December 9 blackout: CEB fails to learn lessons from 2021 disaster
The long power outage last Saturday started when a lightning strike caused both Kotmale-Biyagama transmission lines to trip--but it cascaded countrywide because incorrect protection systems then triggered a breakdown in the two Kotmale-New Habarana transmission lines a full minute after power transferred there.
The shortcomings that led to this should have been corrected after the last countrywide blackout on December 2, 2021, exactly two years ago, power sector sources said.
But a near-identical power system failure followed by an extended restoration delay on December 9 this year proves that technical recommendations gazetted by the Public Utilities Commission of Sri Lanka (PUCSL) have not been implemented.
The blackout of 2021 occurred when one 220kV transmission line connecting the Kotmale grid substation to the Biyagama grid substation tripped followed by the second one.
Then, the total power system collapsed. And, like the blackout of last week, it took six hours for power to be restored to the whole country.
“The occurrence of a blackout due to the failure of one transmission line can be attributed to a failure in the protection system of the transmission network,” a power sector expert said. “This is further highlighted by the fact that it took more than a minute to start the cascading failure after the first tripping of the Kothmale-Biyagama line. Therefore, there was ample time for a properly coordinated protection system to prevent a complete blackout.”
Some power sector sources maintain that the countrywide breakdown could have been avoided if there had been funds to complete the longdelayed Kotmale-Polpitiya transmission line (the stretch from Polpitiya to Padukka is done). But others say that if the recommendations made after the last incident had been implemented, a cascade could have been avoided even without the overdue transmission line being in place.
After the breakdown of the Kotmale-Biyagama lines last week, power shifted to the Kotmale-New Habarana lines and even stabilised, the expert earlier quoted said. But the load current then overshot the existing power setting of those lines and they too tripped, leading to total system failure.
Following a public hearing after the 2021 breakdown, the PUCSL had recommended that the CEB should review and amend its line protection document to avoid “unnecessary tripping of transmission lines”.
“The last blackout was very similar and they have not revisited and adjusted the settings of the protection system to prevent a blackout occurring under comparable circumstances,” the expert said.
Measures have also not been taken to update the restoration manual, which is why it took an identical six hours for power to come back. Colombo city was again the last to receive electricity when it should have been first. And Lakvijaya unit 1 in Norochcholai, which was the only one operational at the time, also powered down as the coal power plant cannot keep running for a long period without grid power for auxiliaries.
The recommendation made after the 2021 breakdown was that the restoration manual be improved “with a review of the priority listing to incorporate critical loads like hospitals, etc., and also considering the adverse effects from industrial/commercial loads having large starting currents”.
The PUCSL also called for a further investigation of the delay in restoring Colombo city power. “But the same thing happened this time, two years later,” the expert said.
The last blackout was very similar and they have not revisited and adjusted the settings of the protection system to prevent a blackout occurring under comparable circumstances