Droop Control of Parallel Dual-Mode Inverters Used in Micro Grid

Abstract: 

Grid-connected and island control of parallel inverters used in micro grid based on a variety of micro-source were introduced in this paper. Micro-grid in the connected mode should be able to operate automatically with the grid frequency and output high quality electricity in PQ control, and in island mode it can realize load power sharing of the parallel DGs in Droop control [1]. Simultaneously, it should also ensure the stability of the load voltage and frequency in island mode with the droop-based controller which can make the micro-grid smoothly switching between the two operation modes. Finally, simulation of two inverters in the connected mode and island mode was introduced in MATLAB / SIMULINK; the simulation results show the effect of droop control and the load-sharing function. The results also indicate the feasibility and correctness of the control strategy.

Keywords

micro grid; parallel dual-mode inverters; PQ control; droop control

Block Diagram:

FIGURE 1. MICRO-GRID STRUCTURE DIAGRAM.

Expected Simulation Results:

FIGURE 2. THE SIMULATION OF GRID-CONNECTED MODE.

FIGURE 3. THE SIMULATION OF ISLAND MODE.

FIGURE 4. THE VOLTAGE AND CURRENT FROM ISLAND TO

                                         GRID-CONNECTED MODE.

FIGURE 5. INVERTER1 AND INVERTER2 OUTPUT ACTIVE POWER.

FIGURE 6. ACTIVE POWER ABSORBED BY THE LOAD.

CONCLUSION

Micro-grid often operates under two typical modes. Micro-grid in the grid-connected mode can make it run automatically with the grid frequency and output high quality electricity. Furthermore, micro-grid in island mode can realize load power sharing of the parallel distributed power of micro-grid. Meanwhile, it can also ensure the stability of the load voltage and frequency. Moreover, the droop controller [5] can make the micro-grid smooth switching between the two kinds of operation modes. Finally, the simulation results indicated that the phase and magnitude of the load voltage are successfully matched to the grid voltage at the point of transfer from island mode to grid-connected operation without any distortions. Similarly, when the mode returns back, the load voltage can quickly approach its desired voltage without voltage and current rush by using the proposed method and strategy.

REFERENCES

[1] Hu, Shang-Hung., Kuo, Chun-Yi., Lee, Tzung-Lin & Josep M.Guerrero, “Droop-Controlled Inverters with Seamless Transition between Islanding and Grid-Connected Operations,” ECCE, Phoenix, AZ, Sept 2011, pp.2196-2201.

[2] Yang Zhan gang, Wang Cheng Shan, Che Yan Bo, “A Small-scale Micro-grid System with Flexible Modes of Operation,” Automation of Electric Power Systems, vol.33, No.14, July 2009, pp.89-92.

[3] Kanellos,F.D., Tsouchnikas,A.I., & Hatziargyriou, N.D., “Micro-grid simulation during grid-connected and islanded modes of operation,” presented at the Int. Conf. Power Systems Transients (IPST), Montreal, QC, Canada, 2005, Paper IPST05-113.

[4] Chen, C.L. Wang, Y.B. & Lai, J.S., “Design of parallel inverters for smooth mode transfer micro-grid applications,” in Applied Power Electronics Conference and Exposition, 2009 IEEE.APEC 2009, pp. 1288-1294.

[5] Lee, C.T. Jiang, R.P. & Cheng, P.T., “A grid synchronization method for droop controlled distributed energy resources converters,” in Energy Conversion Congress and Exposition, 2011 IEEE. ECCE 2011, pp. 743- 749.