What is Power System Islanding and how to detect it

Term power system islanding comes to the picture when their is an interconnection of power grid with distributed generation (DG) like in DC microgrid a common load is shared between Grid and distributed generation such as solar, wind etc, in such setup when there is an outage at the grid side, supply from the grid is stopped whereas distributed generation still supply's the load in such cases DG connected with load is said to be islanded or standalone.

The Grid outage could be due to many factors such as breaker trip, fault, intentional trip, or grid maintenance, environmental factor, equipment failure. the islanding mode could be intentional or unitentional. in case of microgrid system at the time of Grid failure controllers in microgrid acts in such a way that it itself disconnects from grid and allows DG to power the load.

whereas unitentional islanding could be dangerous due to

Personal Safety

1. Generator may back feed power to the disconnected lines
2. Maintenance safety for workers 
3. public safety risk from broken hanging lines

Power Quality

1. Voltage instability
2. frequency instability
3. Reduced power regulation
4. Power disruption

Equipment damage

1. Equipment could be damaged due to sudden change in the power 
2. due to voltage and frequency instability there is more likely chance for equipment damage

How to detect islanding 

there are two methods of islanding detection passive and active methods, passive method includes detection or sensing of related parameters of such we can give an idea of probablistic failure whereas in active method small signals are generated to the grid and further detecting the change in the signal. 

Passive method includes

1. Under/over voltage detection
2. Under/over frequency detection
3. Rate of change of frequency
4. Voltage phase change detection
5. Harmonics detection 

Active method includes

1. Negative sequence current injection - in this method a negative sequence current is injected through a voltage source converter controller, and quantifying the net negative sequence current at the point of common coupling (PCC).
2. Impedance measurement method - in this method impedance is measured fed by the inverter, a small AC current is forced at PCC and net change in the voltage is detected, when the grid is connected there is almost no change in the impedance as grid is infinite voltage source incase of islanding even a small change in the voltage could result in a good over voltage, thus good change in impedance.
3. impedance measurement at a specific frequency - this method is similar to the impedance measurement in this method inverter forces a signal at a particular frequency and the net harmonics is detected in case of islanding the harmonic is more, otherwise in normal operation this frequency is filtered out.
4. Frequency bias - in this method a signal of slightly different frequency is forced to the grid and the phase shift is adjusted at the end of the signal at 0 voltage and the net change in power factor is detected.