Distributed Generation System Control Strategies in Microgrid Operation

 Abstract:

Control strategies of distributed generation (DG) are investigated for different combination of DG and storage units in a microgrid. This paper develops a detailed photovoltaic (PV) array model with maximum power point tracking (MPPT) control, and presents real and reactive power (PQ) control and droop control for DG system for microgrid operation. In grid-connected mode, PQ control is developed by controlling the active and reactive power output of DGs in accordance with assigned references. In islanded mode, DGs are controlled by droop control. Droop control implements power reallocation between DGs based on predefined droop characteristics whenever load changes or the microgrid is connected/disconnected to the grid, while the microgrid voltage and frequency is maintained at appropriate levels. This paper presents results from a test microgrid system consisting of a voltage source converter (VSC) interfacing with a DG, a PV array with MPPT, and changeable loads. The control strategies are tested via two scenarios: the first one is to switch between grid-connected mode and islanded mode and the second one is to change loads in islanded mode. Through voltage, frequency, and power characteristics in the simulation under such two scenarios, the proposed control strategies can be demonstrated to work properly and effectively.

Keywords:

Distributed generation, PV, Microgrid, Droop control, PQ control.

Schematic Diagram:

Fig. 1. Schematic of the microgrid.

Expected Simulation Results:

Fig. 2. PQ control under grid-connected mode.

Fig. 3. Droop control for switching modes.

Fig. 4. Droop control for varying load.

Conclusion

In this paper a detailed PV model with MPPT, and PQ and droop controllers is developed for inverter interfaced DGs. The use of PQ control ensures that DGs can generate certain power in accordance with real and reactive power references. Droop controller is developed to ensure the quick dynamic frequency response and proper power sharing between DGs when a forced isolation occurs or load changes. Compared to pure V/f control and master-slave control, the proposed control strategies which have the ability to operate without any online signal communication between DGs make the system operation cost-effective and fast respond to load changes. The simulation results obtained shows that the proposed controller is effective in performing real and reactive power tracking, voltage control and power sharingduring both grid-connected mode and islanded mode. To fully represent the complexity of the microgrid, future work will include the development of hierarchical controllers for a microgrid consisting of several DGs and energy storage system. The function of primary controller is to assign optimal power reference to each DG to match load balances and the secondary controllers are designed to control local voltage and frequency.

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