From 03–04 March 2026, the 220 kV Singhigaon–Dhamdum feeder (Bay-206) was taken under the Annual Maintenance Plan (AMP) 2026 to improve the reliability and safe operation of the transmission line. The main focus of the maintenance was to investigate and rectify a long-standing **B-phase circuit breaker closing failure and pole discrepancy issue. During the inspection, a loose negative DC return connection at Terminal Block X2-42 was identified and corrected, which restored normal operation of the R-phase and Y-phase breaker poles. Further investigation inside the breaker mechanism revealed that the B-phase cross-roller plunger limit switch (-L4) had been damaged and incorrectly wired to a normally open contact, preventing automatic closing of the B-phase. The wiring was corrected to the proper normally closed terminal, which successfully restored synchronized closing of all three breaker poles and eliminated the unsafe manual operating workaround previously used. In addition, insulation resistance testing of the surge arresters was carried out, confirming that the R and Y phases are in healthy condition, while the B-phase arrester showed comparatively lower insulation values, indicating ageing and requiring close monitoring and future replacement. Overall, the maintenance activities successfully restored reliable breaker operation and improved the operational stability of the 220 kV Singhigaon–Dhamdum transmission line.

On 05.03.2026, the 220 kV Malbase–Singhigaon transmission line was commissioned for the Annual Maintenance Plan (AMP). Based on the tests conducted, the R-phase CVT was found to be technically sound and suitable for continued service, although the absence of Y and B phase CVTs requires the protection scheme to rely on alternate voltage sources such as Bus CVTs to maintain directional and distance protection functions. The surge arrester bank for the line has been fully restored following confirmation of internal dielectric failure in the original R-phase unit; a new OBLUM three-stack surge arrester assembly has been installed and all phases now meet the required insulation resistance standards for 220 kV operation. The Y-phase Current Transformer (Bay-206) is in partial compliance, as cores 1, 2, and 5 meet manufacturer specifications while Core-4 failure poses a potential risk to the protection scheme. Additionally, the CVT voltage ratio test conducted with 244 V primary injection produced consistent secondary readings of 0.144 V across protection and metering cores (approximately 18% deviation from the calculated 0.122 V), which is attributed to normal CVT behaviour under low-voltage testing and does not indicate any defect.

On 05.02.2026, as per the Annual Maintenance Plan (AMP) approved by BPSO, a one-day maintenance program was successfully carried out on the 220 kV Malbase–Singhigaon feeder. The maintenance team commenced the work by isolating the bay equipment to facilitate the required testing and inspection.

Insulation Resistance (IR) testing was conducted on the 198 kV surge arresters for R, Y, and B phases. During the test, the R-phase surge arrester was found to have a low insulation resistance value, below 1000 MΩ, which is not within the permissible limit. Consequently, the defective R-phase surge arrester was replaced with a new surge arrester arranged from Singhigaon Substation.

Further, the secondary winding resistance tests of the Current Transformer (CT) and Capacitive Voltage Transformer (CVT) were carried out and found to be normal. The insulation resistance tests of the CT and line CVT were also performed and CVT Voltage Ratio test has been carried out satisfactorily. All planned maintenance activities were completed as scheduled, and the one-day maintenance program concluded successfully, ensuring the reliability and safe operation of the feeder.

On 24 January 2026, a planned shutdown of the 66 kV Bhutan Concast Feeder was executed by the Construction Project Office (CPO), Phuentsholing, to facilitate critical construction, testing, and commissioning activities associated with the integration of a newly constructed double-circuit 66 kV transmission line at Singhigaon Substation. The shutdown was carried out in accordance with power system operational and safety protocols to ensure secure working conditions for personnel and to prevent any risk to the existing network infrastructure.

During the shutdown period from 09:42 hours to 17:04 hours, the 66 kV Bhutan Concast Pvt. Ltd. and 66 kV Druk Wang Alloys Limited feeders experienced interruption of power supply. The total outage duration was calculated to be 7 hours and 22 minutes. Both industrial consumers are connected through a radial network configuration, and therefore, no alternative supply path was available during the shutdown window. This operational constraint highlights the inherent vulnerability of radial systems in comparison to meshed or ring network configurations within high-voltage distribution systems.

The primary objective of the shutdown was to complete the cable termination, testing, and integration works for the second circuit of the newly constructed double-circuit transmission line at the Singhigaon end. The work scope involved the installation of high-voltage cable terminations, rectification of identified technical deficiencies in the cable system, and verification of electrical and mechanical integrity of the termination assemblies. These activities were mandatory prior to tapping the second circuit from the existing 66 kV Bhutan Concast Feeder, ensuring compatibility and stability of the interconnected network.

