Transformers are essential devices responsible for adjusting available voltages to levels suitable for end users, ensuring reliable power supply. Power utilities frequently install new transformers or upgrade existing ones to meet increasing demand or replace faulty units. Thus, ensuring the effective operation of transformers is crucial for maintaining uninterrupted 24×7 power supply to consumers.
Given their prevalence within the electrical network, transformers are a significant contributor to total technical losses. Faults in them can lead to extensive damage, disrupting power supply to consumers and causing substantial revenue losses for discoms. Continuously in operation, transformers incur technical losses during voltage transformation. Therefore, electric utilities and industries consistently seek diverse methods and technologies to mitigate the inherent losses in transformers.
Need for O&M
Operating beyond specified limits, such as overloading, poses a risk of transformer failure, potentially causing severe disruptions to service, property damage and even endangering lives. Repair or replacement lead times are typically lengthy, underscoring the importance of timely maintenance and adherence to schedules to prevent transformer failures.
Adverse operating conditions, such as harsh weather, overloading, faults, inadequate protection, public interference and poor maintenance, contribute to transformer failures. Variations in load and ambient temperature can lead to moisture ingress, compromising the dielectric strength of transformer oil and causing sludge formation, potentially obstructing oil circulation ducts over time. Therefore, it is imperative to enhance design, enforce quality control during manufacturing, select appropriate components/accessories and conduct effective operations and maintenance (O&M) procedures for these crucial assets to extend their lifespan. Although the natural ageing process cannot be avoided entirely, adhering to these practices can help prevent premature failures.
Key maintenance practices
Standardisation of operational practices
Variations in operational practices between private and government-owned utilities significantly impact system performance and transformer failure rates. High failure rates impose financial financial burden on utilities due to repair and replacement costs and indirect revenue losses from system unavailability. Private utilities have embraced superior operational practices, but both private and government utilities can benefit from further enhancements. They can implement transformer metering with smart metering capabilities to detect and minimise overvoltage, overloading and unbalancing. The use of tap changers in transformers can be minimised to mitigate the risk of failures associated with them. Further, applying special anti-corrosion coatings/paints on the outer tank body can reduce rust and corrosion risks. Fusing efficiency can be enhanced by adopting high rupturing capacity fuse or horn gap fuse arrangements. Incorporating lug/clamp connections between risers and bushings can prevent internal open circuits.
Analysis of cause of failure and maintaining history card for transformers
Transformer failure can stem from manufacturing flaws or inadequate O&M practices. Analysing failure helps in implementing corrective measures and reducing repair/replacement costs. The low tension (LT) side of transformers distributes power to end consumers in India, typically at 11kV/433V levels, though some utilities employ other voltage levels such as 22kV/433V & 33kV/433V. Standard KVA ratings aligned with IS 1180 should be adhered to, with consideration for load forecasting to optimise inventory and minimise costs.
A transformer’s lifespan hinges on its insulation, which deteriorates over time and with temperature fluctuations. Thermal stress from losses in the transformer leads to insulation ageing and gas production, detectable via dissolved gas analysis, similar to a health check for transformers. Rural operating conditions exacerbate failures due to weather, overloading, inadequate protection and poor maintenance of LT and 11 kV lines. Rural transformers face heightened vulnerability to lightning and are often burdened with poor efficiency from unbalanced loading and unauthorised connections. Moisture ingress in these areas weakens insulation, causing sludge buildup and obstructing oil circulation. Neglected maintenance of lines and protective equipment further compounds failure risks.
Inspection
Routine inspections and scheduled maintenance of transformers are essential for early detection of potential issues, allowing utilities to take proactive measures to prevent future problems. Regular checks, particularly for overloading, damaged connectors, worn-out power cords, burning odours, loose plugs or misaligned parts, are critical O&M practices aimed at reducing system failures and fire risks. It is crucial for maintenance crews to adhere to relevant standards, such as ISO 9001-2015/ISO-12000, to ensure a safe working environment for both the equipment and the maintenance staff.
As per the Central Electricity Authority’s guidelines for the O&M of distribution transformers, it is noted that some utilities, which give importance to these maintenance and repair aspects, including training of their O&M staff, are able to achieve a transformer failure rate of less than 2 per cent.
