Technology advancements and new regulatory requirements could reshape how power plant owners operate and maintain large power transformers. Power transformers are notably one of the most expensive and vital components in a power system. While they are typically reliable and relatively low maintenance, sudden failures can result in the loss of power generation, considerable expenditure and potentially wider implications for grid reliability. As such, it is desirable to perform timely maintenance activities to ensure that the transformer maintains a high level of performance and a long functional life. A power transformer requires various routine maintenance tasks such as measurement and testing of different parameters.
A look at some of the transformer operations and maintenance (O&M) practices…
The primary purpose of transformer maintenance is to ensure that the internal and external parts of the transformer and the accessories are kept in good condition and are able to operate safely at all times. A secondary and equally essential purpose is to maintain a historical record of the condition of the transformer.
A protective maintenance approach consists of preventive, predictive and corrective maintenance. Preventive maintenance involves schedule maintenance and regular testing, predictive maintenance involves extra monitoring and testing, whereas corrective maintenance involves repair and restoration. The aim of protective maintenance is to control and prevent serious oil and winding insulation deterioration.
Mineral oil and winding paper insulation are impacted by moisture, oxygen, heat and other agents such as copper, iron and electric stress. The end result is oxidation in the oil, which leads to transformer sludging. In sealed units, the ingress of moisture via the atmosphere or seal leaks needs to be stopped. Moisture decreases the dielectric strength of both oil and winding insulation systems. Also, transformer extra heating causes the winding insulation (paper) to decompose, which, in turn, creates moisture. Increased moisture created in the paper not only decreases the paper insulating strength but also, as temperature increases, migrates from the paper insulation to the oil and decreases its dielectric strength. The most important step is the design of transformers to keep the moisture and oxygen out. The next step is to ensure that transformers are not operated beyond their temperature ratings and limits. Also, the severity of deterioration needs to be controlled by periodically monitoring and testing transformer insulation, and taking mitigation measures to restore the transformer to its original condition.
Inspection has to be done at regular intervals and corrective actions need to be taken when necessary to ensure the satisfactory operation of the equipment. Like other electric devices, transformers need maintenance from time to time to ensure successful service. The frequency of inspection depends on the operating conditions. For clean, dry areas, inspection should be done annually. For other areas, especially where the air is polluted with dust or chemical fumes, an inspection at three or six month intervals would be needed. Typically, after the first few inspection periods, the final schedule can be made based on the existing conditions.
Once the transformer is de-energised inspections have to be made for dirt, especially accumulations on insulating surfaces or those that tend to prevent air flow, for loose connections, for assessing the condition of tap changers or terminal board, and for checking the general transformer condition. Observations have to be made for any signs of overheating and voltage creepage over insulating surfaces, as seen in carbonisation tracking. Signs of rust, corrosion and deterioration of paint need to be checked and mitigation measures taken if required. Fans, motors and other auxiliary equipment should be inspected and maintained properly.
The maintenance practices that utilities typically follow are conventional time-based maintenance (TBM), 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 is a viable 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 to assist in life assessment of the equipment for possible extension, and helps in making run-refurbish-replacement decisions. 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 can also reduce the required maintenance significantly, resulting in a reduction of O&M costs.
Challenges and the way forward
When the oil level of transformers is not maintained, which could result in a flashover, the oil is not filtered, and the breaker for LV protection is either removed or not provided at the time of installation, there are challenges in the O&M of transformers. Ineffective O&M is also caused by inadequate safeguards against overloads and short circuits, non-maintenance of the silica gel breather resulting in moisture intrusion, and a lack of accurate information regarding transformer loading. Other issues could include inadequate earthing of transformers, broken bushings, uneven loading on different phases, long LT lines, loose LT lines, and trees touching LT lines, all of which could lead to overload and frequent short circuits. Further, the non-provision of lightning arresters, improper HV protection, frequent blowing off of HG fuses used for HV protection due to improper fitting, and wind action can cause mechanical damage.
The challenges of maintaining transformers have evolved along with transformer design and construction. Modern transformers are designed to closer tolerances than transformers in the past. Thus, effective, regular maintenance and testing is even more essential to continue operation as traditional overdesign cannot be relied on to overcome abnormal conditions. Moreover, utilities are nowadays increasingly using the industrial internet of things and analytics to improve transformer longevity, reduce maintenance costs, and ultimately, ensure that critical assets have near-zero unplanned downtime.
To conclude, for a smooth transformer performance, the adoption of O&M practices on a regular basis, and reliable and calibrated testing instruments will help ensure long and trouble-free transformer service.