Transformers, used as compensating devices, are essential and expensive assets in a power delivery system. They play a key role not only in terms of investment but also in ensuring reliability, availability of cost-effective uninterrupted (24×7) quality power to all consumers, and smooth operation of the power system. With the expected growth of India’s power generation, transmission and distribution capacity, the need for transformers is bound to increase over the next few years. The reliability and availability of transformers plays an important role in the smooth operation of a power system.
With the objectives of standardising the specifications of power transformers; bringing out critical parameters that affect the quality, reliability, efficiency and cost of such assets; and incorporating the best design practices, state-of-the-art technology, quality control and testing requirements to ensure long and trouble-free service, the Central Electricity Authority recently came up with the “Standard Specifications and Technical Parameters for Transformers and Reactors (66 kV and above voltage class).
Power Line takes a look at some of the operations and maintenance (O&M) practices highlighted in the document…
Need for O&M practices
A key issue that requires industry attention is failure of transformers much before their expected lifespan of about 35 years. The causes for such failure include poor quality of raw material and manufacturing techniques; normal and abnormal stresses on the system during operation such as frequent system faults, overloading, environmental effects, unexpected continuous operating voltage and overvoltage; and poor maintenance practices. The failure of such vital equipment can have a significant economic impact due to its high cost, and long lead time in procurement, manufacturing and installation.
Transformer failure generally follows the bathtub curve. Failures in the initial years of service are mainly caused by manufacturing-related defects. But the continued successful operation of transformers primarily depends on the operating conditions and O&M practices followed by utilities. Improper maintenance or negligence on the part of users, including non-replenishment of saturated silica gel not releasing air trapped after air-cell commissioning, oil seepages, lack of routine maintenance, failure to check tan delta and capacitance of winding and bushing, absence of thermal scanning of terminal interfaces, and lack of dissolved gas analysis monitoring, can lead to serious repercussions on transformer performance. Hence, there is a need to ensure improved design, quality control during manufacturing, and use of the right components/accessories, and undertake proper O&M of these vital assets to increase their life expectancy. While there is no escape from the normal long-term ageing process, premature failure can be avoided by following these practices.
Key maintenance practices
Preventive maintenance is the key to keeping equipment healthy and in service, and is also the most cost-effective maintenance approach. The maintenance practices that utilities usually follow are conventional time-based maintenance (TBM), corrective maintenance (CM), condition-based maintenance (CBM) and reliability-centred maintenance (RCM). Different maintenance strategies have different impacts on the life-cycle cost of equipment.
TBM is based on the concept of preventive maintenance at predefined intervals. Unfortunately, as the maintenance interval is increased, equipment reliability is compromised. CBM offers an attractive option to overcome the shortcomings of traditional TBM. It helps provide information about the health of the equipment in advance for planning major maintenance/overhauls, reduces maintenance cost, defers capital and maintenance expenditure, reduces forced outages of equipment, improves the safety of operating personnel and the reliability and quality of supply for customers, provides valuable information aiding life assessment of the equipment for possible extension, and helps in the run-refurbish-replacement decision. CBM has become the accepted approach worldwide and has gained acceptance from Indian utilities.
Meanwhile, RCM is a structured process that identifies the effects of failures and defines the most technically and economically effective approach to maintenance. It is an optimised strategy that takes into account not only the operation time and/or the technical condition of an asset, but also its position in the network, its operational importance, any potential safety or environmental risk arising from its failure and any likely consequence of its potential outage. RCM may be applied to components either jointly or in isolation. RCM can not only improve the reliability of the system, but also significantly reduce the required maintenance, resulting in a significant reduction in O&M costs.
The extent of maintenance to be performed on a transformer is proportional to the level of risk associated with the unit. In practice, the criticality index is usually combined with a health index to prioritise maintenance activity. Further, monitoring activity can be continuous/ temporary, online or offline. Online measurements offer the advantages of continuous supervision and minimal errors.
Some operational practices
Experience has led to the evolution of various operational practices that can enable utilities to enhance the performance and life of transformers. In the case of liquid fillings for transformers, any leakage of traditional insulating oil must be contained to prevent contamination of the surrounding area. Leakage of oil from transformers is considered a serious quality lapse on the part of the original equipment manufacturer. The utility should monitor and conduct visual inspection of the transformer tank and other body parts such as pipes, flange joints in pipes, valves and stems, oil pumps, radiators, headers, screw joints, gasket joints, weld joints and air bleed plugs regularly to check for any rusting and leakage of oil. Moisture or humidity is a cause of deterioration of transformers. To deal with this, a dehydrating breather is fitted into the transformer to limit the entry of moisture into a free breathing tank.
Dissolved gas analysis is one of the most widely used diagnostic tools for detecting internal faults within oil-filled electrical equipment at an early stage and finding incipient faults such as partial discharge, overheating and arcing. The data obtained from this may give the type, severity, and sometimes, location of the fault. Frequency response analysis is a technique used to diagnose changes in the mechanical condition of a transformer by analysing the transformer winding’s frequency characteristics. Short circuit forces can cause winding movement and changes in winding inductance or capacitance in power transformers. Recording the frequency response to these changes provides information regarding the internal condition of the equipment.
To assess the remaining life of an equipment, paper condition, oil condition and equipment loading need to be assessed separately. In addition to this, furan content; degree of polymerisation; frequency domain spectrometry results; oil parameters such as inhibitor content, acidity and sludge; and temperature and loading profiles are helpful in predicting the residual life of the equipment.
Issues and challenges
The key challenge in maintaining transformer performance has largely been the inability of suppliers/manufacturers to maintain consistency in the material used and the manufacturing process, resulting in variation in the quality of transformers. The change in subvendors and skilled manpower from time to time also requires due diligence, to control and maintain the consistency of the manufacturing process. It is equally important that a transformer is manufactured in a clean, dust-free environment with humidity control. Any compromise on this aspect will have adverse effects on the expected life of the transformer. Besides, on site, the testing options are severely limited. Other issues with transformers pertain to transportation, handling, loading-unloading, erection, testing and commissioning, checks after receipt at site, storage at site, precautions during erection including oil filling, pre-commissioning checks/ tests, and final commissioning checks before energisation.
The way forward
The adoption of these uniform practices across utilities in the country would place all manufacturers on a level playing field and benefit indigenous manufacturers, thereby achieving the ultimate goal of “one nation one specification” in the overall interest of the power system. This initiative would simplify the procurement process, bringing in faster delivery, overall efficiency, quality and productivity in the entire value chain of transformer procurement and operation. Going forward, the adoption of such O&M practices on a regular basis, using reliable and calibrated testing instruments, will help ensure long and trouble-free transformer service.