The electricity supply to the state of Sarawak in Malaysia is based around a long 275kV double circuit line running the length of the state. This line transfers power from northern generation to the state capital, Kuching, in the south. The line is vulnerable to lightning strikes which can trip both circuits and split the Sarawak network, resulting in a generation excess in the north and a generation deficit in the south. Sarawak Energy has installed a System Protection Scheme in the south which is designed to prevent a blackout by shedding large amounts of load in the event of a network split.
In separate incidents in 2015 and 2016, lightning strikes split the network and resulted in blackouts in the southern network around Kuching, despite the load shedding actions of the System Protection Scheme. Soon after each incident, Sarawak Energy contracted PSC to carry out independent investigations to determine the likely cause and possible mitigations. PSC’s investigations were based on interviews with Sarawak Energy engineers, and dynamic stability simulations on PSS/E using network data and synchro-phasor measurements provided by Sarawak Energy.
For the 2015 incident, PSC found that a lightning strike between Kemena and Selangau and single phase auto-reclose action, resulted in a two phase connection between the northern and southern networks with a high positive sequence impedance. Imminent loss of synchronism was only prevented by distance protection tripping the adjacent Selangau to Oya Road circuits (Figure 1). Load shedding acted correctly but the subsequent overvoltage, transformer saturation, and harmonic currents resulted in generation tripping on differential protection (due to the lack of harmonic restraints), or excitation protection (due to a defective circuit board). This additional loss of generation led to the blackout of the southern network. PSC’s recommended mitigations included the completion of an ongoing project to install parallel lines to strengthen the network, application of harmonic restraints on differential protection, and modification of auto-reclose schemes to prevent two phase connections.
For the 2016 incident, PSC found that a lightning strike between Selangau and Oya Road tripped the double circuit 275kV line and split the network. Load shedding acted correctly but the subsequent over-voltage tripped southern generation on overflux protection which had been commissioned with excessively sensitive settings. PSC’s recommended mitigations again included the completion of parallel lines to strengthen the network, plus improved procedures around protection coordination.
PSC presented the findings of the investigation to the electricity regulator and to the Minister of Energy who expedited the implementation of our recommendations.