What is anti-islanding protection and why is it required?

Anti-islanding prevents grid-tied inverters from energizing a de-energized grid section (island). Without it, utility workers could be electrocuted servicing what appears to be a dead line. IEEE 1547 requires inverters to detect islanding and disconnect within 2 seconds. Detection methods: passive (monitor voltage, frequency, phase changes), active (inject disturbances and monitor response - frequency shift, impedance measurement), and communication-based (transfer trip, power line carrier). Challenges increase with high penetration as multiple inverters can support voltage/frequency within limits.