TRANSFORMER AGEING

TRANSFORMER AGEINGSTEP-BY-STEP GUIDE TO PROLONG THE LIFE OF TRANSFORMERSBy monitoring the condition of transformer, the ageing of transformer can be assessed, and suitable remedy can reduce the rate of ageing, prolonging the life of transformerTransformer, high-efficient static electrical device that is used for power transfer from one voltage level to the other, plays a vital role in electrical transmission and distribution system. From the day a transformer is put in service; different electrical, mechanical, chemical, and environmental factors affect it’s performance. In initial stages, degradation of insulation quality occurs slowly, but this deterioration multiplies in due course of time and can lead to failure of the transformer. The primary reasons of transformer failure is directly related to poor manufacturing process and second- grade cold rolled grain oriented (CRGO) sheet steel. To counter these, transformer users should follow certification process to ensure use of primary quality of CRGO sheet steel for a longer life of their transformers.In addition, continuous monitoring and preventive measures should be taken to have trouble-free and continuous running of the transformers. Condition monitoring is a diagnostic process to reduce the rate of ageing, extend the life of transformers and enhance their ability to perform better for longer time.Benefits of transformer condition monitoringThere are numerous benefits of transformer condition monitoring, such as confirmation of fault, provision of obtaining quality control features, and increment of the system stability.It also helps in reducing the rate of ageing, avoiding catastrophic failures, designing more predictable and reliable maintenance schedules, improving overload capacities, and reducing unplanned outages.“The insulation of the transformer mainly consists of cellulose,” said Shantanu Mitra, Head, Service Vertical Transformer Business Unit, EMCO Ltd. “Due to the effect of moisture and temperature, the cellulose chain tends to break down, resulting in a loss of insulating properties. This is normal ageing.”Whereas, Prakash Chandraker, Managing Director, Schneider Electric Infrastructure Ltd., said, “Online condition monitoring helps in knowing about actual health of transformer and ageing behaviour to avoid unexpected outages and  failures.” Ganga P. Shrivastava, Senior General Manager (Works) Transfomers, Bharat Bijlee Ltd., said, “Condition monitoring helps implement reliability driven maintenance strategy to maximise the performance of the transformer at lowest life cycle cost, optimise transformer replacement strategy, and eliminate or reduce catastrophic failure. However, for practicing electrical engineers, implementing condition monitoring is difficult and critical task. Despite numerous studies and many publication of the subject, it always poses challenge to decide when and how to use the various tools to their full potential for early fault detection and corrective action. In condition monitoring, a parameter is measured off and online continuingly. The interpretation and analysis of these data always need expertise on the subject to decide proper corrective or preventive measure.”Tools available for transformer condition monitoringThe major tools available for the condition monitoring of the transformer are dissolved gas analysis of transformer oil, recovery voltage measurement, sweep frequency analysis, partial discharge measurement, direct measurement of oil and winding temperature using fibre optic sensor, measurement of DP of paper insulation and furfural content of oil, tan delta and capacitance measurement, online condition monitoring of bushing, infrared thermography to detect hotspot, and moisture content in oil.There are also various sensors which measure the following information:• Main information: The main information obtained through basic sensors are operating voltage, load current, apparent power and load factor, oil temperature, hotspot temperature, gas-in-oil content, moisture-in-oil content, ageing rate, on-load tap changer position, number of switching operations, sum of switched load current, operating condition of pumps and fans, operating time of pumps and fans, cooling efficiency (RTH), ambient temperature, and auxiliary digital inputs.• Insulation information: These are gas-in-oil gradient, moisture of paper, bubbling temperature and safety margin, predicted top oil temperature, predicted hot spot temperature, and breakdown voltage.• Bushings information: Basic sensors also provides several bushings information like operating voltages, over voltages, number of over voltages, change of capacitance, and online capacitance.• Tap changer information: Tap changer information through basic sensors power consumption of the motor drive, OLTC oil temperature, assessment of mechanical quality, contact wear model, and oil temperature difference OLTC and tank.• Overload information: These are load currents, over currents and short-circuit currents, overload capacity, emergency overloading time, thermal model, and actual losses.Condition monitoring helps implement reliability driven maintenance strategy to maximise the performance of the transformer at lowest life cycle cost, optimise transformer  replacement strategy, and eliminate or reduce catastrophic failure.Innovations in condition monitoringAs the transformer is one of the costliest equipment in the substation, there is greater focus in assessing the condition as well as life enhancement of transformer. Globally, there are many new methodologies being adopted, and better equipment are being developed to get a better picture of the condition of transformer.

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