Sri Lanka’s Blackout Explained: Grid Lessons for India’s Rooftop Solar Push

Immediate disturbance: Sri Lanka’s National System Operator submitted a preliminary investigation to the Public Utilities Commission of Sri Lanka. Reports on its findings say net output from Unit 1 of the Lakvijaya coal plant fell from about 187 MW to 5 MW in roughly 0.45 seconds. The investigation had yet to establish why the unit suddenly lost output.

Possible escalation: The preliminary account discussed the potential loss of non-dispatchable generation, including rooftop solar, after the initial disturbance. The evidence establishes the coal unit’s output drop as the immediate trigger. The extent of rooftop inverter disconnection remains a question for investigation. Automatic load shedding activated, and the grid nevertheless collapsed.

Balancing supply and demand: In an alternating-current grid, frequency measures cycles per second. India and Sri Lanka use a nominal frequency of 50 Hz. If electricity supply suddenly falls below demand, frequency tends to fall. If supply exceeds demand, it tends to rise.

Emergency action: Under-frequency load shedding automatically disconnects selected demand to arrest a frequency fall. Rotating synchronous machines provide inertia, which slows abrupt frequency changes. Fast-responding batteries and suitably controlled inverter-based resources can also support frequency.

Inverter function: Photovoltaic panels produce direct current. An inverter converts it to alternating current compatible with the grid. Its protection settings determine when it remains connected and when it disconnects to protect equipment or people.

Collective response: Thousands of small installations can behave like one large loss of supply if their inverters disconnect together. Fault ride-through lets compliant units remain connected during short voltage or frequency disturbances within prescribed limits. Anti-islanding protection has a separate safety function when part of the network becomes electrically isolated.

Grid visibility: Small rooftop units sit behind consumer meters and connect through local distribution feeders. Operators may see the net effect on demand without seeing the separate generation output. When local production exceeds consumption, electricity may flow back towards upstream equipment and raise local voltage.

Operational flexibility: Solar output varies with weather and time of day. As it falls in the evening, other sources may need to increase supply quickly. Forecasting, batteries, flexible generation and shifting demand can help. An island grid has fewer options for sharing reserves. India’s interconnected grid has wider balancing resources, while local feeders still face their own constraints.

Policy and institutions: PM Surya Ghar: Muft Bijli Yojana supports residential rooftop solar. Distribution companies, or DISCOMs, oversee local connections. The Central Electricity Authority sets technical standards within its remit, while the Central Electricity Regulatory Commission’s Indian Electricity Grid Code provides a framework for secure grid operation.

Practical oversight: Maintain an inventory of installed inverter models and settings, test compliance during commissioning and use feeder-level monitoring where solar penetration is high. Simulate a sudden simultaneous loss of distributed generation before choosing the amount of fast reserves and configuring grid protection.