The development of the double-circuit transmission line was implemented in a phased manner. The first circuit was successfully completed and test-charged on 03 December 2025 at 11:16 hours, supplied from the 66 kV GIS Spare Feeder at Singhigaon Substation. The successful energization of the first circuit confirmed the adequacy of insulation coordination, protection settings, and system stability under operational conditions. The second circuit, completed on 24 January 2026, is designed to be electrically tapped from the existing Bhutan Concast Feeder following successful completion of all commissioning and testing procedures.

During the shutdown, comprehensive electrical testing was conducted on the newly terminated cable system. This included insulation resistance testing, continuity verification, and high-voltage withstand testing to ensure compliance with applicable technical standards. Mechanical and structural inspections were also performed on termination hardware, gantry connections, and associated fittings to confirm mechanical robustness and alignment. Additionally, earthing boxes were installed and integrated into the substation grounding system, and the continuity of the earthing network was verified to ensure effective dissipation of fault currents and enhancement of personnel safety.

The cable termination works were carried out under the existing Bhutan Concast gantry structure, which forms a critical interface between the overhead line system and the underground cable network. Testing under the live system environment required strict adherence to isolation procedures, permit-to-work systems, and safety clearances to avoid operational hazards. The rectification of cable termination points and associated fittings was completed successfully, ensuring that the system meets the required electrical, thermal, and mechanical performance criteria for long-term operation.

The successful completion of the second circuit termination and associated testing activities represents a significant advancement in the reinforcement of the 66 kV network in the Phuentsholing region. The integration of the double-circuit configuration is expected to enhance system reliability, improve operational flexibility, and reduce the risk of extended outages arising from single-circuit failures. From a power system perspective, the project contributes to improved load-sharing capability, enhanced fault tolerance, and strengthened network resilience, particularly for high-demand industrial consumers.

In conclusion, the planned shutdown of the 66 kV Bhutan Concast Feeder on 24 January 2026 was technically justified and operationally essential for the safe execution of cable termination and commissioning works associated with the second circuit of the double-circuit transmission line at Singhigaon Substation. All activities were completed successfully within the approved shutdown window, and the system is now technically prepared for final integration of the second circuit into the existing feeder network. The project demonstrates a systematic and technically sound approach to transmission system expansion and substation infrastructure development within the regional power system.

On 16 January 2026, maintenance work was successfully carried out on the 66 kV Bhutan Concast Feeder between 11:00 hours and 13:14 hours to address a strand cut observed on the line. During the shutdown, the R-phase strand-cut jumper at the cable termination was replaced. This rectification restored the mechanical strength of the connection and ensured its full current-carrying capability, thereby improving the reliability of the feeder. 

Following the repair, Insulation Resistance (IR) testing of the line surge arrester was conducted, and the results were found to be satisfactory, confirming the integrity of the insulation system. In addition, the earthing system was tested, and the measured earth resistance values were within the permissible limits as per substation standards. This verified the effectiveness of the grounding system and ensured the overall operational safety of the installation.

The feeder was subsequently restored to service after completion of all tests, ensuring safe and reliable operation

The 220 V DC Battery Bank–I has been successfully restored to service following the installation and commissioning of a new 220 V DC, 400 Ah battery bank on 07-01-2026. This DC battery system plays a critical role in ensuring uninterrupted and reliable DC supply to protection relays, control circuits, SCADA systems, and emergency tripping operations, especially during AC supply failures. The installation and commissioning activities were carried out with the active involvement of shift operators, the maintenance team, and CST OJT students, providing a valuable hands-on learning opportunity alongside practical exposure to substation DC systems. Key activities included battery placement, polarity checks, inter-cell connections, DC bus voltage verification, charger health checks, insulation resistance testing, and load observation to confirm system stability. Bringing the battery bank back into service has significantly enhanced the reliability and resilience of the substation control and protection system, ensuring safe operation of critical equipment and reinforcing the importance of well-maintained DC auxiliary power systems in power networks.

