In an interconnected power system network, all generators connected in parallel with transmission lines. And each power plant sharing load according to their capacity.
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 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 failed to supply the power, the load of that generator transferred to another generator. And if this generator is not capable to sustain the extra load, it will be overloaded and shut down.
In this way, the load transfer to another generator and in the worst condition, all generators disconnect from the grid. This condition 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 planned to separate from the grid called islanding.
The islanding is the only way to survive from the blackout and save the 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 separated through under frequency or directional power relay and this part may have more than one generating station. It is also necessary to maintain a balance between the power generation and demand in this area.
How does Islanding work?
In a power system network, a set of protective relays connected on 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 isolated from the system by opening the breaker.
The distributed generators also disconnect from the grid. The DG may get damage due to high disturbance and possible shut down.
So, it is necessary to disconnect from the grid to protect the DG in the case of islanding. These DG sets can use to provide the starting power to the generating station while restarting the grid.
Islanding detection techniques
- Undervoltage / overvoltage and Under frequency / over frequency
- Impedance measurement
- The rate of change of frequency
- Voltage phase jump detection
- Harmonic detection
- Save power system from blackout
- Improve the reliability of the system
- Reduce outage area and outage cost
- Islanding is dangerous for the utility crew. The utility workers, who may not realize about live wires.
- The power quality is the main concern in islanding. Because of unsuitable power quality equipment may get damaged.
- If DG is working on islanding, there is a chance to damage the DG and maintenance cost of DG is very high.
- The detection of islanding is a very difficult task. However, there are several schemes and algorithms used to detect islanding.
- If solar power plants are used as DG, the inverter may get confused in islanding condition.