ISLANDING

In an interconnected power system network, all generators are connected in parallel with transmission lines and sharing load according to the capacity of the power plant. 

A large power system network is prone to disturbances. These disturbances are due to various factors like a fault in the major transmission line, sudden shut down of the large capacity power plant and failure in the interconnecting transformer. 

These are the most severe and rare disturbances. In this abnormal condition, the entire system is divided into several parts. Which may or may not alive depending on the availability of generating station and load demand. 

In an interconnected power system, if one generator is failed to supply the power, the load is transferred to another generator and if this generator is not capable to sustain the load, it will be overloaded and shut down. 

By this way, the load is transferred to another generator and in the worst condition, all generators are disconnected from the grid. This condition is known as BLACKOUT or CASCADE TRIPPING.

In the event of severe faults, the part of the power system which has generating station and able to handle a load of that area, that part of the system is planned to separate from the grid is called as islanding. 

The islanding is the only way to survive from blackout and save entire grid in a severe fault condition. The main objective of islanding is to restrict the fault or disturbance to the range and try to save power plants from cascade tripping.

The islanded part of the system is separated through under frequency or directional power relay and this part may have more than one generating stations. It is also necessary to maintain a balance between the power generation and demand in this area.

How does it work?

In a power system network, a set of protective relays are connected at the bus. If a severe fault occurred in the system, these relays will sense the disturbance and give the trip command to the breaker of that bus and the plant is isolated from the system by opening the breaker. 

The distributed generators are also disconnected from the grid. The DG may get damaged due to high disturbance and possible to shut down. So, it is necessary to disconnect from the grid to protect the DG in case of islanding. These DG set can use to provide the starting power to the generating station while restarting the grid.

Islanding detection techniques:

1) Under-voltage / over-voltage and Under frequency / over frequency

2) Impedance measurement

3) The rate of change of frequency

4) Voltage phase jump detection

5) Harmonic detection

Advantages:

1) Save power system from blackout

2) Improve reliability of the system

3) Reduce outage area and outage cost

Disadvantages:

1) Islanding is dangerous for the utility crew. The utility workers, who may not realise about live wires.

2) The power quality is the main concern in islanding. Because of unsuitable power quality equipment may get damaged.

3) If DG is working in islanding, there is a chance to damage the DG and maintenance cost of DG is very high.

4) The detection of islanding is a very complicated task. However, there are several schemes and algorithms used to detect islanding.

5) If solar power plants are used as DG, the inverter may get confused in islanding condition.