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Control
a) Control Strategy for grid-connected Mode
Interface tie-line power control
In order to avoid the impact of large tie-line power variation, the output of energy storage systems and DGs should be under control to guarantee the power flow of the point of common coupling (PCC) within the acceptable range.This makes the microgrid operate as a controllable source/load for a friendly grid-connected distributed system.
Ancillary Services
For higher operation reliability and power quality, the output of ESS and DGs can be controlled to supply the ancillary services, such as frequency and voltage regulation, for power grid based on dispatch enter commands or real-time system operation condition.
Energy storage management
Fluctuation suppression: The intermittent feature of wind generation and solar generation impact the power quality and stability of microgrid greatly. However, the energy storage system has the capability to dynamically absorb or release the power depending upon grid needs. Special strategy is set in the controller to suppress the power output fluctuation of wind generation and solar generation and realize the smooth power output of wind generation and solar generation.
Peak shaving: When microgrid operates in the grid-connected mode, it injects power into the distribution system during the peak-load period and absorbs power from the distribution system during the valley-load period. The peak-valley schedule curve function is set in the controller, which receives the schedule curve issued by the dispatching center and controls the power output to make the microgrid an excellent solution for the distribution system. A peak-valley power output curve can also be set locally to generate power during the peak-price period and absorb power during the valley-price period to obtain the benefit of peak-valley price difference which ultimately enhance the economic efficiency of the microgrid.
b) Control Strategy for Islanded mode
Frequency & voltage emergency control
In islanded mode, the power deficiency or excess may lead to the sharp variation of frequency and voltage. This severely impacts the normal operation of microgrid and causes the system breakdown. The frequency & voltage emergency control is set to balance the power demand within microgrid and to recover the voltage and frequency within the allowable operating ranges by the energy storage output control, fast load shedding and DG disconnection.
PV power limit control
In order to avoid the automatic shutdown or damage of generator (diesel generator, gas engine) due to its too low power output, the PV power output is limited by the automatic generation control to maintain the power output of generator within an allowable range.
c) Transient process control
Switching between grid-connected and islanded
Switchover from grid-connected mode to islanded mode: When the grid is de-energized due to fault, overhaul, etc., the controller quickly detects the islanded condition, trips the circuit-breaker of PCC and sends the islanded signal to the energy storage and DGs for the switching between microgrid's two operating modes.
Synchronous grid-connection
Switchover from islanded mode to grid-connected mode: The controller provides automatic synchronous switching-on function for a safe and reliable grid inter-connection of microgrid.
