a simple grid current control method
for the grid-connected operation, and inverter voltage control method based on
the phase locked loop (PLL) for the intentional islanding operation at the
three-phase grid-connected inverter. The PLL controller based on the pq theory
with a simple P-controller is used to synchronize the phase of inverter output
voltage with a grid voltage at the grid-connected operation or generate a
desired inverter output voltage at the islanding operation. The outputs of
current controller are connected together to those of voltage controller, in
order to prevent a sudden change of the outputs of both controllers during the
transfer instant. The simulation and experimental results are carried out to
verify the effectiveness of the proposed control strategies.
KEYWORDS:
1.
Distributed generation (DG)
2.
Grid-connected operation
3.
Islanding operation
4.
Phase locked loop (PLL)
5.
Three phase inverter.
SOFTWARE: MATLAB/SIMULINK
BLOCK DIAGRAM:
Figure 1. A control structure of
three-phase grid-connected inverter.
EXPECTED SIMULATION RESULTS:
Figure
2. Simulation result for grid current control at grid-connected operation
Figure
3. Simulation result for inverter voltage control at the islanding operation.
CONCLUSION:
This paper described a simple grid current control method for
the grid-connected operation, and output voltage control method based on the
PLL for the intentional islanding operation at the three-phase grid-connected
inverter. The PLL controller based on the pq theory
with a simple P-controller which has no steady-state phase error, was
used to synchronize the phase of inverter output voltage with a grid voltage or
generate a desired voltage. As the outputs of current controller are connected
together to those of voltage controller, the grid connected inverter was able
to change smoothly from the grid connected operation to islanding operation.
The experimental results showed that the proposed control schemes are capable of
obtaining the good grid current response and also maintaining the inverter
voltage within the desired level. The measured THDs of grid current and output
voltage of inverter are only 1.92% and 1.89%, respectively.
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