Incorrect settings at new Habarana caused blackout; it was avoidable

The Public Utilities Commission of Sri Lanka (PUCSL) has published the CEB preliminary report for the total system failure. 

The data shows (refer image 1) that after both lines of Kotmale-Biyagama tripped, the system stabilised with frequency at 49.7 (within the acceptable range of 49.5-50.5) in 20 seconds. The Mahaweli complex generation (approx. 750 MW at the time of tripping) began flowing through Kotmale-New Habarana-New Anuradhapura-Puttalem to Colombo through the 220 kV transmission network.

After stabilisation and operating for 40 seconds, the new Habarana GSS tripped causing the nationwide blackout. The overcurrent setting at new Habarana was set at 800A whereas 850A was flowing which triggered the blackout sequence.

As per CEB Protection Development Unit Chief Engineer the sequence of events is as follows:

(1) Biyagama-Kothmale 220kV line 1 and 2 tripped from both ends due to the operation of Line Differential Protection at 17:02:48 hours. Multiphase (V -B) to earth fault of magnitude in the range of 4kA has occurred in both lines resulting in the tripping of all three phases and clearing the fault within 50ms by the operation of line differential protection of both main 1 and 2 protection relays. Both lines reclosed from Biyagama end after the dead time of 800ms. (Refer figure 2,3,4 & 5) Rec losing from Kothmale end was not attempted since the phase angle of the bus voltage and line voltage exceeded the synchronism check setting value of 20 degrees.

(2) After the tripping of Biyagama Kothmale line 1 and 2, power swing was detected in the 

Kothmale - new Habarana lines lasting approximately 20 seconds and subsequent stabilising while maintaining the system frequency above 49.7Hz. (Refer figure 9, I0 & II)

(3) After the power swing condition, the load current of new Habarana- Kothmale line 1 and 2 have reached around 850A which exceeded the overcurrent setting 800A (with TMS 0.2 1) leading to the tripping of new Habarana - Kothmale line 1 at 17:03:49 hrs. This further overloaded,

new Habarana - Kothmale line 2, resulting in the tripping of the line at 17:03:50 hours. Subsequently, Kothmale end frequency has increased up to 63Hz and the frequency Biyagama - Norochcholai side of the network has dropped to 40Hz resulting in the total system failure.

The full report is available at https://www.pucsl.gov.lk/wp-content/uploads/2023/12/System-Failure-09th-December-2023.pdf

Under the condition 30(10) of Electricity Transmission and Bulk Supply Licence a detailed report on the failure including the actions taken to avoid similar failures in future, should be submitted to the PUCSL on or before 9 January 2024.

 Following is a Q&A with an expert on the subject.

Q: Should CEB have anticipated both lines of Kotmale-Biyagama lines tripping?

Yes, as this event has occurred before, including during the 2021 blackout. In fact, I have reliable information that CEB had an internal high-level discussion on this matter in November.

CEB should have modelled this scenario and understood where the energy will flow and realised that the settings at New-Habarana will create a tripping event and a blackout. 

Q:  Can Kotmale-new Habarana-new Anuradhapura line handle this much power?

Yes. The conductor used is rated for 960A for continuous operation and can take even 1500A for a short period of time. When the Mahaweli complex is generating 750 MW, it does not make sense to keep new Habarana at 800A. 

Q:  If new Habarana overcurrent settings were set at a higher level, would it have saved the grid from a blackout?

 Yes. As the image shows, system frequency stabilised after 20 seconds from Kotmale-Biyagama lines tripping. The system had stable operation for 40 seconds (a really long time in power systems). It could have operated like this until the system operators manually reconnected Kotmale-Biyagama lines. 

Q:  Who is responsible for managing these settings?

The Transmission Protection team of the CEB is responsible for these settings. They are under AGM Transmission. 

The previous two blackouts - one caused by both lines of Kotmale tripping (allegedly by a gentleman of the CEBEU with a jumper cable) and the other caused by a failure at Kerawalapitiya - are similarly caused by incorrect and improperly co-ordinated transmission protection schema. It is important to note that the Transmission Protection head during the previous blackouts is now heading CEB transmission division, and this lack of planning for known contingencies is unacceptable.

But this being CEB and Sri Lanka, there is no repercussions for doing this much damage to the country. 

Q:  Is it true that we need new transmission lines or other protection schemes to avoid this recurring problem?

No. CEB has already increased the overcurrent settings at new Habarana. The system is capable of taking the full generation to load centers using the existing 220kV network.

CEB’s answer to all system issues is to build massive expensive transmission extensions. A local voltage stabilisation issue in Hambantota resulted in CEB building the longest transmission link of the country. Even after Sampur coal plants were cancelled, CEB continued to build the transmission line designed to evacuate power from there. Our transmission network is over designed and underutilised. It’s not cheap either - the new Habarana grid substation is world class with the most sophisticated technology of the world. It is not a technology or infrastructure problem.

Transmission network construction is an oligopoly in Sri Lanka - as well as lending for the same (almost exclusively by ADB with some JICA funds). Lenders happy, construction companies happy, some CEB officials also happy. People pay.

Q:  What happens next?

Who knows with the CEB? This week CEB officials engaged in furious PR exercise (heard it is coming from the Chairman itself) denying CEB is at fault. (Then it must be people’s fault for using electricity). No action was taken against people who were negligent in keeping incorrect protection settings at CEB during previous blackouts - so don’t expect otherwise this time. At least this time, protection settings were changed so the same problem does not recur. It will be no-one’s fault, experts will shift blame away by confusing the public, and that will be the end.

Q:  Is there a better way? 

 Yes, because lightning strikes are common all over the world. One lighting strike in one transmission link in other countries doesn’t take the whole grid down. Because they have proper ‘protection coordination’. The grid has a large number of programmable protection devices, and these have to be set correctly for our grid based on how it operates. In other countries, all of these are modelled in a specialised software and system contingencies are simulated, and protection devices are adjusted to ensure transmission grid stability for all known contingencies. 

CEB does not do that. CEB has the software, but the device data is not updated. Grid substations are rumoured to be running with the commissioning settings, and not updated. As long as we maintain this way of operating, we will continue to have blackouts. 

A USAID project is helping CEB to update the model and train the engineers. But maintaining and adjusting the settings is a CEB task. Whether they will do this is anyone’s guess.