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AS 4777.3-2005
What Is Islanding and Why Is It Dangerous?
Islanding occurs when a grid-connected inverter continues to supply power to a section of the distribution network after the grid has been disconnected — for example, when a fuse blows or a lineworker opens a pole switch for maintenance. The inverter's output energises the "island" of network that is now isolated from the main grid. A lineworker who opens the circuit believing it is de-energised then encounters a live conductor — with fatal consequences.
Anti-islanding protection is therefore one of the most safety-critical requirements in AS 4777.3 and a non-negotiable condition of every grid connection approval in Australia.
How Anti-Islanding Works
Inverter manufacturers implement anti-islanding using one or more of these methods:
Method | Principle | Detection Time |
|---|---|---|
Active frequency shift (AFS) | Inverter deliberately shifts output frequency; without grid, frequency drifts to threshold | <2 s |
Slip-mode frequency shift | Similar to AFS but uses phase slip to accelerate detection | <2 s |
Reactive power variation | Inverter varies reactive power output; without grid, voltage changes to threshold | <2 s |
Rate of change of frequency (ROCOF) | Detects rapid frequency change when large loads disconnect with the grid | <1 s |
The Commissioning Anti-Islanding Test
AS 4777.1 and CEC requirements specify a functional test at commissioning to verify anti-islanding operation. The test procedure:
Ensure the inverter is operating at normal output (minimum 10% of rated power)
Open the AC isolation switch at the main switchboard while the inverter is running
The inverter must cease output within 2 seconds of disconnection
Record the measured disconnection time on the commissioning form
Verify the inverter does not restart within 60 seconds of the switch being closed again (reconnection delay)
Test Safety: Never perform an anti-islanding test with sensitive loads (medical equipment, industrial controls) connected to the same circuit. The disconnection and reconnection events create momentary voltage disturbances.
Engr. Jason Morales — Founder, SolarEnergyPH