On 27 December 2025 (Saturday), a total of 30 technicians from the National Center for Hydrology and Meteorology (NCHM), representing all 20 Dzongkhags of Bhutan, visited the Class C Meteorology Station at Malbase as part of their ongoing workshop program. The visit aimed to provide participants with practical exposure and hands-on understanding of meteorological instruments, station operations, and data collection processes. During the session, technicians were briefed on weather observation techniques, real-time data monitoring, equipment maintenance, and reporting procedures followed at the station. Officials at the Malbase station shared insights on operational challenges, safety practices, and the importance of accurate meteorological data for weather forecasting, hydrology, disaster preparedness, and public safety. The interaction also encouraged knowledge exchange among technicians from different Dzongkhags, strengthening coordination and standardization of meteorological practices nationwide. Overall, the visit was highly informative and productive, enhancing the technical capacity of participants and reinforcing NCHM’s commitment to improving weather and climate services across the country.

On 04 December 2025, two Engineers from the Bhutan Power System Operator (BPSO) visited the Malbase Substation as part of a field familiarization and system study tour. The purpose of their visit was to understand the ground-level operational challenges, review system configurations, and observe the practical aspects of substation operations. During the visit, the Substation Head and  Engineers provided a detailed walkthrough of the substation’s functioning. They highlighted key operational challenges, ongoing technical issues, and shared insights into the real-time constraints faced during day-to-day system operation. The team also reviewed the existing SCADA integration, equipment health conditions, protection coordination, and communication link status. This interactive session was immensely beneficial as it helped the visiting BPSO officials gain a deeper understanding of the site realities, strengthening the coordination between the substation team and system operators. Such field visits play a crucial role in enhancing system reliability, ensuring better decision-making, and improving national grid operations. The familiarization tour stands as a positive step toward collaborative problem-solving and efficient grid management.

On 01 December 2025, the Morning Shift operators, together with the Maintenance team (led by Mr. Dhan Bir Rai), successfully carried out the replacement of the Silica Gel in the 200 MVA ICT breather system. The existing silica gel had completely turned pink, indicating that it had reached its moisture-absorption capacity and required immediate replacement to maintain the health of the transformer.

 Technical Background: Importance and Function of Silica Gel in Transformers

Silica gel is an essential component in the transformer breather assembly. Its primary function is to absorb moisture from the atmospheric air that enters the conservator tank during transformer breathing. When the transformer cools down, it inhales outside air; without a moisture filter, this humid air can lead to:

1. Deterioration of transformer oil quality

2. Reduced dielectric strength

3. Formation of sludge

4. Accelerated ageing of insulation materials

To prevent these issues, silica gel crystals are used. They are usually blue (or orange) when dry and turn pink as they absorb moisture. The color change is a visual indicator that the gel has reached saturation and needs replacement or regeneration.

Work Carried Out

The team performed the following steps:

1. Isolated the breather assembly from the 200 MVA ICT.

2. Removed the saturated pink silica gel, ensuring no contamination entered the breather.

3. Installed newly purchased silica gel, fully activated and dry.

4. Reassembled the breather unit and verified proper sealing and airflow.

 Result: The replacement ensures that the transformer continues to inhale clean, dry air, thereby protecting the insulation system and prolonging the life of the 200 MVA ICT.

Photo Credit @ Mrs. Yeshi Choden | 26-11-2025 | @11:22 Hours

How the 200 MVA ICT Was Brought Back into Service Without Interrupting the 300 MVA ICT under Tala line Shutdown on 26.11.2025

In the 400 kV switchyard, where a one-and-a-half breaker scheme is adopted, we were able to restore the 200 MVA ICT without affecting the supply through the 300 MVA ICT. To maintain continuity, both the 404 Main Breaker (Tala line) and the 405 Tie Breaker were kept closed, while the Tala Line Isolator (404-89L) remained open. With this configuration, 400 kV Bus–I remained energized through the Tala line bay, which in turn continued to feed the 300 MVA ICT and also kept Bus–II energized via the tie breaker. For charging the 200 MVA ICT, it was necessary to close the 409 main bay isolators (409-89M R/Y/B). To do this without causing any interruption, we first opened Breakers 408-52 and 409-52. This operation did not disturb the bus conditions because both Bus–I and Bus–II remained energized via the one-and-a-half breaker configuration. After the area was made safe, all three phases of the 409-89M (R/Y/B) isolators were closed. Once the isolators were secured, Breakers 408-52 and 409-52 were re-closed, thereby charging the 200 MVA ICT on the HV side. Following this sequence, the 200 MVA ICT was successfully energized at 12:07 hours, and subsequently brought on load at 13:01 hours, all accomplished without any interruption to the 300 MVA ICT.