Preventive maintenance            Â
Transformer maintenance involves critical practices to ensure optimal performance and longevity. Regular monitoring of oil levels and prompt leakage rectification is essential. It is crucial to ensure periodic checks for oil breakdown voltage (BDV) and acidity, with observations kept under scrutiny if acidity levels are in the range 0.5-1mg KOH. The breather, equipped with silica gel and an oil seal, requires routine examination for proper functioning. Silica gel dehydration should be conducted as needed. Periodic inspections for loose connections at high voltage (HV) and low voltage (LV) terminations, ensuring integrity and cleanliness, are necessary. Tension on HV-LV terminals caused by cables or conductors should be promptly addressed.
Further, bushings require regular cleaning and inspection for cracks, along with vigilant monitoring for dust, dirt, salt, chemical, cement or acid deposition, which should be promptly rectified. The conservator should be cleaned internally every three years. Additionally, maintaining cleanliness in the substation yard and ensuring the removal of nets, vines and shrubs, is essential.
Each transformer in operation should undergo reconditioning after a specified running cycle, typically every 15 to 20 years. During reconditioning, an active repair approach may be adopted to reduce technical losses, enhance KVA capacity and improve reliability. This approach involves retaining the core while making corrections through winding redesign and/or material change, such as transitioning from aluminium to copper.
Tests
Transformers should undergo rigorous testing procedures both at the manufacturer’s end and in the field, following the established standards. Routine tests include checks for polarity and phase sequence, as well as measurements of no-load current and losses at various voltages, load loss at rated current and frequency. Additionally, tests for impedance voltage, winding resistance, insulation resistance, induced overvoltage withstand test, BDV and moisture content of oil should be conducted.
Special tests include subjecting the transformer to lightning impulse chopped on the tail, short-circuit tests and pressure tests on the transformer tank and its fittings. The pressure relief device should also be tested to ensure proper operation. Additionally, oil samples are taken to comply with relevant standards. Single-phase LV excitation currents are measured across all three phases for reference.
Type tests, conducted on one unit, include temperature rise tests and lightning impulse withstand voltage tests.
Monitoring via IoT
Given the sheer number of operational transformers within power utilities, physical monitoring becomes impractical, necessitating the development of remote monitoring systems. These systems, based on technologies such as general packet radio service, global positioning system, internet of things (IoT) and radio frequency-based smart metering, can be installed on transformers to measure, monitor and record various system parameters. By continuously assessing transformer performance, these meters can promptly alert the control centre via communication channels in the event of abnormal behaviour.
In urban areas, a significant portion of transformers are already equipped with communicable meters featuring automatic meter reading (AMR) capabilities. These meters can be read remotely at data centres, facilitating seamless energy accounting and auditing. However, challenges arise from issues such as day-to-day O&M of AMR systems, modem misalignment or failure, communication disruptions and instances where faulty transformers are replaced but the metering system is not reconnected by utility staff or outsourced agencies.
Challenges
The primary challenge in maintaining transformer performance stems from suppliers/manufacturers’ inconsistency in material usage and manufacturing processes, leading to variations in transformer quality. Additionally, changes in sub-vendors and skilled manpower necessitate careful oversight to ensure consistency in the manufacturing process. It is crucial that transformers are produced in a clean, dust-free environment with humidity control, as any compromise on this front can significantly reduce the transformer’s expected lifespan.
Furthermore, testing options on-site are often limited. Other challenges related to transformers include transportation, handling, loading-unloading, erection, testing and commissioning. This encompasses checks post-receipt at the site, on-site storage, precautions during erection (including oil filling), pre-commissioning checks/tests and final commissioning checks before energisation.
Conclusion
In order to achieve optimal performance and mitigate failures, utilities must adopt best practices for transformer maintenance. Adhering to best practices, conducting rigorous testing and implementing remote monitoring systems are crucial steps in enhancing transformer performance and reliability. Addressing challenges such as manufacturing inconsistencies and limited on-site testing options comprehensively is imperative for maintaining the quality and longevity of transformers in service.