Distribution transformers (DTs) are key assets for any distribution network. As per estimates, nearly 12-15 per cent of DTs fail annually and entail a maintenance cost of Rs 30 billion per annum. This has a huge financial impact on the utilities, besides disrupting power supply to consumers for an extended period. In fact, research indicates that transformers are the second largest loss-making equipment in electricity networks, after transmission lines. In comparison, the DT failure rate globally averages less than 1 per cent. This calls for a concerted effort from power utilities, transformer manufacturers and also those connected with standardisation and quality control for improving operations and maintenance (O&M) practices for better transformer quality and reliability.
A look at some of the best practices in O&M for transformers…
Approaches for transformer O&M
The best way to protect a transformer is to have a good preventive maintenance schedule. Various O&M measures have evolved based on past experiences, which can enable the utilities in enhancing the performance and life of transformers.
Preventive maintenance consists of regular planned inspections and component replacements based on the product-specific maintenance schedule. Meanwhile, condition-based monitoring facilitates the initiation of suitable actions based on the current health condition of the transformer. It dictates that maintenance should be performed only when certain parameters show signs of decreasing performance. There are essentially two methods for condition monitoring, external and insulation. While external monitoring involves the monitoring of parameters such as physical damages, oil leakage, secondary current and voltage, insulation monitoring keeps a check on insulation resistance, electrical discharges and dissolved gas analysis.
Proper handling, loading, unloading and storage of a transformer at a site before assembling also play an important role in the satisfactory operation during its life. Moreover, transformers should always be installed by an experienced technical team under the close supervision of manufacturers. Whenever there is movement of a transformer either from manufacturing works to a site or from one station to another, sweep frequency response analysis (SFRA) should be carried out before the movement of the transformer as well as after its shift to the new location. The SFRA provides information about any deformation in winding/core during the transportation of a transformer.
Dissolved gas analysis (DGA) is one of the most powerful tools that asset managers have to determine the health of their transformers. It is a cost-effective approach that can be used to detect problems in early stages and manage them as the condition evolves. For a transformer, that is operating normally and ageing at an expected rate without developing faults, the test could be done at the established routine intervals based on voltage class, power class and importance to the power system. If a transformer is exhibiting problems such as accelerated ageing or the development of fault conditions, a more frequent DGA or an online monitoring can be applied. Depending on the pattern of gassing, various other tests could be performed. Online DGA monitoring is often recommended for transformers in critical applications, which provide updated information every few hours, making the tracking of changing gassing pattern easier.
Partial discharge analysis is also extensively used for the condition assessment of transformer insulation, given that partial discharges are a major source of insulation failure in power transformers. Also, the majority of transformer failures are caused by a tap changer fault. Several systems for on-load tap changers (OLTCs) for online monitoring are currently available.
Leakage of oil from transformers is a common phenomenon and hence, the oil parameters need to be monitored regularly. It may lead to sludge formation and acidic content, and with the presence of moisture, the life expectancy of a transformer may be drastically affected. Compared to an oil-filled transformer, a dry transformer can withstand more load and has a higher reliability, with a better performance and a longer life.
Another reason for transformer failure is bushing failure. Bushings should be cleaned and inspected for any cracks. Dust and dirt deposition, salt or chemical deposition, cement or acid fume deposition should be carefully noted and rectified. As an alternative to regular cleaning, utilities may choose to protect them by providing a room temperature vulcanisation (RTV) coating. An RTV coating may also be considered by utilities for substation equipment installed in pollution-prone areas.
Another issue which needs attention is the storage of the transformer in case of delay in commissioning. In addition to adhering to the manufacturer’s recommendations, it should not be kept for more than three months with inert gas (nitrogen) filling. After three months, the transformer should be filled with oil under vacuum. It should also be provided with an oil conservator, including an oil level indicator and a breather. During this entire period, the required pressure needs to be maintained in order to avoid any exposure of the active part to the atmosphere.
Active repair approach
Another concept being adopted by utilities is active repair approach to reduce technical losses, thereby saving upon the power procurement costs, to improve energy efficiency of transformers as mandated by the Bureau of Energy Efficiency in PAT II cycle, as well as an increase in the transformer capacity.
The method to transform the transformer basically involves optimisation of the design of the repair unit to reduce the load loss, by replacing both the high voltage (HV) and low voltage (LV) windings by copper (Cu) windings. The use of Cu windings offers various advantages in the method, as it is an inherent low loss material. Further, special skills are not needed during jointing and termination of windings; this is an important benefit as a significant percentage of transformer failures can be attributed to defective joints and termination. It also scores over aluminium in several respects such as conductivity, resistance, thermal conductivity and better withstanding capability during short circuit. Easy availability and competitive pricing also favour Cu usage.
Madhya Pradesh Paschim Kshetra Vidyut Vitaran Company Limited has implemented this technique for 100 kVA and 200 kVA transformers. Following active repair, a significant full load loss reduction was observed. The load loss of the transformer reduced by almost 34 per cent after active repair with a 10 per cent kVA enhancement for the 100 kVA transformer, and 17 per cent kVA enhancement for the 200 kVA transformer. (The capacity of a 100 kVA transformer can be pushed to 130 kVA by using natural ester oil in place of the conventional thermal oil.)
As regards the financial benefits of the method, the payback period will depend on two factors: loading of the transformer and the choice between the average cost of procurement and average cost of supply. If the transformer is loading 70-80 per cent, then the payback is comparatively faster and if the loading is 50 per cent or lower, then the payback is longer. Similarly, the payback period will be longer if the average cost of procurement is chosen over the average cost of supply.
The method is highly useful in constrained capex situations. Further, the method can be used as an alternative to the business-as-usual scenario, where there is double drainage of cash, initially due to the high repair cost of the transformer, and then a high replacement cost of the transformer due to energy loss. In other words, active repair enhances the performance of the old legacy DT to a level of energy efficiency stipulated as per IS 1180, especially at higher loading conditions.
To achieve the desired level of performance and prevent failures, utilities need to follow best practices for the maintenance of transformers. Transformers that are approaching their end-of-service life need to be monitored more closely and action needs to be taken in advance for their replacement in a phased manner. To this end, a reduction of technical losses in DTs through active repair is a feasible method over the currently prevalent reactive repair methods. This proposed active repair enables DTs to bring down to no load or load losses proactively, while also being financially sustainable.