Transformers play a key role in the electricity power system, stepping up and stepping down voltages as per the network requirements. It is a mature technology, and its basic design and principal elements have remained unchanged over the years. Yet, there is significant demand from discoms in India for innovations and improvements in the quality of equipment installed, given the low average life expectancy and high failure rates.
Against the theoretical life of 30 to 35 years, distribution transformers (DTs) in India seldom survive beyond 10 to 20 years. The reliability of DTs varies considerably across utilities. For instance, private utilities like CESC Limited and Tata Power Delhi Distribution Limited (TPDDL) report DT failure rates of 0.5 per cent and 1 per cent respectively, while these rates are substantially higher at 5 per cent for West Bengal Power Development Corporation Limited and 15 per cent for Jaipur Vidyut Vitran Nigam Limited. In general, the failure rate is higher for lower-rating transformers of 16 kV to 25 kV.
Various factors collectively contribute to the low life expectancy and high failure rates of DTs, including shoddy manufacturing, poor maintenance practices and unbalanced load. Utility experience suggests that insulation is the leading cause of failure in the case of three-phase transformers, while overloading is the biggest challenge for single-phase transformers. Addressing these challenges requires the collective efforts of all stakeholders including the manufacturer, regulator and the operator.
At a recent conference organised by Power Line, utilities, equipment manufacturers and government representatives discussed the challenges, various technology solutions and best practices available as well as the standards and regulations that can be implemented to address these challenges. Following are the key highlights…
Best practices
Thorough inspection of transformers at the time of procurement to verify the dielectric strength of the core material is crucial for ensuring their longevity. As a standard practice, utilities in India conduct inspections and seek test reports from vendors at the time of accepting materials. However, these tests are executed only on a sample. Utilities need to gradually build technical and manpower capacity to undertake detailed stage testing of the entire consignment.
Proper sizing is another crucial factor as overloading of transformers is one of the leading causes of equipment failure. Experts prescribe that DTs should be operated at almost half of their rated capacity as this will not only reduce failures, but also reduce technical losses. To enable appropriate balancing of transformer load based on actual data, utilities across the country have increased their focus on improving DT metering. Progressive utilities are also deploying automated meter reading technologies such as data concentrator units in limited pockets to enable accurate data collection.
Meanwhile, condition monitoring of equipment is equally crucial for improving reliability. Most utilities have clearly defined maintenance practices and guidelines. Since oil-filled transformers are most commonly installed across India, utilities have developed oil-tracking maps to test for moisture content and acidity in transformer oil at least once a year. Some utilities also periodically check the partial discharge and infrared thermography of DTs to control failure rates.
Progressive utilities have also started relying on smart technologies to monitor DTs. For instance, TPDDL remotely monitors its transformers installed at 16 kV, 20 kV and 25 kV voltage levels through the supervisory control and data acquisition system. The utility has developed smart devices that capture various parameters like oil temperature, venting temperature, oil level, etc. and transfer the information to the control room via SMS. In another example, CESC has installed ring main units (RMUs), disconnectors, fuses, etc. in the system to ensure uninterrupted power supply in spite of DT failures. Going forward, some utilities are also considering installing self-protecting transformers.
Technology solutions
On the technology front, the failure rate of transformers can be reduced significantly through the installation of dry-type transformers. These transformers are maintenance-free as there is no accumulation of humidity on coils.
Procurement of dry-type transformers in India is limited to urban areas and used primarily for basement or indoor substations or within a building. This is because the Indian Standard regulations do not allow installation of oil-filled transformers inside buildings. Only in the case of urban utilities like CESC, the share of dry-type transformers in the total is higher than 20 per cent. Experts believe that the concerns about costs are myopic, as in the long run the higher costs are compensated through higher life expectancy, lower technical losses and increased reliability.
Meanwhile, utilities have also started using alternating fluid in oil-filled transformers on a pilot basis. Some utilities, like TPDDL, are using natural, ester-filled and hermetically sealed transformers, as well as exploring the option of transformers based on thermally upgraded papers and high temperature fibre-reinforced papers. The utility has also strated using 3D design transformers that offer significant advantages over normal transformers due to less joint core, low losses and compact size.
Standards and regulations
In India, various organisations are engaged in the notification and enforcement of standards and regulations for designing, constructing and installing transformers. The Bureau of Indian Standards (BIS) published the IS 1180 in 1989. In 2014, the standard was revised and IS 1180:2014 was introduced to take into account recent technological developments and minimum energy performance standards set under various regulations.
The Central Electricity Authority (CEA), playing the role of a regulator, has notified that adherence to BIS standards is compulsory. The Bureau of Energy Efficiency (BEE), on the other hand, has defined star labels for transformers corresponding to various levels of efficiency and losses. The existing regulations of the CEA and the BEE are currently being reviewed and revised.
These standards and specifications for transformers have become much more stringent over the years in order to ensure greater operational reliability. The basic considerations while setting the standards for transformers include the availability of indigenous materials and technology, research and development, coordination with other levels of standardisation, consultations involving all stakeholders and access to international technology.
Emerging requirements
Frequent DT failures are one of the key concerns of Indian distribution utilities. Improvements in transformer technology are thus being driven by the demand for efficiency of operations, without significantly adding to the cost of installation. With growing urbanisation, utilities are demanding transformers with more compact design; and ones that can handle higher loads while occupying limited space.
Utilities are looking for smart devices that can help accurately measure parameters such as current, voltage, temperature and oil level. so as to avoid overloading. Thus, the research emphasis for future DTs includes adding capabilities for voltage regulation, effective monitoring, reactive power support and communications.