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.
• Frequency domain spectrograph: To determine the quantum of moisture in insulation• Acoustic emission techniques: To detect internal discharges• Sweep frequency response analysis: To determine if there is any distortion in winding• Dissolved gas analysis: To determine if there is any incipient fault inside the transformer• Checking of degree of polymerisation: To determine the ageing of insulation• Checking of the furan content in oil: To determine the ageing of insulation• Thermal imaging: To determine location of any hot spots on the transformer.While discussing about the innovations in condition monitoring, Mr Chandraker said, “There are a lot of innovations happening in transformer condition monitoring, such as intelligent and adaptive cooling systems based on transformer temperature condition and loading conditions, early warning system based on condition monitoring (i.e. detection of typical fault gases, most preferable if online), online DGA (Dissolved Gas Analysis) from gas samples in Bucholz to know the nature of developing fault conditions. Intelligent and online loss-of-life calculation, reduced moisture ingress effect with intelligent breather heating system (for silica gel activation) based on silica gel condition, vibration monitoring at all crucial core bolt joints based on condition of joints and core laminations, comprehensive online analysis on circulating and leakage currents (between core sections, body etc.).”According to Mr Shrivastava, research is in progress to ascertain the location of fault after proper interpretation and analysis of the data.International standards of condition monitoring techniquesIndia follows  international standards of condition monitoring techniques. According to Mr Chandraker, online DGA and comprehensive transformer operational parameters monitoring are the latest global condition monitoring practices and available in India.In India, several manufacturers have set up process for condition monitoring tests. Several agencies also dedicated to carry out the tests. Mr Mitra explained, “The equipment available with the manufacturers and independent testing agencies are in general up to international standards. Many of the special equipment are imported from reputed manufacturers worldwide. The facilities and the expertise available with EMCO are comparable to the best available in the world. We use the best instruments and equipment available internationally.”Mr Shrivastava also believes that the condition monitoring techniques and equipment of international standards are available in India with major utilities.Condition monitoring: a lengthy processFew years back, some of the condition monitoring techniques — especially DGS — were offline. This often needed a detailed laboratory analysis.Based on the failure nature in transformers — winding failure, core heating, bushing failure, fire and tap changer failure — it’s important to minimise hazardous conditions in transformer. According to Mr Chandraker, “The stress can be minimised by management of loading, voltage levels, cooling and protection system. Transformer specification should be written in better way to get online DGA, prime grade of CRGO, SCADA enabled protection devices etc. Also, the specification should demand for latest manufacturing and design practices to prevent core-bolt failure, core heating and higher hotspot in winding. Fire hazards also can be minimised with ester oil as a substitute of mineral oil which is a new trend now.”Mr Mitra explained, “Contrary to popular belief, the condition monitoring tests are not lengthy process. Most of the offline tests can be done within a day’s shutdown. Only for the measurement of the degree of polymerisation of paper, paper samples have to be taken from inside the transformer. This involves the draining of oil, taking the sample and refilling and filtration. This is a lengthy process.”However, the furan content in oil can be checked to get an estimation of the degradation of the insulation.

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