Voltage source inverters (VSIs) are usually used for all kinds of distributed generation interfaces in a microgrid. It is the microgrid’s superiority to power the local loads continuously when the utility fails. When in islanded mode, the voltage and frequency of the microgrid are determined by the VSIs; therefore the power quality can be deteriorated under unbalanced and non-linear loads. A voltage unbalance and harmonics compensation strategy for the VSIs in islanded microgrid is proposed in this study. This method is implemented in a single synchronous reference frame (SRF) and is responsible for both the voltage unbalance and harmonic compensation. Furthermore, the virtual impedance loop is modified to improve the compensation effect. The impedance model of the VSI is built to explain the compensation ability of the proposed strategy. The whole control system mainly includes power droop controllers, a modified virtual impedance loop and inner SRF-based voltage unbalance and harmonics compensators. The proposed strategy is demonstrated in detail and validated with simulations and experiments.
Fig. 1 Schematic and control of DGs interface in an AC microgrid a) Typical AC microgrid structure with DGs and loads b) Schematic of a VSI as the DG interface c) Power droop control loop of DGs interface
EXPECTED SIMULATION RESULTS:
Fig. 2 Simulation results of the VSI in the islanded microgrid with different control strategies a) Conventional PI controller b) The proposed PI plus multi-resonant controller c) The proposed PI plus multi-resonant controller with modified virtual impedance loop d) Comparison between the output voltage unbalances and THDs under different control strategies
Fig. 3 Bode plots of the closed-loop voltage gain and the output impedance of the VSI a) Closed-loop voltage gain Gu(s) with PI controller when loaded with pure R (solid line), PI controller Gus(s) (dotted line) b) Closed-loop voltage gain with PI plus multi-resonant controller Gur(s) (solid line), PI plus multi-resonant controller GPIR(s) (dotted line) c) The output impedance under PI controller without the virtual impedance loop Zo(s) (solid line), the output impedance under PI controller with the virtual impedance loop ZD(s) (dashed line) and the output impedance under PI plus multi-resonant controller with the virtual impedance loop ZDr(s) (dotted line)
Fig. 4 Proposed voltage control strategy plus modified virtual output impedance loop and the total output impedance a) Voltage control plus LPF virtual impedance loop b) The output impedance under PI plus multi-resonant controller with the regular virtual impedance loop ZDr(s) (dotted line), with the LPF virtual impedance loop ZDf(s) (solid line)
This paper proposes a FPS SRF-based control strategy for voltage unbalance and harmonic compensation of the VSIs used as interfaces in islanded microgrid. The voltage compensation loops are integrated within the power droop loops and the virtual output impedance loop. The proposed strategy is implemented in a single SRF with a PI controller for the voltage’s fundamental component regulation and multi-resonant controller for voltage unbalance and selected harmonics compensation. The impedance model of the DG interface inverter is built when controlled by three different control methods to explain the compensation ability of the proposed strategy, which are the conventional PI voltage controller, the PI plus multi-resonant voltage controller and the PI plus multi-resonant voltage controller with modified virtual impedance loop. The simulation and experimental results of the three different control strategies with balanced load, unbalanced load and diode bridge rectifier load are given to validate the effectiveness of the proposed control strategy.
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