Islanding in Power System:
Islanding is the intentional isolation of a part of power system during external widespread grid disturbance. This isolated part of Grid is called Island. Such a disturbance may lead to black out. Therefore, islanding scheme provides a mean to continue to supply power to the essential services in a zone or area.
We know that Grid is an interconnected system of Generators and Transmission Lines. All the connected Generators run in synchronism. However, if a major disturbance occurs, the Transmission Lines may trip which in turn may create a cascaded effect. This cascaded effect, may eventually lead to collapse of entire Grid and hence black out.
Islanding scheme in power system is designed in such a way that, in case of major Grid disturbance as sensed by the protection element, a portion of system is isolated by tripping the pre-defined tie lines / transmission lines. Thus, isolating the part of system from the remaining Grid. Thus, the effect of Grid disturbance is eliminated to affect this Island.
The objective of islanding are as follows:
- Isolate a part of power system from the Grid to make Island.
- Continue to supply power in Island.
- Avoid tripping of Generators in the Island.
- Quick restoration of remaining system.
There are various method to detect the Grid disturbance to initiate Islanding Scheme. On such method is to sense the Grid frequency. Grid frequency is directly related to load. If the load on Grid increases, the frequency will go down. However, in case of decrease in load, the Grid frequency will increase.
One major cause of Grid disturbance is unbalance in Generation and Load. Mismatch between the two will result in change in frequency from the 50 Hz (assuming Grid voltage to be 50 Hz during normal condition). Therefore, under-frequency relay is employed to sense major Grid disturbance and initiate load shedding by opening designated feeder, prior to islanding, to ensure generation rich island. Low Grid frequency in combination of power flow toward Grid initiates Islanding and hence simultaneous tripping of all pre-defined lines to make island.
Once the island is former, it is very important to control the frequency of Generating Units and maintain load generation balance in the island for successful survival after the islanding.
Advantage of Islanding:
The main advantage of Islanding is that, power supply is not interrupted in the island even during the Grid disturbance. This helps to supply start up power to various Power Plants to restore the system.
Restoration of island is quite easier when compared to restoration of whole system from black out state. The island is synchronized to Grid for integration