Understanding Power System Islanding

Power system islanding occurs when distributed generation becomes isolated from the power system grid and continues to provide power to the portion of the grid it remains connected to. Islanding can occur through the operation of switching devices such as breakers, disconnects or reclosers. Utilities can also experience islanding with system faults, switching operations, environmental causes and equipment failure. For example, a fault causing a recloser to open and lockout causes the generator to become islanded from the source station. 

Power system islands can be intentional and unintentional. When an island is desired in certain circumstances such as micro-grids, utilities will implement intentional islanding and necessary controls. However, unintentional islanding can be considered a risk to personal safety, power quality and equipment.  

How can Unintentional Islanding be Dangerous?

• Personal Safety
  • Generator may back feed power to disconnected lines
  • Maintenance risk for utility personnel working on lines
  • Public safety risk for downed lines
• Power Quality
  • Power regulation may be reduced while islanded
  • Power disruptions and instability more likely
• Equipment Damage
  • Resulting voltage fluctuations may cause damage to utility and customer equipment

IEEE 1547 – Standard for Interconnecting Distributed Resources with Electric Power Systems, recommends that an island be detected and removed within two seconds of an occurrence.