Intentional islanding describes the condition in which a microgrid or a portion of the power grid, which consists of a load and a distributed generation (DG) system, is isolated from the remainder of the utility system. In this situation, it is important for the microgrid to continue to provide adequate power to the load. Under normal operation, each DG inverter system in the microgrid usually works in constant current control mode in order to provide a preset power to the main grid. When the microgrid is cut off from the main grid, each DG inverter system must detect this islanding situation and must switch to a voltage control mode. In this mode, the microgrid will provide a constant voltage to the local load. This paper describes a control strategy that is used to implement grid-connected and intentional-islanding operations of distributed power generation. This paper proposes an intelligent load-shedding algorithm for intentional islanding and an algorithm of synchronization for grid reconnection.
- Distributed generation (DG)
- Grid-connected operation
- Intentional-islanding operation
- Islanding detection
- Load shedding
Fig. 1. Schematic diagram of the grid connected inverter system.
EXPECTED SIMULATION RESULTS:
Fig. 2. From grid-connected to intentional-islanding operation.
Fig. 3. Synchronization for grid reconnection.
Fig. 4. Phase voltage (top) without and (bottom) with the synchronization algorithm.
Fig. 5. Phase voltage Va without the load-shedding algorithm.
Fig. 6.Phase voltage Va with the load-shedding algorithm.
Through this paper, the control, islanding detection, load shedding, and reclosure algorithms have been proposed for the operation of grid-connected and intentional-islanding DGs. A controller was designed with two interface controls: one for grid connected operation and the other for intentional islanding operation. An islanding-detection algorithm, which was responsible for the switch between the two controllers, was presented. The simulation results showed that the detection algorithm can distinguish between islanding events and changes in the loads and can apply the load-shedding algorithms when needed. The reclosure algorithm causes the DG to resynchronize itself with the grid. In addition, it is shown that the response of the proposed control schemes is capable of maintaining the voltages and currents within permissible levels during grid connected and islanding operation modes. The experimental results showed that the proposed control schemes are capable of maintaining the voltages within the standard permissible levels during grid connected and islanding operation modes. In addition, it was shown that the reclosure algorithm causes the DG to resynchronize itself with the grid.
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