This paper deals with robust control strategy for a distributed generation system (DGS), which operates in both islanded and grid-connected modes. Generally, in the low-voltage islanded mode of DGS, the PCC (Point of Common Coupling) voltages are unbalanced due to the unbalanced load connection. Therefore, in an islanded mode of DGS, the LSC is controlled using the IPR (Improved Proportional Resonant) controller to maintain the PCC voltages quality within the IEEE-1547 standard. Moreover, the DGS is capable to synchronize to the grid without any transient current. During the change of modes of DGS, large transients occur in the battery current due to the switching of battery control. This problem is resolved by the presented bidirectional DC-DC converter control strategy and robust ILQSOGI (Inner Loop Quadrature Second Order Generalized Integrator) based PLL. The effectiveness of this DGS control strategy is verified by the corresponding MATLAB/Simulink platform under load unbalance, solar irradiance changes and during mode of switching. Moreover, the simulation results are validated using the test results and show the robustness of the control strategy during abnormal grid voltage condition.
- Solar Photovoltaic Array
- Power Quality
- Bidirectional DC-DC Converter
- Load Side Converter (LSC) and Machine Side Converter (MSC)
Fig.1 Proposed DGS configuration
EXPECTED SIMULATION RESULTS:
Fig. 2 Performance of DGS during mode of switching from IMS to GCM
Fig. 3 Power quality indices of DGS in GCM (a) harmonic spectrum of ig (b) harmonic spectrum of iL
Fig. 4 Performance of BDC control under mode of transition (a) without and with BDC control under grid connection (b) without and with BDC control under grid disconnection.
Fig. 5 Comparison of load voltage waveforms (a) with proposed islanded control technique (b) conventional islanded PI control techniq
Fig. 6 Comparison of load voltage THD (a) with proposed islanded control technique (b) conventional islanded PI control technique
Fig. 7 Performance of DGS (a) solar and wind power variation in IMS (b) at unbalanced load condition in IS mode
The proposed islanded control technique has used the positive sequence load current components with PR control, which has improved the load voltage quality under unbalanced nonlinear load condition and the results have proven the robustness of control technique in islanded mode of DGS. Simulated results have shown the significant difference in PCC load voltage quality using conventional and proposed islanded control technique. Moreover, simulated results have proven the good load voltage quality under unbalanced nonlinear load condition and the range of load voltage quality lies under the IEEE-1547.4 standard. Experimental results show the robustness of the control strategy, which is capable to operate the DGS in different modes such as in grid connected mode and islanded mode. Moreover, the transient free mode change is also presented through test results. The qualities of PCC voltages and currents are also maintained under the IEEE-1547 standard, in the grid connected mode, an islanded mode and during mode transitions. Test results have presented the performance of DGS under different dynamic conditions and validated the robustness and effectiveness of control schemes. Test results have also shown the effectiveness of feed-forward term in grid connected mode and the smooth operation of grid connected mode under battery disconnection.
 B. Zeng, J. Zhang, X. Yang, J. Wang, J. Dong and Y. Zhang, “Integrated Planning for Transition to Low-Carbon Distribution System with Renewable Energy Generation and Demand Response,” IEEE Trans. Power Systems, vol. 29, no. 3, pp. 1153-1165, 2014.
 M. Savaghebi, A. Jalilian, J. C. Vasquez and J. M. Guerrero, “Secondary Control Scheme for Voltage Unbalance Compensation in an Islanded Droop-Controlled Microgrid,” IEEE Trans. Smart Grid, vol. 3, no. 2, pp. 797-807, June 2012.
 IEEE Guide for Design, Operation, and Integration of Distributed Resource Island Systems with Electric Power Systems, IEEE Standard
 Bhutto, Ghullam and Bak, C.L. and Ali, Ehsan, “Controlled Operation of the Islanded Portion of the International Council on Large Electric Systems (CIGRE) Low Voltage Dist. Network”, Energies, 20.17.
 M. E. Baran and F. F. Wu, “Network reconfiguration in distribution systems for loss reduction and load balancing,” IEEE Trans. Power Delivery, vol. 4, no. 2, pp. 1401-1407, April 1989.