The sudden disconnection of 187 megawatts of electricity from the first unit of the Norochcholai Lakvijaya Power Plant triggered the islandwide power system collapse on October 17, according to a preliminary investigation report submitted by the National System Operator (NSO) to the Public Utilities Commission of Sri Lanka (PUCSL).
The report states that the generating unit disconnected at around 11.43 a.m. within 450 milliseconds, causing the system frequency to drop to 48.75 hertz. The entire national power system collapsed within 4.1 seconds, as a result, the report said.
All five stages of the Under Frequency Load Shedding (UFLS) mechanism were activated in an attempt to stabilise the system. However, the protection measures failed to prevent the collapse, resulting in the disconnection of 449.2 megawatts, or 39.6% of the 1,135 megawatts being generated immediately before the outage.
Generating units at several hydropower stations, including Victoria, Kotmale, Randenigala, Samanalawewa and Laxapana, were also shut down following the activation of their protection systems.
The NSO began restoration operations immediately after the incident. Electricity supplies were restored through four systems — Mahaweli, Laxapana, Samanalawewa/Southern and Colombo — using several hydropower plants and the Kelanitissa Power Plant.
All main substations were restored by 4.58 p.m., while the complete restoration of the national transmission system took five hours and 15 minutes, the report said.
The restoration process also encountered technical issues and operational delays at certain generating units. A technical problem in an auxiliary system delayed the restarting of a generating unit at the Yugadanavi Power Plant.
However, the preliminary investigation has not identified the root cause of the sudden disconnection of the Norochcholai unit.
The NSO said a detailed investigation report would be submitted to the PUCSL at a later date. (Newswire)